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Diamond growth by chemical vapour deposition

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This paper reviews the growth of diamond by chemical vapour deposition (CVD). It includes the following seven parts: (1) Properties of diamond: this part briefly introduces the unique properties of diamond and their origin and lists some of the most common diamond applications. (2) Growth of diamond by CVD: this part reviews the history and the methods of growing CVD diamond. (3) Mechanisms of CVD diamond growth: this part discusses the current understanding on the growth of metastable diamond from the vapour phase. (4) Characterization of CVD diamond: we discuss the two most common techniques, Raman and XRD, which have been intensively employed for characterizing CVD diamond. (5) CVD diamond growth characteristics: this part demonstrates the characteristics of diamond nucleation and growth on various types of substrate materials. (6) Nanocrystalline diamond: in this section, we present an introduction to the growth mechanisms of nanocrystalline diamond and discuss their Raman features.This paper provides necessary information for those who are starting to work in the field of CVD diamond, as well as for those who need a relatively complete picture of the growth of CVD diamond.
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Table of Contents
1 Properties and applications of Diamond................................................................................. 3
1.1 CVD diamond applications.............................................................................................4
2 Growth of Diamond by Chemical Vapour Deposition...........................................................6
2.1 Development in Diamond Synthesis by CVD ................................................................ 6
2.2 CVD systems .................................................................................................................. 8
2.3 Filament-assisted thermal CVD.................................................................................... 10
2.4 Plasma-enhanced CVD methods................................................................................... 11
2.5 Combustion-flame-assisted CVD .................................................................................12
2.6 DC plasma jet CVD......................................................................................................13
3 Mechanisms of CVD Diamond Growth ............................................................................... 14
3.1 The gas-phase chemical environment........................................................................... 17
3.1.1 Atomic hydrogen. ................................................................................................. 17
3.1.2 Hydrocarbon chemistry......................................................................................... 19
3.1.3 Effect of oxygen addition...................................................................................... 20
3.2 The growth species and growth mechanisms................................................................ 21
3.3 Diamond doping............................................................................................................ 22
4 CVD Diamond Characterization Techniques .......................................................................23
4.1 Raman spectroscopy .....................................................................................................23
4.2 X-ray diffraction ........................................................................................................... 25
5 Heteroepitaxial CVD Diamond Growth Characteristics....................................................... 26
5.1 Effect of substrate pre-treatment on diamond nucleation............................................. 26
5.2 Effect of deposition parameters on diamond nucleation and growth ........................... 28
5.3 Substrate materials for CVD diamond films................................................................. 29
5.3.1 Materials with little or no carbon solubility.......................................................... 29
5.3.2 Materials with strong carbon dissolving and weak carbide formation ................. 30
5.3.3 Materials with strong carbide formation...............................................................31
6 Nanocrystalline Diamond .....................................................................................................33
6.1 Nanocrystalline and ultrananocrystalline diamond film growth................................... 34
6.1.1 NCD film growth.................................................................................................. 34
6.1.2 UNCD films growth.............................................................................................. 35
6.2 Raman spectroscopy of NCD and UNCD films ........................................................... 36
6.3 NCD and UNCD applications....................................................................................... 36
7 Summary............................................................................................................................... 37
8 Acknowledgement ................................................................................................................38
9 References.............................................................................................................................39
10 Figure Captions................................................................................................................. 47
11 Table Captions..................................................................................................................49
peer-00597830, version 1 - 2 Jun 2011
Author manuscript, published in "Journal of Physics D: Applied Physics 43, 37 (2010) 374017"
DOI : 10.1088/0022-3727/43/37/374017
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Diamond growth by chemical vapour deposition
J.J. Gracio*, Q. H. Fan and J.C. Madaleno
Nanotechnology Research Division (NRD), Centre for Mechanical Technology and Automation,
University of Aveiro, 3810-193, Aveiro, Portugal
* Author for correspondence; electronic mail: jgracio@ua.pt
Abstract
This paper reviews the growth of diamond by chemical vapour deposition (CVD). It includes the
following seven parts. 1. Properties of diamond. This part briefly introduces the unique properties
of diamond and their origin and lists some of the most common diamond applications. 2. Growth
of diamond by chemical vapour deposition. This part reviews the history and the methods of
growing CVD diamond. 3. Mechanisms of CVD diamond growth. This part discusses current
understanding on the growth of metastable diamond from vapour phase. 4. Characterization of
CVD diamond. We discuss two most common techniques, Raman and XRD, which have been
intensively employed for characterizing CVD diamond. 5. CVD diamond growth characteristics.
This part demonstrates the characteristics of diamond nucleation and growth on various types of
substrate materials. 6. Nanocrystalline diamond. In this section, we present an introduction to the
growth mechanisms of nanocrystalline diamond and discuss their Raman features.
This paper provides necessary information for those who are starting to work in the field of CVD
diamond and as well as for those who need a relatively complete picture on the growth of CVD
diamond.
Key words: diamond film, chemical vapour deposition, nanocrystalline diamond
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1 Properties and applications of Diamond
A diamond crystal consists of carbon atoms tetrahedrally bonded with sp
3
hybrid bonds. It has a
body centred cubic structure as shown in Fig. 1. Choose any characteristic property of a material
structural, electrical, optical or mechanical – the value associated with diamond represents almost always
an extremist position among all materials considered for that property. Table 1 lists some of the
remarkable properties of diamond. All the exceptional properties of diamond arise from two basic facts:
(1). Carbon atoms are relatively small and light, with short range bonds in the diamond
structure.
(2). When the carbon atoms bind together with the diamond structure, they form very strong
covalent bonds.
Fig. 1.
Table 1
Due to the high strength of the covalent bond (347 kJ/mole) a lot of energy is required to remove
a carbon atom from the diamond lattice, making diamond very hard and abrasion-resistant. In fact,
diamond can only be scratched by another diamond or polished by diamond powder. In normal wear the
surface of a polished diamond is not scratched by contact with other materials, although the edges can be
chipped because diamond is quite brittle.
The atoms in the lattice vibrate according to the normal mode of vibration of the crystal. The
vibration frequency is proportional to the restoring force and inversely proportional to the mass of the
vibrating atoms. Because the atoms in diamond are both strongly bonded and light, they can vibrate at
unusually high frequencies. The maximum frequency is about 40×10
12
Hz, compared with 16×10
12
Hz of
silicon. The high frequency of vibration results in the diamond fast heat conduction, which is even faster
than in metals: at room temperature diamond conducts heat 4 times better than copper.
It is known that vibration quanta or phonons can be created above the fundamental vibration level
in a crystal by heat or by infrared radiation. In diamond, photon absorption with the creation of only one
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phonon is forbidden by the inversion symmetry of diamond lattice [1]. Absorption with the creation of
two phonons is observed at photon energies up to about 2665 cm
-1
approximately twice the maximum
frequency of lattice vibrations [2]. This leads to a high transparency of diamond to most infrared radiation
it is only slightly opaque in mid-infrared wavelength between 4-5 µm, making diamond a promising
material for window and detector applications.
Due to the large band gap, about 5.47 eV are required to excite an electron from the valence band
to the conduction band, compared with 1.1 eV for Si and 0.7 eV for Ge. Diamond is therefore a wide
bandgap material, since 5.47 eV is much higher than the thermal energy k
B
T~0.025 eV (where k
B
is
Boltzmann’s constant and T is the absolute temperature) and the probability of thermal excitation of
electrons from the valence band to the conduction band is negligible at room temperature. The electric
breakdown field is correspondingly high: 2×10
7
V/cm for diamond, and only 3×10
5
V/cm for Si and
4×10
5
V/cm for GaAs.
While diamond possesses a wide bandgap, the band structure calculation and experiments
demonstrate that the vacuum level lies below the conduction band and diamond is known to show
negative electron affinity (NEA) [3]. This allows the diamond surface to emit electrons to the vacuum
with a low applied electric field and opens the door for the use of diamond for cold cathodes and field-
emitter displays.
1.1 CVD diamond applications
Even though natural diamonds have been known for several thousand years, they are mainly used
as gemstones and as abrasives and cutting tools. In fact, bulk diamond cannot be effectively engineered
into the many physical configurations required to exploit all desired combinations of its properties. The
development in the synthesis of diamond by chemical vapour deposition (CVD) lead to the ability of
growing diamond in the form of thin films or coating on a variety of shapes with controlled grain size and
enabled the exploitation of more combinations of the extreme properties of diamond for specific
applications. Table 2 lists the properties and application or possible application areas of CVD diamond.
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Table 2
Tribological applications come naturally as one of the main and well-established diamond
application areas. Diamond-based tools can be divided into two main areas: (i) tools fabricated using
conventional powder metallurgy techniques, where diamond particles are mixed and sintered together
with a powder metal matrix and (ii) CVD diamond coated tools, where a thin diamond film is deposited
on the tool surface. Commercially available diamond-based tools include drill bits, plates, die blanks,
grinding and cut-off wheels, blades, chain saws and circular concrete saws (www.idr-online.com).
Diamond-coated tools take advantage of diamond’s high wear resistance and chemical inertness and can
be used for ultra-precision- and micro-machining of nonferrous materials and metal matrix composites,
whenever high tolerances and surface finishes are required [4]. In addition, diamond’s high thermal
conductivity and thermal oxidation resistance make it a suitable material for high speed and dry
machining [5] and for the moulding industry [6]. Diamond’s wear resistance, together with the range of
high transmittance, enabled the use of various diamond coatings on wear-resistant windows [7], UV
mirrors [8], IR [9], microwave [10], RF [11] and optical transmission windows [12] and waveguides [13].
Diamond is a wide bandgap material with a large breakdown field and a high hole mobility,
characteristics that make it a promising material for high power and high frequency electronic
applications. Several diamond-based devices have already been developed, such as transistors operating
in the microwave range [14], high-temperature diodes [15], thermistors [16] and transistors [17], laser
windows [18] and solid-state detectors [19]. Diamond field emitters and cold cathodes take also
advantage of the NEA [20]. The excellent thermal properties of diamond are the basis of heat-spreading
films for RF devices thermal management [21] and, more recently, diamond packages for high power
lasers [22]. Taking advantage of diamond high stiffness and corresponding high sound velocity, diamond-
based surface acoustic wave (SAW) devices [23], micromechanical oscillators [24] and tweeter
components [25] have also been successfully developed and commercialized.
In the field of electrochemistry, diamond has again proven to be a unique material. The chemical
inertness of CVD diamond makes it extremely resistant to oxidation and attack by acids, even at high
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temperatures, and boron doped diamond electrodes have been successfully used in electroanalysis [26]
and bulk oxidation of dissolved species in solutions [27]. Diamond electrodes for electrochemical water
treatment are a commercial product readily supplied by several organizations, such as CONDIAS and
CSEM. These electrodes take advantage of the stability and the chemical inertness of the diamond surface
and of the large potentials that can be applied before the water electrolysis takes place.
2 Growth of Diamond by Chemical Vapour Deposition
Deposition of diamond by Chemical Vapour Deposition (CVD) has been largely studied by
different research groups worldwide since the early 1980s. This technique involves the deposition of
carbon atoms that originate from the dissociation of a carbon-containing gas precursor on a solid substrate.
This substrate can be either bulk diamond (either natural or synthetic) or a non-diamond substrate. In the
first case, the resulting films are said to be homoepitaxial or singlecrystalline, in opposition to
heteroepitaxial or polycrystalline films in the latter case. The deposition of diamond on a foreign substrate
usually requires an extra nucleation step, since diamond does not grow spontaneously on non-diamond
materials. This step provides the non-diamond substrate with the necessary diamond seeds that grow
three-dimensionally until the grains coalesce, forming a continuous polycrystalline film with average
grain size increasing with increasing film thickness (van der Drift growth) [28]. The resulting films are
poly, nano or ultra-crystalline, depending on the average grain size, and their macroscopic properties
depend on parameters such as grain size or sp
3
/ non-sp
3
ratio.
This section will outline the development of diamond deposition by CVD and discuss the
technological challenges, issues and achievements of depositing diamond films on foreign substrates.
2.1 Development in Diamond Synthesis by CVD
Artificial diamond synthesis by the high-pressure high-temperature (HPHT) method was first
reported in 1955 by General Electric [29]. With this conceptually simple method, graphite is converted to
diamond by means of applying proper temperature and pressure conditions. At the same time, research
was being made in order to deposit diamond from the gas phase. The first documented report of diamond
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growth at low pressure was that of W. Eversole in 1954 [30]. He conducted a wide range of sophisticated
experiments based on the following reactions:
2CO = C
(diamond)
+ CO
2
, (1)
2CO = C
(graphite)
+ CO
2
(2)
which are thermodynamically possible due to the negative Gibbs free energy. In these experiments, either
diamond or graphite was used as seeds. The experiments were conducted at temperatures from 820 to
1007 ºC and pressures from 5×10
6
to 31×10
6
Pa. It was found that the measured equilibrium ratio CO/CO
2
over diamond (reaction (1)) was different from that over graphite (reaction (2)), which was expected from
thermodynamic data, as the equilibrium is different for C
diamond
and C
graphite
. Furthermore, he found that the
equilibrium and the precipitation of diamond on the diamond seed was metastable. No graphite was
detected, although under these process conditions (pressure and temperature) graphite was the
thermodynamically stable modification. The typical growth rate of diamond over the diamond particles
was in the order of 23% per hour.
In 1956, the former U.S.S.R. scientists B. Spitsyn and B. Deryagin proposed the growth of
diamond at low pressures through the thermal decomposition of carbon tetraiodide [31]. Diamonds were
synthesized by using CBr
4
or CI
4
at temperatures ranging from 800 to 1000 °C and pressures of
approximately 4×10
-4
Pa.
The Soviet group subsequently explored direct CVD from hydrocarbons and in 1969 it was stated
that diamond was synthesized from pure methane at pressures from 13 to 40 Pa and temperatures from
950 to 1050 °C with a growth rate about an order higher than that reported by Eversole.
A major breakthrough on the CVD diamond process was achieved in the early 1970s. It was the
use of atomic hydrogen during growth to remove graphite co-deposits, based on the fact that atomic
hydrogen etches graphite much faster than it does diamond. This gave a much higher growth rate and, of
equal importance, it permitted the nucleation of new diamond crystallites on non-diamond substrates. The
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use of atomic hydrogen was independently pursued by J. Angus in the U.S.A. and by V. Varnin in the
U.S.S.R. [32].
The modern era of CVD diamond started in the early 1980s. A Japanese group, the National
Institute for Research in Inorganic Materials (NIRIM), first published a remarkable series of papers in
which different techniques, hot-filament CVD process, RF-plasma CVD, and microwave plasma CVD
were described [33-36]. They reported that diamond particles and films could be deposited on various
substrates heated around 850 °C, using a mixed gas of methane diluted by hydrogen, and preferred partial
pressures in the range 4×10
3
to 5×10
3
Pa. A growth rate higher than several microns per hour was
achieved. These results were soon confirmed by several research groups in the U.S.A. and Europe [30-32].
The success of the NIRIM group was almost immediately spread and spawned numerous research
programs in the world. These programs included process techniques, understanding the mechanism of
CVD diamond nucleation and growth, diamond doping, investigation of optical, electronic, thermal,
mechanical properties of CVD diamond, diamond coating on various substrates for specific applications,
characterization of CVD diamond, etc.
2.2 CVD systems
The growth of diamond films from vapour phase on non-diamond substrates at practical rates was
accomplished with the development of thermal- and plasma-enhanced CVD methods, in which a
hydrocarbon gas (usually methane) mixed in low concentrations with hydrogen is energized thermally or
in a plasma, prior to contact with a heated substrate. The first of these enhanced CVD methods was the
chemical transport reaction synthesis developed by Soviet workers in the late 1970s [37,[38]. From that
time until the late 1980s, virtually all of the significant developments reported have been due to Japanese
work. These include the development of filament-assisted thermal CVD [33,34], electron-assisted thermal
CVD [39,40], laser-assisted thermal CVD [41], RF-plasma CVD [42], microwave-plasma CVD [35],
combustion flame-assisted CVD [43], direct-current arc plasma jet CVD [44], etc.
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The various enhanced CVD methods, although different in their process details, have a number of
common features, the most important of which are the following:
(1). Growth in the presence of atomic hydrogen
The deposition of polycrystalline diamond films from carbon containing species in the presence
of atomic hydrogen was based on the realization that diamond is more stable towards atomic hydrogen
than graphite. More specifically, if two neighbours of a carbon atom in the diamond structure are replaced
by hydrogen, the sp
3
hybridization is still maintained, while a similar operation in graphite alters the
electronic bonds in the whole graphite ring. Thus, growth of diamond from carbon containing molecules
diluted in hydrogen involves two processes. The first is carbon deposition primarily in the form of
graphite with a small amount of diamond, and the second is selective etching of graphite by atomic
hydrogen. As a result, the various enhanced CVD methods for the growth of diamond are optimized to
produce atomic hydrogen from molecular hydrogen close to the surface of the growing film.
(2). Dissociation of carbon-containing source gases.
In a simple thermal CVD process, the diamond growth rate is very low (<0.1 µm/h). This is due
to the rather high activation energy needed for the decomposition of methane on the surface of diamond
(230-243 kJ/mol). Thus, in the various enhanced CVD methods, the carbon-containing compounds are
dissociated by thermal, plasma or combustion processes to produce the reactant species responsible for
the diamond nucleation and growth. The film growth rate depends on the ability of these reactant species
to be transported to the substrate.
(3). Growth at moderate substrate temperatures
In all enhanced CVD methods, diamond growth takes place at substrate temperatures between
500 and 1200 °C. Growth at temperatures above or below this range often leads to graphite or diamond-
like-carbon (DLC) deposits, respectively. In the following section, we briefly introduce several CVD
methods which are commonly used for the growth of diamond films.
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2.3 Filament-assisted thermal CVD
A schematic diagram of this deposition method is shown in Fig. 2, and Table 3 outlines the
deposition parameters range reported by various works. In this method, diamond particles or films are
deposited on a heated substrate from a mixture of methane and hydrogen dissociated by hot tungsten or
other high-melting-point metal filament placed close to the substrate. The filament temperature may reach
around 2200 ºC during this process. The main role of the hot-filament is to dissociate molecular hydrogen
into atomic hydrogen. At higher pressure (3 4 kPa) the dissociation mainly occurs near the filament
(without adsorption and desorption) due to the high gas temperature. At lower pressure the filament
surface acts as a catalyst for adsorption of molecular hydrogen and desorption of atomic hydrogen. The
dissociation equilibrium is governed by thermodynamics. Detailed plasma studies have been conducted in
order to elucidate the diamond growth mechanism, revealing the presence of different species in the
plasma, such as neutral molecules, ions and electrons [45-47].
Fig. 2.
Table 3
Besides the original role of the tungsten filament, it was discovered that during diamond
deposition the tungsten filament reacts with methane and undergoes carburization. This results in
consumption of carbon from the methane, and thus a specific incubation time is needed for the nucleation
of diamond films. Therefore, this process may affect the early stages of thin film growth. In addition, the
resistance of the filament should be monitored and adjustments to the supplied voltage and current made
in order to maintain the temperature of the filament constant.
Due to the temperature upper limit of the filament material, hot-filament processes operate at
significantly lower gas temperatures than plasma processes, and consequently produce less atomic
hydrogen. The low gas phase concentrations give relatively low growth rates compared to the plasma
methods. Despite these drawbacks, hot-filament assisted deposition has remained popular because of its
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low capital cost and simplicity. Also, hot-filament reactors are directly scalable to large sizes and can be
used to coat complex shapes and internal surfaces.
2.4 Plasma-enhanced CVD methods
Alternative methods of diamond growth involve various forms of plasma-assisted CVD using
carbon-containing species mixed in low concentration with hydrogen. Plasma is generated either by
various forms of electrical discharges or by induction heating. The role of the plasma is to generate
atomic hydrogen and to produce proper carbon precursors for the growth of diamond. Atomic hydrogen is
produced by electron impact dissociation of molecular hydrogen. Although the binding energy of
molecular hydrogen is 4.5 eV, electron energies in excess of 9.5 eV are required due to the mass
difference between electrons and molecular hydrogen. In fact, such dissociation of molecular hydrogen
peaks at electron energies of 25 eV [48]. Thus, atomic hydrogen produced in plasmas generally has high
kinetic energy due to the difference between the hydrogen dissociation energy and the electron kinetic
energy. This is to be contrasted with the thermal-assisted CVD process, where the produced atomic
hydrogen has low kinetic energy.
Similarly, electron impact dissociation processes are responsible for the formation of carbon-
containing neutral and ionic radicals. In general, 1% of the molecules in plasma are converted into neutral
radicals and about 0.01% into ions. Neutral molecules, for example, CH
4
, usually do not participate in the
growth of diamond due to a relatively high Gibbs free energy in the process of their decomposition. Thus,
the growth rate of the diamond film is mainly determined by the concentration of neutral radicals. The
effect of the ionized radicals is not clear. Although their contribution to the growth rate is minimal, one
can not rule out ion-assisted processes. For example, Stoner et al. reported an enhancement in the
diamond nucleation density by microwave plasma-assisted CVD on negatively biased silicon
substrates [49].
The absolute concentration of atomic hydrogen and neutral radicals depends on the pressure of
the plasma. In low-pressure plasma, the electrons acquire high kinetic energies from the electric field;
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however, due to the high mean free path, they do not transfer much energy to molecular species. As a
result, the gas temperature is relatively low and thus atomic hydrogen and neutral radicals are produced in
low concentrations by collisions with high-energy electrons only. This is to be contrasted with high-
pressure plasmas, where, due to the small electron mean free path, the gas and electron temperatures are
about the same. Thus, the concentration of atomic hydrogen and neutral radicals is much higher, since
both electron and molecular collisions contribute to their formation. This accounts for the significantly
higher growth rates reported in high-pressure plasmas.
Among the plasma-enhanced methods, microwave-plasma-assisted CVD has been used much
more extensively than others for the growth of diamond films. Fig. 3 demonstrates a typical microwave-
assisted CVD system.
Fig. 3.
This method of diamond growth has a number of distinct advantages over the other methods.
Microwave deposition, being an electrodeless process, avoids contamination of the films due to electrode
erosion. Furthermore, the microwave discharge at 2.45 GHz, being a higher frequency process than the
RF discharges at 13.5 MHz, produces higher plasma density with higher energy electrons. This should
result in higher concentrations of atomic hydrogen and hydrocarbon radicals. An additional advantage is
that the plasma is confined in the centre of the deposition chamber in the form of a ball and this prevents
carbon deposition on the walls of the chamber.
2.5 Combustion-flame-assisted CVD
This method allows the growth of diamond at atmospheric pressures using combustion flames
from an oxygen-acetylene brazing torch. Because of its simplicity and low cost of the experimental
apparatus as well as the high growth rate, this method has been widely used in diamond growth.
A schematic diagram of the apparatus is shown in Fig 4a [30]. It consists of an oxygen-acetylene
brazing torch supplied with oxygen and acetylene controlled by a mass flow system and a water-cooled
substrate. The substrate temperature is adjusted by varying the substrate surface position relative to the
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water cooled copper mount and its temperature is measured by a two-colour pyrometer which is
insensitive to the flame’s emission. Hydrogen addition to the oxygen-acetylene flame was found to reduce
the amount of amorphous carbon in the diamond films [50]. Normally the oxygen acetylene torch is used
with an excess of oxygen. The acetylene is completely burned to CO
2
and H
2
O. With oxygen depletion,
deposition of carbon (normally graphite) occurs. There is a very distinct concentration ratio of C
2
H
2
/O
2
where this change from carbon deposition and no deposition occurs.
Fig. 4.
Combustion flames operating in the fuel-rich mode have three distinct regions, as shown in Fig.
4b. The inner cone is the primary combustion zone followed by a diffused intermediate region and an
outer zone. The temperature in the primary combustion zone can reach up to 3300 K. The main
combustion reaction at this region leads to the formation of CO and H
2
(eq. (3)):
C
2
H
2
+ O
2
2CO + H
2
(3)
with a number of reactive intermediates (H, OH, C
2
, C
2
H, etc.). In the fuel-rich mode, the un-burnt
hydrocarbons as well as the combustion products form the diffuse intermediate and reaction free region
(called feather). The outer zone, which is also known as a secondary combustion zone, consists of a flame
caused by molecular or turbulent diffusion of oxygen from the surrounding atmosphere. In this region, the
products of the combustion reaction are oxidised to CO
2
and H
2
O. The substrate is usually placed in the
feather region of the flame, where there is an abundance of atomic hydrogen and hydrocarbon radicals as
revealed by in-situ diagnostic studies [51].
2.6 DC plasma jet CVD
A plasma jet, or “arcjet”, is a generic expression for a high pressure direct-current plasma
discharge in which convection plays a significant role in transport processes. Fig. 5 presents a typical
arcjet used as a source for diamond CVD [30].
Fig. 5
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Electrical energy is converted to thermal and kinetic energy of a flowing gas mixture by an
electric arc discharge. Like other CVD methods, a major constituent of the gas mixture is hydrogen, while
methane is most often introduced into the plasma jet to provide a source of carbon. The plasma
temperature is sufficiently high (1000-5000 K on average) to partially dissociate the gas. The plasma jet
containing these reactive species impinges onto a cooled substrate surface (T
s
~1000-1500 K) for film
growth. Normally, the electric discharge is sustained between a concentric cathode rod and a surrounding
cylindrical anode, creating an arc column that is nearly fully-ionized. Positive current flows from the
anode to the cathode, establishing the voltage drop required to dissipate the total power in the arc.
Extremely high diamond growth rates, ~1 mm/hour, have been achieved using DC arcjet CVD
method [52]. In addition, arcjets have a distinct advantage over other diamond CVD methods in that the
deposition of diamond can proceed simultaneously with the deposition of other ceramics and metals by
introducing powders of various types into the plasma stream. Using this approach, Kurihara et al. [53]
deposited well adherent diamond films on tungsten-molybdenum substrates by spraying an interlayer
composed of tungsten carbide, followed by a composite diamond-tungsten-carbide intermediate layer.
Such a functional gradient material reduced the thermal stress between the diamond film and the substrate
and increased the adhesion strength by an order of magnitude over that which was obtained in the absence
of the tungsten-carbide interlayer.
As a summary, Table 4 provides the basic features of the most commonly used methods for
diamond CVD at low pressure.
Table 4
3 Mechanisms of CVD Diamond Growth
The complex chemical processes occurring during the CVD diamond are fascinating and exciting
from many perspectives. First, how does one understand the process of growing a material under
conditions in which it is metastable, e.g. what are the complex gas phase, surface and bulk chemical
processes which lead to diamond vs. graphite or amorphous carbon?
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Secondly, this process deals with many fundamental aspects of CVD, such as gas phase chemistry,
complex heat and mass transport, nucleation, surface chemistry, bulk chemistry and diffusion, and
temporal dynamics as shown in Fig. 6. Understanding the effects of a combination of these features is full
of challenge. And finally, the technological impact and applications enabled by diamond materials are
important in many ways for society and the economy. Industry is already using CVD diamond in
applications such as cutting tools, electronic thermal management, optical windows and radiation
detectors, as has been introduced before.
Fig. 6
The growth of a solid material from gaseous reactants is fundamentally a surface chemical
process which depends on the flux of reactant species to the surface, and products from it, as well as the
surface structure and temperature. For diamond CVD, some general features are summarized as follows.
(1). Gas dissociation.
It is essentially required for achieving appreciable diamond growth rates. Dissociating the gas
prior to deposition increases diamond growth rates from Å/h to µm/h. Electric discharge, microwave, RF,
DC, hot (>2000 °C) filament, and combustion are most commonly used for gas dissociation. Chemical
dissociation in halogen-containing systems may be possible at relatively low temperatures.
(2). Independence of dissociation method.
Good quality CVD diamond has been produced utilizing a variety of the above described methods.
The ease with which diamond is grown depends on the method, but generally accepted explanations for
this influence have not been established.
(3). Independence of carbon-containing precursor.
The chemical nature of the carbon-containing precursor does not determine whether diamond can
be grown. Diamond of similar quality and morphology has been grown using a variety of species such as
aliphatic or aromatic hydrocarbons, alcohols, ketones, carbon monoxide and halocarbons [54].
(4). Hydrogen is required for efficient growth.
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Deryagin and co-workers and Angus first proposed that atomic hydrogen had to be present.
Hydrogen in excess of that introduced as part of a hydrocarbon precursor gas may not be needed.
(5). Oxygen enhances the quality of CVD diamond.
Oxygen added in small amounts to hydrocarbon precursor mixtures enhances the quality of
diamond deposits. Conflicting results are reported on the effect of oxygen on diamond growth rates.
(6). Diamond precursor species.
The most abundant carbon-containing precursors in typical diamond growth systems are methyl
radicals and acetylene molecules.
(7). Co-deposition of diamond and non-diamond carbon.
Graphite and other non-diamond carbons usually deposit simultaneously with diamond.
(8). Diamond growth rate maximum with temperature change.
This maximum occurs around 1000 °C.
(9). Lowering of the growth temperature using halogenated precursor gases.
The addition of halogenated gases to the standard hydrogen and methane vapour phase allows the
deposition of diamond films at considerably lower temperatures (250-750 ºC) [55-57].
(10). Substrate surface treatments.
A substrate pre-treatment of scratching with submicrometer diamond powder is common to
enhance nucleation rates and densities. Electrical biasing of substrates or ultrasonic treatment can also
influence nucleation rates.
(11). Crystallite morphology.
Octahedral {111} and cubic {100} faceted surfaces dominate CVD diamond crystallites, and
twinning frequently occurs on {111} surfaces. Cubo-octahedral crystals composed of both {111} and
{100} surfaces are also common.
In the following section, we present the current understanding of some critical aspects in diamond
CVD growth, including the gas-phase environment, the growth species, diamond doping, diamond
surface chemistry, diamond growth mechanism, and diamond quality.
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3.1 The gas-phase chemical environment
To understand diamond CVD, it is first necessary to characterize the chemical environment that
the diamond film is exposed to during growth. In many other CVD processes this is straightforward, since
the pressure is low enough that gas phase chemistry is negligible (low-pressure CVD of polysilicon) or is
limited to a few precursor decomposition reactions (organometalic CVD of GaAs). However, in diamond
CVD, free radicals – particularly atomic hydrogen – play crucial roles. The presence of radicals ensures
that gas-phase chemistry is an integral part of diamond CVD. A mixture of radicals, molecules, and in
some cases ions, impinges on the substrate and contributes to the growth of diamond, even though the
feed gas composition may be a simple methane/hydrogen mixture.
There have been many studies of the gas-phase chemistry during diamond CVD [58-68]. These
investigations revealed that atomic hydrogen and hydrocarbon are perhaps the most critical determinants
of CVD diamond as well as its quality and growth rate.
3.1.1 Atomic hydrogen.
The production mechanisms, loss mechanisms and concentration profiles of atomic hydrogen are
the basic subjects.
In plasma-enhanced systems such as microwave, RF or DC arcjet reactors, H is produced
homogeneously in the plasma. The external energy input couples directly to the free electrons in the
plasma. The energetic electrons may directly produce H through
+++ eHHeH
2
(4)
while the dissociation of molecular hydrogen by electrons with energy below than 12 eV may occur via
electronic excitation according to Stibbe and Tennyson [69].
In hot-filament systems, at pressures about 4 kPa the dissociation of hydrogen occurs mainly in
the gas phase. The atomic hydrogen produced diffuses rapidly away from nearby the filament, resulting in
a concentration profile near the filament [70].
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The steady state level of atomic hydrogen in the reactor is determined by a balance between the H
atom production rate and the destruction rate due to homogeneous chemistry and wall recombination. For
typical diamond CVD conditions, homogeneous recombination of H is a slow process, and H atoms are
able to diffuse to the walls or to the substrate before recombining in the gas. The rate of the direct
recombination reaction,
MHMHH
2
+++
, (5)
is pressure-dependent, due to the need for a third body (M) to carry away the excess heat of
recombination. At 20 Torr, the characteristic time for this reaction is in the order of 1 sec [59,60]. In time
t an atom may diffuse a distance of order
Dt
, where D is the diffusion coefficient. Using
12.0
=
D
m
2
/s for H in H
2
at 20 Torr implies a diffusion distance of 35 cm in 1 sec. Therefore, since the
substrate is usually at most 1-2 cm from the location of H production, H atoms in an H
2
background are
able to freely diffuse to the substrate without homogeneous recombining.
In the presence of a small amount of hydrocarbon, a second path competes with reaction (5) and,
in many cases, dominates the homogeneous recombination rate. This is due to the two reactions
MCHMCHH
43
+++
(6)
and
234
HCHCHH ++
. (7)
Accounting for these and similar reactions, Goodwin and Gavillet [60] have shown from numerical
simulations that the homogeneous recombination time is reduced to about 50 ms for a gas composition of
0.5% CH
4
in H
2
. However, this still implies a diffusion distance of 8 cm. Therefore, homogeneous
recombination of H may be neglected under typical low pressure CVD conditions. Note, however, that at
hydrocarbon concentrations of several percent, gas-phase H profiles are affected by the presence of the
hydrocarbons, indicating a contribution from homogeneous recombination [70].
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Since homogeneous recombination of H atoms is usually negligible, the loss of H atoms must
occur primarily on reactor walls and on the diamond surface itself. Measurements of the H concentration
profile near a diamond substrate clearly show that diamond is a sink for H at typical substrate
temperatures [71].
3.1.2 Hydrocarbon chemistry
The first hydrocarbon concentration measurements during diamond growth were made by Celii et
al. [58], who used infrared diode laser absorption spectroscopy to detect acetylene (C
2
H
2
), the methyl
radical (CH
3
) and ethylene (C
2
H
4
) in a 3.3 kPa hot-filament reactor with an input gas of 0.5% methane in
hydrogen. Later, a similar measurement was also made for a 2.7 kPa microwave system [72]. The
measured acetylene concentration represented conversion of 10-20% of the initial methane. Since the gas-
phase conversion of methane to acetylene requires several sequential reactions with atomic hydrogen, this
observation clearly showed that significant gas-phase chemistry was occurring. A notable feature of these
results is that methane and acetylene account for the majority of the gas-phase carbon. Ethylene is present
at much lower levels and ethane (C
2
H
6
) is not detected. The only two radical species which are detectable
are atomic hydrogen and the methyl radical CH
3
.
Chemical equilibrium analysis indicates that the distribution of the C
1
species is a function of
only the H/H
2
ratio and the local temperature. Normally the H/H
2
ratio in a diamond CVD system is in a
range of 10
-3
-1, which leads to a result that the most abundant C
1
radicals are CH
3
and atomic carbon.
Therefore, CH
3
and C are most often postulated to be important for diamond growth.
The distribution of species with the C
2
system (C
2
through C
2
H
6
) can not be explained by simple
partial equilibriums of hydrogen shift reactions. This results from the fact that the species C
2
H
2,
C
2
H
4
, and
C
2
H
6
are all stable molecules, and thus reactions such as
HHCHHC
32222
++
(8)
have high activation energies and are consequently very slow.
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The concentrations of all C
2
H
n
species with
n
>
2
are generally low, since these species are
thermodynamically less stable at high temperature and in the presence of atomic hydrogen than acetylene.
Thus C
2
H
n
(
n
>
2
) species are rapidly converted to acetylene. Therefore, C
2
H
2
is still suggested to have a
possible role in diamond growth.
3.1.3 Effect of oxygen addition
Although most diamond CVD is carried out with hydrocarbon/hydrogen gas mixtures, it is also
common to add a small amount of oxygen, or an oxygen-containing compound. Several studies have
reported enhanced growth rates or quality due to oxygen addition and, more importantly, the substrate
temperature can be significantly lowered when oxygen is introduced into the hydrocarbon/H
2
gas
mixtures [73-78].
Because of the rapid gas-phase chemistry, the equilibrium composition provides, once again, a
first estimation of the effect of oxygen addition on the gas composition. Since acetylene is typically one
of the most abundant hydrocarbons, let us consider the oxidation reaction
2222
H2COOHC ++
(9)
This reaction is highly exothermic. The Gibbs free energy is highly negative, resulting in the extent of
reaction near one. The reaction shifts strongly to the right, proceeding until essentially all of one reactant
is depleted. Harris and Weiner [79] have reported mass spectral measurements during diamond CVD
growth with various mixtures of CH
4
, O
2
, and H
2
. In all cases, they found that adding O
2
reduced the
hydrocarbon mole fractions. No O
2
was detected experimentally at the substrate, indicating that all
injected O
2
was rapidly consumed. These results are also consistent with the empirical observations of
Bachmann et al. [80]. As in the case of the acetylene/oxygen flame, for high oxygen content, solid carbon
precipitation in thermodynamically not possible, because CO
2
and H
2
O are more stable and the Gibbs free
energy of the reaction is highly negative. At lower oxygen concentration precipitation of carbon can occur.
The metastable diamond phase is detected at the borderline from no deposition to C (s) precipitation.
These observations suggest that the oxygen rapidly oxidizes any available hydrocarbon. Even though the
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main effect of oxygen addition is to oxidize some of the hydrocarbon to form CO and H
2
, there are other
effects which may be significant for the film growth. For example, it has been shown that oxygen addition
can result in a slight increase in the H level and OH level [79]. Since OH can oxidize pyrolytic, non-
diamond carbon, it may play a role similar to H and thus oxygen addition may aid diamond growth
through creation of OH radicals.
3.2 The growth species and growth mechanisms
The question of which carbon-bearing gas-phase species is most responsible for CVD diamond
growth has attracted much interest. In addition to its fundamental nature, this question is important for
reactor design, since once the “growth species” is identified, a reactor could be designed to maximize the
concentration of this species.
There have been many suggestions for the growth species, including small radicals (C, CH, C
2
,
C
2
H, CH
3
), ions (CH
3+
) and large hydrocarbons which have a similar structure to diamond. Some of these
were suggested based on observation of characteristic emission spectra in plasma or flame environments
during diamond growth. However, some species, such as C
2
and CH, may produce intense visible
emission due to electron-impact excitation or chemiluminescence even at concentrations far too low to
account for measured growth rates, while other abundant species (CH
3
and C
2
H
2
) have no prominent
visible emission bands. Therefore, it is difficult to draw conclusions about the significance of a given
species from its emission spectrum.
The observation that diamond may be readily grown in hot-filament reactors at rates comparable
to plasma systems operating at the same pressure and flow rate indicates that species which might be
found in plasma (ions, electrons, electronically-excited neutrals) but not in a “thermal” environment are
probably not important for diamond CVD. For this reason, most work on determining the growth species
has focused on neutral species.
It is reasonable to assume that any potential growth species must have a collision frequency with
the surface at least as great as the rate at which carbon is incorporating into the film. For a typical growth
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rate of 1 µm/h, this implies a growth species concentration at the surface of at least
3
c
10
cmn103
×
(n
c
is the number of carbons in the growth species) [80]. From measured concentrations of stable species,
it was found that only CH
4
, CH
3
and C
2
H
2
are present in sufficient quantities to account for measured
growth rates. When the low reactivity of methane is taken into account, only CH
3
and C
2
H
2
are left as
likely growth species under typical diamond CVD conditions.
Several mechanisms for diamond CVD have been proposed, including CH
3
-based
mechanisms [81,82], Acetylene-addition mechanisms [83], Combined CH
3
– C
2
H
2
mechanism [83].
However, none of them has been generally accepted as the likely path for incorporation of gas phase
carbon into a bulk diamond structure.
3.3 Diamond doping
CVD diamond p-type and n-type doping can be achieved through the addition of proper elements
in the gas phase.
Boron has been widely used as a dopant for obtaining p-type diamond; boron doping can be
achieved by adding substances such as diborane or trimethyl borane to the plasma. The activation energy
is relatively high, 0.37 eV [84] and this corresponds to a level too deep for room temperature applications
at typical doping levels. However, when the doping level reaches values higher than 10
20
cm
-3
, the
conduction mechanism changes and the activation energy approaches zero [85]. Taking advantage of this
fact, high performance transistors with full activation at room temperature [86] and heavily boron-doped
diamond electrodes for electrochemical applications [87] have been successfully fabricated.
Phosphorous-doped diamond yields n-type conductivity with a donor lever 0.56 eV under the
conduction band [88]. Again, this level is too deep for conventional room temperature applications. The
possibility of n- and p-type diamond doping opens the door for a generation of bipolar devices, such as a
diamond pn junction reported by Koizumi et al. [89].
Other dopants have also been tried, such as sulphur and arsenic [90], however no systematic
results have been obtained so far. A complex involving boron and hydrogen complex has recently been
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proposed as a shallow dopant for n-type diamond [91] but the long-term stability of the complex is still
not known. So far boron and phosphorous remain the most widely accepted and used diamond dopants.
As-grown CVD diamond surfaces, grown under hydrogen-rich atmospheres, also show p-type
surface conductivity due to the termination of the dangling bonds with hydrogen atoms [92] and have
been widely used to fabricate planar devices operating at room temperature [93].
4 CVD Diamond Characterization Techniques
Depending on the applications of the CVD diamond films, various techniques can be employed to
characterize the materials microstructure and their properties, including optical, electrical, thermal and
mechanical characteristics. Two most fundamental techniques, Raman spectroscopy and X-ray diffraction
(XRD), are most intensively used to identify diamond films. In fact, Raman spectra could be found in
almost every published paper related to CVD diamond growth. Due to their special role in characterizing
diamond films, we discuss in more details about Raman and XRD in this section.
4.1 Raman spectroscopy
Raman spectroscopy is most extensively used to characterize CVD diamond because of the
ability to distinguish between different forms of carbon [94-96]. In the Raman spectrometers a laser beam
of certain wavelength can be focused at a spot of 1 µm. The spectrum resolution can reach 0.5 cm
-1
. In
some instruments, a confocal component is installed, which allows a depth profiling of the spectra.
However, the penetrating depth of the laser into the sample depends strongly on the film quality.
It is known that atoms in a crystal vibrate at characteristic frequencies and the lattice vibrations
can be visualized as quanta, called ‘phonons’, with energy
hν
, where ν is the frequency and h is Planck’s
constant. In Raman scattering measurement, monochromatic light with frequency ν
i
interacts with
phonons in the crystal having frequency ν
phonon
. In this process a phonon can be either created or
destroyed, giving rise to either the Stokes Raman line with frequency ν
i
-ν
phonon
or the anti-Stokes Raman
line with frequency ν
i
+ν
phonon
. The anti-Stokes line is much weaker than the Stokes line and depends
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strongly on temperature as it depends on the number of the excited vibration levels at that temperature.
Therefore, at room temperature, it is the Stokes line that is normally observed for diamond. A large
fraction of the incident light is Rayleigh scattered at the same frequency as the incident light. In order that
this Rayleigh scattered light does not impair the Raman spectrum it is necessary to use a high-quality
double monochromator, or a filter that strongly absorbs the exciting radiation but passes the Raman
scattered spectrum.
In CVD films, the presence of diamond is revealed by a sharp line at 1332 cm
-1
that is due to a
vibration of the two interpenetrating cubic sub-lattices of diamond, while graphite usually gives rise to
two broader peaks around 1335 cm
-1
and 1580 cm
-1
with similar intensity. The ratio of the intensities of
the diamond peak to the graphite peak indicates how much of each phase is present, but is also dependent
on the wavelength of excitation. The width of the 1332 cm
-1
line reveals how much random stress is
present, and any directional stress present may give rise to a shift or splitting of the line, as will be
discussed later. The film may also contain amorphous sp
2
-bonded (graphite) carbon and sp
3
-bonded
(diamond-like) carbon. These give rise to broad ill-defined bands with a high-frequency cut-off near
1332 cm
-1
for the sp
3
-bonded carbon, and around 1580 cm
-1
for the sp
2
-bonded carbon. Fig. 7 shows
Raman spectra for two different polycrystalline CVD films deposited on copper. They were measured
using a Raman spectrometer excited at 633 nm. Film (a) shows mainly the 1332 cm
-1
diamond peak and is
of better quality comparing with film (b), which exhibits broad bands, in addition to the diamond Raman
peak. However, the form of the Raman spectra is greatly influenced by the wavelength of laser used for
excitation. Fig. 8 shows Raman spectra taken from the same samples (a) and (b) using a Raman
spectrometer excited at 514 nm. The spectra suggest that even film (b) has quite good quality. This
phenomenon has been attributed to a resonance effect, in which the non-diamond forms of carbon scatter
far more effectively than diamond at longer excitation wavelengths [97].
Fig. 7
Fig. 8
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The message here is clear: if we are concerned with the non-diamond carbon phases or trying to
produce high quality diamond, it is a much more stringent test to measure the Raman spectra with a
longer wavelength laser (633 nm or longer). If we are interested in finding the diamond peak position,
shorter wavelength laser may give a more accurate result.
4.2 X-ray diffraction
In addition to the Raman spectroscopy, XRD can be used to confirm that diamond synthesis has
indeed been achieved. Besides identifying the presence of crystalline phases, the XRD patterns also
provide information on strain, grain size, preferential orientation, etc. Fig. 11 shows XRD patterns of the
two diamonds used in Fig. 7 and Fig. 8. As a reference the positions (<120 °) and relative intensities of
the strongest XRD peaks for powdered diamond are given in Fig. 9 and in Table 5. It is clear that the
XRD is a technique as powerful as the Raman spectroscopy since it also provides a fingerprint of the
presence of diamond phase. Note that the diamond films deposited under different conditions may possess
different preference growth direction. This may contribute to a variation of the relative intensity of the
XRD peaks. For example, the (111) diamond peak is nearly not visible for sample (a), while its (220)
peak is much more intense than in sample (b). It is worth to mention that some substrate materials, like
copper, also have a cubic structure and its lattice parameter is similar to that of diamond. Since X-ray can
penetrate diamond more than 100 µm, the XRD patterns usually exhibit peaks of the substrate, which are
very close to those of diamond and are even stronger than diamond peaks. Special attention is needed on
this. In addition, buffer layers are used in the samples to promote the adhesion of diamond to copper. The
XRD is sensitive enough to reveal the structure of the buffer layers, as evidenced by those un-labelled
peaks that are neither due to the diamond films nor to the copper substrates shown in Fig. 9.
Fig. 9
Table 5
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5 Heteroepitaxial CVD Diamond Growth Characteristics
CVD diamond growth includes a few steps, i.e., nucleation, formation of continuous film,
competition growth of crystallites. The resulting film structure, properties, and surface morphology are
closely related to these three stages. In this section, we briefly discuss the effects of substrate pre-
treatment on diamond nucleation, the effects of deposition conditions on CVD diamond nucleation and
growth, and the growth behaviours of diamond films on various types of substrate materials.
5.1 Effect of substrate pre-treatment on diamond nucleation
The first difficulty that arises from the attempt to grow diamond on foreign substrates is that a
continuous diamond film cannot be deposited unless a proper nucleation step precedes the growth. After a
non-diamond substrate has been exposed to proper growth conditions without a nucleation procedure only
a few isolated diamond crystallites (~10
5
-10
6
cm
-2
) will be found. The control of nucleation density and
film growth is significant for different applications. For instance, a nucleation density higher than 10
8
/cm
2
is required in diamond coating on metals in order to improve adhesion and to reduce carbon diffusion into
the substrate.
Different nucleation procedures have been proposed, the simplest of which is a diamond grit
abrading process, where the flat substrate is pressed against a soft or hard plate that contains diamond
powders from a natural or synthetic source [98]. By the end of this process, small diamond particles of
sizes in the range of 2-10 nm are left on the surface and act as growth sites once the growth cycle is
initiated. It is generally accepted that surface defects such as grain boundaries and dislocations are
favourite sites for diamond nucleation [99]. The nucleation enhancement by scratching can be generally
attributed to (a) seeding effect, (b) minimization of interfacial energy on a sharp convex surface, (c)
breaking of a number of surface bonds or presence of a number of dangling bonds at sharp edges, (d)
rapid carbon saturation (fast carbide formation) at sharp edges.
For many applications, this procedure is sufficiently effective. Fig. 10 shows SEM images of
diamond nucleation on copper with different pre-polishing process to the substrate. The nucleation
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density on copper without pre-treatment (Fig. 10a) is quite low, approximately 5x10
6
/cm
2
, while diamond
powder polishing leads to a significant increase in the nucleation density (Fig. 10c). However, this
method has some limitations: it can be used only with flat surfaces, and if the substrate is coated with
some intermediate layer (for example, by layers that are intended to prevent the film delamination) it may
be damaged by the harsh abrading action.
Fig. 10
The ultrasonic treatment [100] is a gentler method, where the substrate is simply immersed in a
cleaning ultrasonic bath, with the diamond powder properly dispersed in an organic solvent like methanol.
This method can be used with 3D-shaped substrates and the appropriate choice of the diamond particles
size and the seeding time allows a further control of the nucleation procedure. Nanodiamond particles can
also be used; in this case, they can be processed to prevent agglomeration and dispersed in a colloidal
solution with an appropriate solvent [101]. Nucleation densities as high as 10
11
cm
-2
have been obtained
with this method. A modified method, known as NNP (Novel Nucleation Procedure), involves the
deposition of a carbon film prior to the ultrasonic treatment by exposing the substrate to the growth
conditions [102]. Besides increasing the nucleation density, this pre-deposited carbon film acts as a
carbon supply and facilitates the diamond coverage of complex 3D shapes [103].
Another widely known method is the Bias Enhanced Nucleation (BEN) method [104] which
employs an in situ surface bombardment under an applied negative bias on a conductive substrate. The
applied electric field increases the ionization degree of the neutral gas molecules, the energy of the ions
and the surface ion bombardment rate. Different ionic species may be involved in the bombardment, such
as CH
x+
(x = 1 to 5), C, H
+
and H
2+
[105]. During the bombardment, the ionic species alter the surface and
create surface structures that act as the seeds for the growth. The ionic species react with the substrate,
resulting in the formation of a silicon carbide layer with improved adhesion when silicon is used as the
substrate [106]. Nucleation densities higher than 1×10
11
cm
-2
have been obtained with this method [107],
however, Maillard-Schaller et al. reported surface damage induced on a silicon substrate during the BEN
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that happened in the form of holes that could be as deep as 2-3 µm and as large as 200-300 nm in
diameter [108].
Once the small diamond crystallites are present on the non-diamond substrate surface by the
nucleation procedure, they start growing three-dimensionally until the grains coalesce and form a
continuous film; this happens during a time period known as the incubation time. The growth proceeds
with competitive crystal growth between the crystals oriented along the fastest growth direction. This
results in a columnar growth mode oriented parallel to the substrate, with grain sizes coarsening with the
film thickness (van der Drift growth) [28].
5.2 Effect of deposition parameters on diamond nucleation and growth
The CVD process conditions have significant effects on diamond nucleation and growth. The
major process parameters include input power, substrate temperature, methane concentration, gas pressure
and gas flow rates. Detailed study on the effects of these deposition parameters have been systematically
conducted and reported [e.g. 109,113]. The generally observed results are outlined as follows.
(1). Diamond growth rate increases with increasing microwave power, the effect of microwave
power being mainly the effect of plasma density.
(2). Diamond nucleation and growth rate increase with increasing gas pressure and methane
concentration. When the gas pressure reaches a certain value, e.g. 13 kPa, growth stress
starts to be pronounced. The increase in the methane concentration results in monotonic
increase in both growth stress and non-diamond phase.
(3). Gas flow rate has less influence on diamond nucleation and growth.
(4). The substrate temperature influences significantly the film morphology. The diamond
crystals show a (111) face dominating almost in all the cases. (100) faces appear at higher
substrate temperature.
Due to the importance of diamond film’s morphology, we discuss it in more detail here. Most
CVD diamond shows a dominant triangular (111) face. When the substrate temperature becomes
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relatively high, more (100) faces appear [34,110,114]. It is known that the shape of a diamond crystal
depends on the deposition conditions which may influence the relative growth rate of constituent
crystallographic planes. Therefore the shapes can be used to determine the ratio of the growth rate in
different directions. Fig. 11 demonstrates the variation in the crystal shape for diamond crystals supposing
that the limiting faces are (100) and (111) facets but that the crystals grow at different rates V
<100>
and
V
<111>
on the two types of facets. The distance from the centre of the crystal to the centre of each face is
proportional to the growth rate in the direction perpendicular to that face. The longest dimension in the
crystallite defines the fastest growth direction, as indicated by the arrow. It can be seen that (111) plane
grows faster in a cube, while (100) plane is the fastest one in an octahedron.
Fig. 11
5.3 Substrate materials for CVD diamond films
Substrate materials used for diamond deposition may be classified into three major groups in
terms of carbon-substrate interactions as listed in Table 6. According to these interactions, the materials
can be classified as showing (1) little or no solubility carbon reaction, (2) strong carbon dissolving and
weak carbide formation and (3) strong carbide formation. Depending on the carbon-substrate interactions,
the grown diamond exhibits different interface structures and consequently different adhesion behaviours.
Table 6
In the next sections, a few examples of diamond deposition these materials are given.
5.3.1 Materials with little or no carbon solubility
The materials that have little or no C solubility or reaction include some crystals (sapphire, Ge,
diamond and graphite) and a few metals such as Cu, Sn, Pb, Ag, and Au. In the case of diamond
deposition on graphite substrates, the growth conditions induce the graphite etching that happens
concurrently with diamond growth.
Copper can be used as an example to illustrate diamond growth on this kind of materials: as it has
very low carbon affinity, the adhesion of diamond films grown on copper is expected to be very weak.
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This renders copper an interesting substrate material for making free-standing diamond films. In fact,
diamond film can be easily removed from the copper substrate after deposition and becomes free-standing.
Fig. 12 shows SEM images and Raman spectra of the surface and backside of the free-standing
diamond film deposited on copper. It can be seen that the grain size at the film backside is much smaller
than at the surface side. This is because, once the nucleation particles grow up and meet each other, they
can no longer grow in the Cu surface plane but can continue the growth in a perpendicular direction. As a
result of growth competition, some preferential grains become larger and larger until they reach
equilibrium. The Raman spectra show a sharp peak at about 1332 cm
-1
, indicating the existence of good
diamond phase. No carbide transition layer is found, as expected. Although the diamond peaks show
similar width and position, the background of the Raman spectrum of the film backside is obviously
higher than that of the surface side. We suppose that this happens due to the presence of grain boundaries
which are obviously in larger amount in the backside.
Fig. 12
5.3.2 Materials with strong carbon dissolving and weak carbide formation
When the materials are strong carbon dissolving, there is a considerable amount of C diffusion
into the substrate during diamond growth. This class of materials includes some metals such as Pt, Pd, Rh,
Fe and Ni. Under growth conditions, the substrate acts as a carbon sink and the deposited carbon
dissolves into the metal surface to form a solid solution. A large amount of carbon is then transported to
the bulk and this leads to a temporary decrease in the surface C concentration; this, in turn, delays the
onset of nucleation.
CVD diamond coatings on steel, for instance, are attractive for mechanical applications. However,
there are at least two major difficulties that hinder diamond coating on steel. First, iron is a strong carbon-
dissolving element. During CVD diamond process the carbon swiftly diffuses into the steel substrate.
This usually causes poor adhesion of diamond film to the steel substrate. The characteristics of the steel
may also be changed due to the heavy carbon diffusion. Second, the difference in thermal expansion
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coefficients between diamond and steel is very large (at room temperature, α
diamond
~1×10
-6
/K,
α
steel
~16×10
-6
/K. Both of them increase a little with temperature). This causes large residual stress in the
diamond film and influences the adhesion in a negative way.
It is found that diamond film deposited directly on steel substrate can be easily removed from the
steel substrate. Fig. 13 shows SEM images of the surface and backside of the diamond film grown on
high speed steel MG50. The small particles in the film surface are probably some materials diffusing from
the steel substrate. In the backside there is no polycrystalline structure visible. Raman spectra taken from
the two sides are shown in Fig. 13. The spectrum of surface side shows a high background, which is
probably due to those surface particles. The spectrum of the backside shows two broad peaks at
~1335 cm
-1
and ~1580 cm
-1
with similar intensity, being characteristic of graphite. The substrate surface,
where the film is removed, shows a similar structure and Raman spectrum to the film backside,
confirming the idea that, before the diamond starts to grow, graphite layer forms on the steel substrate.
Because of this graphite layer the diamond film exhibits no adhesion. Performing a diffusion calculation,
we can see that the carbon diffusion in iron is very heavy, as shown in Table 7.
Fig. 13
Table 7
A possible approach to gaining adhesion of diamond coatings on steel is the utilization of an
interlayer. The feasibility of this solution has been demonstrated by Chen et al, who employed a Si
interlayer and obtained adherent diamond coating on steel at relatively low deposition temperature [115].
Later Nesladek et al. [116,117] proposed a stress relief multilayer structure (Mo/Ag/Nb) and got good
adhesion. So far, single interlayer materials that have been reported include Si, TiN, W, Mo, Ti, Cr-N etc.
[118-122].
5.3.3 Materials with strong carbide formation
Materials with strong carbide formation include metals such as Ti, Nb, Ta, Cr, Mo, W and some
rare earth metals. B and Si are also materials that form carbide layers, like other Si compounds such as
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32
SiO
2
, quartz and Si
3
N
4
. Carbide materials (for instance SiC, WC and TiC) are also particularly suitable for
diamond deposition.
Silicon is widely used as a substrate for growing CVD diamond. Niobium is commonly used in
boron-doped diamond-coated electrodes (with dimensions 50×100 cm
2
) by the CONDIAS company for
waste water treatment [123] and quartz in optical transparent electrodes [124]. The use of halogenated
precursors also allowed the low-temperature deposition on low melting materials, such as glass [125].
As discussed before, titanium forms strong carbide bond as well as silicon. Therefore, diamond
coating on these types of substrate materials is expected to present good adhesion. Under optimized pre-
treatment and deposition conditions, adherent diamond films can be deposited on Ti substrate. Fig. 14
shows the Raman spectrum of the diamond coating. It is found that the Raman peak shifts to about
1337 cm
-1
. It is noted that free-standing diamond films usually exhibit a Raman peak at 1332 cm
-1
wave
numbers, while the adherent films show the peak shift due to the presence of in-plane stresses caused
mainly by the thermal mismatch between the substrate and the diamond film. The stress σ in the
diamond film can be estimated from
σ
ν
ν
0567
0
. ( )
m
(GPa) for unsplitted Raman peak at ν
m
,
where ν
0
= 1332. Thus, the films can accommodate a compression stress of 2.835 GPa without
delamination.
Fig. 14
Similarly, adherent diamond films can be deposited on Si substrate. Si has a sufficiently high
melting point (1683 ºK), it forms a localised carbide layer and it has a comparatively low thermal
expansion coefficient. The stress in the film is much smaller, as evidenced by the Raman shift in Fig. 15
and Table 8. It is interesting to note that the nature of the stress changes with film growth, which implies
the variation of intrinsic stress along the film depth profile. Fig. 16 shows the profile of a thick diamond
film. The significant change in the size of the crystals can be clearly seen.
As diamond films with strong adhesion can be deposited on Ti and Si, these two materials are
often used as interlayers for obtaining adherent diamond coatings on substrates like steel and copper.
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Fig. 15
Table 8
Fig. 16
6 Nanocrystalline Diamond
In spite of the remarkable properties of diamond, the high surface roughness of CVD diamond
films presents a major roadblock that prevents their widespread use in various applications [126,127],
such as machining and wear, field-emission or optical applications.
In order to overcome this problem, different approaches may be followed; either a post-deposition
polishing procedure or a growth cycle intended to decrease the surface roughness. Since the post-
polishing is an expensive and time-consuming technique [128], a lot of effort has been devoted to
decreasing the surface roughness of the CVD diamond films with a proper control of the gas chemistry
and the deposition parameters. One way to obtain diamond films with considerable thicknesses (several
tens of microns) and low surface roughness is to control the crystalline orientation with (100) facets that
are parallel to the film plane [129]. A different and more flexible approach is the reduction of the film
grain size (from micrometers to nanometers) by means of the growth chemistry and the surface
temperature. These diamond films, commonly referred to as NanoCrystalline Diamond films (NCD), are
grown in hydrogen-rich CVD environments and have grain sizes ranging from a few nanometers up to a
hundred nanometers (increasing with the film thickness) and very low (0.1%) to high (50%) amounts of
sp
2
-bonded carbon, in the form of defects or grain-boundaries [130]. A second category of nanocrystalline
diamond films, known as Ultra-NanoCrystalline Diamond films (UNCD), are grown in argon-rich,
hydrogen-poor CVD environments, and have a typical grain size of 2-5 nm, independent of the film
thickness. The nano grains are embedded in a non-diamond matrix and the films show a significant
content of sp
2
-bonded carbon (up to 5%) [131]
.
NCD and UNCD films have, in general, high Young’s
modulus, high hardness and a low macroscopic friction coefficient, due to their low surface roughness,
and are optically transparent. The UNCD films are also electrically conductive, due to the non-diamond
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34
matrix; both types of film can easily be doped by introducing a gas such as nitrogen or diborane into the
growth chamber.
6.1 Nanocrystalline and ultrananocrystalline diamond film growth
Both NCD and UNCD are usually deposited on non-diamond substrates, such as silicon wafers.
Other materials can also be used, such as SiC, SiO
2
, Si
3
N
4
, etc. The deposition of NCD and UNCD films
involves, like the CVD of the microcrystalline diamond films described above, some kind of nucleation
procedure that will provide the substrate with the necessary diamond seeds for the further film growth
(polishing with diamond powder, pre-coating of a carbon film, ultrasonic treatment, bias-enhanced
nucleation).
6.1.1 NCD film growth
NCD films can be deposited by different methods, such as MPCVD [132-137], Electron
Cyclotron Resonance [138,139], DC Glow Discharge [140,141] and HFCVD [142-146]. They are
typically grown in hydrogen-rich, carbon lean environments, with surface temperatures between 250 and
1000 ºC and pressures higher than 660 Pa [130]. Some nitrogen may also be added during the growth in
order to increase the electrical conductivity of the NCD and the methane-hydrogen ratio can vary between
0.1 and 4%.
NCD films deposited with a high percentage of CH
4
/H
2
ratio (5–20%) usually show cauliflower-
or ballas-type growth morphology [147,148] – Fig. 17a [130]; the higher amount of CH
4
in the gas phase
increases the twinning and non-diamond carbon incorporation [149] (up to 50% non-sp
3
carbon), reducing
the grain size. The addition of N
2
to the gas phase also reduces the NCD grain size (3–30 nm) [150,151]
due to increased micro-twinning and stacking faults that result in the nanocrystalline structure [149].
They can also be deposited by bias-enhanced growth [136] under moderate (2–6%) CH
4
/H
2
ratios
and continuous DC bias (200–320 V) during the deposition. These films show enormous stress, ranging
from 1GPa to 85GPa; apparently, the combination of surface and sub-plantation processes leads to the
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35
formation of a mixed phase containing amorphous tetrahedral carbon and NCD, originating the high
internal stress.
Deposition under extremely low (0.3%) CH
4
/H
2
ratios originates the highest quality NCD films,
with a low content of non-sp
3
carbon and high Young’s modulus, thermal diffusivity and nucleation
density [152].
Fig. 17.
6.1.2 UNCD films growth
In 1994, UNCD films were synthesized in a MPCVD system under hydrogen-poor (1%) and
argon/carbon-rich conditions, using C
60
as the carbon source [153]. Contrary to usual diamond CVD
conditions, there was no excess of atomic hydrogen in the plasma, and it was proposed that the
fragmentation of the C
60
molecule due to Ar
+
collisions lead to the production of the C
2
radicals. These
radicals would directly insert into the C–H bond at the diamond surface, eliminating the need for atomic
hydrogen. Theoretical calculations indicated possible interaction mechanisms of C
2
on the (110) diamond
surface with very low activation barriers (< 5 kcal·mol
-1
) and the formation of a C–C bond between two
adjacent absorbed C
2
being exothermic and occurring without the presence of hydrogen.
However, recent experimental measurements of the C
2
species absolute densities in Ar/CH
4
/H
2
and He/CH
4
/H
2
plasmas, using cavity ring down spectroscopy [154], did not detect ground-state C
2
in the
He/CH
4
/H
2
plasma; in addition, the ground-state C
2
in the Ar/CH
4
/H
2
plasma was too low to account for
the growth rate of UNCD. This suggests that, even though C
2
may play a critical role in UNCD growth, it
cannot account for the bulk growth of UNCD material. More studies are needed in this topic in order to
get a clear view of the UNCD growth mechanism and the role of the different chemical species.
UNCD can be typically deposited in a MPCVD system under low (1%) CH
4
/Ar ratios, at a
substrate temperature between 400 and 800º C – Fig. 17b [130]. Since the amount of atomic H in the
plasma is very low, diamond nucleus renucleate at a very high rate, and grain coarsening does not take
place, resulting in 2–5 nm diamond grains embedded in a non-sp
3
carbon matrix [155] – Fig. 17b. The
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low level of atomic H also minimizes regasification of the grains, and reasonably high growth rates can be
achieved with the formation of low-thickness continuous films.
6.2 Raman spectroscopy of NCD and UNCD films
Raman spectroscopy is also widely used to characterize NCD and UNCD films, however, this
technique is not straightforward due to the different phases present in the films (sp
3
and sp
2
bonding). If a
UV laser is used, the photon energy is shifted closer to the high gap of sp
3
-bonded carbon and this
problem can be somehow overcome. However, the analysis of NCD and UNCD Raman spectra becomes
still more complicated since the nanocrystalline nature of the films causes the breakdown of phonon
selection rules.
Fig. 18 [156] shows the UV Raman spectra of UNCD films grown under different conditions. In
addition to the 1332 (sp
3
-bonded carbon) and 1560 cm
-1
(commonly assigned to the G-band, arising from
the in-plane stretching modes of the sp
2
-bonded carbon at the grain boundaries [157]) peaks, distinct and
broad peaks can usually be seen at 1140, 1330 and 1450 cm
-1
.
Fig. 18.
6.3 NCD and UNCD applications
The electrical and optical properties of UNCD and NCD films make them perfect candidates for
various applications, such as electrochemical electrodes, cold cathode emitters, electrical insulating and
dielectric passivation layers, conducting or insulating layers in MEMS and NEMS devices, support and
transmission windows, etc. In addition, they possess excellent tribological properties, with hardness
values close to polycrystalline films or natural diamond, improved toughness and smooth surfaces with
corresponding low friction coefficients.
One of the first applications of NCD films was as support membranes for absorber patterns in X-
ray photolithography and X-ray transmission windows [133,135]. More recently, they have been
incorporated into silicon on insulator (SOI) wafers [157,158]. NCD-based surface acoustic wave (SAW)
devices have also been fabricated, taking advantage of the improved smoothness and sound velocity of
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NCD films [159]. They have also been used as an optical material to fabricate whispering gallery” mode
optical resonators [160], two dimensional photonic crystals [161,162] and UV transparent electrodes on
SiC [163]. N-doped NDC films have also been used for field-emission [164] and biomedical
applications [165]. Finally, different coating tools have been coated with NCD with promising
results [166-168].
The tribological properties of UNCD conformal coatings were explored in different applications,
such as coating seals of rotating shafts [169], monolithic AFM tips [170] and inkjets for corrosive
liquids [171]. Smooth UNCD films have also been widely used as structural materials [172-178] and
micromechanical switches [179,180] in MEMS and NEMS technology. Efficient field emitters [181] and
field-emitting fibres [182] have also been fabricated with UNCD.
Finally, stable chemical and DNA sensing platforms have also been obtained with chemically
modified NCD and UNCD diamond surfaces [183-186].
7 Summary
This paper provides a general review on the growth of CVD poly and nanocrystalline
diamond, including the material properties, development history, major deposition and
characterization techniques, CVD diamond nucleation and growth mechanisms, characteristics
and applications.
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8 Acknowledgement
The authors would like to thank Foundation for Science and Technology (Portugal), research
project contract number: PTDC/EME-MFE/68042/2006 and research grant number:
SFRH/BPD/24615/2005.
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39
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366 295
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Diamond Films Relat. Mater. (Gaithersburg MD) 133
[151] Corvin R B, Harrison J G, Catledge S A and Vohra Y K 2002 Appl. Phys. Lett. 80 2550
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2003 J. Appl. Phys. 93 2164
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[154] John P, Rabeau J R and Wilson J I B 2002 Diamond Relat. Mater. 11 608
[155] Qin L C, Zhou D, Krauss A R and Gruen D M 1998 Nanostruct. Mater. 10 649
[156] Birrell J, Gerbi J E, Auciello O, Gibson J M, Johnson J and Carlisle J A 2005 Diamond
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[157] Aleksov A, Gobien J M, Li X, Prater J T and Sitar Z 2006 Diamond Relat. Mater. 15 248
[158] Feygelson T, Hobart K, Ancona M, Kub F J and Butler J E 2005 Semicond.Wafer
BondingVIII: Sci. Technol. Appl. (Quebec Canada) p. 439
[159] Lee Y C, Lin S J, Buck V, Kunze R, Schmidt H, Lin C Y, Fang W Land Lin I N 2008
Diamond and Rel. Mater. 17 446
[160] Wang C F, Choi Y S, Lee J C, Hu E L, Yang J and Butler J E 2007 Appl. Phys. Lett. 90
081110
[161] Baldwin J W, Zalalutdinov M, Feygelson T, Butler J E and Houston B H 2006 J. Vac. Sci.
Technol. B 24 50
[162] Wang C F, Hanson R, Awschalom D D, Hu E L, Feygelson T, Yang J and Butler J E
2007 Appl. Phys. Lett. 91 201112
[163] Tadjer M J et al 2007 Appl. Phys. Lett. 91 163508
[164] Corvin R B, Harrison J G, Catledge S A and Vohra Y K 2002 Appl. Phys. Lett. 80 2550
[165] Subramanian K, Kang W P, Davidson J L, Wong Y M and Choi B K 2007 Diamond
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[166] Hu J, Chou Y K and Thompson R G 2008 Int. J. Refract. Metals Hard Mater. 26 135
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[167] Salgueiredo E, Almeida F A, Amaral M, Fernandes A J S, Costa F M, Silva R F and
Oliveira F J 2009 Diamond Relat. Mater. 18 264
[168] Ma Y P, Sun F H, Xue H G, Zhang Z M and Chen M 2007 Diamond Relat. Mater. 16
481
[169] Sumant A V, Krauss A R, Gruen D M, Auciello O, Erdemir A, Williams M, Artiles A F
and Adams W 2005 Tribol. Trans. 48 24
[170] Pacheco S, Zurcher P, Young S R, Weston D, Dauksher W J, Auciello O, Carlisle J,
Kane N and Birrell J 2005 13th GAAS Symposium (Paris) p. 2005
[171] Muller R, Gronmaier R, Janischowsky K, Kusterer J and Kohn E 2005 Diamond Relat.
Mater. 14 504
[172] Kohn E, Adamschik M, Schmid P, Ertl S and Floter A 2001 Diamond Relat. Mater. 10
1684
[173] Kusterer J, Schmid P and Kohn E 2006 New Diamond Frontier Carbon Technol. 16 295
[174] Muller R, Gronmaier R, Janischowsky K, Kusterer J and Kohn E 2005 Diamond Relat.
Mater. 14 504
[175] Jing W, Butler J E, Feygelson T and Nguyen C T C 2004 17th IEEE Int. Conf. Micro
Electro Mechanical Systems p. 641
[176] Sekaric L, Parpia J M, Craighead H G, Feygelson T, Houston B H and Butler J E 2002
Appl. Phys. Lett. 81 4455
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Butler J E 2004 Appl. Phys. Lett. 84 972
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2006 Diamond Relat. Mater. 15 2061
[179] Kusterer J, Alekov A, Pasquarelli A, Muller R, Ebert W, Lehmann-Horn F and Kohn E
2005 Diamond Relat. Mater. 14 2139
[180] Schmid P, Adamschik M and Kohn E 2003 Semicond. Sci. Technol. 18 S72
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94 4079
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023113
[183] Yang W S et al 2002 Nature Mater. 1 253
[184] Yang W S, Butler J E, Russell J N and Hamers R J 2004 Langmuir 20 6778
[185] Yang W S, Baker S E, Butler J E, Lee C S, Russell J N, Shang L, Sun B and Hamers R J
2005 Chem. Mater. 17 938
[186] Yang W S, Butler J E, Russell J N and Hamers R J 2007 Analyst 132 296
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10 Figure Captions
Fig. 1. Structure of diamond.
Fig. 2. A schematic diagram of filament-assisted CVD apparatus [30].
Fig. 3. A schematic diagram of microwave-plasma CVD apparatus [30].
Fig. 4. A schematic diagram of combustion-flame-assisted CVD set-up [30].
Fig. 5. A schematic diagram of a dc plasma jet CVD apparatus [30].
Fig. 6. Schematic of processes occurring during diamond CVD.
Fig. 7. Raman spectra of two different diamond films deposited on copper. Excitation laser
wavelength is 633 nm.
Fig. 8. Raman spectra taken from the samples used in Fig. 7. Excitation laser wavelength is
514 nm.
Fig. 9. XRD patterns of two different diamond coatings on copper.
Fig. 10. SEM images of diamond nucleation on copper substrate with different polishing
treatments. (a) No pretreatment. (b) Polished with Al
2
O
3
powder. (c) Polished with
diamond powder.
Fig. 11. Variation in the crystal shape by the growth ratio of (100) face to (111) face.
Fig. 12. SEM images and Raman spectra of the free-standing diamond film prepared by the
two-step growth method. (a) film surface side, (b) film back side, (c) Raman spectra
taken from the two sides under identical conditions. Wavelength of the laser source:
633 nm.
Fig. 13. Diamond film grown directly on steel at a microwave power of 2500 W for 5 hrs. SEM
images show the film surface side (a) and backside (b). Raman spectra taken from the
two sides are shown in (c).
Fig. 14. Raman spectrum of the diamond film grown on Ti at 2100 W for 3 hrs.
Fig. 15. Raman spectra taken from diamond films grown on 0.3 mm-thick Si substrates. The
film thickness is (a) 1.7 µm, (b) 4.0 µm, (c) 11 µm, (d) 23 µm, and (e) 48 µm.
Fig. 16. SEM image of the profile of a thick diamond film.
Fig. 17. SEM image of (a) NCD and (b) UNCD. J.E. Butler and A.V. Sumant, The CVD of
Diamond Materials, Journal of Chemical. Vapor Deposition 2008, 14, pp. 152.
Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.
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Fig.18. UV Raman spectra of UNCD thin films grown with successive amounts of hydrogen
added to the plasma. Reproduced with permission from [156], Elsevier.
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Fig. 1. Structure of diamond.
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Fig. 2. A schematic diagram of filament-assisted CVD apparatus [30].
Methane
Hydrogen
CH4 + H2
W filamen
t
Substrate
Pum
p
e
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Fig. 3. A schematic diagram of microwave-plasma CVD apparatus [30].
Magnetron
Tuners
Wave
g
uide
Feed
g
as
Substrate holde
r
Substrate
Plasma ball
Vacuum
pump
Microwave
window
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Fig. 4. (a) A schematic diagram of the combustion-flame-assisted CVD set-up [30]. (b) The
combustion regions in an oxygen-acetylene flame.
Primar
y
combustion zone
Intermediate zone
(feather)
Outer zone
Nozzle
(b)
O2
C2H2
Mass flow
controller
Valve
Nozzle
Flame
Substrate
P
y
romete
r
Wate
r
Wate
r
(a)
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Fig. 5. A schematic diagram of a dc plasma jet CVD apparatus [30].
dc power
supply
Cathode
Plasma gas
(Ar,H2, CH4)
Plasma
j
e
t
Substrate
(
Mo
)
Water in
Water ou
t
Anode
Substrate
holder
(copper)
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Fig. 6. Schematic of processes occurring during diamond CVD.
H22H
Diffusion
Substrate
REACTANTS
(HCH
24
+
)
DISSOCIATION
e-, heat
CH H CH H
432
+
+
FLOW AND REACTION
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10000
12000
14000
16000
18000
1180 1230 1280 1330 1380 1430
Raman shift (cm-1)
Intensity (arb. unit)
Sample (b)
Sample (a)
Fig. 7. Raman spectra of two different diamond films deposited on copper. Excitation laser
wavelength 633 nm.
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0
10000
20000
30000
1050 1150 1250 1350 1450 1550
Raman shift (cm-1)
Intensity (arb. unit)
2500
4500
6500
8500
10500
12500
Sample (b)
Sample (a)
Fig. 8. Raman spectra taken from the samples used in Fig. 7. Excitation laser wavelength
514 nm.
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0
1
2
3
35 65 95 125
2 Theta (deg)
Intensity (arb. unit)
Sample (a)
Sample (b)
Reference
(111)
(220)
(311)
(400)
D
Cu Cu
Cu
Cu
Cu CuD
D
D
DD D
D
Fig. 9. XRD patterns of two different diamond coatings on copper.
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Fig. 10. SEM images of diamond nucleation on copper substrate with different polishing
treatments. (a) No pretreatment. (b) Polished with Al2O3 powder. (c) Polished with
diamond powder.
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Fig.11. Variation in the crystal shape by the growth ratio of (100) face to (111) face.
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450
650
850
1050
1250
1100 1200 1300 1400 1500 1600 1700
Wave number (1/cm)
Intensity (arb. unit)
Surface side
Back side
(c)
Fig. 12. SEM images and Raman spectra of the free-standing diamond film prepared by the
two-step growth method. (a) film surface side, (b) film back side, (c) Raman spectra
taken from the two sides under identical conditions. Wavelength of the laser source:
633 nm.
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8000
11000
14000
17000
20000
1100 1300 1500 1700
Wavenumber (1/cm)
Intensity (arb. unit)
0
20
40
60
80
back side
surface side
(c)
Fig. 13. Diamond film grown directly on steel at a microwave power of 2500 W for 5 hrs. SEM
images show the film surface side (a) and backside (b). Raman spectra taken from the
two sides are shown in (c).
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1337
7600
8100
8600
9100
9600
1250 1300 1350 1400 1450
Wavenumber (1/cm)
Intensity (arb. unit)
Fig. 14. Raman spectrum of the diamond film grown on Ti at 2100 W for 3 hrs.
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1100 1200 1300 1400 1500
Wave number (cm
-1
)
Intensity (arb. unit)
(a)
(
b
)
(c)
(d)
(e)
Fig. 15. Raman spectra taken from diamond films grown on 0.3 mm-thick Si substrates. The
film thickness is (a) 1.7 μm, (b) 4.0 μm, (c) 11 μm, (d) 23 μm, and (e) 48 μm.
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Fig. 16. SEM image of the profile of a thick diamond film.
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Fig. 18. UV Raman spectra of UNCD thin films grown with successive amounts of hydrogen
added to the plasma. Reproduced with permission from [156], Elsevier.
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11 Table Captions
Table 1. Outstanding properties of diamond.
Table 2. Properties and application areas of CVD diamond.
Table 3. Parameter range for diamond synthesis by filament-assisted thermal CVD method [30].
Table 4. Present status of low pressure diamond CVD methods.
Table 5. XRD patterns for powdered diamond (Cu Kα radiation, λ=1.5405 Å).
Table 6. Classification of metal substrates for CVD diamond.
Table 7. Diffusion depth p of carbon in iron, where the concentration C(p,T) of carbon is one
thousandth of its value C(0,T). Diffusion temperature is 1100 K.
Table 8. Raman shift and corresponding residual stress in diamond films of different thickness
deposited on Si substrates. The Raman shift is an average value of five different points
in each sample.
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Table 1. Outstanding properties of diamond.
1. Extreme mechanical hardness (~100 GPa).
2. Strongest known material, highest bulk modulus (1.2x10
12
N/m
2
).
3. Highest known value of thermal conductivity at room temperature
(2x10
3
W/mK).
4. Thermal expansion coefficient at room temperature (0.8x10
-6
/K)
comparable with that of invar.
5. Broad optical transparency from the deep UV to the far IR region of
the electro-magnetic spectrum.
6. Good electrical insulator (room temperature resistivity ~10
16
·cm).
7. Very resistant to chemical corrosion.
8. High radiation hardness.
9. High bandgap (5.47 eV).
10. High breakdown field (~2×10
7
V/cm).
11. High carrier mobility (2400 cm
2
/(V·s) for electrons, 2100 cm
2
/(V·s)
for holes.
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Table 2. Properties and application areas of CVD diamond.
Property Comments and
competing materials Possible applications
Vicker’s hardness
(kg/mm
2
)
Friction coefficient
12000-15000
~0.1 (in air)
As hard as bulk
diamond
Depends on the grain
size
Drill bits, polishing materials,
cutting tools, sintered or brazed
diamond compacts, wear resistant
coatings on windows and moulds
and bearing under vacuum
Young’s modulus
(N/m
2
)
Sound propagation
velocity (km/s)
1.2×10
12
18.2
Twice the value of
alumina, high
mechanical strength
1.6x the value of
alumina
Stiff membrane for lithography
masks, tweeter components,
micromechanical oscillators
SAW filters
Chemical inertness Inert At room temp.
resistant to all acids
bases and solvents
Coating for reactor vessels, diamond
containers, diamond electrodes
Range of high
transmittance (µm)
Refractive index
0.22-0.25 and
>6
2.41
In the IR orders of
magnitude lower than
other materials;
1.6x the value of silica
UV-VIS-IR windows and coatings,
microwave windows, optical filters,
optical wave guides
Band gap (eV)
Electron/hole
mobility (cm
2
/Vs)
Dielectric constant
5.47
2400/2100
5.5
1.1 for Si; 1.43 for
GaAs; 3 for B-SiC
1500/600 for Si
8500/400 for GaAs
11 for Si
12.5 for GaAs
High power electronics, high
frequency devices, high temperature
devices, solid-state detectors
Thermal conductivity
(W/cmK) 20 ~4x the value of Cu or
Ag Heat sinks for electronic devices,
heat spreading films on RF devices,
laser packages
Thermal expansion
coef. (1/K) 0.8×10
-6
At room temp. close to
silica value of
0.57×10
-6
Thermal stable substrates, e.g. for x-
ray lithography masks
Work function Negative The vacuum level lies
below the conduction
band
Light emitters, displays
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Table 3. Parameter range for diamond synthesis by filament-assisted thermal CVD method [30].
Gas mixture Total pressure Temperature (°C)
(Torr) Substrate Filament
H
2
+ CH
4
(0.5-2%) 10-100 700-1000 2000-2300
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Table 4. Present status of low pressure diamond CVD methods.
Method Results
Rate
(µm/h) Area
(cm
2
) Quality
(Raman) Substrates Advantages Drawbacks
Combustion
flame
Hot filament
DC plasma
jet
Microwave
plasma
DC
discharge
(low P)
DC
discharge
(medium
pressure)
RF plasma
(thermal, 1
atm)
Microwave
plasma
(ECR
2.45GHz)
30-100
0.5-8
930
3 (low P)
30 (high P)
<0.1
20-250
180
0.1
<2
>250
<2
100
70
<2
3
100
+ +
+ + +
+ + +
+ + +
+
+ + +
+ + +
- / +
Si, Mo,
TiN
Si, Mo,
silica etc.
MO, Si
Si, Mo,
silica, WC,
Cu etc.
Si, Mo,
silica etc.
Si. MO,
alumina
Mo
Si
Simple
Simple,
large area
Rate, quality
Quality,
stability
Simple,
large area
Rate, quality
Rate, quality
Area, low P,
low T
Area, stability
Contaminations,
stability
Contamination,
homogeneity,
stability
Rate, area
Quality, rate
Area
Area, stability,
homogeneity
Quality, rate,
contaminations
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Table 5. XRD patterns for powdered diamond (Cu Kα radiation, λ=1.5405 Å).
hkl 2θ relative intensity
111 43.9 100
220 75.3 25
311 91.5 16
400 119.5 8
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Table 6. Classification of metal substrates for CVD diamond.
1. Little or no solubility or reaction Cu, Sn, Pb, Ag, Au, etc
2. Strong carbon dissolving and
weak carbide formation Pt, Pd, Rh, Fe, Ni
3. Strong carbide formation Si, Nb, Ta, Cr, Mo, W, etc.
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Table 7. Diffusion depth p of carbon in iron, where the concentration C(p,T) of carbon is one
thousandth of its value C(0,T). Diffusion temperature is 1100ºK.
Diffusion time (min) 5 10 30
Diffusion depth (µm) 804 1138 1971
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Table 8. Raman shift and corresponding residual stress in diamond films of different thickness
deposited on Si substrates. The Raman shift is an average value of five different points
in each sample.
Film thickness
(µm) Raman shift
(cm
-1
) Stress evaluated from
Raman shift (GPa)
1.7 1332.43 -0.244
4 1332.33 -0.189
11 1331.93 0.038
23 1331.73 0.151
48 1331.60 0.227
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... The gas-phase deposition by thermal activation of precursor gases (Hot Filament Chemical Vapor Deposition (HFCVD) or Hot-Wire Chemical Vapor Deposition (HWCVD)) is widely used for the diamond structures' synthesis [1][2][3][4]. This method is based on the use of mixtures of hydrogen and carbon-containing gases activated on hot metal surfaces and interacting with a relatively cold substrate surface. ...
... At present, the research interest in the field of HFCVD diamond deposition stays high. The area of diamond coatings' application is constantly expanding [3], therefore, the majority of these studies are focused on obtaining the diamond coatings that meet the specific applications' requirements. At the same time, the development of the method itself clarifying the individual processes' role in the diamonds' synthesis, stays topical. ...
... In the HFCVD synthesis of diamonds, methane is most often used as a carbonaceous source [1][2][3]. At the same time, the diamond structures' synthesis using active fragments formed during the activation of gas mixtures of hydrogen and other hydrocarbons studies are still relevant. ...
Article
The gas-jet deposition method (a modification of the Hot Filament Chemical Vapor Deposition method (HFCVD)) was used to deposit some diamond structures on a molybdenum substrate from a hydrogen-ethylene mixture activated on hot tungsten. The experiments were performed for two lengths of activation reactors at different ethylene flow rates. For comparison, some diamond structures were synthesized from a hydrogen-methane mixture under similar conditions. The resulting structures were studied with the scanning electron microscopy and Raman spectroscopy methods. The reacting mixture flow through the heated catalytic channel was simulated by solving the Navier-Stokes equations accounting for homogeneous and heterogeneous chemical reactions. The reactor (hot tungsten tube) length and diameter effects as well as carbonaceous admixture supply effects were analyzed. The main diamond precursor's formation pathways were analyzed and the reaction chains' complexity was demonstrated. The simulation results are in good agreement with the experimental data. The numerical experiments made it possible to reveal the growth process key driving parameters.
... Эпитаксиальные слои и отдельные объемные кристаллы алмаза получают путем химического осаждения из газовой фазы (CVD). В реализации данного метода следует отметить работы [35][36][37][38][39][40][41]. Для выращивания монокристаллов с помощью технологии CVD требуется алмазная монокристаллическая подложка, в качестве которой обычно используется пластина, полученная методом HPHT. ...
Article
На базе как результатов собственных исследований, так и работ других авторов проведен критический анализ существующих методов контроля концентрации примесей и основных носителей заряда в широкозонных полупроводниках, а также рассмотрены вопросы совершенствования современной диагностики основных электрофизических свойств монокристаллического алмаза. Установлено, что для полупроводникового алмаза принципиально важным оказывается раздельное определение концентрации примеси и концентрации свободных носителей заряда, что является следствием очень малой (менее 1%) степени ионизации внедренной примеси. Показаны преимущества и перспективность спектроскопии адмиттанса как диагностического метода. При исследовании образцов алмаза, легированных бором, обнаружено уменьшение энергии активации дырок с примесного уровня бора от 325 до 100 meV при увеличении концентрации бора с 2·10 ¹⁶ до 4·10 ¹⁹ cm ⁻³ . Сделано предположение, что причиной различия в энергиях активации, получаемых измерениями на постоянном и переменном токе, являются измеряемые соответствующим прибором токи проводимости либо смещения. Для сильно легированных образцов монокристаллического алмаза с концентрацией бора N A ≥5·10 ¹⁸ cm ⁻³ при температурах 120-150 K зарегистрирован переход к прыжковому механизму проводимости по примесной (акцепторной) зоне с термической энергией активации 10-20 meV. На базе результатов собственных исследований и работ других авторов проведен критический анализ существующих методов контроля концентрации примеси и основных носителей заряда в широкозонных полупроводниках, а также рассмотрены вопросы совершенствования современной диагностики основных электрофизических свойств монокристаллического алмаза. Установлено, что для полупроводникового алмаза принципиально важным оказывается раздельное определение концентрации примеси и концентрации свободных носителей заряда, что является следствием очень малой (менее 1%) степени ионизации внедренной примеси. Показаны преимущества и перспективность спектроскопии адмиттанса как диагностического метода применительно к сверхширокозонным полупроводникам, предлагаются решения, направленные на корректную интерпретацию экспериментальных данных. Большая энергия ионизации примеси бора в алмазе (370 meV) приводит к сильной частотной дисперсии измеряемой барьерной емкости. Показано, что для корректных измерений концентрации носителей заряда методом ВФХ в условиях нарушения квазистатичности необходимо использование низких частот и высоких температур. Проводится сопоставление результатов электрофизических исследований с традиционными измерениями концентрации примеси в алмазе оптическими методами. При температурных измерениях адмиттанса образцов монокристаллического алмаза, легированного бором, обнаружено уменьшение энергии активации дырок с примесного уровня бора от 325 до 100 meV при увеличении концентрации бора N A с 2·10 ¹⁶ до 4·10 ¹⁹ cm ⁻³ , а также при N A ≥5·10 ¹⁸ cm ⁻³ при температурах 120-150 K зарегистрирован переход к прыжковому механизму проводимости по примесной (акцепторной) зоне с термической энергией активации 10-20 meV. Ключевые слова: монокристаллический алмаз, примесь бора, концентрация носителей заряда, энергия активации, адмиттансная спектроскопия, вольт-фарадные измерения.
... However, they are difficult to be used in the applications of semiconductors and other modern technologies, for the small size and random shape of natural diamonds. Since the first synthetic diamond was synthesized in 1954 [6], high-pressure and high-temperature synthesis, chemical vapor deposition, and other synthetic diamond technologies began to develop rapidly [7][8][9][10]. They have solved the limitation of the small size and random shape of natural diamonds and greatly promoted the application of diamonds. ...
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Diamond is an important material in today and future industry, for its relatively advantages in hardness, optics, heat, and electricity. Strongly impeted by the synthetic low-cost and large-scale manufacturing feasibility, applications of diamond material quickly developed to optical windows, heat transfer, electronic semiconductors, and other high-tech fields, from the traditional field of cutting tools and jewelry. However, these applied fields require for superior surface quality, while great difficulties in the polishing and smooth processing are brought by the excellent physical and chemical characteristics of the diamond. In this paper, the research status of diamond mainstream polishing methods and dynamic friction polishing is summarized and analyzed, which proved that dynamic friction polishing is superior to other polishing methods in cost, efficiency, and precision. The progress of dynamic friction polishing technology was discussed from the aspects of equipment innovation, process parameter optimization, and material removal mechanism exploration. Dynamic friction polishing possesses vividly considerable application prospects, for its simple equipment, low cost of the processing, and can realize micro/nanoscale polishing at room temperature and no protective gas conditions. More strikingly, it can quickly reduce the surface roughness from microlevel to nanolevel.
... CVD, as the name suggests, involves the deposition of chemical vapour onto a substrate, resulting in the formation of diamond films. Nanocrystalline and ultra-nanocrystalline films with grain sizes of 5-100 and 3-5 nm, respectively, can be prepared [6]. NDs are more commonly prepared by HPHT or detonation of carbon explosives [7]. ...
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Nanodiamonds, due to their chemical inertness and biocompatibility, have found extensive uses in drug delivery and biomedical applications. Fluorescent nanodiamonds with fluorescent properties generated by nitrogen-vacancy defects have been intensively investigated for bioimaging, due to their high quantum yield and high photobleaching stability. In addition, the surface properties and particle size of nanodiamonds have significant impacts on cellular uptake and imaging quality. In this study, nitrogen-vacancy nanodiamonds with different particle sizes (40 nm and 90 nm) have been physicochemically characterised and investigated for their cytotoxicity and potential in fluorescence imaging. The nanodiamonds (with concentrations up to 100 µg/mL) showed cell viability >70% with mesenchymal stromal cells. The number of nanodiamonds was observed to have a larger impact on cell viability than the mass of nanodiamonds. Larger nanodiamonds (90 nm) exhibited a lower level of cytotoxicity, higher cellular uptake and fluorescence intensity. The results indicate the potential of using fluorescent nanodiamonds as a nanoprobe for effective bioimaging and cell tracking.
Article
Carbon fibre reinforced polymers (CRFP) are extensively used in many industrial applications thanks to its mechanical properties and its low weight. Nevertheless, the orthotropic character of CRFP highly reduces its applications. The transversal electrical conductivity in CRFP is two orders poorer than in the longitudinal direction. To improve their electrical properties, this work proposes the use of polycrystalline boron doped diamond (BDD) as coating of the carbon fibres (CF). BDD coating is deposited on CF surface using microwave plasma enhanced chemical vapor deposition (MPCVD) system. The BDD coating forms a rigid conductive coating around the CF as a core–shell structure. Here, an electrical characterization of both, 12,000 filaments (a tow) and a single coated filament, are carried out in the longitudinal and cross-section directions. Macro, micro and local analysis using the Kelvin method, Conductive Atomic Force Microscopy (C-AFM), and Scanning Microwave Impedance Microscopy (sMIM), were carried out to evidence the improvement of the electrical properties. Macro measurement reveals that the BDD coating decreases to half the resistivity of the CF. The BDD coating raises the local electrical conductivity of the CF by an order of magnitude with respect to the uncoated ones. sMIM maps identified BDD locations in ring-like configurations.
Article
As one of the representatives of carbon-based semiconductors, diamond is called the “Mount Everest” of electronic materials. To maximize its properties and realize its industrial applications, the fabrication of wafer-scale high-quality diamonds is critical. To date, heteroepitaxy is considered as a promising method for the growth of diamond wafers with considerable development. In this review, fundamentals of diamond heteroepitaxy is firstly introduced from several perspectives including nucleation thermodynamics and kinetic, nucleation process at the atomic level, as well as the interplay between the epitaxial film and substrate. Second, the bias enhanced nucleation (BEN) method is reviewed, including BEN setup, BEN process window, nucleation phenomenology (mainly on Iridium), nucleation mechanism by ion bombardment, and large-scale nucleation realization. Third, the following textured growth process is presented, as well as grain boundary annihilation, and dislocation and stress reduction technologies. Fourth, the applications of diamonds in electronic devices are studied, showing its excellent performances in the future power and electronic devices. Finally, prospects in this field are proposed from several aspects.
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The incorporation of impurity atoms into diamonds has been an important issue for the application in the area of electronics, opto-electrics, and quantum optics with color centers. To date, it remains a challenge to explore the impurity distribution in diamond films owing to the low incorporation efficiency. In this work, Si-doped diamond films were deposited in microwave CVD system. Thermal oxidation was employed to selectively etch the non-diamond phase to study the impurity distribution and evolution. For micro-/nano-sized diamond films, the micro-sized grains remain intact, while the diamond nanocrystals are oxidized into porous oxides. The diamond needles exhibit strong silicon-vacancy center optical emission at 738 nm, implying that the Si atoms are incorporated into the lattice. Detailed microstructure characterizations reveal that the porous oxides are crystallized in amorphous state, consisting of silicon, oxygen, and carbon elements. Such abundance of Si in the amorphous porous oxides suggests that the Si atoms segregate at the grain boundaries. Therefore, this work provides a new path to reveal the impurity distribution along diamond crystalline defects. Moreover, the in-situ formed silicon oxide can act as an anti-reflection coating to enhance the optical emission of color centers, which is important for their optical applications.
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Diamond-silicon carbide (SiC) composite stacks is composed of two kinds of wide bandgap materials, each of which has excellent thermal, electronic, optical and mechanical properties, and is considered as an ideal material for heat dissipation. For optimal application, the interface between the two materials needs to be almost void free and of high-quality growth. Traditional methods such as sintering and liquid/vapor phase infiltration have many defects, but the preparation of diamond-SiC composites by microwave plasma chemical vapor deposition (MPCVD) method can effectively solve these problems, overcome the interface defects, and break through the size limitation. In this review, various techniques for preparing diamond-SiC composites by MPCVD will be discussed. It mainly includes co-deposition of diamond and cubic polytype β-SiC, deposition of diamond films on β-SiC/Si substrates and deposition of diamond films on 4H-SiC and 6H-SiC substrate. The implementation methods, research progress and application trend of each technique are reviewed in detail.
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To improve the adhesion of diamond film, we worked on forming functionally gradient diamond film, which have intermediate composition gradient layer consisting of diamond synthesized by dc plasma jet CVD and plasma sprayed material. Diamond film is synthesized by spraying a plasma jet of hydrogen and methane onto a water-cooled substrates. Intermediate layers is formed by feeding powdered materials to the plasma for diamond synthesis. The composition gradient is controlled by changing the powder feed rate. WC and Mo powders were plasma sprayed at the same time with diamond synthesis. The formation of the mixture of diamond and plasma sprayed materials was observed by XMA analysis. TEM, X-ray diffraction, and Raman spectroscopy analysis showed that intermediate layer does not contain graphite. The adhesion strength of the diamond film with an intermediate layer exceeded 150 kg/cm²,compared to less than 10 kg/cm² for that without an intermediate layer.
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Diamond can offer a number of advantages in MEMS structures owing to its outstanding material properties. Furthermore, it is a multifunctional material and can be used as a substitute for a variety of materials such as metals and ceramics, particularly for actuator structures. In this review, microstructures with electromechanical activation are evaluated with emphasis on two questions: firstly, what is the best actuation principle to obtain the full benefit from diamond material properties and secondly, what are the technological possibilities that can be used to develop advanced components based on these activation principles? These features are highlighted by a hybrid fabrication scheme for RF switches. It has already been shown that there is a tool box of technological materials and processing steps that allows the establishment of fabrication routines. Equally, simulation has progressed such that such structures can be designed and optimized, and performance advantages over conventional technologies can be predicted.
Chapter
Amongst the varieties of applications of CVD diamond, the Surface Acoustic Wave (SAW) device, using its high SAW velocity, is supposed to be one of the practical applications, because polycrystalline film can be utilized. This is also followed by some other advantages: impurity control is not required; and thicknesses of only several microns are sufficient because the energy concentration is on the surface, which enables low-cost manufacturing.
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Nanocrystalline diamond films and e-beam patterning techniques have been used to fabricate visible to near-infrared photonic slab crystals (PhCs) with deep submicron feature sizes. Two methods of fabrication, both based on electron-beam lithography, have been explored and are detailed in this Communication. The first method uses direct patterning of flowable oxide as a hard mask for a subsequent highly anisotropic oxygen plasma reactive ion etching of the nanocrystalline diamond film. The second method involves image inversion and employs an organic-inorganic bilayer resist structure that planarizes the surface and provides for a well-controlled undercut. The subsequent metal evaporation and lift-off creates a metal mask with 100 nm features demonstrating fine control over edge roughness that is not compromised by the nanocrystalline roughness of the diamond film. Chromium etch mask and oxygen plasma were used to fabricate the diamond PhC. With the proper choice of metal mask and reactive ion etch, this technique can be applied to a wide range of nanocrystalline and polycrystalline films and will enable further scaling into the sub-100 nm regime.
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We studied diamond film nucleation phenomena using d.c. plasma jet chemical vapour deposition (CVD). Pt and Cu, which have no carbon atom interaction, generate high density diamond nucleation. W, Mo, Ti, Ta and Nb, which form carbide compounds, generate carbide before nucleation. Ni, Co and Fe, which have a negative energy of carbide formation, generate low density nucleation. These results suggest that diamond nucleation needs a stable surface for the carbon radicals. We developed a new process to increase the nucleation density for low nucleation density substrates. A plasma-sprayed layer, which promotes nucleation, was formed on the substrate surface before diamond synthesis. Both diamond synthesis and the formation of the plasma-sprayed layer can be done by d.c. plasma jet CVD without vacuum leakage.
Article
A method of surface treatment was found to enhance diamond chemical vapor deposition nucleation on nondiamond substrates such as Si, SiO2, and Al2O3. The nucleation density obtained by ultrasonic abrasion with diamond powder alone was found to be enhanced by a few orders of magnitude using a mixed slurry consisting of diamond and metal powders. No strong nucleation enhancement was observed using a metal slurry only for surface treatment. The metal powders used were W, Ta, Mo, Nb, Ti, Al, Fe, Ni, Cu, and Si. It is concluded that the enhanced nucleation is associated with a physico‐chemical modification of the substrate surface, attained through a cooperative effect of both the metal and diamond particles during the ultrasonic abrasion process.
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The effects of halogenated precursor associated with O2 addition in the gas phase for diamond growth have been studied. Optical emission spectroscopy (OES) and exhaust gas mass spectrometry (MS) were used in a microwave plasma-assisted chemical vapor deposition (MWPACVD) reactor. The relative emission intensity of atomic hydrogen line Hα (656.3 nm) has been measured with CF4 and CCl2F2 concentrations in the range of 0–5%. Our results show an increase of atomic hydrogen concentration up to 80 and 200% when CF4 and CCl2F2 are added in the mixture, respectively. The input gases were chosen in order to discern the oxygen effects for mixtures with different concentrations of CF4 and CCl2F2. An additional increase of 50% H generation was observed when O2 was added in the range of 0–3% for both mixtures with 3% of CF4 or CCl2F2. Mass spectrometry was used to analyze the dissociation process of CF4 and CCl2F2 associated with HF and HCl formation. These dissociation processes are correlated with the increase of atomic hydrogen concentration. For oxygen addition, CO formation is the main final product from the halocarbon. The higher thermodynamic stability of CO enables us to obtain a gas phase with a high concentration of atomic hydrogen and inhibits the formation of solid non-diamond carbon observed in mixtures with high halocarbon concentrations.
Article
A stress relief multilayer (SRM) structure to improve the adhesion of chemical vapour deposition (CVD) diamond films to steel and WC-Co substrates is described. This structure contains silver as a stress-relaxing layer and a refractory metal (RM) layer as a diamond-bonding layer. Raman analysis shows that the thermal stress in diamond films deposited on steel and WC-Co substrates coated with SRM structures is relaxed after a thermal treatment, performed during the last stage of the CVD. The results of tests of diamond film adhesion to the SRM structures are presented. An X-ray investigation of the diamond-RM bonding interface shows where adhesion failure occurs.