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Morphology and optoelectrical properties study of nano/micro structures silicon layer prepared by photo electrochemical and electrochemical etching

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  • Mustansiriyah University
  • Alnukhba University College

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In this work, Electrochemical Etching, (ECE) and Photo Electrochemical Etching, (PECE) were used to produce porous silicon for p-type and n-type (111) orientation. The Root-mean-square (RMS) surface roughness is a commonly accepted parameter to describe surface by imaging techniquesAtomic Force Microscopic (AFM) was used to analyse the surface sample. The effect of type substrate on a surface porous morphology by optical microscope have been examined. the dependence of porous silicon morphology on fabrication conditions and the link between morphology, photocurrent, and energy gap of porous silicon layer (PS)have been determined.
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Morphology and optoelectrical properties study of nano/micro structures
silicon layer prepared by photo electrochemical and electrochemical etching
Nadir F.Habubi1*, Raid A.Ismail2, Hasan A.Hadi3
1Dep. Physics-Education faculty-The University of Mustanseriyah-Baghdad, (IRAQ)
2Dep. of Applied Sciences-The University of Technology Baghdad, (IRAQ)
3Dep. Physics-Education faculty-The University of Mustanseriyah-Baghdad, (IRAQ)
E-mail : nadirfadhil@yahoo.com, hadihasan@hotmail.co.uk, raidismail@yahoo.com
ABSTRACT
In this work, Electrochemical Etching, (ECE) and Photo Electrochemical Etching, (PECE) were used to produce
porous silicon for p-type and n-type (111) orientation. The Root-mean-square (RMS) surface roughness is a
commonly accepted parameter to describe surface by imaging techniques Atomic Force Microscopic (AFM) was
used to analyse the surface sample. The effect of type substrate on a surface porous morphology by optical
microscope have been examined. the dependence of porous silicon morphology on fabrication conditions and the
link between morphology, photocurrent, and energy gap of porous silicon layer (PS)have been determined.
2013 Trade Science Inc. - INDIA
An Indian Journal
Trade Science Inc.
Volume 7 Issue 3
Nano Science and Nano Technology
Nano Science and Nano Technology
NSNTAIJ, 7(3), 2013 [108-113]
ISSN : 0974 - 7494
INTRODUCTION
Porous silicon (PS) has many unique characteristics
such as direct and wide modulated energy band gap,
high resistivity, vast surface area-to-volume ratio and the
same single-crystal structure as bulk. Physical features
of porous silicon are connected with quantum confine-
ment effects, i. e., with a change of the band diagram and
increase of effective band gap. Distinction in absorption
of the light by PS and crystalline silicon is that in PS the
pores can play a role of waveguides. The light got in a
pore, after repeated reflections from the pore walls will
penetrate far deep into PS. At the expense of it, PS ab-
sorbs light more strongly than bulk silicon. Therefore, it is
promising for creation of photodetectors on the PS base.
These advantages make it a suitable material for photo-
detectors[1]. Silicon nanocrystal have been shown to pos-
sess intriguing properties, such as band gap control with
nanocrystal size, very fast optical transition, and multiple
carrier generation[2].
EXPERIMENTAL DETAILS
Electrochemical etching
The silicon wafer serves as the anode. The cath-
ode is made of platinum or any HF-resistant and con-
ducting material. The cell body itself is, in general, made
of highly acid-resistant polymer such as Teflon. Since
the entire silicon wafer serves as the anode, PS is formed
on any wafer surface in contact with the HF solution,
including the cleaved edges. Figure 1 shows a sche-
matic design of PS system.
Photo electrochemical etching
As shown in Figure 2, in this case by illuminating n-
type substrates, surface of the wafer with sufficiently
energetic photons, holes can be photo-generated in the
bulk. Illuminating n-type substrates surface by the di-
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ode laser (red-650nm) and power of 30mW To get etched area , a double-concave lens is used.
Figure 1 : Schematic diagram for the designed porous silicon fabrication system.
Sample preparation
The experiments, Porous silicon films were produced
using monocrystalline silicon wafers p and n-type, with
resistivitys (14-22) .cm, and 10 .cm respectively
having a (111) orientation. Samples were made of po-
rous silicon produced with a standard technique of an-
odizing p and n-Si silicon substrates in an electrolyte HF
(40%) :(99.8) % CH2OH (with a 1:1 volume ratio) un-
der constant etching current density of 60 mA/cm² at
10min etching time. Methanol and alcohol are used com-
monly to clean the wafer by immersing it in these chemi-
cals in turn, in the ultrasonic bath for few second. The
average porous layer thickness and the porosity were
measured by gravimetric methods. The samples were
thermally oxides in air (at 300 °C for 30 min). top Al/PS/
c-Si/bottom Al was shown in Figure 3. To ensure an
uniform current distribution as possible, the samples were
coated with 800 nm layer of aluminum on the back-
side. The evaporation was performed in a vacuum pres-
sure of 10-6 torr, using an evaporation plant model E306
A manufactured by Edwards high vacuum. In this work,
an AA 3000 Scanning Probe Microscope AFM system
in School of applied sciences, University of Technology
has been used. The current passing through the device
was measured using a UNI-T UT61E Digital Multimeters.
This measurements was done under light of different illu-
mination power densities supplied by a Halogen lamp
150W which is connected to a variac and calibrated by
a power-meter. The photosensitivity of the photodetec-
tor was investigated in the wavelength range of 400-1000
nm with the aid of Joban-Yvon monochromatic and stan-
dard Si power-meter.
Figure 3 : Cross-sectional view Al/PS/c-Si/Al sandwich struc-
ture.
RESULTS AND DISCUSSION
X-ray diffraction
Figures 4 and 5 show the X-ray diffraction pat-
terns of the porous structure on p-Si and n-Si substrate
Figure 2 : Schematic diagram of the porous silicon fabrica-
tion system.
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at different etching current density respectively. A peak
becomes very broad with varying full-width at half maxi-
mum as shown in Figures 4-5 which confirm the forma-
tion of porous structure on the crystalline silicon sur-
face at 40 mA/cm2. At high etching current density large
than 60mA/cm2 XRD spectra showed that the struc-
ture is amorphous.
on n- and p- type for 10 min etching time 60 mA/cm2
etching current density are shown in the following fig-
ures. The PS layer thickness and roughness are not
monotonically proportional to the anodization time. The
surface morphology measured by AFM is given in Fig-
ures 7 and 8, which show that the surface of the PS
layer consists of inhomogeneous and large number of
irregularly shaped distributed randomly over the en-
tire surface. Representative 5 µm x 5 µm images two
and three dimension of porous silicon with various
etching time are shown in Figures 7 and 8. The sur-
face of the etched PS layer consists of a matrix of
randomly distributed nanocrystalline Si pillars which
have the same direction and AFM images also show
voids that the uniformity. Root-mean-square (RMS)
surface roughness is a commonly accepted parameter
to describe surface. It is typically used to quantify varia-
tions in surface elevation, the RMS roughness for p-
type is found to be 24 nm and n-type was 5.52 nm;
also the Sz.(Ten Point height) was 50.9 nm for n-type
and 199 nm for p-type, With large irregular upright
structure of silicon crystallites. It is clear that the for-
mation porous layer depends on the substrate type.
The change of these values in RMS for n and p-type
agree with Hong and Lee[4]. These different morpholo-
gies are the result of different pore formation mecha-
nisms. Many mechanisms are thought to contribute to
the electrochemical pore growth process in silicon,
and the morphology resulting from a given experiment
is usually determined by a combination of several of
these[5].
Figure 4 : XRD spectra of c-Si and PS samples anodized for
10 min at b) 40mA.cm-2, and c) 60mA.cm-2.
Figure 5 : XRD spectra of c-Si and PS samples anodized for
10 min at b) 40mA.cm-2, and c) 60 mA.cm-2.
According to the theory of propulsion the essential
tensile stresses are produced both in porous silicon and
in Si substrates. Therefore, the micro cracks are formed
in PS and that serve as easy path for further pore
growth[9].
SEM
The structural properties of PSi layer such as sur-
face morphology, specific surface area, pore width, pore
shape, thickness of walls between pores, and layer thick-
ness have been studied in this work by using SEM.
Figure 6 shows the electron micrograph of inclined sili-
con surface etched with 40mA/cm2. This figure con-
firms formation of columnar grains arranged periodi-
cally along the etched surface.
AFM
Two and Three-dimensional AFM image of the
as-anodized porous silicon surface structure formed Figure 6 : SEM micrograph of PS prepared with J=40 mA/
cm2.
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Optical microscopy
From Figure 9, optical micrograph shows that the
distinct variation between the fresh silicon surface and
the porous silicon surfaces formed at 40mA/cm2 etch-
ing current density for 10 min etching time. The sample
exhibited high density of small pores distributed over
the etched region where there is big difference between
the non-etched and etched silicon surfaces as shown in
Figures 6 and 7. The porous surface shows different
colors as shown in Figure 7; also some time close to
red resulting may be sub oxide of silicon. This confirms
the anodic dissolution of the silicon surface leading to
porous structure formation and the visual observation
of the silicon surface is considered as a very important
feature gave photoluminescence.
Figure 7 : 2D and 3D AFM images of PS surface at constant
current density 60mA/cm-2 and at 10min etching time for n-
type (5ìm×5ìm).
Figure 8 : 2D and 3D AFM images of PS surface at constant
current density 60mA/cm-2 and at 10min etching time for p-
type (5ìm×5ìm).
Photoluminescence in simple terms is a reverse pro-
cess of absorption, so the different color resulting from
broadening of the band gap energy occurs when there
is a decrease in the crystallite size. (PL) at room tem-
perature when compared to a fresh c-Si surface for p-
Si and n-Si.
Illuminated current voltage
Figures 10 and 11 show the reversed current-volt-
age characteristics of the device measured in dark and
under different light intensity illumination, the photocur-
rent under a (1.2-20) mW/cm2 tungsten lamp illumina-
tion, also this figure explains the J-V characteristics
sandwiches Al/PS/p-Si/Al and Al/PS/n-Si/Al structure
under illumination with constant etching time. It can be
Morphology and optoelectrical properties study of nano/micro structures silicon layer
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seen that the reverse current value at a given voltage for
the Al/PS/c-Si/Al heterojunction under illumination is
higher than that in the dark. It increases with the in-
crease of light intensity. Increasing the bias voltage in-
creases the photocurrent. The photocurrent decreases
with increasing preparation current density, etching time,
etc. The increasing value of resistivity is due to increas-
ing the PS layer thickness, but as shown in the figures,
there are increase in photocurrent at increasing PS layer
thickness in p-type. This may due to the excessive etch-
ing process which leads to increase of porosity of the
porous silicon layer and hence improve the sensitivity of
the formed junction between the crystalline silicon and
the PS layer[3]. Figure 12 demonstrates the dependence
of photocurrent on light power density. It is clear that the
relationship is linear and the Al/PS/p-Si/Al photodetec-
tor has good linearity characteristics compare with the
Al/PS/n-Si/Al photodetector.
Figure 9 : [a] and [b] optical micrograph of PS surface on p-Si and n-Si formed at 60mA/cm2 etching current density for
10min etching time; [c] Optical images of cross-sections of porous silicon PS/Si interface.
Figure 10 : Photocurrent of PS/p-Si heterojunction as a func-
tion of reverse bias illuminated for different power density at
10 min etching time, 60mA/cm2.
Figure 11 : Photocurrent of PS/n-Si heterojunction as a func-
tion of reverse bias illuminated for different power density at
10 min etching time 60mA/cm2.
Figure 12 : Photocurrent density of PS/p-Si and PS/n-Si
heterojunction as a function power density for different etch-
ing time at 5v reverse bias.
Energy gap of porous silicon
The value of energy gap is determined by the
photoresponse spectrum curve between photocurrent
and energy of quanta of the incident light. In the case of
nano- or micro-porous silicon, quantum confinement
causes spatial fluctuations of the effective band gap as
can be seen in Figures 13 and 14 for p-type and n-
type, so as[6,7] reported that the porous layer behaves
as wide band semiconductor sensitive to the visible light.
Figures 13 and 14 show the energy gap for the investi-
gated sample is large than 1.1eV, and for nanostructure
PS layer was 2.26eV, while it was 1.9eV for micro-
structure PS layer. The enhancing of band gap in PS is
related to quantum-size effect. The main quantum con-
finement effect is represented by the appearance of new
energy levels in the silicon band gap. The increased band
gap resulting from quantum - confinement excludes va-
lence band holes from these smallest regions of the po-
rous silicon matrix[8].
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CONCLUSION
The dimensions and morphology of the pores can
be controlled by anodization parameters. Porous sili-
con was anodized on n-type silicon with laser light and
on p-type silicon in dark, using a current density of 60
mA/cm2 etching current density for 10 min etching time.
We demonstrated that it is possible to use the AFM to
obtain information of the surface of porous layers, such
as surface roughness and thickness. The Al/PS/n-Si/Al
photodetector has the performance of photocurrent ~
1079ìA under 20 mW/m2 illuminations and dark cur-
rent ~ 0.08ìA, while the Al/PS/p-Si/Al photodetector
has the performance of photocurrent ~ 1500ìA and
dark current ~ 26 ìA at the same condition. The low-
energy gap at 1.9 eV was obtained in PS with character-
ized by the thickness 222.53 nm for p-type and the high-
energy gap at 2.26 eV was obtained in PS with charac-
terized by the thickness 66.76 nm for n-type. The peak
shifts towards the higher energy side, which supports the
quantum confinement effect in porous silicon.
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Article
The structure and optical properties of n-type and p-type porous silicon (PS) prepared by the chemical etching in the light and the dark, respectively, are reported in this paper. Micro-structural features of the samples are mainly investigated by SEM, AFM, XRDGI techniques. Also, their optical properties are investigated by photoluminescence (PL) and Fourier transform infrared absorption measurements. In the n-type PS, the room temperature photoluminescence is observed in a visible range from 500 nm to 650 nm in contrast to that in the blue region (400-650 nm) in p-type PS. Further, semi-transparent, Cu films in thickness range of ∼ 40 nm are deposited by rf-magnetron sputtering on PS to investigate the I-V characteristics of the samples.
Chapter
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Electrical and photoelectrical properties of Al/porous silicon/ monocrystalline silicon sandwich- structures (Al/PS(c-Si)) based on nanostructured porous silicon, obtained by electrochemical anodization of monocrystalline silicon wafers are reported. The photosensitivity of Al/PS(c-Si) structures is determined by PS layer photoconduction. The photoelectrical method of denition of eectiv e band gap in the vicinity of PS(c-Si) heterojunction is proposed. The opportunity of denition of the eectiv e diameter of quantum wires is shown. Diusion length of the minority charge carriers in porous silicon is determined by the method of reverse photocurrent.
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Chapter
Porous silicon (PS) is a material formed by anodic dissolution of single crystalline silicon in HF containing solutions. Since its discovery more than four decades ago, a large number of investigations have been undertaken, the results of which revealed that PS has extremely rich morphological features and the formation process of PS is a very complex function of numerous factors.1 Accordingly, many theories have been proposed on the various mechanistic aspects on formation and morphology of PS. Figure 1 is a summary of the progress of research on PS with respect to the discovery of major PS features and development of theories.
Article
The present state-of-the-art in understanding the mechanisms of the formation of porous silicon (PS) and its physical properties is reviewed, with special emphasis on problems which were not much in the focus of existing review literature: mechanisms of the pore growth, stability of the PS properties in environment and electrical properties of PS layers. Emerging applications of porous silicon in different fields of technology are outlined.
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A highly sensitive (metal/nanostructure silicon /metal) photodiode has been fabricated from rapid thermal oxidation (RTO) and rapid thermal annealing (RTA) processes,Of nanostructures porous silicon prepared by laser assisted etching . Photoresponse was investigated in the wavelength rang of (400-850nm) . A responsivity of (3A/w) was measured at (450 nm) with low value of dark current of about ( 1 µA /cm2 ) at 5 volt reverse bias.
  • D F Timokhov
  • F P Timokhov
D.F.Timokhov, F.P.Timokhov; Semiconductor Physics, Quantum & Optoelectronics, 6(3), 307-310 (2003).
Porous silicon in practice: Preparation, characterization and applications, 1 st Edition
  • Michael
Michael; Porous silicon in practice: Preparation, characterization and applications, 1 st Edition, Published 2012 by Wiley-VCH Verlag GmbH & Co. KGaA, 15 (2012).