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Compositional dependence of epitaxial Tin+1SiCn MAX-phase thin films grown from a Ti3SiC2 compound target
Abstract
We investigate sputtering of a Ti3SiC2 compound target at temperatures ranging from RT (no applied external heating) to 970 oC as well as the influence of the sputtering power at 850 oC for the deposition of Ti3SiC2 films on Al2O3(0001) substrates. Elemental composition obtained from time-of-flight energy elastic recoil detection analysis shows an excess of carbon in all films, which is explained by differences in angular distribution between C, Si and Ti, where C scatters the least during sputtering. The oxygen content is 2.6 at.% in the film deposited at RT and decreases with increasing deposition temperature, showing that higher temperatures favor high purity films. Chemical bonding analysis by X-ray photoelectron spectroscopy shows C-Ti and Si-C bonding in the Ti3SiC2 films and Si-Si bonding in the Ti3SiC2 compound target. X-ray diffraction reveals that the phases Ti3SiC2, Ti4SiC3, and Ti7Si2C5 can be deposited from a Ti3SiC2 compound target at substrate temperatures above 850 oC and with growth of TiC and the Nowotny phase Ti5Si3Cx at lower temperatures. High-resolution scanning transmission electron microscopy shows epitaxial growth of Ti3SiC2, Ti4SiC3, and Ti7Si2C5 on TiC at 970 oC. Four-point probe resistivity measurements give values in the range 120 to 450 mucro-Ohm-cm and with the lowest values obtained for films containing Ti3SiC2, Ti4SiC3, and Ti7Si2C5.
... MAX-phase ternary carbides possess excellent electrical conductivity and corrosion resistance. However, their fabrication requires a high-temperature condition [23][24][25][26][27]. Bipolar plates have been coated with ternary carbide using a two-step process [28], including low-temperature film deposition and subsequent vacuum annealing. ...
... Interestingly, they found that the differences in the electronic structure are strongly related to the bonding nature between Ti-C and Ti-A layers. Later, they also investigated the substrates temperature growth of these films and found that films grown at room temperature possess almost 2.6 at% of oxygen content [63], which deceases with the increase in growth temperature. The film also shows room temperature resistivity of 120 -cm. ...
Layered nanolaminate ternary carbides, nitrides and carbonitrides with general formula Mn+1AXn or MAX (n = 1, 2, or 3, M is an early transition metal, A is mostly group 13 or 14 element, and X is C and/or N) has revolutionized the world of nanomaterials, due to the coexistence of both ceramic and metallic nature, giving rise to exceptional mechanical, thermal, electrical, chemical properties and wide range of applications. Although several solid-state bulk synthesis methods have been developed to produce a variety of MAX phases, however, for certain applications, the growth of MAX phases, especially in its high-quality epitaxial thin films form is of increasing interest. Here, we summarize the progress made thus far in epitaxial growth and property evaluation of MAX phase thin films grown by various deposition techniques. We also address the important future research directions to be made in terms of thin-film growth. Overall, in the future, high-quality single-phase epitaxial thin film growth and engineering of chemically diverse MAX phases may open up interesting new avenues for next-generation technology.
... Deposition of Group 4-6 TMB2 films from compound targets is further complicated by the difference in atomic masses between the constituents of the target: B with 10.81 and the TM ranging from Ti with 47.867, to W with 183.84. When emitted by the sputtering process, this mass effect results in a different angular distribution with a larger probability of lighter atoms such as boron to be sputtered along the target normal [245] as investigated by Olson et al. [246] For TMB2 a significant deviation of the film composition in films containing elements with significant mass difference in the target has previously been observed for WBx films where, Willer [247] et al. in 1990 deposited W-rich films from a WBx compound target (27 at .% B). ...
We review the thin film growth, chemistry, and physical properties of Group 4-6 transition-metal diboride (TMB2) thin films with AlB2-type crystal structure (Strukturbericht designation C32). Industrial applications are growing rapidly as TMB2 begin competing with conventional refractory ceramics like carbides and nitrides, including pseudo-binaries such as Ti1-xAlxN. The TMB2 crystal structure comprises graphite-like honeycombed atomic sheets of B interleaved by hexagonal close-packed TM layers. From the C32 crystal structure stems unique properties including high melting point, hardness, and corrosion resistance, yet limited oxidation resistance, combined with high electrical conductivity. We correlate the underlying chemical bonding, orbital overlap, and electronic structure to the mechanical properties, resistivity, and high-temperature properties unique to this class of materials. The review highlights the importance of avoiding contamination elements (like oxygen) and boron segregation on both the target and substrate sides during sputter deposition, for better-defined properties, regardless of the boride system investigated. This is a consequence of the strong tendency for B to segregate to TMB2 grain boundaries for boron-rich compositions of the growth flux. It is judged that sputter deposition of TMB2 films is at a tipping point towards a multitude of applications for TMB2 not solely as bulk materials, but also as protective coatings and electrically conducting high-temperature stable thin films.
... Deposition of Group 4-6 TMB 2 films from compound targets is further complicated by the difference in atomic masses between the constituents of the target: B with 10.81 and the TM ranging from Ti with 47.867, to W with 183.84. When emitted by the sputtering process, this mass effect results in a different angular distribution with a larger probability of lighter atoms such as boron to be sputtered along the target normal [245] as investigated by Olson et al. [246] For TMB 2 a significant deviation of the film composition in films containing elements with significant mass difference in the target has previously been observed for WB x films where, Willer [247] et al. in 1990 deposited W-rich films from a WB x compound target (27 at.% B). ...
We review the thin film growth, chemistry, and physical properties of Group 4–6 transition-metal diboride (TMB2) thin films with AlB2-type crystal structure (Strukturbericht designation C32). Industrial applications are growing rapidly as TMB2 begin competing with conventional refractory ceramics like carbides and nitrides, including pseudo-binaries such as Ti1-xAlxN. The TMB2 crystal structure comprises graphite-like honeycombed atomic sheets of B interleaved by hexagonal close-packed TM layers. From the C32 crystal structure stems unique properties including high melting point, hardness, and corrosion resistance, yet limited oxidation resistance, combined with high electrical conductivity. We correlate the underlying chemical bonding, orbital overlap, and electronic structure to the mechanical properties, resistivity, and high-temperature properties unique to this class of materials. The review highlights the importance of avoiding contamination elements (like oxygen) and boron segregation on both the target and substrate sides during sputter deposition, for better-defined properties, regardless of the boride system investigated. This is a consequence of the strong tendency for B to segregate to TMB2 grain boundaries for boron-rich compositions of the growth flux. It is judged that sputter deposition of TMB2 films is at a tipping point towards a multitude of applications for TMB2 not solely as bulk materials, but also as protective coatings and electrically conducting high-temperature stable thin films.
... Therefore, the carbide deposited after 30 min could be considered as an interlayer that promotes the nucleation of the MAX phase. Indeed, a substoichiometric carbide interlayer has been found to undergo a solid state reaction with the [44], which was also the case of the target used in our study (C-rich target). Representative SEM micrographs of the produced thin films in Fig. 5 show the increase in grain size when the deposition temperature increased from 500 to 800 °C. ...
This work reports on sputter depositions carried out from a compound (Ti,Zr)2AlC target on Al2O3(0001) substrates at temperatures ranging between 500 and 900 °C. Short deposition times yielded 30 to 40 nm-thick Al-containing (Ti,Zr)C films, whereas longer depositions yielded thicker films up to 90 nm which contained (Ti,Zr)C and intermetallics. At 900 °C, the longer depositions led to films that also consisted of solid solution MAX phases. Detailed transmission electron microscopy showed that both (Ti,Zr)2AlC and (Ti,Zr)3AlC2 solid solution MAX phases were formed. Moreover, this work discusses the growth mechanism of the thicker films, which started with the formation of the mixed (Ti,Zr)C carbide, followed by the nucleation and growth of aluminides, eventually leading to solid state diffusion of Al within the carbide, at the highest temperature (900 °C) to form the MAX phases.
... Another important aspect is the use of compound targets, which is generally preferred in industrial applications for its simplicity and repeatability. Yet, it has been recognized that the elemental composition of MAX phase films obtained by magnetron sputtering can deviate from the target composition depending on the process parameters [55,56,60,61]. This deviation is frequently observed for compound targets containing elements with large differences in atomic masses [62,63] and, especially in HiPIMS, is exacerbated by differences in ionization energies. ...
Cr2AlC coatings were synthesized by high power impulse magnetron sputtering (HiPIMS) from Cr2AlC compound target and subsequent thermal annealing in Ar atmosphere. The effect of HiPIMS duty cycle and substrate bias potential (UB) on the thin film composition were investigated. All initial Cr-Al-C coatings exhibit similar compositions close to the target stoichiometry, and dense and amorphous structure independently of the duty cycle. Meanwhile, Al deficiencies up to 15 % are observed with UB increasing to-200 V. Based on the measured fraction of ionized metal species flux at the substrate, highly energetic bombardment of the coating with ionized inert gas and metal plasma species causes preferential resputtering of Al. Partially crystallized Cr2AlC thin films were obtained by annealing as-deposited Cr-Al-C coatings at 550°C for 4 h. The annealed coating is made of an amorphous inner layer and a crystalline Cr2AlC outer layer. A higher annealing temperature of 650°C led to complete transformation from amorphous phase to crystallized Cr2AlC, and to micro-cracking. These results indicate that the synthesis temperature of MAX phase could be reduced, and the annealing time increased, to obtain protective coatings of Cr2AlC on heat-sensitive components without alteration of the substrate metallurgical properties.
... One noticeable evidence is that a TiC(111) seed layer was frequently deposited on single crystal substrates to stimulate the epitaxial growth and improve the crystalline quality of MAX phase thin films initially. [64][65][66] Additionally, lower growth rates along the c axis for typical hexagonal crystal structures 67,68 likely further promote the textural growth of the MAX phase films. ...
Mn + 1AXn (MAX; n = 1–3) phases are ternary layered nitride and carbide compounds featuring a combination of metallic and ceramic properties. Highly basal-plane textured and polycrystalline Cr2AlC, Ti2AlC, and Ti3AlC2 single-phase coatings have been synthesized on both amorphous and polycrystalline substrates via controlled thermal annealing of magnetron-sputtered nanoscale multilayers built by individual transition metal, carbon, and aluminum layers. Formation of substitutional solid solution carbide phases was triggered via solid-state diffusion reactions during annealing. Lower ordered Ti2AlC initially crystallized at an intermediate temperature range and was recognized as an intermediate reactant in the case of synthesizing the Ti3AlC2 312 MAX phase via annealing corresponding stoichiometric multilayers. The crystallization onset temperatures identified via in-situ high-temperature x-ray diffraction measurements were approximately 480, 660, and 820 °C for Cr2AlC, Ti2AlC, and Ti3AlC2, respectively. Contrary to the usually observed columnar structure representative of magnetron-sputtered coatings, the coatings synthesized via the current approach are composed of plateletlike, elongated crystallites. The nanoscale multilayered design stimulates the textured growth of MAX structures during thermal annealing. More specifically, the preferred crystallographic orientation relationships among the as-deposited transition metal layers, the intermediate solid solution phases, and the end-product MAX phases facilitate the growth of textured MAX phase films.
In recent decades, MAX phases have attracted considerable attention from the scientific community due to their unique combination of metallic and ceramic properties, which provide exceptional mechanical, thermal, electrical and chemical characteristics. The synthesis of MAX phases in the form of coatings is of increasing interest for many applications. The aim of this review is to summarize the progress made in the synthesis of coatings based on MAX phases using different methods. The advantages and characteristics of the implementation of ion-plasma physical vapor deposition methods are discussed. The use of ion-plasma methods allows to significantly reduce the synthesis temperature of MAX phases due to the high energy of the particles forming the coating. The effect of deposition parameters on the composition, structure and properties of the coatings is analyzed. Coatings with high protective properties and prospects for their application in industry are considered. This part of the review focuses on methods for depositing MAX phase based coatings.
This article is a review of the Mn+1AXn phases (“MAX phases”, where n = 1, 2 or 3), their MXene derivatives and the reinforcement of polymers with these materials. The MAX phases are a class of hexagonal-structure ternary carbides and nitrides ("X") of the transition metal ("M") and the A-group element. The unique combination of chemical, physical, electrical and mechanical properties that combine the characteristics of metals and ceramics is of interest to researchers in the MAX phases. For example, MAX phases are typically resistant to oxidation and corrosion, elastic, but at the same time, they have high thermal and electrical conductivity and are machinable. These properties stem from an inherently nanolaminated crystal structure, with Mn+1Xn slabs intercalated with pure A-element layers. To date, more than 150 MAX phases have been synthesized. In 2011, a new family of 2D materials, called MXene, was synthesized, emphasizing the connection with the MAX phases and their dimension. Several approaches to the synthesis of MXene have been developed, including selective etching in a mixture of fluoride salts and various acids, non-aqueous etching solutions, halogens and molten salts, which allows the synthesis of new materials with better control over the chemical composition of their surface. The use of MAX phases and MXene for polymer reinforcement increases their thermal, electrical and mechanical properties. Thus, the addition of fillers increases the glass transition temperature by an average of 10%, bending strength by 30%, compressive strength by 70%, tensile strength up to 200%, microhardness by 40%, reduces friction coefficient and makes the composite material self-lubricating, and 1 % wt. MAX phases increases thermal conductivity by 23%, Young’s modulus increases. The use of composites as components of sensors, electromagnetic protection, wearable technologies, in current sources, in aerospace and military applications, etc. are proposed.
Layered nanolaminate ternary carbides, nitrides and carbonitrides with general formula Mn+1 AXn or MAX (n = 1, 2, or 3, M is an early transition metal, A is mostly group 13 or 14 element, and X is C and/or N) has revolutionized the world of nanomaterials, due to the coexistence of both ceramic and metallic nature, giving rise to exceptional mechanical, thermal, electrical, chemical properties and wide range of applications. Although several solid-state bulk synthesis methods have been developed to produce a variety of MAX phases, however, for certain applications, the growth of MAX phases, especially in its high-quality epitaxial thin films form is of increasing interest. Here, we summarize the progress made thus far in epitaxial growth and property evaluation of MAX phase thin films grown by various deposition techniques. We also address the important future research directions to be made in terms of thin-film growth. Overall, in the future, high-quality single-phase epitaxial thin film growth and engineering of chemically diverse MAX phases may open up interesting new avenues for next-generation technology.
Ti3SiC2 has the unique properties integrating the advantages of metals and ceramics, and good open pore structure when alloyed with Al. In this work, porous Ti3SiC2 compounds with different Al/Si atom ratios were prepared through the reactive synthesis of elemental powders at 1300 °C. The results indicate that the phase compositions are determined by Al element mole number, and that the pore structure can be controlled through varying Ti particle size. The MAX phase transits from Ti3SiC2 with Al element mole number no more than 0.6 to Ti3AlC2 with Al element mole number in the range of 0.8–1.2. When Al element mole number is 0.6, the porous compound has a single MAX phase of Ti3SiC2 with uniform microporous structure and high bending strength. Porous Ti3SiC2 alloyed with 0.6Al has a slow linear increase rate of 0.0083%/μm in open porosity with increasing Ti particle size, and a strict linear relationship between the maximum aperture and Ti particle size with the increase rate of 0.0342 μm/μm. The pore structure formed by the phase transition mechanism for porous MAX phase has the smallest tortuosity factor compared with that formed by the clearance mechanism and the Kirkendall effect.
We have studied the formation of the Ti4SiC3 MAX phase during the vacuum carbosilicothermic reduction of TiO2 with a combined reducing agent consisting of SiC and elemental Si and analyzed the effects of the synthesis temperature, heat treatment time, and percentage of elemental silicon in the starting mixture on the Ti4SiC3 yield. Optimal Ti4SiC3 synthesis conditions are as follows: temperature from 1550 to 1650°C, isothermal holding time of 360 min, and the starting-mixture composition TiO2 + 1.2SiC + 0.6Si. The Ti4SiC3 yield then reaches 92 wt %.
The effect of target power density, substrate bias potential and substrate temperature on the thin film composition was studied. A Cr-Al-C composite target was sputtered utilizing direct current (DCMS: 2.3 W/cm²) and high power pulsed magnetron sputtering (HPPMS: 373 W/cm²) generators. At floating potential, all Cr-Al-C thin films showed similar compositions, independently of the applied target power density. However, as substrate bias potential was increased to -400 V, aluminum deficiencies by a factor of up to 1.6 for DCMS and 4.1 for HPPMS were obtained. Based on the measured ion currents at the substrate, preferential re-sputtering of Al is suggested to cause the dramatic Al depletion. As the substrate temperature was increased to 560 °C, the Al concentration was reduced by a factor of up to 1.9 compared to the room temperature deposition. This additional reduction may be rationalized by thermally induced desorption being active in addition to re-sputtering.
Inherently and artificially layered materials are commonly investigated both for fundamental scientific purposes and for technological application. When a layered material is thinned or delaminated to its physical limits, a two-dimensional (2D) material is formed and exhibits novel properties compared to its bulk parent phase. The complex layered phases known as 'MAX phases' (where M = early transition metal, A = A-group element, e.g. Al or Si, and X = C or N) are an exciting model system for materials design and the understanding of process-structure-property relationships. When the A layers are selectively etched from the MAX phases, a new type of 2D material is formed, named MXene to emphasize the relation to the MAX phases and the parallel with graphene. Since their discovery in 2011, MXenes have rapidly become established as a novel class of 2D materials with remarkable possibilities for composition variations and property tuning. This article gives a brief overview of MAX phases and MXene from a thin-film perspective, reviewing theory, characterization by electron microscopy, properties and how these are affected by the change in dimensionality, and outstanding challenges.
This is a critical review of MAX-phase carbides and nitrides from an electronic-structure and chemical bonding perspective. This large group of nanolaminated materials is of great scientific and technological interest and exhibits a combination of metallic and ceramic features. These properties are related to the special crystal structure and bonding characteristics with alternating strong M\ \C bonds in high-density MC slabs, and relatively weak M\ \A bonds between the slabs. Here, we review the trend and relationship between the chemical bonding, conductivity , elastic and magnetic properties of the MAX phases in comparison to the parent binary MX compounds with the underlying electronic structure probed by polarized X-ray spectroscopy. Spectroscopic studies constitute important tests of the results of state-of-the-art electronic structure density functional theory that is extensively discussed and are generally consistent. By replacing the elements on the M, A, or X-sites in the crystal structure, the corresponding changes in the conductivity, elasticity, magnetism and other material properties make it possible to tailor the characteristics of this class of materials by controlling the strengths of their chemical bonds.
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Ti3SiC2 thin films were synthesized by magnetron sputtering from Ti3SiC2 and Ti targets. Sputtering from a Ti3SiC2 target alone resulted in films with a C content of ∼50 at. % or more, due to gas-phase scattering processes and differences in angular and energy distributions between species ejected from the target. Addition of Ti to the deposition flux from a Ti3SiC2 target is shown to bind the excess C in TiCx intergrown with Ti3SiC2 and Ti4SiC3. Additionally, a substoichiometric TiCx buffer layer is shown to serve as a C sink and enable the growth of Ti3SiC2.
Carbon diffusion is observed in single crystal alpha-Al2O3 substrates from carbon and Ti2AlC thin films synthesized via pulsed cathodic arc deposition. Diffusion was found to occur at substrate temperatures of 570 °C and above. The diffusion coefficient of carbon in alpha-Al2O3 is estimated to be of the order 3×10-13 cm2/s for deposition temperatures in the 570-770oC range by examining elastic recoil detection analysis (ERDA) elemental depth profiles. It is suggested that an appropriate diffusion barrier may be useful when depositing carbon containing thin films on alpha-Al2O3 substrates at high temperatures.
Thin films of Mn+1AXn layered compounds in the Ti-Si-C system were deposited on MgO(111) and Al2O3(0001) substrates held at 900°C using dc magnetron sputtering from elemental targets of Ti, Si, and C. We report on single-crystal and epitaxial deposition of Ti3SiC2 (the previously reported MAX phase in the Ti-Si-C system), a previously unknown MAX phase Ti4SiC3 and another type of structure having the stoichiometry of Ti5Si2C3 and Ti7Si2C5. The latter two structures can be viewed as an intergrowth of 2 and 3 or 3 and 4 M layers between each A layer. In addition, epitaxial films of Ti5Si3Cx were deposited and Ti5Si4 is also observed. First-principles calculations, based on density functional theory (DFT) of Tin+1SiCn for n=1,2,3,4 and the observed intergrown Ti5Si2C3 and Ti7Si2C5 structures show that the calculated difference in cohesive energy between the MAX phases reported here and competing phases (TiC, Ti3SiC2, TiSi2, and Ti5Si3) are very small. This suggests that the observed Ti5Si2C3 and Ti7Si2C5 structures at least should be considered as metastable phases. The calculations show that the energy required for insertion of a Si layer in the TiC matrix is independent of how close the Si layers are stacked. Hardness and electrical properties can be related to the number of Si layers per Ti layer. This opens up for designed thin film structures the possibility to tune properties.
Photoemission studies using synchrotron radiation have been performed on epitaxial Ti3SiC2(0001) and compound nanocrystalline (nc-)TiC/amorphous (a-)SiC thin films deposited by magnetron sputtering. As-introduced samples were found to be covered by surface oxides, SiOx and TiOx. These oxides could be removed by in-situ annealing to ∼1000 °C. For as-annealed Ti3SiC2(0001), surface Si was observed and interpreted as originating from decomposition of Ti3SiC2 through Si out-diffusion. For nc-TiC∕a-SiC annealed in situ to ∼1000 °C, the surface instead exhibited a dominant contribution from graphitic carbon, also with the presence of Si, due to C and Si out-diffusion from the a-SiC compound or from grain boundaries.
The Mn+1AXn (MAX) phases are ternary compounds comprising alternating layers of a transition metal carbide or nitride and a third “A-group” element. The effect of substrate orientation on the growth of Ti2AlC MAX phase films was investigated by studying pulsed cathodic arc deposited samples grown on sapphire cut along the (0001), (100), and (102) crystallographic planes. Characterization of these samples was by x-ray diffraction, atomic force microscopy, and cross-sectional transmission electron microscopy. On the (100) substrate, tilted (108) growth of Ti2AlC was found, such that the TiC octahedra of the MAX phase structure have the same orientation as a spontaneously formed epitaxial TiC sublayer, preserving the typical TiC–Ti2AlC epitaxial relationship and confirming the importance of this relationship in determining MAX phase film orientation. An additional component of Ti2AlC with tilted fiber texture was observed in this sample; tilted fiber texture, or axiotaxy, has not previously been seen in MAX phase films.
We report on the synthesis and characterization of epitaxial single-crystalline Ti3SiC2 films (Mn+1AXn-phase). Two original deposition techniques are described, (i) magnetron sputtering from Ti3SiC2 compound target and (ii) sputtering from individual titanium and silicon targets with co-evaporated C60 as carbon source. Epitaxial Ti3SiC2 films of single-crystal quality were grown at 900 °C with both techniques. Epitaxial TiC(111) deposited in situ on MgO(111) by Ti sputtering using C60 as carbon source was used to nucleate the Ti3SiC2 films. The epitaxial relationship was found to be Ti3SiC2(0001)//TiC(111)//MgO(111) with the in-plane orientation Ti3SiC2[100]//TiC[101]//MgO[101]. © 2002 American Institute of Physics.
The more than 60 ternary carbides and nitrides, with the general formula Mn+1AXn—where n = 1, 2, or 3; M is an early transition metal; A is an A-group element (a subset of groups 13–16); and X is C and/or N—represent a new class of layered solids, where Mn+1Xn layers are interleaved with pure A-group element layers. The growing interest in the Mn+1AXn phases lies in their unusual, and sometimes unique, set of properties that can be traced back to their layered nature and the fact that basal dislocations multiply and are mobile at room temperature. Because of their chemical and structural similarities, the MAX phases and their corresponding MX phases share many physical and chemical properties. In this paper we review our current understanding of the elastic and mechanical properties of bulk MAX phases where they differ significantly from their MX counterparts. Elastically the MAX phases are in general quite stiff and elastically isotropic. The MAX phases are relatively soft (2–8 GPa), are most readily mac...
The MAX phases are a group of layered ternary compounds with the general formula Mn+1AXn (M: early transition metal; A: group A element; X: C and/or N; n = 1‐3), which combine some properties of metals, such as good electrical and thermal conductivity, machinability, low hardness, thermal shock resistance and damage tolerance, with those of ceramics, such as high elastic moduli, high temperature strength, and oxidation and corrosion resistance. The publication of papers on the MAX phases has shown an almost exponential increase in the past decade. The existence of further MAX phases has been reported or proposed. In addition to surveying this activity, the synthesis of MAX phases in the forms of bulk, films and powders is reviewed, together with their physical, mechanical and corrosion/oxidation properties. Recent research and development has revealed potential for the practical application of the MAX phases (particularly using the pressureless sintering and physical vapour deposition coating routes) as well as of MAX based composites. The challenges for the immediate future are to explore further and characterise the MAX phases reported to date and to make further progress in facilitating their industrial application.
We have synthesized Ti-Si-C nanocomposite thin films by dc magnetron sputtering from a Ti(3)SiC(2) compound target in an Ar discharge on Si(100), Al(2)O(3)(0001)- and Al substrates at temperatures from room temperature to 300 degrees C. Electron microscopy, x-ray diffraction, and x-ray photoelectron spectroscopy showed that the films consisted of nanocrystalline (nc-) TiC and amorphous (a-) SiC, with the possible presence of a small amount of noncarbidic C, The growth mode was columnar, yielding a nodular film-surface morphology. Mechanically, the films exhibited a remarkable ductile behavior. Their nanoindentation hardness and E-modulus values were 20 and 290 GPa, respectively, The electrical resistivity was 330 mu Omega cm for optimal Ar pressure (4 mTorr) and substrate temperature (300 degrees C). The resulting nc-TiC/a-SiC films performed well as electrical contact material. These films' electrical-contact resistance against Ag wits remarkably low, 6 mu Omega at a contact force of 800 N compared to 3.2 mu Omega for Ag against Ag. The chemical stability of the nc-TiC/a-SiC films was excellent, as shown by a Battelle flowing mixed corrosive-gas test, with no N, Cl, or S contaminants entering the bulk of the films.
The electronic structures of epitaxially grown films of Ti(3)AlC(2), Ti(3)SiC(2), and Ti(3)GeC(2) have been investigated by bulk-sensitive soft x-ray emission spectroscopy. The measured high-resolution Ti L, C K, Al L, Si L, and Ge M emission spectra are compared with ab initio density-functional theory including core-to-valence dipole matrix elements. A qualitative agreement between experiment and theory is obtained. A weak covalent Ti-Al bond is manifested by a pronounced shoulder in the Ti L emission of Ti(3)AlC(2). As Al is replaced with Si or Ge, the shoulder disappears. For the buried Al and Si layers, strongly hybridized spectral shapes are detected in Ti(3)AlC(2) and Ti(3)SiC(2), respectively. As a result of relaxation of the crystal structure and the increased charge-transfer from Ti to C, the Ti-C bonding is strengthened. The differences between the electronic structures are discussed in relation to the bonding in the nanolaminates and the corresponding change of materials properties.
Epitaxial Ti3SiC2(0001) thin films have been deposited by dc magnetron sputtering from three elemental targets of Ti, C, and Si onto MgO(111) and Al2O3(0001) substrates at temperatures of 800-900 degreesC. This process allows composition control to synthesize M(n+1)AX(n) (MAX) phases (M: early transition metal; A: A-group element; X: C and/or N; n=1-3) including Ti4SiC3. Depositions on MgO(100) substrates yielding the Ti-Si-C MAX phases with (10 (1) over bar5), as the preferred orientation. Samples grown at different substrate temperatures, studied by means of transmission electron microscopy and x-ray diffraction investigations, revealed the constraints of Ti3SiC2 nucleation due to kinetic limitations at substrate temperatures below 700 degreesC. Instead, there is a competitive TiCx growth with Si segregation to form twin boundaries or Si substitutional incorporation in TiCx. Physical properties of the as-deposited single-crystal Ti3SiC2 films were determined. A low resistivity of 25 muOmega cm was measured. The Young's modulus, ascertained by nanoindentation, yielded a value of 343-370 GPa. For the mechanical deformation response of the material, probing with cube corner and Berkovich indenters showed an initial high hardness of almost 30 GPa. With increased maximum indentation loads, the hardness was observed to decrease toward bulk values as the characteristic kink formation sets in with dislocation ordering and delamination at basal planes.
In the present research, single-phase Ti2AlN coatings were prepared on polycrystalline Al2O3 from cost-efficient hot-pressed Ti-Al-N targets by D.C. magnetron sputtering and subsequent annealing treatment. SEM, TEM and XRD analyses were performed to characterize the microstructure and phase constituents of the Ti-Al-N target and coating. The results reveal that phase constituents of hot-pressed Ti-Al-N targets were affected by the synthesis temperature. When the synthesis temperature was 700 °C, the target mainly comprised α-Ti, TiN and Ti-Al intermetallics. While in the target synthesized at 900 °C, TiN, Ti-Al intermetallics and Ti2AlN were the main phase constituents. The as-deposited coatings from both targets exhibited typical amorphous crystal structure．Crystallization of the coating initiated between 500 °C and 600 °C. After annealing at 700 °C for 1 h, the amorphous coating was transformed to single-phase polycrystalline Ti2AlN coating. In general, single-phase Ti2AlN coatings could be obtained from the target whether with Ti2AlN or not. It was the element ratio of Ti:Al:N = 2:1:1 (at%) rather than the phase contents in the targets that was critical to fabricate single-phase Ti2AlN coating.
Ti2AlC and Ti3AlC2 coatings were successfully prepared via a two-step method with initial DC magnetron sputtering at ambient temperature and post annealing at 800 °C for 1 h. Particularly, cost-effective targets synthesized by hot-pressing Ti/Al/C powders at low temperature (800 °C) were used. The phase components and microstructure of the coatings were characterized by XRD, laser Raman spectroscopy, SEM and TEM. It was found that the phase components of the as-deposited and annealed coatings varied with the molar ratio of Ti, Al and C powders in the cost-effective targets. Phase-pure Ti2AlC and Ti3AlC2 could be obtained from the target with the Ti:Al:C molar ratios of 2:1.5:1 and 3:2:2, respectively. These two coatings were consisted of plate-like grains. Due to the high internal stress resulting from crystallization- or phase transformation-induced volume change, both of the Ti2AlC and Ti3AlC2 coatings cracked during the annealing process.
We report x-ray photoelectron spectroscopy (XPS) core level binding energies (BE's) for the widely-applicable groups IVb-VIb transition metal carbides (TMCs) TiC, VC, CrC, ZrC, NbC, MoC, HfC, TaC, and WC. Thin film samples are grown in the same deposition system, by dc magnetron co-sputtering from graphite and respective elemental metal targets in Ar atmosphere. To remove surface contaminations resulting from exposure to air during sample transfer from the growth chamber into the XPS system, layers are either (i) Ar⁺ ion-etched or (ii) UHV-annealed in situ prior to XPS analyses. High resolution XPS spectra reveal that even gentle etching affects the shape of core level signals, as well as BE values, which are systematically offset by 0.2–0.5 eV towards lower BE. These destructive effects of Ar⁺ ion etch become more pronounced with increasing the metal atom mass due to an increasing carbon-to-metal sputter yield ratio. Systematic analysis reveals that for each row in the periodic table (3d, 4d, and 5d) C 1s BE increases from left to right indicative of a decreased charge transfer from TM to C atoms, hence bond weakening. Moreover, C 1s BE decreases linearly with increasing carbide/metal melting point ratio. Spectra reported here, acquired from a consistent set of samples in the same instrument, should serve as a reference for true deconvolution of complex XPS cases, including multinary carbides, nitrides, and carbonitrides.
Ti3SiC2 and Ti4SiC3 MAX phase ceramics were fabricated through high-temperature vacuum reduction of TiO2 using SiC as a reductant, followed by hot pressing of the products under 25 MPa of pressure at 1600 °C. It was found that both Ti3SiC2 and Ti4SiC3 may be obtained in good yields, depending on the annealing time during the reduction step. In addition to MAX phases, the products contained some amounts of TiC. The hot pressing step did not significantly affect the composition of the products, indicating good stability of Ti3SiC2 and Ti4SiC3 under these conditions. Analysis of the densification behavior of the samples revealed lower ductility in Ti4SiC3 compared to Ti3SiC2. The samples prepared herein exhibited the flexural strength, fracture toughness and microhardness typical of coarse-grained MAX-phase ceramics.
In this comprehensive yet compact monograph, Michel W. Barsoum, one of the pioneers in the field and the leading figure in MAX phase research, summarizes and explains, from both an experimental and a theoretical viewpoint, all the features that are necessary to understand and apply these new materials. The book covers elastic, electrical, thermal, chemical and mechanical properties in different temperature regimes. By bringing together, in a unifi ed, self-contained manner, all the information on MAX phases hitherto only found scattered in the journal literature, this one-stop resource offers researchers and developers alike an insight into these fascinating materials.
Atomic layer deposition (ALD) was used to grow TixAlyN and TixAlyC thin films using trimethylaluminum (TMA), titanium tetrachloride and ammonia as precursors. Deposition temperature was varied between 325 °C and 500 °C. Films were also annealed in vacuum and N2-atmosphere at 600–1000 °C. Wide range of characterization methods was used including time-of-flight elastic recoil detection analysis (ToF-ERDA), X-ray diffractometry (XRD), X-ray reflectometry (XRR), Raman spectroscopy, ellipsometry, helium ion microscopy (HIM), atomic force microscopy (AFM) and 4-point probe measurement for resistivity. Deposited films were roughly 100 nm thick and contained mainly desired elements. Carbon, chlorine and hydrogen were found to be the main impurities.
This is a critical review of MAX-phase carbides and nitrides from an electronic-structure and chemical bonding perspective. This large group of nanolaminated materials is of great scientific and technological interest and exhibits a combination of metallic and ceramic features. These properties are related to the special crystal structure and bonding characteristics with alternating strong MC bonds in high-density MC slabs, and relatively weak MA bonds between the slabs. Here, we review the trend and relationship between the chemical bonding, conductivity, elastic and magnetic properties of the MAX phases in comparison to the parent binary MX compounds with the underlying electronic structure probed by polarized X-ray spectroscopy. Spectroscopic studies constitute important tests of the results of state-of-the-art electronic structure density functional theory that is extensively discussed and are generally consistent. By replacing the elements on the M, A, or X-sites in the crystal structure, the corresponding changes in the conductivity, elasticity, magnetism and other material properties make it possible to tailor the characteristics of this class of materials by controlling the strengths of their chemical bonds.
The bulk MAX phase Ti4SiC3 was first synthesized with a yield of 86% by a long-time thermal treatment of TiO2 and SiC powder mixture with a molar ratio of 2:3 at 1600 °C under vacuum conditions. It was found that the appearance of Ti4SiC3 was preceded by the formation of TiC and Ti3SiC2 as a result of the following reactions: (1) combined carbothermic and silicothermic reduction of TiO2 to TiC accompanied by evolution of SiO and CO gases; (2) silicidation of TiC with gaseous SiO, leading to the growth of Ti3SiC2. It was suggested that, apart from TiC and Ti3SiC2 solids, sublimed gaseous species such as Ti, Si, Si2C, SiC2, etc., could take part in the Ti4SiC3 formation that occurred in the next stage. The crystal structure of synthesized Ti4SiC3 was refined by X-ray diffraction Rietveld analysis and confirmed by high-resolution scanning transmission electron microscopy. The measured structural characteristics of bulk Ti4SiC3 are in good agreement with those predicted by ab initio calculations reported in the literature.
MAX phases are promising materials for protective coatings on steel due to their unique combination of properties like corrosion and oxidation resistance, good electrical conductivity, low friction coefficient, damage tolerance, and high temperature stability. Here the deposition of large area MAX phase coatings on steel was investigated by magnetron sputtering of a Ti2AlC compound target. It was found that the chemical composition of the produced films was different from the composition of the target. The deposited films contained about half as much Ti as the target. Consequently the dependence of the film composition on temperature and pressure was investigated. The film composition prooved to be independent of pressure but the lack of Ti increased with increasing temperature. The stoichiometry of the films was then adjusted by adding Ti to the growing coating from a separate plasma source. This method provides a pathway to stoichiometric Ti2AlC coatings with an equilibrium volume that is in excellent agreement with our ab initio calculations.
The bonding sensitivities of Si–N and Si–C bonds in the a-SiCN thin films were investigated experimentally and theoretically. It was found that a sharp phase transition from the predominantly Si–C bonded structure to the Si–N bonded structure occurs during the deposition of SiCN thin films without and with N incorporation. The stronger affinity of silicon to bond with nitrogen than to bond with carbon results in the complete absence of Si–C bonds in a-SiCN thin films. These were further verified by the analyses of crystal structures and local bonding configurations, morphology, and optical properties. Finally, the relative stabilities of Si–C and Si–N bonds in the a-SiCN network were studied by the ab initio calculations for the simple SiCN clusters. The local atomic structure with Si–N–C (or Si–NMC) bonding exhibits a considerably lower total energy of 0.65 (0.52) eV than that with Si–C–N (Si–CMN) bonding, providing the explanation of the stronger affinity of Si–N bonds than Si–C bonds in a-SiCN thin films. D 2004 Elsevier B.V. All rights reserved.
We investigate ultrathin silicide formation during a solid-state reaction between Ni layers and Si(001) substrates by aberration-corrected electron microscopy. Interdiffusion of two nm thick (equivalent) Ni layers with Si during magnetron-sputter deposition results in an amorphous Ni–Si solid solution. Upon annealing at 150–350 °C, a novel body-centered cubic (bcc) NiSix phase is found to grow epitaxially with a crystallographic relationship {100}<001>bcc-NiSix//{100}<001>Si. bcc-NiSix belongs to the space group I4̅3m (217) with random Ni and Si distribution. The cell parameter is 0.272 nm, which is approximately half that of NiSi2. Further annealing transforms bcc-NiSi to NiSi2 with an activation energy of 0.6 ± 0.1 eV.
Titanium Silicon Carbide films were deposited from three separate magnetrons with elemental targets onto Si wafer substrates. The substrate was moved in a circular motion such that the substrate faces each magnetron in turn and only one atomic species (Ti, Si or C) is deposited at a time. This allows layer-by-layer film deposition. Material average composition was determined to Ti0.47Si0.14C0.39 by energy-dispersive X-ray spectroscopy. High-resolution transmission electron microscopy and Raman spectroscopy were used to gain insights into thin film atomic structure arrangements. Using this new deposition technique formation of Ti3SiC2 MAX phase was obtained at a deposition temperature of 650 °C, while at lower temperatures only silicides and carbides are formed. Significant sharpening of Raman E2g and Ag peaks associated with Ti3SiC2 formation was observed.
Ti3SiC2 layers were grown by reactive chemical vapor deposition (RCVD) of a H2/TiCl4 gaseous mixture on previously deposited SiC layers. A comparison was made between classical RCVD in which the gases continuously flow at a constant low pressure during several minutes in the reactor and pressure-pulsed RCVD (P-RCVD) in which the reactor is (periodically) (re)filled with the H2/TiCl4 gas and (re)emptied every few seconds. Long duration single treatments resulted in similar thick multi-phased coatings growing by solid state diffusion with both RCVD and P-RCVD methods. Conversely, in relation with the steps of nucleation and growth by surface reaction, the repetition of short duration SiC deposition/RCVD sequences with or without pressure pulses gave rise to Ti3SiC2 coatings with different textures.
A time of flight-energy recoil telescope system for mass and energy dispersive recoil spectrometry has been applied to study the formation of Mg2Si layers and depth profiling of Ga1−xAlxAs quantum well structures. Measurements of the energy (depth) dependence of the mass resolution showed that the telescope could be used over the energy range from 5 to 18 MeV to distinguish between recoils of 1 amu mass difference up to mass 28 amu. The energy dependence of the detection efficiency was found to be independent of the recoil energy for 12C and 28Si recoils and no strong evidence for a recoil species dependence of the detection efficiency for recoils heavier than 16O was found.
The interaction of yttrium overlayers with an atomically clean Si(100) surface has been studied by X-ray photoemission. Also studied, were the effects of Y overlayers on the oxidation of silicon. It is found that the growth of yttrium on silicon can be explained by a three-step mechanism. At low coverage (θ ⩽ 2 ML) yttrium deposits on top of silicon associated with charge transfer from the Y overlayer to the silicon. Between 2 and 5 ML coverage silicon diffuses through the Y overlayer, and above 5 ML a metallic Y is deposited on top of the silicon. In addition, yttrium overlayers were found to significantly enhance the oxidation of the Si(100) surface. At 5 ML Y coverage the obtained SiO2 oxides were up to a factor of 15 thicker than those obtained for the clean unpromoted Si(100) surface.
Chemical vapor deposition in the system TiSiC has been studied with the reaction system TiCl4(g) + SiCl4(g) + CCl4(g) + H2(ex). A complete deposition diagram (xa(s) = f(xi(g); xj(g))) at 1200 °C and normal pressure has been constructed. The morphology and intergrowth in two- or three-phase equilibria, and especially the properties of the complex carbide Ti3SiC2 have been studied. In analogy to the deposition of binary systems, in which the phases are deposited within a linear concentration range (“vapor deposition range” Δxi(g)), the ternary phases (Ti3SiC2 or Ti5Si3Cx) are deposited within two-dimensional concentration fields (Δxi(g); Δxj(g)).
The WTi films were deposited by an unbalanced magnetron sputtering of a WTi (70:30 at. %) alloy target. The influence of the working gas (Ar) pressure, substrate bias, and substrate location on the composition of films was studied. The films deposited at low working gas pressures (≪1 Pa) onto electrically floating substrates were largely depleted in Ti while the composition of films deposited at high argon pressure (25 Pa) was close to that of the target. The ion bombardment of the growing film resulted in a decrease of the Ti content in the films. The composition of the films deposited simultaneously onto a pair of substrates placed at the axis and at the periphery of the target did not depend on the substrate position at both low and high pressure. Further studies were carried out for a better understanding of the underlying processes affecting the film composition. Namely, the mass-resolved ion energy distribution function at the substrate position was measured for various pressures. Further, the composition of the flux towards the target (backward flux) was studied as a function of pressure by Rutherford backscattering. Finally, the direct simulation Monte Carlo (DSMC) computer simulation of the gas-phase transport of sputtered species was carried out. The results of the DSMC simulation (film composition, backward atomic flux, and ion energy distribution at the substrate) were compared with the experimental results. The formation mechanism includes the simultaneous action of two competing factors. One factor is the resputtering by fast argon neutrals reflected from the cathode and/or by plasma ions accelerated by the substrate bias, resulting in films deficient in Ti. The other factor is the gas phase scattering on the background gas with a twofold effect. On one hand the scattering leads to the reduction of energy of fast neutrals and consequently to diminishing of the resputtering effect. On the other hand, under conditions of comparable values- - of the mean free path and the substrate-to-target distance the difference in scattering of various sputtered species can lead to the alteration of the composition of deposited films. © 2001 American Vacuum Society.
Epitaxial predominantly phase-pure Ti7Si2C5 thin films were grown onto Al2O3(0 0 0 1) by reactive magnetron sputtering. The c-axis lattice constant is ∼60.2 Å; the Ti7Si2C5 unit cell comprises alternating Ti3SiC2-like and Ti4SiC3-like half-unit-cell stacking repeated three times. Elastic recoil detection analysis showed a few percent of nitrogen in the films from the acetylene gas used. The nitrogen-induced stabilization mechanism for Ti7Si2C5 relative to Ti3SiC2 and Ti4SiC3 is discussed. Electrical-transport measurements showed metallic temperature dependence and a room-temperature resistivity of ∼45 μΩ cm.
Sputter deposition from a Ti2AlC target was found to yield Ti–Al–C films with a composition that deviates from the target composition of 2:1:1. For increasing substrate temperature from ambient to 1000°C, the Al content decreased from 22at.% to 5at.%, due to re-evaporation. The C content in as-deposited films was equal to or higher than the Ti content. Mass spectrometry of the plasma revealed that the Ti and Al species were essentially thermalized, while a large fraction of C with energies >4eV was detected. Co-sputtering with Ti yielded a film stoichiometry of 2:0.8:0.9 for Ti:Al:C, which enabled growth of Ti2AlC. These results indicate that an additional Ti flux balances the excess C and therefore provides for more stoichiometric Ti2AlC synthesis conditions.
Ti 2 AlC thin films deposited onto Al 2 O 3 by magnetron sputtering were used as model for studying the early stages (<15 min) of relatively low-temperature (500 • C) oxidation of Ti 2 AlC. The well-defined microstructure of these films forms a surface of valleys, hillocks and plateaus comprised of basal-plane-oriented grains with a fraction of nonbasal-plane-oriented grains with out-of-plane orientation of (1 0 ¯ 1 3) and (1 0 ¯ 1 6) as shown by X-ray diffraction and s electron microscopy. During oxidation, Al 2 O 3 clusters and areas of C-containing titania (TiO x C y) are formed on the surface. A mechanism is proposed in which the locations of the Al 2 O 3 clusters are related to the migration of Al atoms diffusing out of Ti 2 AlC. The Al 2 O 3 is initially formed in valleys or on plateaus where Al atoms have been trapped while TiO x C y forms by in-diffusion of oxygen into the Al-deficient Ti 2 AlC. At 500 • C, the migration of Al atoms is faster than the oxidation kinetics; explaining this microstructure-dependent oxidation mechanism.
The electronic structure of the nanolaminated transition metal carbide Ti2AlC has been investigated by bulk-sensitive soft x-ray emission spectroscopy. The measured Ti L, C K, and Al L emission spectra are compared with calculated spectra using ab initio density-functional theory including dipole matrix elements. The detailed investigation of the electronic structure and chemical bonding provides increased understanding of the physical properties of this type of nanolaminates. Three different types of bond regions are identified: The relatively weak Ti 3d–Al 3p bond 1 eV below the Fermi level and the Ti 3d–C 2p and Ti 3d–C 2s bonds which are stronger and deeper in energy are observed around 2.5 and 10 eV below the Fermi level, respectively. A strongly modified spectral shape of the 3s final states in comparison to pure Al is detected for the intercalated Al monolayers indirectly reflecting the Ti 3d–Al 3p hybridization. The differences between the electronic and crystal structures of Ti2AlC, Ti3AlC2, and TiC are discussed in relation to the number of Al layers per Ti layer in the two former systems and the corresponding change of the unusual materials properties.
Sputter‐deposited Ti1−xWx diffusion barriers in microelectronic devices have been reported by many groups to be Ti deficient with respect to the target composition. In the present experiments, polycrystalline TixW1−x alloys were grown on oxidized Si(001) substrates at temperatures Ts between 100 and 600 °C by ultrahigh‐vacuum magnetron cosputter deposition from pure W and Ti targets in 5 mTorr (0.65 Pa) Ar and Xe discharges. Films deposited in Ar were found by Rutherford backscattering and Auger electron spectroscopies to be increasingly Ti deficient with increases in the Ti sputtering rate and/or Ts at a constant W sputtering rate. TRIM calculations and Monte Carlo gas‐transport simulations were used, in combination with the experimental results, to show that the Ti loss was due primarily to differential resputtering of the growing film by energetic Ar particles backscattered from the heavier W target. This effect is exacerbated at elevated film growth temperatures by Ti surface segregation in the alloy. The use of Xe, rather than Ar, as the sputtering gas greatly reduces both the flux and the average energy of backscattered particles incident at the substrate such that measurable Ti loss is no longer observed. © 1995 American Institute of Physics.
The reported experimentally determined bulk modulus for Cr2AlC is 28-40% lower than the value obtained by ab initio calculations. To identify the origin of this extensive difference between theory and experiment, the elastic modulus of thin Cr2AlC films was measured by nanoindentation and the temperature dependence thereof was studied by ab initio molecular dynamics. Our experimental and theoretical elastic modulus data are within the expected error margin and hence consistent with the published ab initio data.
The electronic structure in the new transition-metal carbide Ti(4)SiC(3) has been investigated by bulk-sensitive soft x-ray emission spectroscopy and compared to the well-studied Ti(3)SiC(2) and TiC systems. The measured high-resolution Ti L, C K, and Si L x-ray emission spectra are discussed with ab initio calculations based on density-functional theory including core-to-valence dipole matrix elements. The detailed investigations of the Ti-C and Ti-Si chemical bonds provide increased understanding of the physical properties of these nanolaminates. A strongly modified spectral shape is detected for the intercalated Si monolayers due to Si 3p hybridization with the Ti 3d orbitals. As a result of relaxation of the crystal structure and the charge-transfer from Ti (and Si) to C, the strength of the Ti-C covalent bond is increased. The differences between the electronic and crystal structures of Ti(4)SiC(3) and Ti(3)SiC(2) are discussed in relation to the number of Si layers per Ti layer in the two systems and the corresponding change of materials properties.
The formation of ternary compounds within the Ti-Al-C system was studied by magnetron sputtering for thin-film deposition and first-principles calculations for phase stability. As-deposited films were characterized with X-ray diffraction (XRD) and high-resolution transmission electron microscopy (TEM). The hardness and Young's moduli of the material were studied by nanoindentation. Epitaxial and phase-pure films of M(n+1)AX(n) phases Ti3AlC2 and Ti2AlC as well as the perovskite phase Ti3AlC were deposited on Al2O3(00l) wafers kept at temperatures between 800 and 900 degrees C. The only ternary phases observed at low temperatures (300 degrees C) were Ti3AlC and cubic (Ti,Al)C, the latter can be described as a metastable solid solution of Al in TiC similar to the more studied (Ti,Al)N system. The difficulties to form MAX phases at low substrate temperatures were attributed of requirement for a sufficient diffusivity to partition the elements corresponding to the relatively complex crystal structures with long c-axes. While MAX-phase synthesis at 800 degrees C is significantly lower than contemporary bulk sintering processes, a reduction of the substrate temperature towards 300 degrees C in the present thin-film deposition experiments resulted in stacking sequence variations and the intergrowth of (Ti,AI)C.
We have deposited Ti-Si-C thin films using high-power impulse magnetron sputtering (HIPIMS) from a Ti3SiC2 compound target. The as deposited films were composite materials with TiC as the main crystalline constituent. X-ray diffraction and photoelectron spectroscopy indicated that they also contained amorphous SiC, and for films deposited on inclined substrates, crystalline Ti5Si3Cx. The film morphology was dense and flat, while films deposited with direct-current magnetron sputtering under comparable conditions were rough and porous. We show that, due to the high degree of ionization of the sputtered species obtained in HIPIMS, the film composition, in particular the C content, depends on substrate inclination angle and Ar process pressure.
In advanced silicon technology, tungsten-rich W-Ti alloy films are frequently used as diffusion barriers and etch-stop layers in combination with aluminium-based interconnections. We have studied both microstructure and properties of magnetron sputtered W80Ti20 (atomic per cent) alloy films with and without nitrogen incorporated in the film structure. The efficiency of these films as a barrier layer has been studied by the interaction between interconnecting Al-(alloy) and WTi(N) using the following samples: Al(Cu,Si)/WTi(N)/Si02/Si. The barrier films have been characterized with the help of cross-sectional transmission electron microscopy, electron diffraction, electrical resistivity probing, Rutherford back-scattering spectrometry and X-ray diffraction.The W80Ti20 films exhibit a columnar microstructure. As the recrystallizationtemperature of these (refractory) barrier metals is far above the anneal temperature of 450°C, the columnar growth morphology is preserved on annealing. The intercolumnar material (or low density network) provides favourite sites for vacancy condensation and forms short-circuit diffusion paths in a direction perpendicular to the plane of the films. Such W80Ti20 films therefore behave as rather poor diffusion barriers. As shown in the literature by several researchers, both the incorporation of nitrogen into the structure of W-Ti films and the application of a thin oxide film at the barrier-interconnect interface may improve the barrier properties considerably. The present investigations of the film properties resulted in a better understanding of the barrier function of both binary W-Ti and ternary WTiN alloy films.
We report that magnetron sputtering can be applied to synthesize MAX-phase films of several systems including Ti–Si–C, Ti–Ge–C, Ti–Al–C, and Ti–Al–N. In particular, epitaxial films of the known phases Ti3SiC2, Ti3GeC2, Ti2GeC, Ti3AlC2, Ti2AlC, and Ti2AlN as well as the newly discovered thin film phases Ti4SiC3, Ti4GeC3 and intergrown structures can be deposited at 900–1000 °C on Al2O3(0001) and MgO(111) pre-seeded with TiC or Ti(Al)N. From XTEM and AFM we suggest a growth and nucleation model where MAX-phase nucleation is initiated at surface steps or facets on the seed layer and followed by lateral growth. Differences between the growth behavior of the systems with respect to phase distribution and phase stabilities are discussed. Characterization of mechanical properties for Tin+1Si–Cn films with nanoindentation show decreased hardness from about 25 to 15 GPa upon penetration of the basal planes with characteristic large plastic deformation with pile up dependent on the choice of MAX material. This is explained by cohesive delamination of the basal planes and kink band formation, in agreement with the observations made for bulk material. Measurements of the electrical resistivity for Ti–Si–C and Ti–Al–N films with four-point probe technique show values of 30 and 39 μΩ cm, respectively, comparable to bulk materials.
The ternary compund Ti3SiC2 is a prominent representative of a new class of layered ceramics whose extraordinary physical properties has attracted much attention in recent years. Ti3SiC2 is electrically and thermally highly conductive, elastically rigid, lightweight, and maintains its strength to high temperatures. It is furthermore damage tolerant and oxidation resistant. We have studied fractured surfaces of coarse-grained Ti3SiC2 by means of photoelectron spectroscopy at the MAX-lab synchrotron radiation facility in Lund, Sweden. High-resolution C 1s, Si 2p, Ti 2p, Ti 3s and Ti 3p core-level spectra are reported and interpreted in terms of crystallographic and electronic structure. Valence band spectra confirm the validity of recent band calculations.
In the ternary system Ti-Si-C, the ternary compound Ti3SiC2 seems to exhibit promising thermal and mechanical properties. Its synthesis as a thin film from the vapour phase is very difficult owing to the complexity of the system. A contribution to the knowledge of the CVD of Ti3SiC2 from a TiCl4-SiCl4-CH4-H2 gas mixture is proposed on the basis of a thermodynamic approach. This approach is based on a reliable estimation of Ti3SiC2 thermodynamic data in good accordance with recent experimental results on its thermal stability. A first equilibrium calculation for the deposition on an inert substrate shows the influence of the experimental parameters on the composition of both the deposit and the gas phase. As a result, the deposition of Ti3SiC2 can be favoured by an excess of TiCl4 ( 45%), a rather low pressure (10–20 kPa), high temperature ( 1273 K) and low H2 dilution ratio. On the basis of equilibrium calculations for various reactive substrates, complex mechanisms of Ti3SiC2 deposition are pointed out, with intermediate steps of substrate consumption, e.g. the formation of TiC from a carbon substrate or TiSi2 from a silicon substrate.
Die Struktur von Ti3SiC2 wird aus Einkristallaufnahmen bestimmt. Die Gitterparameter der hexagonalen Zelle sind:a=3,068,c=17,669 undc/a=5,759. Die Titan-Atome besetzen die Punktlagen 2a) und 4f) (zTi=0,135), die Silicium-Atome die Punktlage 2b) und die Kohlenstoff-Atome die Punktlage 4f) (zC=0,5675) in der Raumgruppe D
6h
4
–P63/mmc. Die Struktur gehrt zu den Komplexcarbiden mit oktaedrischen Bauelementen [T
6C].The crystal structure of Ti3SiC2 has been determined by means of single crystal photographs; the lattice parameters of the hexagonal cell were found to be:a=3.068,c=17.669 andc/a=5.759. The titanium atoms occupy the positions 2a) and 4f) (zTi=0.135), the silicon atoms 2b) and the carbon atoms 4f) (zC=0.5675) of the space group D
6h
4
–P63/mmc. The crystal structure type belongs to the class of complex carbides having octahedral groups [T
6C].
Electron spectroscopic comparison of the C‐rich SiC(0001¯) and Si‐rich SiC(0001) surfaces after cleaning and disordering by Ar+ ion sputtering and subsequent annealing is reported. The chemical behavior of the two disordered surfaces differs significantly. Three distinct temperature regions with different carbon surface segregation kinetics are discernible on SiC(0001¯). On SiC(0001) only one temperature region for C‐segregation is observed. Below 900 K, no spectroscopic differences between the two crystal surfaces are observed. Between 900 and 1300 K, both faces are terminated by a surface graphite layer and the C‐rich face shows an additional carbon surface segregation process. Above 1300 K, the C‐terminated surface graphitizes at a higher rate than the Si‐terminated surface. Massive graphitization on both surfaces above 1300 K is attributed to Si(g) sublimation from the SiC surfaces. The results demonstrate that extensive surface disordering of polar SiC faces does not destroy the memory for the polarity of the original crystal insofar as high‐temperature surface chemistry is concerned.