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Interfacial Reactions in GaSb/Co Metallization Contacts during Thermal Processing A. A. Kodentsov,*,† S. L. Markovski,† C. Cserha´ti,‡ and F. J. J. van Loo† COBRA Inter-University Research Institute Communication Technology, Basic Research and Applications, Laboratory of Solid State and Materials Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands, and Department of Solid State Physics, Kossuth Lajos University, P.O. Box 2, H-4010 Debrecen, Hungary Received July 17, 2002. Revised Manuscript Received October 10, 2002
The utility of thermodynamic potential (activity) diagrams in predicting the reaction zone morphology developed in the GaSb/Co metallization contacts during thermal processing is demonstrated. A number of experiments were designed to test the model. These are aimed at determining phase equilibria in the Ga-Sb-Co system and studying the microstructural evolution of the reaction zone in bulk as well as thin-film diffusion couples. Interfacial reactions between cobalt and single-crystal (001) GaSb have been investigated at 500 °C. No ternary phases exist in the system at this temperature. The cubic CoGa and CoSb3 phases were observed to be dominant growing compounds in the semi-infinite bulk as well as in thin-film reaction couples, the latter intermetallic being formed next to the GaSb substrate. When the Co film is consumed by the reaction, the final configuration of the metallization layer GaSb/CoSb3/CoGa was found. This information is important in designing uniform, stable contacts for the metallization of gallium antimonide.
Introduction Recently, GaSb has begun receiving more attention as an attractive semiconducting material for 2-4-µm wavelength range optoelectronic applications. An important criterion for the functioning and durability of electronic devices is the quality of their contacts to the external electrical circuit, which must be morphologically uniform, thermodynamically stable, and “ohmic”, i.e., nonrectifying, and with a negligible voltage drop at the contact interface. Furthermore, such a contact must retain these properties up to the rather high temperatures of device fabrication (up to 700 °C) and operation (still up to 400 °C).1 Although GaSb is already being used in the development of photodetectors and lasers, metallization of this III-V compound semiconductor is one of the key technology issues that still must be addressed before this material can realize its great potential for advanced electronic and photonic applications. It is important to realize that development of metallization schemes to semiconducting materials, which have become well-known as a substantial endeavor among physicists and electrical engineers, in fact also involves many chemical aspects. The metal contact cannot be treated as a simple inert junction between the metallization constituent and compound semiconductor. New phases are generally formed at the metal/ semiconductor interface during the fabrication proce* Corresponding author. Tel: +31 40 2472314. Fax: +31 2445619. E-mail:
[email protected]. † COBRA Inter-University Research Institute. ‡ Kossuth Lajos University. (1) Shen, T. C.; Gao, G. B.; Morkoc¸ , H. J. Vac. Sci. Technol. B 1992, 10, 2113.
dure (annealing) which will have different carrier conduction properties. This can be a significant factor affecting the quality (electrical properties and structural integrity) of the metal contact, and, hence, the overall performance of the semiconductor device. In view of the intimate relationship between microstructure and electrical characteristics of the contacts, more work on understanding and controlling the reaction phenomena in metal/compound semiconductor systems is, therefore, highly desirable. In general, the chemical interaction between inorganic solids is governed by thermodynamics and diffusion kinetics of the system under consideration. It was repeatedly demonstrated that the optimal starting point for research on any metal/semiconductor interactions is the investigation of the phase equilibria and reactive phase formation in the relevant material systems.2,3 In our previous work4,5 we have explored some thermodynamic and kinetic aspects of solid-state interactions in the Ga-X-Co (X ) P, As, Sb) systems using the bulk diffusion couple technique. Since the time of these publications, substantial progress has been made in understanding the reaction behavior of Co films on GaSb substrates. In the present paper we concentrate on the results of a comparative study of chemical reactions between GaSb and Co in thin-film and bulk couples in an effort to develop a phenomenological model that can explain (2) Sands, T. J. Metals 1986, 10, 31. (3) Lin, J.-C.; Schulz, K. J.; Hsieh, K.-C.; Chang, Y. A. J. Electrochem. Soc. 1989, 136, 3006. (4) Markovski, S. L.; Van Beek, J. A.; Schiepers, R. C. J.; Kodentsov, A. A.; Van Loo, F. J. J. J. Chim. Phys. 1997, 94, 992. (5) Kodentsov, A. A.; Markovski, S. L.; Cserha´ti, C.; Van Loo, F. J. J. Defect Diffus. Forum 2001, 194-199, 1619.
10.1021/cm021265a CCC: $25.00 © 2003 American Chemical Society Published on Web 12/10/2002
Heat Reaction of GaSb/Co Metallization Contact
(and predict) morphology evolution in the reaction zone. The aims of this communication are as follows: (1) to investigate the interfacial reactions occurring in the Cobased metallization to GaSb during thermal annealing; and (2) to demonstrate that thermodynamic potential (activity) diagrams in combination with knowledge of the relative mobilities of the components offer a framework for rationalizing phase formation in GaSb/Co contacts. Experimental Procedure A 200-nm thick cobalt film was deposited onto an undoped GaSb 〈001〉 oriented wafer (MCP Wafer Technology Ltd. UK) by electron-beam evaporation in a vacuum (10-6 mbar). The temperature of the substrate was kept at 54 °C during deposition. Before the deposition the GaSb wafer was degreased in acetone and etched with a 1:1 aqueous solution of HCl (36%), rinsed in deionized water, and blown dry with nitrogen. The Co-deposited GaSb wafer was cleaved into 5 × 5 mm2 pieces, sealed in silica ampules filled with 150 mbar of high purity argon (H2O < 5 ppm; O2 < 2 ppm), and annealed at 500 °C in an electroresistance furnace. The temperature was controlled within (3 °C. After heat treatment, the samples were quenched in cold water. Cross-section specimens for transmission electron microscopy (TEM) were prepared by laminating the thin-film samples together with epoxy, then mechanically thinning them, and finally low-angle Ar ion milling to accomplish electron-beam transparency. The same commercially available GaSb wafers and Co foils of 0.7 mm thickness, supplied by Goodfellow (UK), were used for studying reactions between GaSb and Co in bulk diffusion couples. The “sandwich” samples GaSb/Co/GaSb were prepared and heat-treated in a vacuum (∼10-6 mbar) or in high purity argon under an external load of about 2 MPa. The temperature control was carried out within an accuracy of (3 °C. After annealing and standard metallographic preparation the bulk diffusion couples were examined by optical microscopy, scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). For constructing the isothermal cross-section through the ternary Ga-Sb-Co phase diagram, samples were prepared using cobalt powder (99.99% with particle size of ∼40 µm) supplied by Goodfellow (UK) and powder made of commercial semiconductor grade GaSb (Alfa Product, Germany). A number of powder mixtures of Co with GaSb were uniaxially pressed into pellets (of about 1 g each). Nominal compositions of the mixtures were: (GaSb)0.425Co0.15, (GaSb)0.35Co0.3, (GaSb)0.3Co0.4, (GaSb)0.265Co0.47, and (GaSb)0.15Co0.7. The samples were sealed in silica ampules under 150 mbar of argon and first annealed at 500 °C for 400 h. Then, the sintered compacts were pulverized, again pressed into pellets, and annealed in argon for another 400 h. Heat treatment was conducted in an electroresistance tube furnace. The temperature was controlled within (3 °C. The weight change of the samples after the equilibration was less than 1 wt % relative. After annealing, the pellets were examined by X-ray diffraction (XRD) with a cylindrical texture camera using manganese-filtered Fe KR radiation. Then, one-half of each pellet was ground into a fine powder and studied with a conventional powder diffractometer. The second halves of the equilibrated compacts were impregnated with epoxy resin (EPO-KWICK, Buehler Ltd., Lake Bluff, IL) and imbedded in special plastic. These samples were ground and polished to a final finish with 0.25-µm diamond slurry and examined with polarized light microscopy. To improve the electrical conductivity of the samples and make them suitable for electron-microscopic investigations, a thin layer of carbon was deposited in a vacuum on the metallographically prepared surfaces. Further analysis of the powder compacts was carried out with SEM and EPMA.
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Results and Discussion Phase Equilibria in the Ga-Sb-Co System at 500 °C. To facilitate analysis of the interfacial reactions in Co-based metallization to GaSb, the previously reported 4,5 phase relations and solubility ranges in the ternary Ga-Sb-Co system were reexamined. This is essential in view of the uncertainties in the available thermodynamic information which renders the theoretical calculations impossible. After equilibration at 500 °C, X-ray diffraction analysis and EPMA gave positive identification of CoSb3, CoGa, and gallium antimonide in the GaSb+Co powder compact with nominal composition (GaSb)0.35Co0.3. The same reaction products were also found in the annealed sample with nominal composition (GaSb)0.425Co0.15. Obviously, a three-phase equilibrium GaSb + CoGa + CoSb3 exists in the Ga-Sb-Co system at this temperature. The solubility of cobalt in GaSb is negligible and a maximum antimony concentration of about 2.5 at. % was measured in the CoGa- intermetallic. Under these conditions, the binary compound CoSb3 dissolves up to 4 at. % of Co. It was found that the annealed powder compact (GaSb)0.265Co0.47 exhibited a three-phase morphology composed of CoSb2, CoGa, and CoSb, whereas only two phases, CoSb2 and CoGa, were detected in the sample (GaSb)0.3Co0.4 equilibrated at 500 °C. This indicates the existence of a three-phase equilibrium CoSb2 + CoGa + CoSb on the ternary isotherm. The presence of another three-phase equilibrium CoGa + CoSb + Co-based (fcc) solid solution (R-CoSS) in this system at 500 °C was established by analyzing a powder compact with nominal composition (GaSb)0.15Co0.7. In addition, the R-CoSS/CoGa phase boundary was reestablished by analyzing an equilibrated Co 20 at. % Ga two-phase alloy. The solid-state solubility of gallium in cobalt at this temperature was found to be ∼17 at. %. The R-Co-based solid solution equilibrates with the CoGa phase containing approximately 31 at. % of Ga. The results of the examination of equilibrated powder compacts in combination with the information on binary systems6,7,8,9 led to the cross-section of the Ga-Sb-Co phase diagram at 500 °C as represented in Figure 1. No ternary compounds are present on this ternary isotherm. An interesting point here is that CoGa intermetallic (B2; Pm3m) is in thermodynamic equilibrium with all intermediate phases stable in the corresponding binary systems. Morphological Development During Reaction Between Co and GaSb. To our best knowledge, no attempt has been made to investigate the reaction behavior of Co thin films on GaSb substrates. In this work, primary attention was directed toward studies of systems with unreacted cobalt present. In other words, the samples to be described in this section are “classical” semi-infinite reaction couples, and after the diffusion annealing the couple ends still have their original (6) Ngai, T. L.; Sharma, R. C.; Chang, Y. A. Bull. Alloy Phase Diagrams 1988, 9, 586. (7) Ishida, K.; Nishizawa, T. Bull. Alloy Phase Diagrams 1990, 11, 243. (8) Schubert, K.; Lukas, H. L.; Meissner, H.-G.; Bhan, S. Z. Metallkd. 1959, 50, 534. (9) Feschotte, P.; Eggimann, P. J. Less-Common Met. 1974, 63, 15.
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Figure 1. Isothermal cross-section through the ternary phase diagram Ga-Sb-Co at 500 °C based on the results of the present study and available data on the binary systems.6-9 The compositions of the experimentally investigated powder mixtures are depicted as points.
Figure 2. (a) Typical electron micrograph (bright-field image) of the cross-section of the as-deposited sample and (b) an electron diffraction pattern revealing a fine polycrystalline structure of the metal film. (The ring pattern is consistent with the low-temperature hexagonal -Co).
compositions. If surface and interface contributions to the free energies are negligible and if these systems are considered to be closed, one would expect the same reaction products as mandated by the bulk Ga-Sb-Co phase diagram. Figure 2a shows a cross-section bright-field TEM micrograph of the as-deposited sample. The Co layer has a very fine grain structure and no reaction products were detected at the GaSb/Co interface. The electron diffraction verified the expected formation of polycrystalline -Co (hexagonal) during deposition (Figure 2b). Another important feature can be seen on this micrograph. There exists a light-contrasted layer between GaSb substrate and the Co metallization, which is too thin to be properly characterized by the analytical techniques used. Possibly, this layer can be associated with the “native oxides” that might remain on the original substrate surface prior to Co deposition. The presence of an oxide film at the GaSb/Co interface may influence nucleation (and growth) of the new phases during an initial stage of interaction. However, it does not completely suppress the chemical reaction between GaSb and cobalt at 500 °C, although the product layer, formed after annealing at this temperature in argon for 18 min, has a laterally nonuniform structure as one can appreciate from Figure 3. The reaction zone, developed
Kodentsov et al.
Figure 3. Cross-sectional bright-field electron micrograph of the annealed GaSb/Co contact (500 °C; 18 min; 150 mbar of Ar) viewed along the [011] direction in the lattice of GaSb. The moire´ patterns are formed due to overlapping between the faceted crystals of the product phase CoSb3 and the GaSb substrate.
under these circumstances, basically consists of two layers, and some of the faceted crystals of the reaction product protrude into the GaSb. It was not possible to establish unambiguously the phase composition of the product layers using energy dispersive X-ray spectroscopy (EDS) and electron diffraction. Nevertheless, when imaging conditions are appropriate, the faceted domains within the resulting microstructure are seen to be associated with the formation of moire´ fringes that may facilitate phase identification. This translational (parallel) moire´ pattern is formed due to overlapping between the product crystals and the GaSb substrate. Because the fringes run perpendicular to the [111h ]-direction in the GaSb lattice, the parallel moire´ pattern can be interpreted with respect to the correspondence between the reflecting planes of type {111} in the cubic GaSb and a lattice of one of the possible product phases.10 The experimentally determined mean spacing of the moire´ fringes of ∼1.5 nm is consistent with the calculated one between d{111} for GaSb (B3; F4 h 3m; a ) 0.6095 nm) and d{310} for CoSb3 (DO2; {Im}3 h ; a ) 0.9034 nm), i.e., the reflecting planes are {111}GaSb and {310}CoSb3, and 〈111〉 GaSb // 〈310〉 CoSb3. (The lattice parameter values for GaSb and CoSb3 were taken from the JCPDS-files no. 7-215 and 11-336, respectively.) Increasing of the annealing time up to 1 h results in a diffusion-zone morphology shown in Figure 4. After interaction under these conditions, the thin-film couple is still semi-infinite. The product layer developed during reaction is rather uniform in thickness and consists essentially of two sublayers. As determined by electron diffraction, the more coarse-grained product adjacent to the GaSb substrate is the CoSb3, and the fine-grained layer next to the unreacted Co metallization appears to be the CoGa phase. A somewhat similar reaction pattern was also observed in bulk GaSb/Co couples after much longer annealing times (up to 600 h). As an example, a representative microstructure of the central part of the couple annealed at 500 °C for 600 h in a vacuum is given in Figure 5. From the micrograph it is apparent that the dominant phases formed in the reaction zone are again CoGa and CoSb3. In addition, a fragmented layer (10) Hirsch, P. B.; Nicholson, R. B.; Howie, A.; Pashley, D. W.; Whelan, M. J. Electron Microscopy of Thin Crystals; Butterworth: London, 1965.
Heat Reaction of GaSb/Co Metallization Contact
Figure 4. Cross-section TEM micrograph (bright-field image) of the GaSb/Co contact after annealing at 500 °C for 1 h under 150 mbar of argon.
Figure 5. Backscattered electron image of the central part of the reaction zone in a bulk GaSb/Co diffusion couple after reaction at 500 °C for 600 h in a vacuum.
Figure 6. Microstructure of the central part of the reaction zone in a binary Sb/Co diffusion couple after annealing at 500 °C for 49 h in a vacuum (backscattered electron image.)
of CoSb can be seen next to the cobalt, and in some areas of the CoSb3/CoGa interfacial region the formation of CoSb2 intermetallic was detected. Closer inspection also reveals the presence of small CoSb precipitates along the grain boundaries in the CoGa layer. The reaction layer grows parabolically with time, underlining a diffusion-controlled process. The results of the present study demonstrate that CoSb3 is the product phase formed next to the GaSb substrate, and CoGa is the second fastest growing intermetallic. The same CoSb3 compound was found to be the fastest growing phase in binary Sb/Co diffusion couples as well (Figure 6). However, the relative growth rates of this intermetallic in the binary and ternary couples differ significantly (cf. Figures 5 and 6). This observation can be qualitatively explained by the fact that in the binary Sb/Co reaction couple, CoSb3 grows over its entire (very narrow) homogeneity range, whereas in the ternary GaSb/Co couple it may grow only over a
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Figure 7. Optical micrograph of a reaction zone developed between liquid Ga and Co after heat treatment at 500 °C for 23 h in flowing argon. A very thin layer of the CoGa phase is present between the Co substrate and the CoGa3 compound.
concentration range that is in thermodynamic equilibrium with GaSb. This means that the chemical potential (activity) gradient across the product layer, and, hence, the driving force for the diffusion-controlled growth of CoSb3 intermetallic in the reaction zone, will be different in both couple types. At first sight, it might also be surprising that the CoGa3 phase does not form at all during interaction between GaSb and Co, because in a binary Ga/Co couple this compound is a dominant reaction product at 500 °C (Figure 7). On the contrary, the CoGa intermetallic, which is a fast-growing phase in the ternary couples, forms in the reaction zone of the binary couple in a very minute quantity. It was estimated that, at this temperature, the interdiffusion in CoGa3 is at least 8 orders of magnitude faster than that in the CoGa phase.11 Therefore, the CoGa occurs in the diffusion zone only as a very thin layer between the CoGa3 product phase and Co. In next section we will see that the absence of CoGa3 is due to thermodynamic reasons. Obviously, the diffusion-zone morphology evolved during chemical interaction between metal and compound semiconductor is defined by type, structure, number, shape, and topological arrangement of product phases, which are kinetically, as well as thermodynamically, constrained. Therefore, the central theoretical question here is: “To what extent is it possible to rationalize and to predict the interfacial reactions from the more or less easily accessible thermodynamic information, which can be obtained or estimated by independent methods?” Analysis of Interfacial Reactions in the GaSb/ Co System Using Chemical Potential Diagrams. It may be recalled that the resulting microstructure of the reaction zone between the end-members of the couple can be visualized by a diffusion (reaction) path. This is a line on the ternary isotherm representing the locus of the average composition in planes parallel to the original interface throughout the diffusion zone. Naturally, the diffusion path must fulfill the law of conservation of mass. If no material is lost or created during the reaction, then the diffusion path is forced to cross the straight line between the end-members of the reaction couple (the so-called mass balance line) at least once. Under conditions of local equilibria in the diffusion zone (11) Markovski, S. L. Chemical Interaction between Metals and Compound Semiconductors, Ph.D. Thesis,; Eindhoven University of Technology, The Netherlands, 1999.
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of a semi-infinite couple, the reaction path involves a time-independent sequence of intermediate layers. Undoubtedly, the Ga-Sb-Co phase diagram provides guidance in understanding solid-state interactions in this system. However, from the isothermal section alone it is not unequivocally clear how the phase sequence in the reaction zone between GaSb and Co will evolve. At 500 °C, for example, a diffusion path (reaction layer sequence) such as GaSb|CoGa3|CoGa|CoSb3|CoSb2|CoSb| R-Co cannot be excluded when looking only at the experimentally determined phase equilibria in the GaSb-Co system. In principle, all neighboring phases in this hypothetical sequence can coexist in equilibrium and the mass balance can be preserved. It should be mentioned that for a diffusion-controlled process when local equilibrium is supposed to exist within the reaction zone, the chemical potential (thermodynamic activity) of the species varies continuously through the product layers and has the same value at both sides of an interphase interface. The concentration of each component, on the other hand, will change discontinuously across phase boundaries from one end-member of the reaction couple to the other. The diffusion paths should obey the rules extensively explained in earlier papers.12,13 Intrinsic fluxes of atoms in the reaction zone are controlled by the direction and magnitude of their chemical potential gradients. Therefore, the nature of the diffusion phenomena can be better interpreted by superimposing the diffusion path on the potential (activity) diagram rather than on the Gibbs compositional triangle. Intrinsic diffusion of an element takes place only in a direction in which it decreases its own chemical potential (in other words, in the direction in which its own thermodynamic activity is lowered). In this context, it is important to emphasize the word “intrinsic”. The equations for the interdiffusion fluxes in a ternary diffusion couple are dependent on the chemical potential gradients of two components. By contrast, the intrinsic flux of a component, measured in the Kirkendall frame of reference, depends only upon its own potential gradient as shown by Lane and Kirkaldy.14 The often observed maxima, not only in the concentration but also in the activity plots of a component, arise from the mobility of the other two components. This leads to some confusion in the literature about the meaning of an intrinsic flux in multicomponent systems.15 It should be clear that intrinsic fluxes can be measured experimentally only with reference to inert markers throughout the entire diffusion zone. Before proceeding further with the discussion, it is appropriate to consider the available thermodynamic information on the Ga-Sb-Co system required to rationalize the interfacial reactions. Because no evidence was found for the existence of ternary phases in the Ga-Sb-Co system at 500 °C, the corresponding potential diagrams can be calculated (or better, reliably estimated) using pertinent thermodynamic data on the binary systems and the measured solubility ranges. (12) Van Loo, F. J. J. Prog. Solid State Chem. 1990, 20, 47. (13) Van Loo, F. J. J.; Kodentsov, A. A. Pure Appl. Chem. 1998, 70, 501. (14) Lane, J. E.; Kirkaldy, J. S. Can. J. Phys. 1964, 42, 164. (15) Ro¨nka¨, K. J.; Kodentsov, A. A.; Van Loon, P. J. J.; Kivilahti, J. K.; Van Loo, F. J. J. Metall. Mater. Trans. A 1996, 27A, 2229.
Kodentsov et al. Table 1. Standard Gibbs Energy of Formation for the Sb-Containing Compounds of the Ga-Sb-Co System at 500 °C Used in the Present Study for the Activity Calculations compound
free energy of formation (kJ/mol)
reference
GaSb CoSb3 CoSb2 CoSb
-36.01 -59.11 -49.45 -38.97
16 17 17 17
Thermodynamic parameters for GaSb used for the activity calculations were adopted from the compilation of Kubaschewski et al.16 Values for the standard Gibbs energy of formation for the binary Co-Sb intermetallics were taken from ref. 17. All these phases were treated as line compounds. Free energies of formation for different Sb-containing compounds of the Ga-Sb-Co system used are given in Table 1. Katayama et al.18 performed a comprehensive study of the thermodynamic properties of Co-Ga alloys containing 0-58.9 at. % of Ga over the temperature range 800-1000 °C. The emf method was employed using galvanic cells with the solid electrolyte ZrO2 + CaO. Later, Mikula et al.19,20 conducted some additional emf measurements on this system. These authors also derived thermodynamic parameters for CoGa phase using a theoretical model developed by Neumann et al.21 for thermodynamic description of B2-type structures exhibiting triple defects. In the present work, the reported emf values for different Co-Ga alloys were extrapolated to 500 °C within the single-phase regions of the R-Co(Ga) solid solution and of the CoGa intermetallic. In this manner, thermodynamic activities of Ga in these composition ranges were obtained, and the corresponding values of the Co activity were computed using the Gibbs-Duhem integration. These results are presented graphically in Figure 8. It is interesting to notice that at this temperature the R-Co(Ga) solid solution exhibits a large negative deviation from Raoultian behavior for the activity of Ga in the whole concentration range studied. Activities of Co, on the other hand, showed very small positive deviations from Raoult’s law in the R-Co-based solid solution, but very large negative ones in the CoGa phase. To assess the thermodynamic stability of the binary phase CoGa3, the following procedure was used. This evaluation is based on the fact that at the phase boundary CoGa/CoGa+CoGa3, the thermodynamic activity of gallium in the CoGa equals the Ga activity in the CoGa3 compound, because these phases coexist in the binary Co-Ga system at 500 °C.8,9 The same is also true for the activity of cobalt. Then, the Gibbs energy of the formation of the CoGa3 phase can be found using (16) Kubaschewski, O.; Alcock, C. B.; Spencer, P. J. Materials Thermochemistry; Pergamon Press: Oxford, 1993. (17) Barin, I.; Knacke, O.; Kubaschewski, O. Thermochemical Properties of Inorganic Substances; Springer-Verlag: Berlin, 1977. (18) Katayama, I.; Kemori, N.; Kozuka, Z. Trans. JIM 1975, 16, 423. (19) Mikula, A.; Chang, Y. A.; Neumann, J. P. Trans. JIM 1978, 19, 307. (20) Mikula, A.; Schuster, W.; Chang, Y. A.; Henig, E.-T. Z. Metallkd. 1987, 78, 172. (21) Neumann, J. P.; Chang, Y. A.; Lee, C. M. Acta Met. 1976, 24, 593.
Heat Reaction of GaSb/Co Metallization Contact
Figure 8. Thermodynamic activity of the components in the Co-Ga system as a function of composition at 500 °C, derived from the measurements of Katayama et al.18 (b) and Mikula et al.19,20 (2).
the Gibbs-Duhem relation. This value was estimated at 500 °C as ∼26.83 kJ/mol of atoms (with liquid Ga and solid Co as a reference state). With the thermodynamic information on binary phases considered above, the experimentally determined phase relations in the ternary Ga-Sb-Co system are in agreement with those predicted theoretically. Now, potential diagrams for antimony and gallium can be constructed, and the diffusion-activity model can be further extended to reactions in the GaSb/Co couples. It is simple to see from the isothermal construction shown in Figure 9a that the phase CoSb3 can never be formed in the diffusion zone at the position in the hypothetical sequence given above, because the intrinsic
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diffusion of antimony should then take place through CoGa3 and CoGa toward CoSb3. The intrinsic diffusion of Sb should then go into a direction of a higher Sb activity which is thermodynamically forbidden. In fact, the formation of the CoGa3 phase is exluded on the basis of these thermodynamic constraints. In the experiments described in the preceding section (Figures 4 and 5), a continuous layer of CoSb3 phase was always found next to GaSb in the transition zone of the GaSb/Co couples after annealing at 500 °C in a vacuum or argon. In the case of the bulk diffusion couple, the reaction path proceeds in the phase sequence GaSb|CoSb3|CoSb2|CoGa|CoSb|R-Co which is, indeed, kinetically allowed and thermodynamically possible as may be concluded from the isothermal section and potential diagrams for Sb and Ga (Figures 1 and 9). The presence of small inclusion of CoSb along the grain boundaries of the CoGa does not violate the model as set above, but it may influence the thickness of the reaction layer. In fact, using this simple model, one can only predict the reaction layer sequences that are not allowed. Before ending this part of the discussion, it should be mentioned that the formation of the GaSb/CoSb3/ CoGa structure was found to be the final stage of the reaction in the thin-film samples, as predicted by the bulk phase diagram. In Figure 10 a representative microstructure of the contact after reaction at 500 °C for 9 h is shown as an example. The product phases were readily identified by means of electron diffraction. It is, however, important to point out that owing to the detection limitation we can only conclude that similar to the bulk samples, interdiffusion in the thin-film samples shows the dominant growth of CoSb3 and CoGa phases, but we are uncertain about the absence of the CoSb2 and CoSb compounds.
Figure 9. Thermodynamic potential (activity) diagrams for antimony (a) and gallium (b) in the Ga-Sb-Co system at 500 °C. The hypothetical diffusion path between GaSb and Co showing the sequence GaSb|CoGa3|CoGa|CoSb3|CoSb2|CoSb|R-Co is given as dotted lines. Note from Figure 9a that this hypothetical path in is not allowed.
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Figure 10. Cross-section TEM micrograph (bright-field image) of the GaSb/Co thin-film sample annealed in argon at 500 °C for 9 h showing an equilibrium structure GaSb/CoSb3/CoGa after completion of the reaction.
Concluding Remarks From a theoretical as well as a practical point of view, it is very tempting to have a phenomenological approach that enables us to rationalize, and to a certain extent predict, the reactive phase formation at the interfaces using just data that are relatively easily attainable in experiments. However, caution should be exercised when applying the proposed general deductions for evaluating interaction between III-V compound semiconductors and metals. In this respect, several points are to be noted. First, one must be careful not to attribute too much precision to the potential diagrams used in the present discussion. These isothermal constructions are no better than the thermodynamic data used to construct them and these are, very often, quite poor. Nevertheless, the chemical potential diagrams are useful for interpreting solid-state reactions, and by identifying the domains where data are poor, they can be systematically improved. Second, it is also important to keep in mind that the analysis employed in the present paper essentially relies on the assumption of local equilibria in the diffusion zone. The latter implies that each infinitely thin layer of such a reaction zone is in thermodynamic equilibrium with the neighboring layers. In a translated sense, this means that chemical reactions at the interfaces are very rapid compared with the rate of diffusion in the product layers. It was, however, often reported that in planar binary thin-film diffusion couples not all of the compound phases predicted by the equilibrium phase diagram have been observed to be present simultaneously.22,23 In such cases, the formation of the product phases is said to be controlled by the reaction at the interfaces. The reaction behavior of thin-film structures can be further complicated by surface phenomena, epitaxial relations, and anisotropy associated with single-crystal substrates, and diffusion fluxes can be (22) Go¨sele, U.; Tu, K. N. J. Appl. Phys. 1983, 53, 3252. (23) Thompson, C. V. J. Mater. Res. 1992, 7, 367.
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dominated by grain boundary and other extended defect mechanisms. In this context, it is necessary to stress that from our experimental results no evidence can be found to support the idea of sequential appearance of the product phases in reaction between Co film (200 nm) and GaSb. In both bulk and thin-film GaSb/Co diffusion couples studied, as long as a supply of Co was maintained, the reaction zone morphology was found to be dominated by the binary CoGa and CoSb3 phases, the latter intermetallic growing next to the compound semiconductor. These reaction layers are consistent with the thermodynamic data and the phase diagram. CoSb2 and CoSb were present in small amounts in bulk diffusion couples but were not detected in the thin-film samples. This might be due to the difficulties in nucleation, although it is also possible that these phases are present in the reaction zone in such minute quantities that they were overlooked by the experimental techniques available. Another question that might cause uneasiness in evaluating the interfacial phenomena in III-V compound semiconductor/metal systems is connected with the influence of the annealing atmosphere on the reactive phase formation. It is important to realize that the isothermal section given in Figure 1 implicitly assumes that the partial pressure of antimony in ambient environment is higher than the dissociation pressure of CoSb3. Actually, such a situation was attained in our experiments, which were carried out in small, argon-filled silica ampules. However, under circumstances of, for example, a dynamic vacuum, the CoSb3 phase can be on the “border” of its thermodynamic stability. From the reaction equation CoSb2 + 1/4 Sb4 (g) ) CoSb3, the value of the equilibrium partial pressure of the gaseous species Sb4 was estimated at 500 °C as about 7.4‚10-6 mbar. The necessary thermodynamic data were taken from ref. 17. Accordingly, the equilibrium vapor pressure of Sb4 gas above the CoSb3 is higher than that usually maintained during vacuum anneals (∼10-6 mbar). It is evident that under these conditions, the topology of the Ga-Sb-Co diagram will differ from that shown in Figure 1. This was also confirmed experimentally: no CoSb3 compound was detected by means of X-ray diffraction in a sample with nominal composition (GaSb)0.35Co0.30 annealed at 500 °C in dynamic vacuum. Instead, a three-phase equilibrium involving GaSb, CoSb2, and CoGa was found to be present on the isothermal section. This situation profoundly affects the phase composition and morphology of the diffusion zone in the vicinity of the “triple point” in the GaSb/Co couple, where reaction layers meet at ambient atmosphere,24 and has also to be taken into consideration when reproducibility of the metal contacts is an issue. However, despite all the interesting problems mentioned above, this type of research provides a link between bulk and thin-film interdiffusion studies which is required in order to arrive at a comprehensive understanding of the mechanisms of reactive-phase formation at the interfaces in inorganic materials. CM021265A (24) Markovski, S. L.; Van Dal, M. J. H.; Verbeek, M. J. L.; Kodentsov, A. A.; Van Loo, F. J. J. J. Phase Equilib. 1999, 20, 373.
