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On the constitution of some Ga-M-P systems (where M represents Co, Rh, Ir, Ni or Pt)
MATERIALS
SCIENCE ENCIHEERIWC
&
B
Materials Scienceand Engineering B39 (1996)52-61 ELSEVIER
On the constitution of some Ga-M-P systems (where M represents Co, Rh, Ir, Ni or Pt) D. Swensona,*, Y.A. Changb “Chetnistry bDeparittlent
and Materials of Materials
Science, Science
Lawrence Livermore National and Engineering, University
Laboratory,
of Wisconsin,
P.O. Box 808, L-370, Liwrtuore, 1509 University Areme, Icfodimn,
CA 94551, WI 53706,
USA USA
Received3 October 1995;in revisedform 10October1995
Abstract Phase equilibria are established in the Gap-rich regions of five Ga-M-P systems (where M represents Co, Rh, Ir, Ni, or Pt) at 700 “C (or 600 “C for the Ni-bearing sample) using X-ray diffraction analysis. The results of the present study, in conjunction with previous work on the Ga-Pd-P system, give a complete picture of phase equilibria between GaP and the gallides and phosphides of the Co and Ni groups. Based on these data, it is concluded that many binary metal gallides, including CoGa3, CoGa, RhzGas, RhGa,, Rh,,Ga,,, RhGa, Ir,Ga,, IrGa,, Ni,Ga,, PdGa, Pt,Ga,, PtGa, and Pt,Ga,, may potentially serve as contact materials for use in high-temperature Gap-based electronic devices. Keywords:
Metal gallides; Phaseequilibria
1. Introduction Historically, the practical applications of gallium phosphide (Gap) have essentially been limited to the fabrication of light emitting diodes (LEDs) [I]. In recent years, however, there has been an increased level of interest in this compound semiconductor. Gallium phosphide has been identified as a candidate for use in photonic or photonic-like devices, including near-UV energy conversion photodetectors and “betavoltaic” cells [2]. Additionally, owing to its wide bandgap (2.26 eV) and the low leakage current of GaP p-n junctions, it shows great promise as a material for use in highelectronic temperature ( > 300 “C) minority-carrier devices, especially diodes, bipolar junction transistors and power-switching devices [3]. Moreover, sufficient experimental data are available to suggest that even 500 “C GaP electronic devices, both majority- and minority-carrier, are well within the realm of possibility c3,41. Zipperian et al. [3] summarized the state of GaP technology through 1981 as it applies to high-temperature electronics applications. In their review, they specifically identified GaP contact technology as an area in which additional research was required if viable high-
temperature GaP devices were to be realized. Since 1981, however, relatively little advancement in GaP contact technology has been achieved. The most serious problem facing any contact material for use in high-temperature electronic devices is that of thermal stability. Even if a contact exhibits acceptable electrical properties immediately after device fabrication, it must maintain these properties over long periods of time during routine operation at elevated temperatures. Interdiffusion between the metallization layer and the semiconductor may not be tolerated, as it will almost certainly alter the electrical properties of the contact and hence affect the performance of the device. This implies that a successful contact material for hightemperature devices must be nonreactive with respect to the semiconductor, since at elevated temperatures thermodynamically allowed chemical reactions are not kinetically hindered. To date, few metallurgical investigations of contacts to GaP have been reported in the literature. Based on these available data, however, it is apparent that in general metals are not’chemically inert with respect to Gap. For example, using X-ray diffraction analysis, Kumar [5] has found that Pt films react with GaP at temperatures as low as 2.50 “C. Employing in-situ
D. Swemor~,
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1 Materials
Auger electron spectroscopy in an ultrahigh vacuum deposition system, Hiraki et al. [6] have observed the formation of an interdiffused layer which was tens of nanometers in thickness after annealing a Au/Gap contact for just 7 min at 200 “C. Finally, using transmission electron microscopy, Mohney et al. [7] observed a 10 nm reacted layer between Pd and GaP in as-deposited contacts, and found that a 500 “C, 30 s heat treatment was sufficient to completely consume a 50 nm Pd film. Potential chemical reactions between metal contacts and a semiconductor substrate may be circumvented by selecting metallizations which are in thermodynamic equilibrium with the semiconductor. Such an approach was first suggested by Lince and Williams [8], who demonstrated the utility of ternary phase diagrams for selecting thermodynamically stable, Au-based contacts to GaSb. This approach to metallization selection has been applied with great success to the development of contacts to GaAs (see, for example, Ref. [9]). However, only recently has such an approach been extended to other III-V semiconductor systems. In particular, there is a paucity of data pertaining to Ga-transition metal(M)-P phase equilibria; indeed, the very recent determination by Mohney et al. [7] of partial phase equilibria in the Ga-Pd-P system represents the only such information that has ever been reported in the literature. Based on the preceding discussion, it is quite clear that more Ga-M-P phase diagrams must be determined if advances are to be made in GaP contact technology. Therefore, in the present investigation, phase equilibria have been established in the Gap-rich regions of five Ga-M-P systems, where M denotes Co, Rh, Ir, Ni or Pt. These data are then used to identify potential thermodynamically stable contact materials to GaP.
2. Experimental
procedure
Semiconductor grade GaP powder, and Co, Rh, Ir, Ni and Pt of at least 99.95% purity were weighed in appropriate ratios to produce samples with nominal total masses of 0.3 g. The Pt sample was significantly smaller, owing to a limited supply of the metal. The elements Co, Ni and Pt were in powder form, and were mixed directly with GaP powder and pressed into pellets. These samples were sealed in quartz ampoules which were evacuated to a pressure of 10e2 Torr. The Co and Pt samples were annealed at 700 “C for 14 d. They were then quenched in iced water and ground into powders, and were again pressed into pellets and encapsulated in evacuated quartz ampoules. The samples were annealed for an additional two months. A similar procedure was adopted for the Ni sample, except that it was annealed at 600 “C for a total time of five months.
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The elements Rh and Ir were in the form of small pieces. To facilitate chemical reaction between the metal pieces and GaP, an initial rigorous heat treatment was adopted. The Rh and Ir pieces were combined with GaP powder, and the samples were sealed in evacuated quartz ampoules and annealed at 1000 “C for one week. The annealing temperature was then slowly lowered to 700 “C over the course of a second week. The samples were subsequently quenched in iced water, ground into homogeneous powders, pressed into pellets, sealed in evacuated quartz ampoules, and annealed at 700 “C for two additional months. Following their final heat treatments, all samples were quenched in iced water. Each sample was crushed and analyzed by X-ray powder diffraction, using a Scintag PAD V automated diffractometer and Cu Kcr radiation. For the Co sample, silicon (NIST standard 640b) was used as an internal standard, and lattice parameters of the phases present were refined using a nonlinear least-squares computer program included in the Scintag software package. One of the phases in the Ir sample could not be identified by X-ray diffraction analysis. Therefore, the sample was sintered in an evacuated quartz ampoule at 700 “C for an additional month, metallographically cross-sectioned and analyzed for composition by electron probe microanalysis (EPMA), using a Cameca electron microprobe and employing wavelength-dispersive spectroscopy. Elemental Ir and commercially produced GaAs and InP wafers were used as standards.
3. Results and discussion Prior to discussing the results obtained in the present investigation, phase equilibria in the constituent binary Ga-P, Ga-M, and M-P systems will be reviewed. Additionally, a survey of all previous experimental investigations pertaining to the ternary Ga-M-P systems will be presented. 3.1. Phase equilibria in the constituent binary systems The phases present in the constituent binary Ga-P, Ga-M and M-P systems, their ranges of homogeneity, crystal structures and temperature stabilities are given in Table 1. Unless otherwise noted, these data are taken from Ref. [lo]. The constituent binary systems of each Ga-M-P ternary system are briefly discussed in the following five subsections. 3.1.1. Ga-Co-P The system Ga-P contains one intermediate GaP, which is thermbdynamically stable only stoichiometric composition. Below about 800 solubility of P in liquid Ga is negligible (much than 1 at.%).
phase, at the “C, the smaller
D. Swenson, Y.A. Chang / Maaterials Science and Engineering 839 (1396) 52-61
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Table 1 Compositional stabilities, crystal structures and temperature stabilities of constituent binary phases in Ga-M-P
systems”
Phase
Composition (at.% Ga or P)
Structure
Temperature W)
GaP
50.0
cubic, ZnS, cF8, B3
1467(c)b
CoGa, CoGa COP, COP Co,P
75.0 32.0-67.0 75.0 50.0 33.3
tetragonal, CoGa,, tP16 cubic, CsCl, cP2, B2 cubic, CoAs,, ~132, DO, orthorhombic, MnP, oP8, B32 orthorhombic, Co,Si, oP12, C23
855(P) 1210(P) unknown unknown 1386(c)
Rh,Ga, RhGa, Rhd% RhGa [13] RhP, RhP, RhJ’, RM’, Rh,P
81.8 75.0 [12] 63.0 49.0-50.0 [12] 75.0 66.7 42.9 40.0 33.3
monoclinic, Co,Al,, mP22, D8d tetragonal, CoGa,, tP16 tetragonal, Rh,,Ga,,, tP108 cubic, C&l, cP2, B2 [13] cubic, CoAs,, cI32,DO, monoclinic, CoSb,, mP12 orthorhombic, Rh,P,, oP28 tetragonal, Rh,P,, tP5 cubic, arzti-CaF,, cF12, Cl
unknown, unknown, unknown, unknown, unknown unknown unknown unknown - 1500(c)
Ir,Ga, IrGa,(a) IrGadJ) Ir,Ga, IrGa IrP, IrP, Ir,P
81.8 75.0 74.0-77.0 [14] 61.0-65.0 [14] 48.0-50.0 [14] 75.0 66.7 66.7
monoclinic, Co2Al,, mP22, D8, unknown tetragonal, CoGa,, rP16 tetragonal, Ir,Ga,, tP32 cubic, CsCl, cP2, B2 cubic, CoAs,, ~132, DO, monoclinic, CoSb,, ~zP12 cubic, anti-CaFz, cF12, Cl
unknown unknown unknown unknown unknown unknown 1230(d) - 1335(c)
NiGa, Ni,Ga, Ni,Ga, NiGa Ni,3Ga, Ni,Ga, Ni,Ga, Ni,Ga Nip3 NiP, NiP N&P, Ni,P Ni12P5(y) NiddJ) Ni,P,(u) W’~U) N&P
80.0 60.0 56.6-57.6 30.4-57.0 39.2-42.2 35.0-41.0 36.2-38.0 22.6-30.0 75.0 66.7 50.0 44.4 33.3 29.4 29.4 28.6 28.6 25.0
cubic, NiGa,, ~140 [15] trigonal, Ni,Al,, hP5, D513 cubic, Ni,Ga,, ~I112 cubic, CsCl, cP2, B2 monoclinic, Ni,,Gap, mC44 hexagonal, NiAs, hP4, B8, orthorhombic, Pt,Ga,, oC16 cubic, Cu,Au, cP4, Ll, cubic, CoAs,, ~132, DO, monoclinic, Nip,, mC12 orthorhombic, Nip, oP16 hexagonal, N&P,, hP36 hexagonal, Fe,P, hP9, C22 tetragonal, Ni,,P,, t134 unknown hexagonal, Ni,P,, hP168 unknown tetragonal, Ni,P, tI32, DO,
363(p) 895@) 542(pd) 1220(c) 680@d) 680(ed) < T < 949(pd) 741 W) 1212(P) 697(ed) < T< unknown unknown 850(ed) < T c unknown unknown 1100(c) 1OOO(pd) lOOO(ed) < Tc 1125(p) 1025(pd) 1025(cd) < T < 1170(c) 970(P)
PtGa, Pt,Ga, PtGa,(a) PtGa,(P) Pt,Ga, PtGa Pt,Ga, Pt,Ga(cc)
85.7 70.0 66.7 [16] 65.5-66.7 [16] 60.0 50.0 37.0-42.0 33.3 33.3 33.3 24.5-26.5 26.5-27.5 24.0-32.0 66.7 28.6
orthorhombic, PtGa, cubic, Ir,Ge,, ~140, D8, tetragonal, PtGa,, tI96 [16] cubic, CaF,, cF12, Cl trigonal, Ni,Al,, hP5, D5,, cubic, FeSi, cP8, B20 orthorhombic, Pt,Ga,, oC16 tetragonal, AuCu, tP2, L10 orthorhombic, Pt,Ga(P) orthorhombic, RhsGe,, oP16 tetragonal, Pt,Ga, tP16 tetragonal, SiU,, t116, DO, cubic, Cu,Au, cP4, Ll, cubic, Fe& cP12, C2 monoclinic, Pt,P,, mC28
290(P) 822(p) 1W4 153(ed) < T < 922(p) 937(P) 1104(c) 1142(p) unknown unknown 1149(p) 210@d) Wpd) 1374(c) unknown, but > 1500(c) WP)
Pt,Ga(P) WWy) Pt,Ga(a) PVNP) WWy) PtP, w*
but but but but
<: 1000 < 1000 < 1000 > 1000
[II] [Ill [ll] [I l]
aUnless otherwise noted all data have been taken from Ref. [lo]. bThe symbols c, p, ed, pd and d denote congruent melting, peritectic melting, eutectoid decomposition, peritectoid decomposition and dissociation, respectively.
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The Co-Ga system is also very simple. It comprises two intermediate phases, CoGa, and CoGa. While CoGa, exists only at the stoichiometric composition, CoGa exhibits a very wide range of homogeneity, extending from 32 to 67 at.% Ga. There are, however, slight discrepancies among researchers about the range of homogeneity of CoGa as a function of temperature [17-201. The solubility of Ga in solid Co is quite extensive. The Co-P system has been assessed by Ishida and Nishizawa’ [21]. Three intermediate phases have been found in this system: COP,, COP and Co,P. Each of the cobalt phosphides is essentially stoichiometric, although a slight range of homogeneity has been observed for Co,P at high temperatures. Phosphorus is reported to be insoluble in solid Co. 3.1.2. Ga-R/z-P
No Ga-Rh phase diagram has been reported in the literature. Four intermediate phases are known to exist in the Ga-Rh system: Rh,Ga,, RhGa,, Rh,,Ga,, and RhGa. The phase RhGa has been overlooked in Ref. [lo], although its existence has been confirmed by several researchers [l 1,13,17,22,23]. The X-ray diffraction work of GuCrin et al. [ll] suggests that the phases Rh,Gag, RhGa, and RhGa are stoichiometric. This is essentially in agreement with the work of Schulz et al. 1121,who used EPMA to measure the ranges of homogeneity of RhGa, and RhGa at 600 “C in bulk Ga-Rh diffusion couples. In the latter investigation, RhGa, was found to be stoichiometric, whereas RhGa exhibited a small (1 at.%) range of homogeneity. Ga does not seem to be soIuble in solid Rh [11,12,17]. The temperature stabilities of the rhodium gallides are unknown. However, GuCrin et al. have demonstrated that RhGa is the only rhodium gallide that is stable at 1000 “C. It is also noteworthy that G&in et al. were not able to confirm the existence of Rh,,Ga,, at 700 “C. The Rh-P system has been assessed by Okamoto [24]. As was the case for the Ga-Rh system, the Rh-P phase diagram is unknown. The existence of five rhodium phosphides has been reported. These include RhP3, RhP,, Rh,P3, Rh,P, and Rh*P. The rhodium phosphides are considered to be stoichiometric, although a small, unquantified range of homogeneity has been suggested for RhP,. Phosphorus is probably insoluble in solid Rh. With the exception of Rh,P, the melting or decomposition temperatures of the rhodium phosphides are unknown. 3.1.3. Ga-Ir-P
Again, no phase diagram is available for the Ga-Ir system. It is known that at least four intermediate phases exist in the system: Ir2Ga9, IrGaj, Ir,Ga, and IrGa. According to Schubert et al. [17], the phase IrGa, is dimorphic, and transforms from the CoGa, structure at
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high temperatures into an unspecified structure at low temperatures. They have also reported the existence of a phase with the approximate composition IrGa6, although it seems likely that they were in fact referring to the phase Ir,Ga,, the existence of which was unknown at the time. The ranges of homogeneity of the phases IrGax, Ir,GaS and IrGa were determined at 600 “C by Schulz et al. [14], using EPMA and bulk Ga-Ir diffusion couples. From their phase diagram work on the Ga-IrAs system, Ga appears to be insoluble in Ir at 600 “C. Little is known about the temperature stabilities of the iridium gallides. The Ir-P system has been assessed by Okamoto [25]. Three iridium phosphides, IrP,, IrP, and Ir,P, are known to exist. All of these phosphides are stoichiometric. The melting point of Ir,P and the dissociation temperature of IrP2 are known approximately. The solubility of P in solid Ir is negligible. 3.1.4. Ga-Ni-P
More detailed assessments of both the Ga-Ni system and the Ni-P system than those found in Ref. [lo] may be found in Ref. [26]. The Ga-Ni binary system is very complicated. The system comprises seven phases: NiGa,, Ni,Ga,, Ni,Ga,, NiGa, Ni,,Ga,, Ni,Ga, and Ni,Ga. The phase Ni,,Ga, is dimorphic. Above 680 “C, it has a partially filled NiAs (B8,) structure. Below this temperature, it transforms into a monoclinically distorted NiAs superlattice structure. The phases NiGa and Ni,Ga have significant ranges of homogeneity. Solid Ni exhibits a large solubility of Ga as well. The system Ni-P is also very complex, and is known incompletely. There are nine intermediate phases in this system: NiP,,^NiP,, NIP, Ni,,,,P, N&P,, N&P, Ni,,P,, N&P*, and N&P. The phases Ni12Pj and Ni,P, undergo structural transformations at high temperatures into phases with slightly different compositions. Only the phase N&P exhibits a significant range of homogeneity. The solubility of P in solid Ni is very small, reaching a maximum of 0.32 at.% at 870 “C. 3.1.5. Ga-Pt-P
The system Ga-Pt comprises eight intermediate phases: PtGa,, Pt,Ga,, PtGa,, Pt,Ga,, PtGa, Pt,Ga,, Pt,Ga, and Pt,Ga. Pt,Ga is polymorphic, and at low temperatures, Pt,Ga exists in three different structural modifications at various compositions. PtGa, reportedly decomposes eutectoidally into Pt,Ga, and Pt,Ga, at 153 “C, although recent experiments by Swenson and Morosin [16] suggest that instead it undergoes a structural transformation at this temperature. With the exceptions of Pt,Ga,, Pt,Ga, and possibly the high-temperature form of PtGa,, the ranges of homogeneity of the platinum gallides are negligible. Solid Pt exhibits some solubility of Ga. The Pt-P system [27] consists of only two intermediate phases: PtP, and Pt,P,. Above 683 “C, there is also a
56
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region of liquid-liquid immiscibility in the composition range of 26-52 at.% P. The maximum solubility of P in solid Pt has been reported to be 0.03 at.%. 3.2. Review of phase equilibria in the Ga-M-P systems
ternary
3.2.1. Ga-Ni-P
Several researchers have investigated the electrical properties of Ni/n-GaP contacts. Goldberg et al. [28] and Lei et al. [29,30] have determined the Schottky barriers of Ni/n-GaP contacts deposited by electrodeposition and electron beam evaporation, respectively. Nannichi and Pearson [4] measured the electrical properties of Ni/n-GaP Schottky diodes as a function of temperature between room temperature and 500 “C. However, they did not explicitly report their electrical data, stating only that the properties of the contacts were similar to those of Cr/n-GaP contacts, the electrical properties of which were investigated in detail. Nakatsuka et al. [31] studied the morphology of Ni thin films on GaP as a function of temperature using high temperature optical microscopy. They found that the films exhibited signs of melting at 760 f 20 “C. It is noteworthy that the molten phase did not wet the GaP substrate. Jan [32] attempted to produce a NiAs @,)-type phase with the composition Ni,GaP from a mixture of Ni and GaP at 600 “C, employing experimental procedures very similar to those of the present investigation. However, his attempt was unsuccessful, and resulted in the formation of NiGa and N&P. Nevertheless, Jan’s experiment demonstrates that Ni is not in thermodynamic equilibrium with Gap.
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analysis to characterize chemical reactions between Pt thin films and GaP. Kumar noted the rapid dissolution of Ga into thin Pt films on GaP at 250 “C, as well as the subsequent formation of Pt,Ga and PtP, at the metalsemiconductor interface. He also observed the formation of the phases Pt,Ga, P&Gas, PtGa and PtP, upon annealing a thick film Pt/GaP couple at 400 “C for 16 h. From these experimental observations, as was mentioned in the Introduction, it is clear that Pt is not in thermodynamic equilibrium with GaP. El-Boragy and Schubert [35] have reported the existence of a ternary phase with a composition close to Pt,GaP after annealing a sample of the appropriate composition at 600 “C for 1 d. Most of the X-ray diffraction peaks could be indexed assuming the tetragonal Pd,TlAs structure, although 10 mol.% of the diffraction peaks were ascribed to other unidentified phases. 3.3. Experimental phase equilibria irz the Go-M-P terns
sys-
Experimentally determined phase equilibria in the Ga-M-P systems are depicted in Figs. l(a)-l(f). In these figures, experimental tie-lines are represented by solid lines, whereas tie-lines that were inferred using the phase rule are dashed. The gross sample compositions employed, and the phases present in each sample as determined by X-ray diffraction analysis, are given in Table 2. A diagram of the Ga-Pd-P system, based upon the work of Mohney et al. [7], is depicted in Fig. l(e), in order to provide a complete picture of phase equilibria in Ga-near-noble-metal-P systems. The constitution of each of these systems will be discussed below. 3.3.1. Ga-Co-P
3.2.2. Ga-Pt-P
Several investigations of Pt thin films on GaP have been reported in the literature. However, most of these studies [30,33-361 have entailed only the measurement of the Schottky barrier heights of Pt/n-GaP diodes at room temperature. Wronski [34] has compared the Schottky barrier heights of Pt contacts deposited on room temperature GaP substrates with those of Pt contacts deposited on 200-300 “C substrates. Although differences in electrical properties were noted, no attempts were made to correlate the change in electrical properties with a possible chemical reaction at the Pt-GaP interface. Ruth et al. [37] and Kumar [5] have investigated the chemical interaction of Pt thin films with GaP. Ruth et al. observed the formation of unspecified platinum gallides and PtP, between Pt and GaP after annealing the contacts at 250-350 “C. No characterization technique was mentioned in Ref. [37]. Their findings are in accordance with those of Kumar, who used X-ray diffraction
In the system Ga-Co-P, depicted i+ Fig. l(a), the phases CoGa and COP were found to be in thermodynamic equilibrium with GaP. According to the phase rule, the phases CoGa, and COP, must also be in thermodynamic equilibrium with GaP, barring the existence of ternary phases. Because the phase CoGa exhibits a wide range of homogeneity, it is important to determine the composition range over which CoGa is in thermodynamic equilibrium with GaP. The lattice parameter of CoGa is known to be a strong function of composition [ 17,19,20]. In principle, therefore, the composition of CoGa in the Ga-Co-P sample may be determined using X-ray diffraction analysis. Unfortunately, discrepancies among researchers in the composition of the Ga-rich phase boundary of CoGa make this determination somewhat ambiguous, as discussed below. The lattice parameter of CoGa in the Ga-Co-P sample was found to be a = 0.28424(3) nm, where the number in parentheses represents the estimated standard
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Ni,Ga, NiGa
/ Ni,Ga, Ni,Ga Ni,:Ga,
Cd)
(a) P
Ga
Ga
Ga
Fig. 1. Isothermal phase equilibria in Ga-M-P systems, where M represents Co, Rh, Ir, Ni, Pd or Pt. All diagrams portray phase equilibria at 700 “C except for (d) and (e), which represent thermodynamic equilibrium at 600 “C. Experimentally determined tie-lines are depicted as solid lines, whereas tie-lines inferred from the phase rule are drawn as dashed lines. The symbol X within a Gibbs isotherm represents the gross composition of a sample employed in the present investigation. These gross compositions, along with phases identified in each sample by X-ray diffraction analysis, are given in Table 2. (e) is based on the work of Mohney et al. [7].
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deviation of the least significant digit. Within experimental error, this lattice parameter is identical to that of CoGa at the Ga-rich phase boundary at 800 “C, as determined by Lindeberg and Andersson [39] in their investigation of the Ga-Co-As phase diagram. Based on the work of Ipser et al. [20], which represents the most complete compilation to date of data relating the lattice parameter of CoGa with its composition, this lattice parameter corresponds to a composition of 61.5 at.% Ga. This composition is in good agreement with the accepted Ga-rich phase boundary of CoGa at 800 “C [lo], and thus corroborates the X-ray diffraction data of Lindeberg and Andersson. However, according to the assessed diagram, the Ga-rich phase boundary of CoGa should only be 59.5 at.% Ga at 700 “C, the temperature employed in the present investigation. At the latter composition, the lattice parameter of CoGa would be substantially larger than that which was found in the present investigation. Therefore, the results of the present investigation imply that the Ga-rich phase boundary of CoGa is more vertical than it has been depicted in the assessed diagram, and hence the boundary is somewhat more Ga-rich at lower temperatures than has been thought previously. It is noteworthy that Feschotte and Eggimann [18] found the phase boundary of CoGa to be at 63.0 at.% Ga at 600 “C and 64.2 at.94 Ga at 800 “C using EPMA, although their data have not been accepted in the assessed diagram. Clearly, this issue cannot be resolved until a definitive investigation of the Ga-rich phase boundary of CoGa is undertaken. The lattice parameters of COP (a = 0.5102(l) nm, b = 0.3288(l) nm, c = 0.5699(2) nm) as determined in the present investigation are significantly larger than those reported in the literature [40]. This suggests that COP exhibits some ternary solubility of Ga, and is more accurately denoted Co(Ga,P). It is interesting to note that the essentially isostructural phase CoAs exhibits significant ternary solubility of Ga as well [39,41].
Table 2 Gross sample compositions and phases identified by X-ray diffraction in Ga-M-P samples Phases by X-ray diffraction co Rh Ir Ni Pt
0.35 0.35 0.40 0.35 0.35
0.30 0.30 0.20 0.30 0.30
0.35 0.35 0.40 0.35 0.35
CoGa, COP, GaP RhGa, RhP?, GaP Ir0.34Ga0.66a,IrP2, GaP N&Ga,, Ni,P, GaP Pt,Ga,, PtGa, PtP,, GaP
aDiffraction pattern could not be indexed; phase identified by EMPA (Please refer to text).
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It is possible to estimate the composition of Co(Ga,P) found in the present investigation from its lattice parameters. Previous researchers [39,41] have observed that the lattice parameters of CoAs are not significantly altered by the replacement of As atoms with Ga atoms. Therefore, the molar volume of Ga in CoAs must be approximately equal to the molar volume of As in CoAs. If it is assumed that the molar volume of Co(Ga,P) increases linearly as a function of Ga content between the molar volume of COP [40] and the molar volume of CoAs [42], a composition of CoGa,,,,P,,,, is obtained for Co(Ga, P) with the lattice parameters found in the present investigation. The lattice parameters of COG~,,,~P~.~~, as well as its composition, are noteworthy, in light of previous observations made pertaining to phases crystallizing with this structure. COP possesses the MnP (B31) structure, which may be regarded as an orthorhombic distortion of the hexagonal NiAs (B8,) structure [43]. In fact, the NiAs structure may be described in terms of the MnP structure via the following transformation of lattice parameters: aMnP
=
CNiAs>
bep
= INCAS,
and
CM,,~
=
@
nNiAs.
Pfisterer and Schubert [44] have noted that the c/b ratio of phases with the MnP structure appears to be dependent upon electron concentration, Rundqvist [40] has explored this possibility by studying the effects of the substitution of several elements, including Cr, Fe, Mn, B and Si, on the c/b ratio of COP. In all cases, Rundqvist found that when substitution was sufficient such that the c/b ratio was equal to @, the electron concentration of the phase was approximately 13.6 electrons/formula unit (assuming that all electrons outside the last completely filled shell are valence electrons. It is also noteworthy that when c/b = fit the MnP structure is pseudohexagonal, as is implied by the preceding formulas relating the lattice parameters of the MnP structure to those of the NiAs structure). Upon examination of the lattice parameters of COG~~,,,P~,~~, one finds that c/b is almost exactly @, Furthermore, the composition CoGa 0,,7P,,.83 corresponds to an electron concentration of 13.7 electrons/formula unit, in excellent agreement with the observations of Rundqvist. 3.3.2. Gn-Rh-P In the system Ga-Rh-P, shown in Fig. l(b), RhGa and RhP, were found to be in thermodynamic equilibrium with GaP. By employing the phase rule, it may be shown that all of the rhodium gallides, as well as the phase RhP, are also in equilibrium with GaP. Moreover, this one phase diagram sample, in conjunction with the phase rule, is sufficient to determine the entire Ga-Rh-P phase diagram isotherm, provided that no ternary phases exist in the system.
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3.3.3. Ga-h-P The Ga-Ir-P phase diagram isotherm is presented in Fig. l(c). IrP, and GaP were identified in the phase diagram sample by X-ray diffraction analysis. However, the remaining diffraction peaks did not correspond to any of the known iridium gallides or phosphides, nor could they be indexed by the present investigators. Therefore, the sample was analyzed by EPMA. Unfortunately, the grain size of the sample was very small, which made probing individual phases difficult. While the measured amounts of Ga and Ir remained relatively constant, the amount of P detected in the phase, though small in magnitude, varied considerably. In a few cases, however, compositional data indicated the total absence of P, and thus the erratic detection of P within the phase may be attributed to overlapping X-ray signals generated from neighboring phases. Based on EPMA, the unidentified phase appears to be an iridium gallide of the approximate stoichiometry This composition lies close to the range of Ir,&ao.,,. homogeneity reported by Schulz et al. [14] for Ir,Ga,. Nevertheless, as was mentioned previously, the X-ray diffraction pattern of the phase does not correspond to that of Ir,Ga, [45]. Therefore, it seems likely that this phase is a previously unknown iridium gallide of the approximate stoichiometry IrGa,. Further investigation of the Ga-Ir phase diagram is required to clarify this issue. According to the phase rule, Ir,Ga,, IrGa, and IrP, should also be in thermodynamic equilibrium with GaP, based upon the known ternary phase equilibria in the system. 3.3.4. Ga-Ni-P
In the Ga-Ni-P system (Fig. l(d)), a region of three-phase equilibrium was found to exist between Ni,Ga,, GaP and N&P. According to the phase rule, NIP,, NiP and N&P, must also coexist with GaP at 600 “C. The tie-line determined by Jan [32] between NiGa and N&P is consistent with the results of the present investigation. The topology of the Ga-Ni-P phase diagram depicted in Fig. l(d) suggests an explanation of the melting phenomenon at 760 + 20 “C observed by Nakatsuka et al. [31]. It is likely that partial melting occurs near 760 “C through a class II ternary reaction: L + Ni,P + Ni,Ga, + Gap. Such a reaction would account for the formation of a liquid phase, yet represents a single tie-line shift from the 600 “C isotherm. Moreover, the possible existence of such a ternary reaction is supported by thermodynamic data. Experimental Gibbs energies of formation [46,47] are available for all four of the phases at 1100 K (827 “C), which is very close to the melting temperature observed by Nakatsuka et al. Using these data, one may calculate the Gibbs energy of reaction, AG:, for the reaction
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given above to be only about - 0.6 kJ g-l essentially zero within experimental error.
atom-‘,
or
3.3.5. Gcr-P&P Fig. l(e) portrays the Ga-Pd-P system at 600 “C, based on the work of Mohney et al. [7]. The phases PdGa and PdP, are the only intermediate phases in thermodynamic equilibrium with GaP. Three ternary phases [7,38,48], for which the equilibrium relationships with the constituent binary phases are unknown, have been included in this diagram for the sake of completeness. 3.3.6. Ga-Pt-P
Finally, in the Ga-Pt-P system, shown in Fig. l(f), Pt,Ga, and PtP, were found to coexist with GaP at 700 “C. The phases Pt,Ga,, PtGa, must be in thermodynamic equilibrium with GaP as well, according to the phase rule. The phase Pt,GaP has been included in the diagram for the sake of thoroughness, although its equilibrium relationships with the remaining phases are unknown. It should be noted that the Ga-Pt-P phase diagram sample contained a small amount of PtGa ( < 5 mol.%), and therefore did not reach thermodynamic equilibrium. 3.4. Tlzermodyiynamically stable contacts
to GaP
Clearly, the results of the present investigation indicate that none of the Co or Ni group metals may be used as electrical contacts to GaP for high-temperature device applications; all of these metals are reactive with respect to GaP. However, the present investigation indicates that many gallides or phosphides of the Co and Ni groups may potentially serve as thermodynamically stable contacts to GaP. Although thermodynamic stability is of utmost importance, there are other criteria which must be considered when selecting contact materials. Primary among these is electrical conductivity; the contact material must be metallic. In this regard, most of the metal phosphides in equilibrium with GaP are unsuitable as contact materials, because they are semiconducting. These include COP,, RhP,, RhP,, IrP,, IrP,, NiP2, PdPz and PtP, [49]. The phases COP, Ni5P4, N&P are reportedly metallic [49], and hence could be used as contact materials. Another concern is the ease of fabrication of a contact via conventional processing methods, such as sputter deposition or electron beam evaporation. It is unlikely that stoichiometric phosphides could be deposited by electron beam evaporation, owing to the fact that the vapor pressure of P will undoubtedly be much higher than that of the metal. Likewise, one might be concerned with the potential loss of P during sputtering through the formation of a gaseous phosphorus species.
60
D. Swenson, Y.A. Gang / Materials Science and Engineering B39 (1996) 52-61
Such losses are of concern both because of the ensuing nonstoichiometry of the deposited films and because of the physical dafigers associated with free phosphorus gas. Nevertheless, Appelbaum et al. [50] have successfully deposited N&P on InP by sputter deposition, apparently without the significant loss of P. However, one must also consider the potential gradual loss of phosphorus from the contact material at the high temperatures experienced during device operation. These practical concerns pertaining to contact fabrication and long-term stability may be allayed if one employs metal gallides rather than metal phosphides as contact materials to GaP. While only a few metallic metal phosphides are in thermodynamic equilibrium with GaP, over one dozen metal gallides have been identified which possess the thermodynamic compatibility required of contacts to Gap. These phases include CoGa,, CoGa, Rh,Ga,, RhGa,, Rh10Ga17, RhGa, Ir,Gag, IrGa,, Ni,Ga,, PdGa, Pt,Ga,, PtGa, and Pt,Ga,. Moreover? many of the metal gallides mentioned above, including CoGa [51-531, RhGa [54], Ni,Ga, [55] and PtGa, [56] have been fabricated previously as contacts to GaAs, primarily by molecular beam epitaxy or metallorganic chemical vapor deposition. Therefore, the feasibility of utilizing Co and Ni group gallides as practical contact materials has already been demonstrated.
4. Conclusions Phase equilibria are established in the Gap-rich sections of several Ga-M-P systems, where M denotes Co, Rh, Ir, Ni and Pt. When combined with previous experimental phase diagram investigations of the GaPd-P system, these data provide an overview of phase equilibria between GaP and the gallides and phosphides of the Co and Ni group metals. It is found that most of the Ga-rich metal gallides of these systems, as well as most of the P-rich phosphides, are in thermodynamic equilibrium with GaP. From a chemical standpoint, each of these gallides and phosphides is suitable as a contact material to GaP. However, upon consideration of other criteria by which contact materials must be judged, such as high electrical conductivity and ease of fabrication by conventional deposition methods, it becomes clear that the metal gallides are in general more suitable than the metal phosphides as potential contact materials. Numerous metal gallides are viable candidates for use as contact materials, including CoGa,, CoGa, Rh,Ga,, RhGa,, Rhdh, RhGa, Ir,Ga,, IrGa3, Ni,Ga,, PdGa, Pt,Ga,, PtGa, and Pt,Ga,. Moreover, several of these gallides have been fabricated previously as contacts to GaAs, demonstrating the feasibility of their use as practical contact materials.
Acknowledgements The present investigators wish to thank I,. Pike for determining the lattice parameters of CoGa and COP and D.Y. Chen for performing EPMA of the Ga-Ir-P sample. This work was performed at the University of Wisconsin and was supported by the National Science Foundation through Grant No. DMR-94-24478. References [I] A.A. BerghandP.J. Dean,Liglrf-mithlg Diodes, Oxford UniversityPress,London,1976. [2] R.C. Hughes,T.E. Zipperian,L.R. Dawson,R.M. Biefeld,R.J. WalkoandM.A. Dvorack,J. Appl. Phys., 69 (1991) 6500. [3] T.E. Zipperian,R.J. Chaflinand L.R. Dawson,IEEE Ttwu. Indm. Electron., IE-29 (1982) 129.
[4] Y. Nannichiand G.L. Pearson,Solid-Stale EMron, 12 (1969) 341. [5] V. Kumar,J. Ploys.C’/wn.Solids, 36 (1975) 535. [6] A. Hiraki, K. Shuto,S.Kim,W. KammuraandM. Iwami,Appl. Phys. Lelt., 31 (1977) 611.
[7] S.E.Mohney,CF. Lin andY.A. Chang,Appl. P/IJ*s. Lell., 63 (1993)1255. [8] J.R. LinceandR.S. Williams,Thiu Solid Fibs, 137 (1986)251. [9] C.J. Palmstrom andT.D. Sands,in L.J. Brillson(ed.),Corltacts to Setniconducrors: Fmlatmn~als and Techolog~l, NoyesPublications,Park Ridge,NJ, 1993, (ed.),Birlary Alloy Phase Diagram, ASM Inter[lOI T.B. Massalski national,MaterialsPark,OH, 1990. Rec.Phys. u 11 R. Guitrin,A. Guivarc’h,Y. BalliniandM. Secoub, Appl., 25 (1990) 412. L7.1K.J. Schulz,O.A. MusbahandY.A. Chang,J. P/rose Equil., 12 (1991)10. data, 1954,cited in W.B. Pearson,A v31 E. Hellner,unpublished
Handbook of Lattice Spacing,r awl Smtclwes of Metals attd Alloys, Pergamon, Oxford, UK, 1958,p. 670. u41 K.J. Schulz,O.A. MusbahandY.A. Chang,&r/l. /il/o~*Phase Diagr., II (1990)211. [I51 J.K. LiangandS.S. Xie, Sciwtia Sbha A, 26 (1983)1305. andB, Morosin,.I. Alloys Corr~p,, submitted. [161D. Swenson P71 K. Schubert,H.L. Lukas,H.-G. Meissncrand S. Bhan, Z. Metallkd., 50 (1959)534. andP.Eggimann, J. Less-Corrwort hfet., 63(1974) V81 P. Feschotte
15.
VI K.M. WunschandE. Wachtel,Z. Merallkd, 73 (1982)311. VI H. Ipser,A. Mikula and W. Schuster,illor~atslr. Chm., 120 (1989)283.
&r/l. Ailo~Phase Dingy., 11 (1990) VI K. IshidaandT. Nishizawa, 555.
;Morrolslt. Chem., Pa H. Holleck,H.’ Nowotnyand P. Benesovsky, 94(1963)841. ~31 B.H. Verbeek,H.W. A.M. Rompa,P.K. Larsen,MS. Methfesse1and F.M. Mueller,Phys. Rec. B, 28 (1983)6774. 1241H. Okamoto,Bul1. Alloy Phase Diagr., II (1990)415. [251 H. Okamoto,Bull. Alloy Phase Dingy., II (1990)413. [261 P. Nash(ed.), Phase Diagram of Bhaty Nickel Alloys, ASM International,MaterialsPark,OH, 1991. ~71 H. Okamoto,BUN. Alloy Phase Diagr., 11 (1990)511. P81Yu.A. Goldberg,E.A. Posseand B.V. Tsarenkov,Electrori. Lell., 7 (1971)601. P91 T.F. Lei, CL. Leeand C.Y. Chang,Solid-SlnteElecfrorr.,21 (1978)385. 1301 T.F. Lei, CL. Lee and C.Y. Chang,Solid-Slate Electron., 22 (1979)1035.
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[31] H. Nakatsuka, A.J. Domenico and G.L. Pearson, Solid-State Electron., 14 (1971) 849. 1321 C-H. Jan, unpublished data (1991). [33] A.M. Cowley and S.M. Sze, J. Appl. Phys., 36 (1965) 3212. [34] CR. Wronski, J. Appl. Phys., 41 (1970) 3805. [35] D.A. Evans, T.P. Chen, Th. Chassi, K. Horn, M. von der Emde and D.R.T. Zahn, Surf. Sci., 2691270 (1992) 979. [36] M. von der Emde, D.R.T. Zahn, Ch. Schultz, D.A. Evans and K. Horn, Appl. Sur$ Sci. 70/71 (1993) 507. [37] R.L. Ruth, J.M. Poate and A.K. Sinha, unpublished data (1975); cited by A.K. Sinha and J.M. Poate in (J.M. Poate, K.N. Tu and J.W. Mayer (eds.)), Thin Fi/ms - Inter@iision and Reactions,Wiley, New York, 1978, 407. 1381 M. El-Boragy and K. Schubert, Z. fvletallkd., 61 (1970) 579. [39] I. Lindeberg and Y. Andersson, J. Less-Cowmorl Met., 175 (1991) 155. [40] S. Rundqvist, Acfa Chem. Stand., 16 (1962) 287. [41] F.-Y. Shiau, Y. Zuo, J.-C. Lin, X.-Y. Zheng and Y.A. Chang, 2. Metallkd., 80 (1989) 544. [42] K. Selte and A. Kjekshus, Acta Chem. &and., 25 (1971) 3277. [43] K.-E. Fylking, Ark. Kemi, Mineral. Geol. B, II (1935) No. 48. [44] H. Pfisterer and K. Schubert, Z. Metallkd., 41 (1950) 358.
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[45] H. Viillenkle, A. Wittmann and H. Nowotny, ~~o~nrsir. C/ie,?l., 97 (1966) 506. 1461 I. Barin, Thermochemical Data of Pure Substances, VCH Publishing, Weinheim, FRG, 1989. [47] J.N. Pratt and J.M. Bird, J. Phase Equil., 14 (1993) 465. [48] M. Ellner, M. El-Boragy and B. Predel, J. Less-Common Met. 114 (1985) 175. [49] F. Hulliger, Struct. Bonding, 4 (1968) 83. [50] A. Appelbaum, M. Robbins and F. Schrey, IEEE Trans. Electron. Dev., ED-34 (1987) 1026. [51] C.J. PalmstrBm, B.-O. Fimland, T. Sands, K.C. Garrison and R.A. Bartynski, J. Appl. Phys., 65 (1989) 4753. 1521 T.C. Kuo, T.W. Kang and K.L. Wang, J. Ciyst. Growth, 111 (1991) 996. [53] F. Maury, A.A. Talin, H.D. Kaesz and R.S. Williams, Chem. Mater., 5 (1993) 84. [54] A. Guivarc’h, M. Secoui: and B. Guenais, Appl. Phys. Lett., 52 (1988) 948. [55] A. Guivarc’h, J. Caulet, B. Guenais, Y. Ballini, R. Gukrin, A. Poudoulec and A. Regreny, J. Cryst. Growth, 95 (1989) 427. [56] Y.K. Kim, D.A. Baugh, D.K. Shuh, R.S. Williams, L.P. Sadwick and K.L. Wang, J. Marer. Res., 5 (1990) 2139.
SCIENCE ENCIHEERIWC
&
B
Materials Scienceand Engineering B39 (1996)52-61 ELSEVIER
On the constitution of some Ga-M-P systems (where M represents Co, Rh, Ir, Ni or Pt) D. Swensona,*, Y.A. Changb “Chetnistry bDeparittlent
and Materials of Materials
Science, Science
Lawrence Livermore National and Engineering, University
Laboratory,
of Wisconsin,
P.O. Box 808, L-370, Liwrtuore, 1509 University Areme, Icfodimn,
CA 94551, WI 53706,
USA USA
Received3 October 1995;in revisedform 10October1995
Abstract Phase equilibria are established in the Gap-rich regions of five Ga-M-P systems (where M represents Co, Rh, Ir, Ni, or Pt) at 700 “C (or 600 “C for the Ni-bearing sample) using X-ray diffraction analysis. The results of the present study, in conjunction with previous work on the Ga-Pd-P system, give a complete picture of phase equilibria between GaP and the gallides and phosphides of the Co and Ni groups. Based on these data, it is concluded that many binary metal gallides, including CoGa3, CoGa, RhzGas, RhGa,, Rh,,Ga,,, RhGa, Ir,Ga,, IrGa,, Ni,Ga,, PdGa, Pt,Ga,, PtGa, and Pt,Ga,, may potentially serve as contact materials for use in high-temperature Gap-based electronic devices. Keywords:
Metal gallides; Phaseequilibria
1. Introduction Historically, the practical applications of gallium phosphide (Gap) have essentially been limited to the fabrication of light emitting diodes (LEDs) [I]. In recent years, however, there has been an increased level of interest in this compound semiconductor. Gallium phosphide has been identified as a candidate for use in photonic or photonic-like devices, including near-UV energy conversion photodetectors and “betavoltaic” cells [2]. Additionally, owing to its wide bandgap (2.26 eV) and the low leakage current of GaP p-n junctions, it shows great promise as a material for use in highelectronic temperature ( > 300 “C) minority-carrier devices, especially diodes, bipolar junction transistors and power-switching devices [3]. Moreover, sufficient experimental data are available to suggest that even 500 “C GaP electronic devices, both majority- and minority-carrier, are well within the realm of possibility c3,41. Zipperian et al. [3] summarized the state of GaP technology through 1981 as it applies to high-temperature electronics applications. In their review, they specifically identified GaP contact technology as an area in which additional research was required if viable high-
temperature GaP devices were to be realized. Since 1981, however, relatively little advancement in GaP contact technology has been achieved. The most serious problem facing any contact material for use in high-temperature electronic devices is that of thermal stability. Even if a contact exhibits acceptable electrical properties immediately after device fabrication, it must maintain these properties over long periods of time during routine operation at elevated temperatures. Interdiffusion between the metallization layer and the semiconductor may not be tolerated, as it will almost certainly alter the electrical properties of the contact and hence affect the performance of the device. This implies that a successful contact material for hightemperature devices must be nonreactive with respect to the semiconductor, since at elevated temperatures thermodynamically allowed chemical reactions are not kinetically hindered. To date, few metallurgical investigations of contacts to GaP have been reported in the literature. Based on these available data, however, it is apparent that in general metals are not’chemically inert with respect to Gap. For example, using X-ray diffraction analysis, Kumar [5] has found that Pt films react with GaP at temperatures as low as 2.50 “C. Employing in-situ
D. Swemor~,
Y.A. Gang
1 Materials
Auger electron spectroscopy in an ultrahigh vacuum deposition system, Hiraki et al. [6] have observed the formation of an interdiffused layer which was tens of nanometers in thickness after annealing a Au/Gap contact for just 7 min at 200 “C. Finally, using transmission electron microscopy, Mohney et al. [7] observed a 10 nm reacted layer between Pd and GaP in as-deposited contacts, and found that a 500 “C, 30 s heat treatment was sufficient to completely consume a 50 nm Pd film. Potential chemical reactions between metal contacts and a semiconductor substrate may be circumvented by selecting metallizations which are in thermodynamic equilibrium with the semiconductor. Such an approach was first suggested by Lince and Williams [8], who demonstrated the utility of ternary phase diagrams for selecting thermodynamically stable, Au-based contacts to GaSb. This approach to metallization selection has been applied with great success to the development of contacts to GaAs (see, for example, Ref. [9]). However, only recently has such an approach been extended to other III-V semiconductor systems. In particular, there is a paucity of data pertaining to Ga-transition metal(M)-P phase equilibria; indeed, the very recent determination by Mohney et al. [7] of partial phase equilibria in the Ga-Pd-P system represents the only such information that has ever been reported in the literature. Based on the preceding discussion, it is quite clear that more Ga-M-P phase diagrams must be determined if advances are to be made in GaP contact technology. Therefore, in the present investigation, phase equilibria have been established in the Gap-rich regions of five Ga-M-P systems, where M denotes Co, Rh, Ir, Ni or Pt. These data are then used to identify potential thermodynamically stable contact materials to GaP.
2. Experimental
procedure
Semiconductor grade GaP powder, and Co, Rh, Ir, Ni and Pt of at least 99.95% purity were weighed in appropriate ratios to produce samples with nominal total masses of 0.3 g. The Pt sample was significantly smaller, owing to a limited supply of the metal. The elements Co, Ni and Pt were in powder form, and were mixed directly with GaP powder and pressed into pellets. These samples were sealed in quartz ampoules which were evacuated to a pressure of 10e2 Torr. The Co and Pt samples were annealed at 700 “C for 14 d. They were then quenched in iced water and ground into powders, and were again pressed into pellets and encapsulated in evacuated quartz ampoules. The samples were annealed for an additional two months. A similar procedure was adopted for the Ni sample, except that it was annealed at 600 “C for a total time of five months.
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The elements Rh and Ir were in the form of small pieces. To facilitate chemical reaction between the metal pieces and GaP, an initial rigorous heat treatment was adopted. The Rh and Ir pieces were combined with GaP powder, and the samples were sealed in evacuated quartz ampoules and annealed at 1000 “C for one week. The annealing temperature was then slowly lowered to 700 “C over the course of a second week. The samples were subsequently quenched in iced water, ground into homogeneous powders, pressed into pellets, sealed in evacuated quartz ampoules, and annealed at 700 “C for two additional months. Following their final heat treatments, all samples were quenched in iced water. Each sample was crushed and analyzed by X-ray powder diffraction, using a Scintag PAD V automated diffractometer and Cu Kcr radiation. For the Co sample, silicon (NIST standard 640b) was used as an internal standard, and lattice parameters of the phases present were refined using a nonlinear least-squares computer program included in the Scintag software package. One of the phases in the Ir sample could not be identified by X-ray diffraction analysis. Therefore, the sample was sintered in an evacuated quartz ampoule at 700 “C for an additional month, metallographically cross-sectioned and analyzed for composition by electron probe microanalysis (EPMA), using a Cameca electron microprobe and employing wavelength-dispersive spectroscopy. Elemental Ir and commercially produced GaAs and InP wafers were used as standards.
3. Results and discussion Prior to discussing the results obtained in the present investigation, phase equilibria in the constituent binary Ga-P, Ga-M, and M-P systems will be reviewed. Additionally, a survey of all previous experimental investigations pertaining to the ternary Ga-M-P systems will be presented. 3.1. Phase equilibria in the constituent binary systems The phases present in the constituent binary Ga-P, Ga-M and M-P systems, their ranges of homogeneity, crystal structures and temperature stabilities are given in Table 1. Unless otherwise noted, these data are taken from Ref. [lo]. The constituent binary systems of each Ga-M-P ternary system are briefly discussed in the following five subsections. 3.1.1. Ga-Co-P The system Ga-P contains one intermediate GaP, which is thermbdynamically stable only stoichiometric composition. Below about 800 solubility of P in liquid Ga is negligible (much than 1 at.%).
phase, at the “C, the smaller
D. Swenson, Y.A. Chang / Maaterials Science and Engineering 839 (1396) 52-61
54
Table 1 Compositional stabilities, crystal structures and temperature stabilities of constituent binary phases in Ga-M-P
systems”
Phase
Composition (at.% Ga or P)
Structure
Temperature W)
GaP
50.0
cubic, ZnS, cF8, B3
1467(c)b
CoGa, CoGa COP, COP Co,P
75.0 32.0-67.0 75.0 50.0 33.3
tetragonal, CoGa,, tP16 cubic, CsCl, cP2, B2 cubic, CoAs,, ~132, DO, orthorhombic, MnP, oP8, B32 orthorhombic, Co,Si, oP12, C23
855(P) 1210(P) unknown unknown 1386(c)
Rh,Ga, RhGa, Rhd% RhGa [13] RhP, RhP, RhJ’, RM’, Rh,P
81.8 75.0 [12] 63.0 49.0-50.0 [12] 75.0 66.7 42.9 40.0 33.3
monoclinic, Co,Al,, mP22, D8d tetragonal, CoGa,, tP16 tetragonal, Rh,,Ga,,, tP108 cubic, C&l, cP2, B2 [13] cubic, CoAs,, cI32,DO, monoclinic, CoSb,, mP12 orthorhombic, Rh,P,, oP28 tetragonal, Rh,P,, tP5 cubic, arzti-CaF,, cF12, Cl
unknown, unknown, unknown, unknown, unknown unknown unknown unknown - 1500(c)
Ir,Ga, IrGa,(a) IrGadJ) Ir,Ga, IrGa IrP, IrP, Ir,P
81.8 75.0 74.0-77.0 [14] 61.0-65.0 [14] 48.0-50.0 [14] 75.0 66.7 66.7
monoclinic, Co2Al,, mP22, D8, unknown tetragonal, CoGa,, rP16 tetragonal, Ir,Ga,, tP32 cubic, CsCl, cP2, B2 cubic, CoAs,, ~132, DO, monoclinic, CoSb,, ~zP12 cubic, anti-CaFz, cF12, Cl
unknown unknown unknown unknown unknown unknown 1230(d) - 1335(c)
NiGa, Ni,Ga, Ni,Ga, NiGa Ni,3Ga, Ni,Ga, Ni,Ga, Ni,Ga Nip3 NiP, NiP N&P, Ni,P Ni12P5(y) NiddJ) Ni,P,(u) W’~U) N&P
80.0 60.0 56.6-57.6 30.4-57.0 39.2-42.2 35.0-41.0 36.2-38.0 22.6-30.0 75.0 66.7 50.0 44.4 33.3 29.4 29.4 28.6 28.6 25.0
cubic, NiGa,, ~140 [15] trigonal, Ni,Al,, hP5, D513 cubic, Ni,Ga,, ~I112 cubic, CsCl, cP2, B2 monoclinic, Ni,,Gap, mC44 hexagonal, NiAs, hP4, B8, orthorhombic, Pt,Ga,, oC16 cubic, Cu,Au, cP4, Ll, cubic, CoAs,, ~132, DO, monoclinic, Nip,, mC12 orthorhombic, Nip, oP16 hexagonal, N&P,, hP36 hexagonal, Fe,P, hP9, C22 tetragonal, Ni,,P,, t134 unknown hexagonal, Ni,P,, hP168 unknown tetragonal, Ni,P, tI32, DO,
363(p) 895@) 542(pd) 1220(c) 680@d) 680(ed) < T < 949(pd) 741 W) 1212(P) 697(ed) < T< unknown unknown 850(ed) < T c unknown unknown 1100(c) 1OOO(pd) lOOO(ed) < Tc 1125(p) 1025(pd) 1025(cd) < T < 1170(c) 970(P)
PtGa, Pt,Ga, PtGa,(a) PtGa,(P) Pt,Ga, PtGa Pt,Ga, Pt,Ga(cc)
85.7 70.0 66.7 [16] 65.5-66.7 [16] 60.0 50.0 37.0-42.0 33.3 33.3 33.3 24.5-26.5 26.5-27.5 24.0-32.0 66.7 28.6
orthorhombic, PtGa, cubic, Ir,Ge,, ~140, D8, tetragonal, PtGa,, tI96 [16] cubic, CaF,, cF12, Cl trigonal, Ni,Al,, hP5, D5,, cubic, FeSi, cP8, B20 orthorhombic, Pt,Ga,, oC16 tetragonal, AuCu, tP2, L10 orthorhombic, Pt,Ga(P) orthorhombic, RhsGe,, oP16 tetragonal, Pt,Ga, tP16 tetragonal, SiU,, t116, DO, cubic, Cu,Au, cP4, Ll, cubic, Fe& cP12, C2 monoclinic, Pt,P,, mC28
290(P) 822(p) 1W4 153(ed) < T < 922(p) 937(P) 1104(c) 1142(p) unknown unknown 1149(p) 210@d) Wpd) 1374(c) unknown, but > 1500(c) WP)
Pt,Ga(P) WWy) Pt,Ga(a) PVNP) WWy) PtP, w*
but but but but
<: 1000 < 1000 < 1000 > 1000
[II] [Ill [ll] [I l]
aUnless otherwise noted all data have been taken from Ref. [lo]. bThe symbols c, p, ed, pd and d denote congruent melting, peritectic melting, eutectoid decomposition, peritectoid decomposition and dissociation, respectively.
D. Swenson,
Y.A. Gang
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The Co-Ga system is also very simple. It comprises two intermediate phases, CoGa, and CoGa. While CoGa, exists only at the stoichiometric composition, CoGa exhibits a very wide range of homogeneity, extending from 32 to 67 at.% Ga. There are, however, slight discrepancies among researchers about the range of homogeneity of CoGa as a function of temperature [17-201. The solubility of Ga in solid Co is quite extensive. The Co-P system has been assessed by Ishida and Nishizawa’ [21]. Three intermediate phases have been found in this system: COP,, COP and Co,P. Each of the cobalt phosphides is essentially stoichiometric, although a slight range of homogeneity has been observed for Co,P at high temperatures. Phosphorus is reported to be insoluble in solid Co. 3.1.2. Ga-R/z-P
No Ga-Rh phase diagram has been reported in the literature. Four intermediate phases are known to exist in the Ga-Rh system: Rh,Ga,, RhGa,, Rh,,Ga,, and RhGa. The phase RhGa has been overlooked in Ref. [lo], although its existence has been confirmed by several researchers [l 1,13,17,22,23]. The X-ray diffraction work of GuCrin et al. [ll] suggests that the phases Rh,Gag, RhGa, and RhGa are stoichiometric. This is essentially in agreement with the work of Schulz et al. 1121,who used EPMA to measure the ranges of homogeneity of RhGa, and RhGa at 600 “C in bulk Ga-Rh diffusion couples. In the latter investigation, RhGa, was found to be stoichiometric, whereas RhGa exhibited a small (1 at.%) range of homogeneity. Ga does not seem to be soIuble in solid Rh [11,12,17]. The temperature stabilities of the rhodium gallides are unknown. However, GuCrin et al. have demonstrated that RhGa is the only rhodium gallide that is stable at 1000 “C. It is also noteworthy that G&in et al. were not able to confirm the existence of Rh,,Ga,, at 700 “C. The Rh-P system has been assessed by Okamoto [24]. As was the case for the Ga-Rh system, the Rh-P phase diagram is unknown. The existence of five rhodium phosphides has been reported. These include RhP3, RhP,, Rh,P3, Rh,P, and Rh*P. The rhodium phosphides are considered to be stoichiometric, although a small, unquantified range of homogeneity has been suggested for RhP,. Phosphorus is probably insoluble in solid Rh. With the exception of Rh,P, the melting or decomposition temperatures of the rhodium phosphides are unknown. 3.1.3. Ga-Ir-P
Again, no phase diagram is available for the Ga-Ir system. It is known that at least four intermediate phases exist in the system: Ir2Ga9, IrGaj, Ir,Ga, and IrGa. According to Schubert et al. [17], the phase IrGa, is dimorphic, and transforms from the CoGa, structure at
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high temperatures into an unspecified structure at low temperatures. They have also reported the existence of a phase with the approximate composition IrGa6, although it seems likely that they were in fact referring to the phase Ir,Ga,, the existence of which was unknown at the time. The ranges of homogeneity of the phases IrGax, Ir,GaS and IrGa were determined at 600 “C by Schulz et al. [14], using EPMA and bulk Ga-Ir diffusion couples. From their phase diagram work on the Ga-IrAs system, Ga appears to be insoluble in Ir at 600 “C. Little is known about the temperature stabilities of the iridium gallides. The Ir-P system has been assessed by Okamoto [25]. Three iridium phosphides, IrP,, IrP, and Ir,P, are known to exist. All of these phosphides are stoichiometric. The melting point of Ir,P and the dissociation temperature of IrP2 are known approximately. The solubility of P in solid Ir is negligible. 3.1.4. Ga-Ni-P
More detailed assessments of both the Ga-Ni system and the Ni-P system than those found in Ref. [lo] may be found in Ref. [26]. The Ga-Ni binary system is very complicated. The system comprises seven phases: NiGa,, Ni,Ga,, Ni,Ga,, NiGa, Ni,,Ga,, Ni,Ga, and Ni,Ga. The phase Ni,,Ga, is dimorphic. Above 680 “C, it has a partially filled NiAs (B8,) structure. Below this temperature, it transforms into a monoclinically distorted NiAs superlattice structure. The phases NiGa and Ni,Ga have significant ranges of homogeneity. Solid Ni exhibits a large solubility of Ga as well. The system Ni-P is also very complex, and is known incompletely. There are nine intermediate phases in this system: NiP,,^NiP,, NIP, Ni,,,,P, N&P,, N&P, Ni,,P,, N&P*, and N&P. The phases Ni12Pj and Ni,P, undergo structural transformations at high temperatures into phases with slightly different compositions. Only the phase N&P exhibits a significant range of homogeneity. The solubility of P in solid Ni is very small, reaching a maximum of 0.32 at.% at 870 “C. 3.1.5. Ga-Pt-P
The system Ga-Pt comprises eight intermediate phases: PtGa,, Pt,Ga,, PtGa,, Pt,Ga,, PtGa, Pt,Ga,, Pt,Ga, and Pt,Ga. Pt,Ga is polymorphic, and at low temperatures, Pt,Ga exists in three different structural modifications at various compositions. PtGa, reportedly decomposes eutectoidally into Pt,Ga, and Pt,Ga, at 153 “C, although recent experiments by Swenson and Morosin [16] suggest that instead it undergoes a structural transformation at this temperature. With the exceptions of Pt,Ga,, Pt,Ga, and possibly the high-temperature form of PtGa,, the ranges of homogeneity of the platinum gallides are negligible. Solid Pt exhibits some solubility of Ga. The Pt-P system [27] consists of only two intermediate phases: PtP, and Pt,P,. Above 683 “C, there is also a
56
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region of liquid-liquid immiscibility in the composition range of 26-52 at.% P. The maximum solubility of P in solid Pt has been reported to be 0.03 at.%. 3.2. Review of phase equilibria in the Ga-M-P systems
ternary
3.2.1. Ga-Ni-P
Several researchers have investigated the electrical properties of Ni/n-GaP contacts. Goldberg et al. [28] and Lei et al. [29,30] have determined the Schottky barriers of Ni/n-GaP contacts deposited by electrodeposition and electron beam evaporation, respectively. Nannichi and Pearson [4] measured the electrical properties of Ni/n-GaP Schottky diodes as a function of temperature between room temperature and 500 “C. However, they did not explicitly report their electrical data, stating only that the properties of the contacts were similar to those of Cr/n-GaP contacts, the electrical properties of which were investigated in detail. Nakatsuka et al. [31] studied the morphology of Ni thin films on GaP as a function of temperature using high temperature optical microscopy. They found that the films exhibited signs of melting at 760 f 20 “C. It is noteworthy that the molten phase did not wet the GaP substrate. Jan [32] attempted to produce a NiAs @,)-type phase with the composition Ni,GaP from a mixture of Ni and GaP at 600 “C, employing experimental procedures very similar to those of the present investigation. However, his attempt was unsuccessful, and resulted in the formation of NiGa and N&P. Nevertheless, Jan’s experiment demonstrates that Ni is not in thermodynamic equilibrium with Gap.
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analysis to characterize chemical reactions between Pt thin films and GaP. Kumar noted the rapid dissolution of Ga into thin Pt films on GaP at 250 “C, as well as the subsequent formation of Pt,Ga and PtP, at the metalsemiconductor interface. He also observed the formation of the phases Pt,Ga, P&Gas, PtGa and PtP, upon annealing a thick film Pt/GaP couple at 400 “C for 16 h. From these experimental observations, as was mentioned in the Introduction, it is clear that Pt is not in thermodynamic equilibrium with GaP. El-Boragy and Schubert [35] have reported the existence of a ternary phase with a composition close to Pt,GaP after annealing a sample of the appropriate composition at 600 “C for 1 d. Most of the X-ray diffraction peaks could be indexed assuming the tetragonal Pd,TlAs structure, although 10 mol.% of the diffraction peaks were ascribed to other unidentified phases. 3.3. Experimental phase equilibria irz the Go-M-P terns
sys-
Experimentally determined phase equilibria in the Ga-M-P systems are depicted in Figs. l(a)-l(f). In these figures, experimental tie-lines are represented by solid lines, whereas tie-lines that were inferred using the phase rule are dashed. The gross sample compositions employed, and the phases present in each sample as determined by X-ray diffraction analysis, are given in Table 2. A diagram of the Ga-Pd-P system, based upon the work of Mohney et al. [7], is depicted in Fig. l(e), in order to provide a complete picture of phase equilibria in Ga-near-noble-metal-P systems. The constitution of each of these systems will be discussed below. 3.3.1. Ga-Co-P
3.2.2. Ga-Pt-P
Several investigations of Pt thin films on GaP have been reported in the literature. However, most of these studies [30,33-361 have entailed only the measurement of the Schottky barrier heights of Pt/n-GaP diodes at room temperature. Wronski [34] has compared the Schottky barrier heights of Pt contacts deposited on room temperature GaP substrates with those of Pt contacts deposited on 200-300 “C substrates. Although differences in electrical properties were noted, no attempts were made to correlate the change in electrical properties with a possible chemical reaction at the Pt-GaP interface. Ruth et al. [37] and Kumar [5] have investigated the chemical interaction of Pt thin films with GaP. Ruth et al. observed the formation of unspecified platinum gallides and PtP, between Pt and GaP after annealing the contacts at 250-350 “C. No characterization technique was mentioned in Ref. [37]. Their findings are in accordance with those of Kumar, who used X-ray diffraction
In the system Ga-Co-P, depicted i+ Fig. l(a), the phases CoGa and COP were found to be in thermodynamic equilibrium with GaP. According to the phase rule, the phases CoGa, and COP, must also be in thermodynamic equilibrium with GaP, barring the existence of ternary phases. Because the phase CoGa exhibits a wide range of homogeneity, it is important to determine the composition range over which CoGa is in thermodynamic equilibrium with GaP. The lattice parameter of CoGa is known to be a strong function of composition [ 17,19,20]. In principle, therefore, the composition of CoGa in the Ga-Co-P sample may be determined using X-ray diffraction analysis. Unfortunately, discrepancies among researchers in the composition of the Ga-rich phase boundary of CoGa make this determination somewhat ambiguous, as discussed below. The lattice parameter of CoGa in the Ga-Co-P sample was found to be a = 0.28424(3) nm, where the number in parentheses represents the estimated standard
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Ni,Ga, NiGa
/ Ni,Ga, Ni,Ga Ni,:Ga,
Cd)
(a) P
Ga
Ga
Ga
Fig. 1. Isothermal phase equilibria in Ga-M-P systems, where M represents Co, Rh, Ir, Ni, Pd or Pt. All diagrams portray phase equilibria at 700 “C except for (d) and (e), which represent thermodynamic equilibrium at 600 “C. Experimentally determined tie-lines are depicted as solid lines, whereas tie-lines inferred from the phase rule are drawn as dashed lines. The symbol X within a Gibbs isotherm represents the gross composition of a sample employed in the present investigation. These gross compositions, along with phases identified in each sample by X-ray diffraction analysis, are given in Table 2. (e) is based on the work of Mohney et al. [7].
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deviation of the least significant digit. Within experimental error, this lattice parameter is identical to that of CoGa at the Ga-rich phase boundary at 800 “C, as determined by Lindeberg and Andersson [39] in their investigation of the Ga-Co-As phase diagram. Based on the work of Ipser et al. [20], which represents the most complete compilation to date of data relating the lattice parameter of CoGa with its composition, this lattice parameter corresponds to a composition of 61.5 at.% Ga. This composition is in good agreement with the accepted Ga-rich phase boundary of CoGa at 800 “C [lo], and thus corroborates the X-ray diffraction data of Lindeberg and Andersson. However, according to the assessed diagram, the Ga-rich phase boundary of CoGa should only be 59.5 at.% Ga at 700 “C, the temperature employed in the present investigation. At the latter composition, the lattice parameter of CoGa would be substantially larger than that which was found in the present investigation. Therefore, the results of the present investigation imply that the Ga-rich phase boundary of CoGa is more vertical than it has been depicted in the assessed diagram, and hence the boundary is somewhat more Ga-rich at lower temperatures than has been thought previously. It is noteworthy that Feschotte and Eggimann [18] found the phase boundary of CoGa to be at 63.0 at.% Ga at 600 “C and 64.2 at.94 Ga at 800 “C using EPMA, although their data have not been accepted in the assessed diagram. Clearly, this issue cannot be resolved until a definitive investigation of the Ga-rich phase boundary of CoGa is undertaken. The lattice parameters of COP (a = 0.5102(l) nm, b = 0.3288(l) nm, c = 0.5699(2) nm) as determined in the present investigation are significantly larger than those reported in the literature [40]. This suggests that COP exhibits some ternary solubility of Ga, and is more accurately denoted Co(Ga,P). It is interesting to note that the essentially isostructural phase CoAs exhibits significant ternary solubility of Ga as well [39,41].
Table 2 Gross sample compositions and phases identified by X-ray diffraction in Ga-M-P samples Phases by X-ray diffraction co Rh Ir Ni Pt
0.35 0.35 0.40 0.35 0.35
0.30 0.30 0.20 0.30 0.30
0.35 0.35 0.40 0.35 0.35
CoGa, COP, GaP RhGa, RhP?, GaP Ir0.34Ga0.66a,IrP2, GaP N&Ga,, Ni,P, GaP Pt,Ga,, PtGa, PtP,, GaP
aDiffraction pattern could not be indexed; phase identified by EMPA (Please refer to text).
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It is possible to estimate the composition of Co(Ga,P) found in the present investigation from its lattice parameters. Previous researchers [39,41] have observed that the lattice parameters of CoAs are not significantly altered by the replacement of As atoms with Ga atoms. Therefore, the molar volume of Ga in CoAs must be approximately equal to the molar volume of As in CoAs. If it is assumed that the molar volume of Co(Ga,P) increases linearly as a function of Ga content between the molar volume of COP [40] and the molar volume of CoAs [42], a composition of CoGa,,,,P,,,, is obtained for Co(Ga, P) with the lattice parameters found in the present investigation. The lattice parameters of COG~,,,~P~.~~, as well as its composition, are noteworthy, in light of previous observations made pertaining to phases crystallizing with this structure. COP possesses the MnP (B31) structure, which may be regarded as an orthorhombic distortion of the hexagonal NiAs (B8,) structure [43]. In fact, the NiAs structure may be described in terms of the MnP structure via the following transformation of lattice parameters: aMnP
=
CNiAs>
bep
= INCAS,
and
CM,,~
=
@
nNiAs.
Pfisterer and Schubert [44] have noted that the c/b ratio of phases with the MnP structure appears to be dependent upon electron concentration, Rundqvist [40] has explored this possibility by studying the effects of the substitution of several elements, including Cr, Fe, Mn, B and Si, on the c/b ratio of COP. In all cases, Rundqvist found that when substitution was sufficient such that the c/b ratio was equal to @, the electron concentration of the phase was approximately 13.6 electrons/formula unit (assuming that all electrons outside the last completely filled shell are valence electrons. It is also noteworthy that when c/b = fit the MnP structure is pseudohexagonal, as is implied by the preceding formulas relating the lattice parameters of the MnP structure to those of the NiAs structure). Upon examination of the lattice parameters of COG~~,,,P~,~~, one finds that c/b is almost exactly @, Furthermore, the composition CoGa 0,,7P,,.83 corresponds to an electron concentration of 13.7 electrons/formula unit, in excellent agreement with the observations of Rundqvist. 3.3.2. Gn-Rh-P In the system Ga-Rh-P, shown in Fig. l(b), RhGa and RhP, were found to be in thermodynamic equilibrium with GaP. By employing the phase rule, it may be shown that all of the rhodium gallides, as well as the phase RhP, are also in equilibrium with GaP. Moreover, this one phase diagram sample, in conjunction with the phase rule, is sufficient to determine the entire Ga-Rh-P phase diagram isotherm, provided that no ternary phases exist in the system.
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3.3.3. Ga-h-P The Ga-Ir-P phase diagram isotherm is presented in Fig. l(c). IrP, and GaP were identified in the phase diagram sample by X-ray diffraction analysis. However, the remaining diffraction peaks did not correspond to any of the known iridium gallides or phosphides, nor could they be indexed by the present investigators. Therefore, the sample was analyzed by EPMA. Unfortunately, the grain size of the sample was very small, which made probing individual phases difficult. While the measured amounts of Ga and Ir remained relatively constant, the amount of P detected in the phase, though small in magnitude, varied considerably. In a few cases, however, compositional data indicated the total absence of P, and thus the erratic detection of P within the phase may be attributed to overlapping X-ray signals generated from neighboring phases. Based on EPMA, the unidentified phase appears to be an iridium gallide of the approximate stoichiometry This composition lies close to the range of Ir,&ao.,,. homogeneity reported by Schulz et al. [14] for Ir,Ga,. Nevertheless, as was mentioned previously, the X-ray diffraction pattern of the phase does not correspond to that of Ir,Ga, [45]. Therefore, it seems likely that this phase is a previously unknown iridium gallide of the approximate stoichiometry IrGa,. Further investigation of the Ga-Ir phase diagram is required to clarify this issue. According to the phase rule, Ir,Ga,, IrGa, and IrP, should also be in thermodynamic equilibrium with GaP, based upon the known ternary phase equilibria in the system. 3.3.4. Ga-Ni-P
In the Ga-Ni-P system (Fig. l(d)), a region of three-phase equilibrium was found to exist between Ni,Ga,, GaP and N&P. According to the phase rule, NIP,, NiP and N&P, must also coexist with GaP at 600 “C. The tie-line determined by Jan [32] between NiGa and N&P is consistent with the results of the present investigation. The topology of the Ga-Ni-P phase diagram depicted in Fig. l(d) suggests an explanation of the melting phenomenon at 760 + 20 “C observed by Nakatsuka et al. [31]. It is likely that partial melting occurs near 760 “C through a class II ternary reaction: L + Ni,P + Ni,Ga, + Gap. Such a reaction would account for the formation of a liquid phase, yet represents a single tie-line shift from the 600 “C isotherm. Moreover, the possible existence of such a ternary reaction is supported by thermodynamic data. Experimental Gibbs energies of formation [46,47] are available for all four of the phases at 1100 K (827 “C), which is very close to the melting temperature observed by Nakatsuka et al. Using these data, one may calculate the Gibbs energy of reaction, AG:, for the reaction
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given above to be only about - 0.6 kJ g-l essentially zero within experimental error.
atom-‘,
or
3.3.5. Gcr-P&P Fig. l(e) portrays the Ga-Pd-P system at 600 “C, based on the work of Mohney et al. [7]. The phases PdGa and PdP, are the only intermediate phases in thermodynamic equilibrium with GaP. Three ternary phases [7,38,48], for which the equilibrium relationships with the constituent binary phases are unknown, have been included in this diagram for the sake of completeness. 3.3.6. Ga-Pt-P
Finally, in the Ga-Pt-P system, shown in Fig. l(f), Pt,Ga, and PtP, were found to coexist with GaP at 700 “C. The phases Pt,Ga,, PtGa, must be in thermodynamic equilibrium with GaP as well, according to the phase rule. The phase Pt,GaP has been included in the diagram for the sake of thoroughness, although its equilibrium relationships with the remaining phases are unknown. It should be noted that the Ga-Pt-P phase diagram sample contained a small amount of PtGa ( < 5 mol.%), and therefore did not reach thermodynamic equilibrium. 3.4. Tlzermodyiynamically stable contacts
to GaP
Clearly, the results of the present investigation indicate that none of the Co or Ni group metals may be used as electrical contacts to GaP for high-temperature device applications; all of these metals are reactive with respect to GaP. However, the present investigation indicates that many gallides or phosphides of the Co and Ni groups may potentially serve as thermodynamically stable contacts to GaP. Although thermodynamic stability is of utmost importance, there are other criteria which must be considered when selecting contact materials. Primary among these is electrical conductivity; the contact material must be metallic. In this regard, most of the metal phosphides in equilibrium with GaP are unsuitable as contact materials, because they are semiconducting. These include COP,, RhP,, RhP,, IrP,, IrP,, NiP2, PdPz and PtP, [49]. The phases COP, Ni5P4, N&P are reportedly metallic [49], and hence could be used as contact materials. Another concern is the ease of fabrication of a contact via conventional processing methods, such as sputter deposition or electron beam evaporation. It is unlikely that stoichiometric phosphides could be deposited by electron beam evaporation, owing to the fact that the vapor pressure of P will undoubtedly be much higher than that of the metal. Likewise, one might be concerned with the potential loss of P during sputtering through the formation of a gaseous phosphorus species.
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Such losses are of concern both because of the ensuing nonstoichiometry of the deposited films and because of the physical dafigers associated with free phosphorus gas. Nevertheless, Appelbaum et al. [50] have successfully deposited N&P on InP by sputter deposition, apparently without the significant loss of P. However, one must also consider the potential gradual loss of phosphorus from the contact material at the high temperatures experienced during device operation. These practical concerns pertaining to contact fabrication and long-term stability may be allayed if one employs metal gallides rather than metal phosphides as contact materials to GaP. While only a few metallic metal phosphides are in thermodynamic equilibrium with GaP, over one dozen metal gallides have been identified which possess the thermodynamic compatibility required of contacts to Gap. These phases include CoGa,, CoGa, Rh,Ga,, RhGa,, Rh10Ga17, RhGa, Ir,Gag, IrGa,, Ni,Ga,, PdGa, Pt,Ga,, PtGa, and Pt,Ga,. Moreover? many of the metal gallides mentioned above, including CoGa [51-531, RhGa [54], Ni,Ga, [55] and PtGa, [56] have been fabricated previously as contacts to GaAs, primarily by molecular beam epitaxy or metallorganic chemical vapor deposition. Therefore, the feasibility of utilizing Co and Ni group gallides as practical contact materials has already been demonstrated.
4. Conclusions Phase equilibria are established in the Gap-rich sections of several Ga-M-P systems, where M denotes Co, Rh, Ir, Ni and Pt. When combined with previous experimental phase diagram investigations of the GaPd-P system, these data provide an overview of phase equilibria between GaP and the gallides and phosphides of the Co and Ni group metals. It is found that most of the Ga-rich metal gallides of these systems, as well as most of the P-rich phosphides, are in thermodynamic equilibrium with GaP. From a chemical standpoint, each of these gallides and phosphides is suitable as a contact material to GaP. However, upon consideration of other criteria by which contact materials must be judged, such as high electrical conductivity and ease of fabrication by conventional deposition methods, it becomes clear that the metal gallides are in general more suitable than the metal phosphides as potential contact materials. Numerous metal gallides are viable candidates for use as contact materials, including CoGa,, CoGa, Rh,Ga,, RhGa,, Rhdh, RhGa, Ir,Ga,, IrGa3, Ni,Ga,, PdGa, Pt,Ga,, PtGa, and Pt,Ga,. Moreover, several of these gallides have been fabricated previously as contacts to GaAs, demonstrating the feasibility of their use as practical contact materials.
Acknowledgements The present investigators wish to thank I,. Pike for determining the lattice parameters of CoGa and COP and D.Y. Chen for performing EPMA of the Ga-Ir-P sample. This work was performed at the University of Wisconsin and was supported by the National Science Foundation through Grant No. DMR-94-24478. References [I] A.A. BerghandP.J. Dean,Liglrf-mithlg Diodes, Oxford UniversityPress,London,1976. [2] R.C. Hughes,T.E. Zipperian,L.R. Dawson,R.M. Biefeld,R.J. WalkoandM.A. Dvorack,J. Appl. Phys., 69 (1991) 6500. [3] T.E. Zipperian,R.J. Chaflinand L.R. Dawson,IEEE Ttwu. Indm. Electron., IE-29 (1982) 129.
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[7] S.E.Mohney,CF. Lin andY.A. Chang,Appl. P/IJ*s. Lell., 63 (1993)1255. [8] J.R. LinceandR.S. Williams,Thiu Solid Fibs, 137 (1986)251. [9] C.J. Palmstrom andT.D. Sands,in L.J. Brillson(ed.),Corltacts to Setniconducrors: Fmlatmn~als and Techolog~l, NoyesPublications,Park Ridge,NJ, 1993, (ed.),Birlary Alloy Phase Diagram, ASM Inter[lOI T.B. Massalski national,MaterialsPark,OH, 1990. Rec.Phys. u 11 R. Guitrin,A. Guivarc’h,Y. BalliniandM. Secoub, Appl., 25 (1990) 412. L7.1K.J. Schulz,O.A. MusbahandY.A. Chang,J. P/rose Equil., 12 (1991)10. data, 1954,cited in W.B. Pearson,A v31 E. Hellner,unpublished
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[31] H. Nakatsuka, A.J. Domenico and G.L. Pearson, Solid-State Electron., 14 (1971) 849. 1321 C-H. Jan, unpublished data (1991). [33] A.M. Cowley and S.M. Sze, J. Appl. Phys., 36 (1965) 3212. [34] CR. Wronski, J. Appl. Phys., 41 (1970) 3805. [35] D.A. Evans, T.P. Chen, Th. Chassi, K. Horn, M. von der Emde and D.R.T. Zahn, Surf. Sci., 2691270 (1992) 979. [36] M. von der Emde, D.R.T. Zahn, Ch. Schultz, D.A. Evans and K. Horn, Appl. Sur$ Sci. 70/71 (1993) 507. [37] R.L. Ruth, J.M. Poate and A.K. Sinha, unpublished data (1975); cited by A.K. Sinha and J.M. Poate in (J.M. Poate, K.N. Tu and J.W. Mayer (eds.)), Thin Fi/ms - Inter@iision and Reactions,Wiley, New York, 1978, 407. 1381 M. El-Boragy and K. Schubert, Z. fvletallkd., 61 (1970) 579. [39] I. Lindeberg and Y. Andersson, J. Less-Cowmorl Met., 175 (1991) 155. [40] S. Rundqvist, Acfa Chem. Stand., 16 (1962) 287. [41] F.-Y. Shiau, Y. Zuo, J.-C. Lin, X.-Y. Zheng and Y.A. Chang, 2. Metallkd., 80 (1989) 544. [42] K. Selte and A. Kjekshus, Acta Chem. &and., 25 (1971) 3277. [43] K.-E. Fylking, Ark. Kemi, Mineral. Geol. B, II (1935) No. 48. [44] H. Pfisterer and K. Schubert, Z. Metallkd., 41 (1950) 358.
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