Calculated phase diagrams of ternary Ga-As-metal systems for the metals Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Re, Os, Rh, Ir, Ni, Pd, Pt, Cu, Ag and Au are presented. The predictive calculations are based on the following simplifications: Ternary phases and solid solubilities are disregarded and the Gibbs energy of formation of binary compounds is estimated by the enthalpy of formation and calculated from Miedema’s model. The predicted diagrams agree surprisingly well with experimental data and they may be useful for the many cases where data are lacking or fragmentary. The phase diagrams and the thermodynamic data are shown to be a powerful tool for the understanding of interface reactions of metallic contacts to GaAs and hence for the development of improved contact materials.

Stability of metal/GaAs-interfaces: a phase diagram survey

Reactions between Pd and GaAs have been studied using bulk-diffusion couples of Pd (approx.0.6 mm thick)/GaAs and thin-film Pd (50 and 160 nm)/GaAs samples. The sequence of phase formation at 600 /sup 0/C between bulk Pd and GaAs was established. Initial formation of the solution phase ..mu.. and the ternary phase T does not represent the stable configuration. The stable configuration is GaAs chemically bondepsilonchemically bondlambdachemically bond..gamma..chemically bond..nu..chemically bondPd and is termed the diffusion path between GaAs and Pd. The sequence of phase formation for the bulk-diffusion couples is similar at 500 /sup 0/C. Phase formation for the thin-film Pd/GaAs specimens was studied at 180, 220, 250, 300, 350, 400, 450, 600, and 1000 /sup 0/C for various annealing times. The sequence of phase formation obtained from the thin-film experiments is rationalized readily from the known ternary phase equilibria of Ga--Pd--As and the results from the bulk-diffusion couples of Pd/GaAs. The thin-film results reported in the literature are likewise rationalized. The diffusion path concept provides a useful guide in understanding the phase formation in Pd--GaAs interface or any other M--GaAs interface. This information is important in designing a uniform, stable contact for the metallization of GaAs.

Reactions between palladium and gallium arsenide: Bulk versus thin-film studies

The thermal reaction of a vacuum‐evaporated Ti film with substrate GaAs has been investigated using Auger electron spectroscopy and x‐ray diffractometry. As accumulation and Ga depletion at the Ti/GaAs interface and Ga pileup in front of the As‐rich layer have been observed. The As‐rich and the Ga‐rich layers have been found to be composed of TiAs/Ti5As3 and Ti2Ga3/Ti5Ga4, respectively. It has also been found that the rate of the Ti‐GaAs reaction is governed by the interdiffusion process of Ti and As at the interface with an activation energy of 1.70±0.05 eV. The low diffusivity and relatively high Schottky barrier height of the present system facilitate the formation of useful Schottky barrier junctions.

Thermal reaction of Ti evaporated on GaAs

The ohmic contacts to p-type GaAs formed by GaAs/Pt/Ti/Pt/Au systems were investigated. The specific contact resistance below 8×10-7 Ωcm2 was achieved when the interface Pt between GaAs and Ti/Pt/Au was thicker than 50 A, which is about one-fourth of the conventional Ti/Pt/Au contact. The activation energies of the initial degradation of the Pt/Ti/Pt/Au electrodes correspond to the reaction of GaAs and Pt to form PtAs2. However, even after the initial degradation, Pt/Ti/Pt/Au with the thin Pt interface layer still shows lower contact resistivity. These systems are promising for practical p-tye ohmic contacts for AlGaAs/GaAs heterojunction bipolar transistors.

https://iopscience.iop.org/article/10.1143/JJAP.30.L558/pdf

Electrical characteristics and reliability of Pt/Ti/Pt/Au ohmic contacts to p-type GaAs

It has been proposed that interdiffusion of Pt, Au, and GaAs constitutes one major source of instability in GaAs microwave devices with Pt/Au metallization. In order to determine the extent of interdiffusion of these materials, preparations of Pt/GaAs and Au/Pt/GaAs were heat treated over the range 250–500 °C, and chemical depth profiles were obtained by measuring the surface composition with Auger electron spectroscopy while ion milling. Platinum films were 125–9000 A thick. When Pt/GaAs was heated in vacuum, the Pt‐GaAs interaction initiated with a rapid migration of Ga into Pt and simultaneous formation of an As‐rich layer at the Pt/GaAs interface. Gallium eventually traveled the entire thickness of even the 9000‐A Pt film while arsenic stopped abruptly at a distance ∼2/3 of the way into the Pt. Little or no Au was detected (<1 at.%) in the Pt or GaAs after extensive Pt/GaAs reaction in Au/Pt/GaAs. The reacted Pt film displayed a multilayered structure with each layer containing different amounts of Ga...

Interdiffusions in thin‐film Au on Pt on GaAs (100) studied with Auger spectroscopy

The rate at which GaAs reacts with Pt has been determined at selected temperatures between 300 and 400 °C. The amount of GaAs reacted is determined by measuring the movement of the original Pt Schottky‐barrier contact into the GaAs. Data are presented which show the amount of GaAs reacted as a function of anneal time at several different temperatures. The reaction follows a parabolic rate law with activation energy of 1.6 eV. Some change in the electrical characteristics of the Schottky barrier is observed.

D. J. Coleman

Papers

Reaction rates for Pt on GaAs