Thermally stable ohmic contacts to n-type GaAs. I. MoGeW contact metal

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Abstract
Thermally stable, low‐resistance ohmic contacts on n‐type GaAs are required to fabricate high‐speed GaAs integrated circuits. MoGeW contacts prepared by annealing at high temperature around 800 °C in an InAs overpressure are attractive, because the contact is expected to be thermally stable during subsequent annealing at 400 °C, which is required by several process steps following ohmic contact formation. In the present experiment, the contact resistance measurements and microstructural analysis of MoGeW contacts were carried out to establish a fabrication process which forms ohmic contacts with low contact resistance. The contact metals were prepared by sequentially depositing Ge, Mo, Ge, and W, with various Mo/Ge layer thickness ratios, onto (100)‐oriented GaAs wafers. The conducting channels were formed by doping GaAs with about 1×1018 cm−3 Si. Contact resistances were determined by the transmission line method, and microstructural analysis was carried out by x‐ray diffraction, Auger electron spectroscopy, secondary ion mass spectroscopy (SIMS), and transmission electron microscopy. Contact resistance (Rc) was found to be strongly influenced by the Mo/Ge layer thickness ratio and annealing temperature. Rc values lower than 0.5 Ω mm were obtained for samples with a Mo/Ge thickness ratio in the range 0.6–1.3 and annealed at around 800 °C. The lowest mean Rc value obtained in the present experiment was 0.3 Ω mm. The major compound formed in this contact was identified to be Mo5As4, which has a high melting point. No changes in the microstructure and the Rc values were observed after annealing the contacts at 400 °C for more than 100 h. Finally, an attempt to understand the carrier transport mechanism was carried out by correlating the electrical behavior with the film microstructure. For this purpose the samples were annealed in an InAs overpressure with or without a Si3N4 cap, by flash annealing, and in an arsine atmosphere. The ohmic behavior was observed only in the samples annealed in an InAs overpressure. The SIMS analysis indicated that a small amount of In, less than 1 at. %, was segregated at the metal/GaAs interfaces in this sample. The In could form compounds with GaAs and reduce the barrier height, resulting in reduction of the contact resistances.

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