PT/TI OHMIC CONTACTS TO ULTRAHIGH CARBON-DOPED P-GAAS FORMED BY RAPID THERMAL-PROCESSING

Increasing the concentration of the carbon dopants in p-GaAs layers grown on semi-insulating substrates to levels of 1×1020 to 5×1020 cm-3 enables the formation of an ohmic contact with low resistance using the refractory Pt/Ti metallization. These contacts showed ohmic behavior prior to any heat treatment with specific contact resistance as low as 7×10-6 Ω cm2 (0.08 Ω mm) for the lower doping level and 8×10-7 Ω cm2 (0.04 Ω mm) for the higher level. Small improvements in the specific resistance of the former contact were achieved by rapid thermal processing at a temperature of 450°C for 30 s, which yielded a value of 4.9×10-6 Ω cm2. The electrical nature of the contact to the heavily doped GaAs was not affected by heat treatments at temperatures up to 450°C. Rapid thermal processing of these contacts at higher temperatures, however, caused an increase in the contact resistance which was correlated to the expanded Ti/GaAs and Pt/GaAs interfacial reactions. Current-voltage characteristics were found to be temperature independent. This suggested that the field emission quantum-mechanical tunneling was the dominant carrier transport mechanism in these contacts.