Electrical evaluation of Al-p⁺-Ge_xSi_1-x ohmic contacts

The electrical properties of Al-p⁺-Ge_xSi_1-x contacts were studied using transmission line measurements. For this study the GeSi alloy layers were selectively formed with 30 or 90 keV Ge implantation into Si, a technique which offers a simple, self-aligned process for the fabrication of such layers. Measurements of the current-voltage characteristics showed that the metal-alloy contacts were ohmic over the voltage range examined. The specific contact resistivity was found to be a function of Ge concentration, decreasing with increasing Ge concentration for concentrations below a critical value and increasing with increasing Ge concentration above this value. The initial decrease in specific contact resistivity is attributed to the effect of Ge on the contact barrier height and width, an effect which is caused by the reduction in the band gap of the alloy. The subsequent increase in specific contact resistivity at higher Ge concentrations is believed to be due to the presence of a high concentration of dislocations in the alloy layer. The thermal stability of contacts is also reported.