OXIDATION OF POLYCRYSTALLINE-SIGE ALLOYS

Polycrystalline-SiGe (poly-SiGe) films with Ge concentrations ranging from 5% to 30% were wet oxidized with trichloroethane at temperatures ranging from 700 to 1000-degrees-C. For oxidation temperatures greater-than-or-equal-to 800-degrees-C, the oxidation rate of poly-SiGe depends only weakly on the Ge concentration. At 700-degrees-C, the oxidation rate increases with Ge concentration and can exceed that at 800-degrees-C. Rutherford backscattering spectra show that, in samples oxidized at or above 800-degrees-C, Ge is completely rejected from the oxide, resulting in a pileup at the interface and diffusion into the poly-SiGe. At 700-degrees-C, however, Ge is partially incorporated into the growing oxide when layers with high Ge concentration (greater-than-or-equal-to 20%) are oxidized. Most of the Ge is still rejected from the oxide and diffuses into the poly-SiGe layer. This behavior differs from that observed during the oxidation of epitaxial SiGe. Our results can be explained by assuming that diffusion of oxidant through the oxide is the rate controlling step. The oxide composition in turn, depends on the degree with which Ge is rejected from the oxide. The Ge removal rate from the interface exceeds that of single crystal films because of the enhanced diffusion of Ge along grain boundaries.