KINETICS AND MECHANISM OF WET OXIDATION OF GEXSI1-X ALLOYS

The thermal oxidation of polycrystalline GexSi1-x (0.10 less than or e qual to x less than or equal to 0.47) and pseudomorphic Ge0.2Si0.8 has been studied in wet ambient at 550 to 900 degrees C. A uniform GexSi1 -xO2 oxide is observed by backscattering spectrometry for a high Ge co ntent at low oxidation temperatures; a SiO2 oxide is obtained for a lo w Ge content at high temperatures; a GeySi1-yO2 oxide with reduced Ge content (y<x) is found in between. Ge piles up behind the oxide when S iO2 or GeySi1-yO2 form. The transition between these three types of ox ides also depends on the crystallinity of the GeSi alloy. When a unifo rm GexSi1-xO2 oxide grows, its thickness is proportional to the square root of the oxidation duration, which indicates that the rate-limitin g process is the diffusive transport across the oxide of, most probabl y, the oxidant. The rate increases with the Ge content in the alloys. The proportionality constant, B, for this process is B(T)=[(1.0+/-0.2) X10(11) nm(2)/h]exp[(-1.1+/-0.2 eV)/kT] for Ge0.47Si0.53. It is propos ed that, in general, the oxidation behavior is determined by the compe tition between the speed of the diffusive process in the unoxidized Ge Si alloy and the velocity at which the oxidation front progresses. The controlling factors are the oxidation temperature, the composition, a nd the structure of the GexSi1-x alloy. A model is proposed that is ba sed on these three factors. Analogies with this system exist where all three elements are solid.