Oxidation of Si1−xGex alloys at atmospheric and elevated pressure

Abstract

The oxidation of alloys of Si1−xGex differs significantly from that of pure Si in both the thermodynamics of the process and in the kinetics of the oxidation reaction. In this paper these fundamental differences are explored and are used to explain experimental observations of Si1−xGex oxidation that are presented herein and elsewhere in the literature. Alloys of Si1−xGex (with x=5.4, 11.6, and 17 at. %) approximately 200 nm thick were oxidized using the following two processes: (i) dry oxygen at 680 atm at a temperature of 550 °C and (ii) conventional, 1-atm steam at 800 °C. The wet oxidation conditions were chosen to produce an oxide thickness comparable (≊100 nm for xGe=11.6 at. %) to that obtained during high-pressure oxidation at 550 °C. Auger sputter depth profiling, x-ray photoelectron spectroscopy (XPS), and cross-sectional transmission electron microscopy were used to characterize the as-grown oxides. XPS studies reveal that high-pressure oxides formed from alloys of Si1−xGex chemically incorporate Ge directly into the oxide. In contrast, atmospheric oxidation of the same alloy composition produces oxides with no Ge incorporation that is detectable using XPS. It is shown that the selective removal of Si from alloys of Si1−xGex is predicted from Si-Ge-O ternary-phase equilibrium considerations. High-pressure oxidation conditions, however, allow the growth of oxides at much lower temperatures which minimizes Si-Ge interdiffusion and, as a consequence, prevents the selective oxidation of Si from the alloy. These results have direct significance to future device fabrication in Si1−xGex and to the issues of oxidation of multicomponent systems in general.