Oxidation of relaxed Si0.5 Ge0.5 alloy.

Oxidation is a fundamental process in the fabrication of microelectronic devices and in order to promote the incorporation of SiGe into Si technology we have investigated the thermal oxidation of Si1-yGey alloy (y≈0.5) at high temperatures (mainly 900°C and 1000°C) using Rutherford backscattering spectroscopy, infrared transmission spectroscopy and X-ray photoelectron spectroscopy. It has been observed that three distinct regions form during oxidation of Si0.5Ge0.5 alloy, which are (I) a mixed oxide layer Si0.5Geg0.5O2, (II) a pure SiO2 layer and (III) a Si1-yGey (y≠0.5) alloy layer. These are formed during both wet and dry oxidation when the sample is not preheated, whilst only two regions (II) and (III) form when the sample is preheated in a non oxygen ambient up to the oxidation temperature prior to oxidation. Enhancement of the rate of wet oxidation of SiGe compared with bulk Si and the accumulation of Ge in region III just below SiO2 layer (region II) has also been observed. This behaviour is in good agreement with other results, however, some abnormal behaviour during wet and dry oxidation has been observed. The rate of oxidation during short wet oxidations (15 minutes) of Si0.5 Ge0.5 decreases as the oxidation temperature increases from 800°C to 1000°C for 15 minutes, which has not been previously reported, A reduced rate of oxidation of Si1-yGey (y≈0.5) in a dry environment has been observed and is discussed. In order to investigate the thermodynamics of the oxidation process new experiments have been carried out which involve the synthesis of a buried oxide layer, by O+ implantation followed by a high temperature anneal. The implanted oxygen atoms preferentially bond to silicon atoms and the oxygen atoms are found to bond to germanium atoms only after all of the silicon atoms are fully oxidised. Germanium tends to be rejected from the growing oxide during a subsequent higher temperature (>900°C) anneal. The different behaviour of the Si and Ge atoms during both thermal and internal oxidations is described in terms of the thermodynamics and kinetics of the SiGe alloy system. There are three important conclusions which emerge from these analyses: (i) in order to adequately control the composition of thermal oxides (T>800°C) grown on Si0.5Ge0.5 material it is necessary to preheat the samples in a non oxidising atmosphere prior to oxidation. By so doing the entrapment of Ge in the near surface layer (region I) is inhibited; (ii) during the initial stage of dry oxidation the process is described by the reaction Si+O2?Si2 and we suggest that the rate is controlled by the availability of oxygen atoms and is numerically the same as for bulk Si; (iii) in contrast, during the initial stage of wet oxidation, the process is described by the reaction Si+2OH→SiO2+H2 and the rate is controlled by the areal density of Si atoms, which we propose is high due to the weak Si-Ge binding energy and thus an enhanced oxidation rate occurs