SELECTIVE THERMAL - AS OPPOSED TO NONSELECTIVE PLASMA - NITRIDATION OF SI-GE RELATED MATERIALS EXAMINED BY IN-SITU PHOTOEMISSION TECHNIQUES

NH3 thermal and N-2 plasma reactivity with Si(001), Ge(001), Si1-xGex(001) surfaces has been studied by means of in situ X-ray photoelectron spectroscopy (XPS) in a temperature domain (T similar to 600 degrees C) compatible with the MBE growth of Ge-Si-based heterostructures. Si(001) surfaces present a strong initial thermal reactivity against NH3, contrary to Ge(001) which is totally inert. The selectivity against thermal nitridation, which may be anticipated for thermodynamical reasons, has been verified by nitrogen uptake measurements of N 1s core level intensities as a function of NH3 exposure, both for Si(001) and Ge(001) surfaces. As a consequence of this strong reactivity difference, an exclusive Si3N4 formation and Ge phase separation result from nitridation attempts of Si1-xGex alloys. Thus, an important finding is the indispensable utilization of plasma-assisted nitridation methods in order to achieve low-temperature Ge nitridation, either on clean Ge(001) surfaces or simultaneously with Si on SiGe alloys. In this paper, the first results relevant to Ge and SiGe alloy nitridation by irradiation of these surfaces by electron cyclotron resonance (ECR) nitrogen (N-2) plasmas are presented. These alloys are thermally unstable as nitridation transfer from Ge to Si occurs after annealing, in accordance with thermally favored Si nitridation. In addition, Ge3N4 (Si3N4) thick layers were grown using ECR N-2 plasma treatment associated with a concomitant Ge (Si) atom-supply on the substrate, performed by Ge evaporation (SiH4 reacting gas).