STRICT THERMAL NITRIDATION SELECTIVITY BETWEEN SI AND GE USED AS A CHEMICAL PROBE OF THE OUTERMOST LAYER OF SI1-XGEX ALLOYS AND GE SI(001) OR SI/GE(001) HETEROSTRUCTURES/

The thermal reactivity of NH3 with Si(001), Ge(001), Si1-xGex(001), an d thinly Ge-covered Si(001) or Si-covered Ge(001) surfaces has been st udied by means of in situ x-ray photoelectron spectroscopy in a temper ature domain (T approximately 600-degrees-C) compatible with the usual growth of Ge-Si based heterostructures. A very marked difference betw een Si(001) and Ge(001) initial sticking coefficients is found, the la tter surface being totally inert against nitridation by NH3 in contras t with the Si(001) surface. This nitridation selectivity provides easy access to information about surface termination in different situatio ns where Si and Ge are potentially mixed, the reactivity depending on whether Si appears at the reactive interface or not. As a test, compar ing the nitrogen uptake curves in the early stages of NH3 exposure, fo r all the studied structures except Si(001), a Ge-like initial stickin g coefficient or a non-nitriding behavior is found. This is fairly exp lained on the basis of the well-known concepts of the Stranski-Krastan ov growth mode for the Ge/Si system and by Ge surface segregation for the Si1-xGex and Si/Ge systems. Our result also implies the need of pl asma-assisted treatments in order to achieve simultaneous Si and Ge ni tridation of SiGe alloys.