Thermal nitridation of Si(100)-2 × 1 surface by NH 3: XPS results

Abstract

Abstract Using essentially X-ray photoelectron spectroscopy (XPS), the adsorption of NH 3 on the Si(100) surface is examined in wide ranges of substrate temperature ( T s between room temperature and 800° C), gas pressure ( p NH 3 ) or exposure ( E ). Much effort has been devoted to elucidating: (i) The general T S dependent evolution of the chemisorbed species. Successively, with increasing T S , NH 2 or NH fragments are first bound to the Si surface, which is followed by a hydrogenation stage, and finally, after H desorption above 350 ° C, the start of atomic nitrogen chemisorption is evidenced. (ii) The particular exposure and T S behaviour of the latter nitridation regime, occurring in the 350–800° C range, mostly studied in this article, (a) At high T S (600–800° C) and low NH 3 pressure, growth of Si 3 N 4 islands on the Si(100) surface is deduced from comparative determinations of the nitrogen coverage, of its in-depth repartition (by angular variations) and of local Si 4 environments (by binding energy shifts). Very low nitrogen uptake, governed by N thermal desorption increasing with T S , is found in this domain. For higher NH 3 pressures or exposures ( E > 200 L) complete nitridation in a thin homogeneous Si 3 N 4 overlayer is achieved. Further growth becomes all the more limited (slow nitridation stage) as the reacting Si is more separated from the gaseous nitrogen source by the previously formed nitride diffusion barrier. By giving the empirical laws for the nitride growth we are able to separate clearly for the first time the pressure and time effects during the exposure. We conclude that the exposure only expressed in langmuirs, or in integrated numbers of impinging molecules cannot be a good parameter for the description of Si nitridation. (b) At lower T S (350–600° C) the nitrogen is chemisorbed at the surface in a more homogeneous way and, even if the initial coverage for low exposure is now higher, very slow bulk diffusion confines the interaction within the top layer and leads to a more rapid saturation of the nitrogen uptake with exposure. In this regime mainly intermediate silicon subnitride environments (Si 3 Si) are found. Despite a T S shift, accounting for the room temperature dissociative chemisorption of O 2 on Si, whereas that of atomic nitrogen only starts above 350 ° C, a qualitative similarity can be remarked for the main features involved in the initial, fast silicon nitridation stage with those occurring in oxidation.