Nucleation and growth of low-temperature fine-crystalline silicon: a scanning probe microscopy and Raman spectroscopy study of the influence of hydrogen and different substrates

Abstract

Low-temperature fine-crystalline silicon films grown by plasma-enhanced chemical vapour deposition (PECVD) on different substrates are investigated by scanning probe microscopy and Raman spectroscopy. By this, more insight into nucleation and growth of crystallites is provided. For this purpose deposition conditions within the transition regime from fine-crystalline to amorphous growth are chosen leading to the growth of individual crystallites embedded in an amorphous matrix. Effects of the type of substrate and of hydrogen dilution are studied. Films grown on naturally oxidized Si(100) and on graphite(0001) show a clear correlation between the area density of crystallites as inspected from surface micrographs and the volume fraction of crystalline phase as detected by Raman spectroscopy. Nucleation of crystallites takes place within a narrow range of film thickness (the first few 10 nm for growth on silicon oxide) whereas the subsequent growth stage exclusively promotes the continuous growth of existing crystallites, i.e. nucleation of new crystallites is suppressed. The initial nucleation strongly depends on the type of substrate, for instance purely amorphous films can grow instead of two-phase ones if only the oxidized Si(100) substrate is replaced by Si(100) which has been hydrogen passivated by a HF treatment. A pronounced influence of hydrogen on crystallite nucleation is observed: if the source gas ratio R=SiH 4/(SiH 4+H 2) is reduced by only 0.3% the nucleation density increases by about one order of magnitude. In addition to presently discussed kinetic growth models the results indicate the applicability of a thermodynamical concept for explaining the structural evolution.