Isotopic tracing of Si during thermal growth of Si3N4 ultrathin films

We investigated the mobility of Si atoms during the thermal growth of silicon nitride films in ammonia using Si isotopic labeling, together with nuclear resonant reaction analysis for depth profiling. A thin Si-29-enriched layer of silicon with nominal thickness of 1.4 nm was deposited on a Si(001) wafer with natural isotopic composition (92.2% Si-28, 4.7% Si-29). After epitaxial recrystallisation of the enriched layer, a Si3N4 film with nominal thickness of 2.86 nm was thermally grown in ammonia atmosphere. Excitation curves of the narrow resonance in the Si-29(p,gamma)P-30 nuclear reaction at E(R) = 324 keV were measured using a high efficiency gamma-ray detection system (solid angle around 4 pi). The very close similarity between the excitation curves obtained for two pieces of the same sample, one without and one with thermal nitridation led us to conclude that the Si atoms are not mobile during thermal growth.