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 dep osited on a Si(001) wafer with natural isotopic composition (92.2% Si- 28, 4.7% Si-29). After epitaxial recrystallisation of the enriched lay er, a Si3N4 film with nominal thickness of 2.86 nm was thermally grown in ammonia atmosphere. Excitation curves of the narrow resonance in t he 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 aroun d 4 pi). The very close similarity between the excitation curves obtai ned for two pieces of the same sample, one without and one with therma l nitridation led us to conclude that the Si atoms are not mobile duri ng thermal growth.