Temporal evolution of ion implanted CdSe distribution in SiO2 on silicon during annealing

The temporal and spatial evolution of Cd and Se concentration distribution during formation of buried CdSe nanocrystals in thermally grown 500 nm thick SiO2 on Si(1 0 0) was investigated. For that Cd and Se was implanted at liquid nitrogen temperature. Diffusion and chemical reaction was initiated ex situ by a rapid thermal annealing process in Ar at atmospheric pressure at 800 deg. C and annealing times ranging from 30 s up to 32 min. The evolution of the concentration distributions of Cd and Se were quantitatively analyzed by Rutherford backscattering spectrometry (RBS) and dynamic secondary ion mass spectrometry (SIMS) by detecting the diatomic secondary ions SeCs+ and CdCs+. With RBS the absolute implanted dose was determined and used to calibrate the corresponding SIMS depth profiles. It was found, that CdSe formation takes place immediately during the first 30 s of the annealing procedure. For longer annealing times Cd and Se material diffuses nearly congruently to greater depths accompanied by CdSe precipitation. Once material has reached the SiO2/Si interface accumulation of CdSe happens inside the SiO2 at the interface in form of CdSe nanocrystallites. In the case of higher Cd over-stoichiometry and long annealing times self-organized nearly periodic concentration variations over the SiO2 thickness were observed. Details of the crystallite size and their spatial distribution were imaged by cross-sectional TEM for individual samples