Structural analyses of thermal annealed SRO/SiO2 superlattices

Abstract

AbstractNanocrystalline silicon embedded in dielectric matrices is currently studied for Si‐photonics, memory devices and solar cells. A common method for the preparation of silicon nanocrystals embedded in oxides is the phase separation of silicon rich oxide (SRO) in SiO2 and Si via thermal annealing. Phase separation, nucleation and crystallization of SRO are known to depend on the thickness of the SRO layer. Here we investigate the structural changes in a sample consisting of alternated nanometer‐thick SRO and SiO2 layers—a so‐called superlattice (SL)—during thermal annealing. Under a thermal treatment the material undergoes a series of modifications due to sintering, phase separation, crystallization and layer mixing. In this work we investigate these transformations in an SL grown by plasma enhanced chemical vapor deposition (PECVD) with several analytical techniques: XPS, variable angle ellipsometric spectroscopy (VASE), SIMS, and X‐ray reflectivity (XRR). Both SIMS and XRR measurements clearly reveal the periodicity of the samples. XPS analysis reveals that phase separation of SRO in silicon and SiO2 occurs in the annealing temperature range 600–925 °C. The process is accompanied by reduction in overall thickness of the samples (ongoing also at higher temperatures) as evidenced from the ellipsometric spectra. A maximum form birefringence is achieved at 925 °C and stays nearly constant until 1100 °C. Eventually, the form birefringence decreases at the highest annealing temperature of 1150 °C, which according to SIMS measurements is caused by a partial oxidation of silicon in the outermost SRO layer. Copyright © 2010 John Wiley & Sons, Ltd.