Structural and optical characterization of two-dimensional arrays of Si nanocrystals embedded in SiO2 for photovoltaic applications

We report on the structural and optical characterization of two-dimensional arrays of silicon nanocrystals (SiNCs) suitable for photovoltaic applications. Single and multiple SiNC layers were grown on quartz by low pressure chemical vapor deposition of Si and subsequent thermal oxidation steps. The single SiNC layers consisted of one SiNC layer embedded in two silicon dioxide (SiO2) layers, whereas the multi-layered structure consisted of five SiNC layers of equal thickness separated by SiO2 layers. SiNC layers with thicknesses ranging from 2 to 25 nm were investigated. A thorough structural characterization of the films was carried out by combining grazing incidence x-ray diffraction, x-ray reflectivity, and transmission electron microscopy (TEM). Both XRD and TEM measurements revealed that the SiNC layers were polycrystalline in nature and composed of SiNCs, separated by grain boundaries, with their vertical size equal to the SiNC layer and their lateral size characterized by a narrow size distribution. The high resolution TEM (HRTEM) images showed that oxidation of the SiNC layers proceeded by consumption of Si from their top surface, without any detectable oxidation at the grain boundaries. Only in the case of the thinnest investigated SiNC layer (2 nm), the SiNCs were well separated by SiO2 tunnel barriers. From transmission and reflection optical measurements, energy band gaps of the SiNCs were estimated. These results were correlated with the sizes of the SiNCs obtained by HRTEM. A shift of the estimated band gaps with decreasing SiNC size was observed. This was consistent with quantum size effects in the SiNCs. The film containing the smallest SiNCs (2 nm in the growth direction), besides a significant shift of the absorption edge to higher energies, showed light emission at room temperature which is due to radiative recombination of photo-generated carriers in localized SiNCs separated by SiO2 tunnel barriers. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4707939]