Raman spectroscopy of silicon-nanocrystals fabricated by inductively coupled plasma chemical vapor deposition

Size dependent properties of nanostructures – quantum confinement effect, optical phonon confinement, multiexciton generation – prompted extensive research of good old silicon, at nanoscale dimensions, for potential applications in next generation device applications. However, for realizing functional devices, a thorough understanding of size related properties and a viable fabrication method are crucial. In this study, we present a thorough Raman analysis of silicon-nanocrystals (Si-NCs) of various sizes fabricated by inductively coupled plasma enhanced CVD (ICPCVD). Si-NCs were realized using a two-step process for tight size control; initially alternating multilayers of SiOx<2 (SRO) and SiO2 were deposited by ICPCVD, followed by high temperature annealing for phase-separation and crystallization. To study the optical phonon confinement, a series of five multilayer samples with thickness of SRO sublayer (TSRO) varying from 10 nm to 2 nm were fabricated. Raman spectra for ML samples with View the MathML source exhibited three notable features; a red-shifted sharp peak compared to c-Si, a broad asymmetric shoulder on the lower frequency side and a grain boundary band related to the interface between Si-NCs and SiO2. These features confirm the formation of Si-NCs in SRO sublayers. ML samples with View the MathML source showed incomplete crystallization indicating SRO thickness dependent crystallization. To further study this behavior, the degree of crystallization was quantified by estimating the crystalline volume fraction which was observed to decrease with decreasing TSRO. Red shift of the Raman peak, in accordance with the optical phonon confinement model, was used to estimate the size of Si-NCs. The sizes so obtained were observed to be in good agreement with the TSRO values. The study is relevant and encouraging for further understanding of Si-NC based composite material for various applications including next generation photovoltaics.