Spectroscopic studies on nanocrystalline silicon thin films prepared from H_2-diluted SiH_4-plasma in inductively coupled low pressure RF PECVD

摘要

A comprehensive analysis on the evolution of the microstructure as well as optical constants and dielectric functions of intrinsic hydrogenated nano-crystalline silicon thin films prepared by highly H_2 diluted SiH4 plasma in a planar inductively coupled RF plasma chemical vapor deposition (ICP-CVD) reactor has been performed by spectroscopic ellipsometry. Films are assumed to have a three-layer structure, with a thin incubation layer at substrate/bulk interface, the bulk layer and a thin growth zone and surface roughness layer. Individual composition and the thickness of each layer have been estimated from the simulation of the ellipsometry data using Bruggeman effective medium approximation (BEMA). The ellipsometry results are correlated with atomic force microscopy and micro-Raman data of these films. The effect of the flow rate of SiH_4 and the key role of hydrogen dilution on growth dynamics, optical constants and dielectric functions of highly crystalline nanosilicon films is discussed elaborately. The bulk crystalline volume fraction of the deposited films varies considerably (~67-84%) with the change in flow rate of SiH_4. With increasing SiH_4 flow rate the overall bulk crystallinity reduces; however the ultra-nanocrystalline component (X_(unc)) enhances substantially that helps reducing the porosity and surface roughness.