Optimization in the nanostructural evolution of hydrogenated silicon germanium thin film in RF-PECVD

Abstract

Abstract: In view of the grossly different chemical reaction behaviors of the precursors generated from two different very commonly used source gases, e.g., SiH4 and GeH4, development of nc-Si1-xGex:H alloy material wherein both Si and Ge could organize the crystalline configuration is a challenging task. Low temperature (Ts ∼220 °C) growth of nc-SiGe:H thin films has been pursued by optimizing the 13.56 MHz RF power applied to the H2-diluted (SiH4 + GeH4) plasma in PECVD. Systematic structural evolution in the network identifies mostly Ge-populated and Si-deficient amorphous dominated SiGe matrix having low magnitudes of structure factors, RSi ∼0.29 and RGe ∼0.41 at low level of RF power, P = 50 W. Crystallinity evolves in the network at P = 75 W. However, superior crystallinity comprised of nanocrystals of average size ∼15 nm with significant crystallographic orientations are attained at P = 100 W. Homogeneous distribution of the Si and Ge atoms of density ∼96 at.% and ∼4 at.%, respectively, with signatures of Si 2p3/2, Si 2p1/2 and Ge 3d5/2 and Ge 3d3/2 sublevels are attained. Accompanying sharp increase of RGe to RSi ratio identifies an overpopulation of the network by polyhydrides of Ge at elevated RF power that leads to a sharp deterioration in crystallinity, accommodating significantly smaller size of the nanocrystals, at P = 150 W. Accordingly a moderate RF power is needed to attain an optimum nc-SiGe:H network at which Ge in sensible population could be accommodated with Si organized in a crystalline configuration of the alloy network with controlled amount of polyhydrogenation.