Thermodynamics of Amorphous and Nanocrystalline Si and Si:H Thin Films

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

NON-TECHNICAL ABSTRACT For over 40 years, it has been known that the low temperature properties, such as thermal conductivity and specific heat, of glasses and amorphous materials differ dramatically from their crystalline counterparts and show a surprising universality for a wide range of materials. The Tunneling Level Systems (TLS) model can successfully describe this behavior below 1K but the physical mechanism that gives rise to the TLS has yet to be identified. Amorphous silicon is a unique system in that silicon?s tetrahedral bonding is predicted to preclude the presence of TLS yet the density of these states is found to vary by orders of magnitude; from the densities typically found in glasses to almost undetectable levels. Light soaking can increase the density of TLS and the presence of TLS has been linked to the decrease in efficiency of amorphous silicon solar cells over time. In this project, we will use the unique ability to tune the density of TLS in amorphous silicon, both by preparation conditions and light soaking, to systematically probe the origin of the thermodynamic universality. In particular, we will measure the low temperature specific heat and thermal conductivity and look for correlations to changes in the local bonding in the amorphous matrix. Identifying the mechanism that gives rise to the TLS has a direct impact on quantum information processing where the TLS give rise to decoherence in qubits and also to the use of amorphous silicon as a low cost photovoltaic material. TECHNICAL ABSTRACT This project will investigate the thermodynamic properties of amorphous vapor deposited films. Amorphous materials exhibit a characteristic set of thermodynamic behaviors that differ substantially from their crystalline counterparts. This universal behavior includes: low temperature properties (below ~1K) that are explained by the model of tunneling level states (TLS); higher temperature properties which include excess heat capacity C (well above what is calculated from sound velocity), a ?boson peak?, and a plateau in thermal conductivity k. The focus will be on a-Si and a-Si:H films where changes in growth conditions and H content are known to reduce the density of TLS that give rise to these universal properties at low temperature. Dangling bond passivation with H will be probed via FTIR and these results will be coupled with EXAFS and XANES measurements to see how the local structural order of the amorphous matrix correlates to any changes in the specific heat. Specific heat measurements will be made with a MEMS based nanocalorimeter that is specifically designed for thin films. Comparisons between as deposited, light soaked, and annealed films will be made to further our understanding of the mechanisms that give rise to the light induced degradation of the efficiency of a-Si photovoltaic devices known as the Staebler-Wronski Effect. The project will provide research training and education for two graduate students and an estimated 5-6 undergraduates, including exposure to the national laboratories through the PI's collaborations with the National Renewable Energy Laboratory and Lawrence Berkeley National Laboratory.