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

40多年来，人们已经知道，玻璃和非晶材料的低温特性，如导热系数和比热，与晶体材料有很大的不同，并且在广泛的材料中显示出惊人的普遍性。隧道级系统（TLS）模型可以成功地描述1K以下的这种行为，但产生TLS的物理机制尚未确定。非晶硅是一种独特的体系吗？我们预测四面体成键会排除TLS的存在，但发现这些态的密度会以数量级变化；从玻璃中常见的密度到几乎检测不到的水平。光浸泡可以增加TLS的密度，随着时间的推移，TLS的存在与非晶硅太阳能电池的效率下降有关。在本项目中，我们将利用独特的能力，通过制备条件和光浸泡来调节非晶硅中TLS的密度，系统地探索热力学普适性的起源。特别是，我们将测量低温比热和导热系数，并寻找非晶基体中局部键合变化的相关性。确定产生TLS的机制对量子信息处理有直接影响，其中TLS会产生量子比特的退相干，也会使用非晶硅作为低成本光伏材料。本项目将研究非晶气相沉积薄膜的热力学性质。非晶材料表现出一组与晶体材料截然不同的热力学行为特征。这种普遍的行为包括：用隧道能级态（TLS）模型解释的低温特性（低于~1K）；高温特性，包括过剩热容量C（远高于由声速计算得到的），a ？玻色子高峰?重点将放在a- si和a- si:H薄膜上，已知生长条件和H含量的变化会降低TLS的密度，从而在低温下产生这些通用特性。悬空键与H的钝化将通过FTIR进行探测，这些结果将与EXAFS和XANES测量结果相结合，以了解非晶基体的局部结构顺序如何与比热的任何变化相关。比热测量将使用专门为薄膜设计的基于MEMS的纳米热计进行。将对沉积膜、光浸泡膜和退火膜进行比较，以进一步了解引起光诱导的a-Si光伏器件效率退化的机制，即所谓的Staebler-Wronski效应。该项目将为两名研究生和约5-6名本科生提供研究培训和教育，包括通过PI与国家可再生能源实验室和劳伦斯伯克利国家实验室的合作，接触国家实验室。

NON-TECHNICAL ABSTRACTFor 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 ABSTRACTThis 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.