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

非晶和纳米晶 Si 和 Si:H 薄膜的热力学

• 批准号: 0907724
• 负责人: Frances Hellman
• 金额: $ 37.5万
• 依托单位: University of California-Berkeley
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2009
• 资助国家: 美国
• 起止时间: 2009-06-01 至 2014-11-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0907724&HistoricalAwards=false
• 关键词: Thermodynamics; Amorphous; Nanocrystalline; Si; Thin

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.

非技术摘要 40 多年来，人们都知道玻璃和非晶材料的低温性能（例如导热率和比热）与其晶体材料有很大不同，并且对于各种材料显示出令人惊讶的通用性。隧道层系统 (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系统，硅的四面体键合预计会排除 TLS 的存在，但发现这些状态的密度存在数量级的变化；从通常在玻璃中发现的密度到几乎无法检测到的水平。光浸泡可以增加 TLS 的密度，并且 TLS 的存在与非晶硅太阳能电池效率随时间的下降有关。在这个项目中，我们将利用通过制备条件和光浸泡来调节非晶硅中TLS密度的独特能力，系统地探讨热力学普遍性的起源。特别是，我们将测量低温比热容和导热系数，并寻找与非晶基体中局部键合变化的相关性。识别产生 TLS 的机制对量子信息处理有直接影响，其中 TLS 会引起量子位的退相干，并且还会使用非晶硅作为低成本光伏材料。技术摘要该项目将研究 TLS 的热力学性质非晶气相沉积薄膜。非晶态材料表现出一组特征性的热力学行为，与晶态材料有很大不同。这种普遍行为包括： 由隧道能级状态 (TLS) 模型解释的低温特性（低于 ~1K）；更高温度的特性，包括过剩热容 C（远高于根据声速计算得出的值）、“玻色子峰”和热导率 k 的平台。重点将放在 a-Si 和 a-Si:H 薄膜上，其中生长条件和 H 含量的变化已知会降低 TLS 的密度，从而在低温下产生这些通用特性。将通过 FTIR 探测用 H 进行的悬空键钝化，并将这些结果与 EXAFS 和 XANES 测量相结合，以了解非晶基体的局部结构顺序与比热的任何变化之间的关系。比热测量将使用专为薄膜设计的基于 MEMS 的纳米量热仪进行。我们将对沉积薄膜、光浸薄膜和退火薄膜进行比较，以进一步了解导致非晶硅光伏器件效率光致下降（称为 Staebler-Wronski 效应）的机制。该项目将为两名研究生和大约 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来接触国家实验室。