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，人们已经知道玻璃和非晶材料的低温性能，例如热导率和比热，与它们的晶体对应物有很大的不同，并且对于广泛的材料显示出令人惊讶的普遍性。Tunnel Level Systems（TLS）模型可以成功地描述低于1 K的这种行为，但产生TLS的物理机制尚未确定。非晶硅是硅中的一个独特体系吗？的四面体键预测排除TLS的存在，但这些状态的密度被发现不同的数量级，从密度通常发现在玻璃中几乎无法检测到的水平。光浸泡可以增加TLS的密度，TLS的存在与非晶硅太阳能电池效率随时间的降低有关。在这个项目中，我们将利用独特的能力来调整TLS在非晶硅中的密度，无论是通过制备条件还是光浸泡，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热导率，并寻找与非晶基质中局部键合变化的相关性。识别的机制，产生的TLS有一个直接的影响，量子信息处理的TLS引起量子位的退相干，也使用非晶硅作为一种低成本的光伏material.Technical AbstrCTThis项目将调查的热力学性质的非晶气相沉积薄膜。无定形材料表现出一组特征性的热力学行为，这些行为与它们的结晶对应物有很大的不同。这种普遍的行为包括：低温性质（低于~ 1 K），由隧道能级态（TLS）模型解释;高温性质，包括过剩热容C（远高于声速计算），a？玻色子峰？以及热导率k的平台。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性能。悬挂键钝化与H将通过FTIR探测，这些结果将与EXAFS和XANES测量相结合，以了解无定形基质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MEMS纳米热量计进行。沉积，光浸泡，和退火的薄膜之间的比较，将进一步我们的理解的机制，引起光致退化的a-Si光伏器件的效率称为Staebler-Wronski效应。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接触国家实验室。