Localized Electronic States in Amorphous Semiconductors

这项研究是一个关于无序半导体的电子和结构性质的现有项目的继续和重新定向。一个主要的重点是光变暗效应，这是在高于光学带隙能量的光激励下，光吸收边亚稳态地向较低的能量移动。这个项目的主要目标是开发一份详细的显微描述，描述在光变暗过程中砷和硫化物位置发生的变化。第二个推力涉及非晶态半导体中光激发的电子和空穴的衰变，这种衰变发生在低温和光激发停止后的很长一段时间。这些问题将通过熔体淬火、溅射和PECVD生长技术、磁共振光谱(ESR、核磁共振和NQR)光学光谱、光学和磁共振技术的各种组合、应用于特定硫化物玻璃系统的电学和声学测量来解决。这项工作的具体任务是：了解硫化物玻璃光暗前后的局域结构有序的细节；确定硫化物原子对光激发电子和空穴衰变的影响，这些电子和空穴似乎在四面体配位的非晶态半导体中普遍存在；在纳米尺度上检查部分晶化和各向异性硫化物玻璃；利用声速和衰减法研究刚性渗流阈值；研究硫化物玻璃从低平均配位数到高平均配位数时的本征缺陷；以及研究分子氢所起的作用以及与IV族硫化物玻璃亚稳态的可能关系。这项工作将与学生和博士后研究人员一起进行。%非晶态、微晶态和纳米晶态半导体目前正在或正在考虑用于许多应用，包括平板显示器、红外光纤和探测器以及太阳能电池。尽管有这些成功的应用，但很少有通用性质来提供对这些材料的一般理论理解。这项研究集中于研究某些类型的玻璃的电和光激发如何在很长一段时间内衰减。这些研究的目标是发展对这些过程和相关过程的原子/分子水平的理解。这项研究将与学生和博士后研究助理一起进行，从而为在工业、政府和学术界就业做好准备。

This research is a continuation and redirection of an existing project on the electronic and structural properties of disordered semiconductors. One major emphasis is the photodarkening effect, which is a metastable shift of the optical absorption edge to lower energies after optical excitation at energies greater than the energy of the optical band gap. The primary goal of this project is to develop a detailed microscopic description of the changes that occur at arsenic and chalcogen sites on photodarkening. A second thrust concerns the decay of optically excited electrons and holes in amorphous semiconductors that occurs at low temperatures and at long times after cessation of the optical excitation. These problems will be attacked by means of melt quenching, sputtering and PECVD growth techniques, magnetic resonance spectroscopy (ESR, NMR, and NQR) optical spectroscopies, various combinations of optical and magnetic resonance techniques, electrical and acoustic measurements applied to specifically selected chalcogenide glass systems. The specific tasks of this work are: to develop an understanding of the details of the local structural order before and after photodarkening in chalcogenide glasses; to ascertain the influence of chalcogen atoms on the decay of photo-excited electrons and holes that appears to be universal in the tetrahedrally-coordinated amorphous semiconductors; to examine partially crystallized and anisotropic chalcogenide glasses on a nanometer scale; to investigate the rigidity percolation threshold using sound velocity and attenuation methods; to study the intrinsic defects in chalcogenide glasses as the structure goes from low average to high average coordination number; and to study the role played by molecular hydrogen and the possible relationship with metastabilities in group IV-chalcogenide glasses. This work will be performed with students and postdoctoral research associates.%%%Amorphous, microcrystalline and nanocrystalline semiconductors are currently used, or are contemplated for use, in many applications, including flatpanel displays, infrared optical fibers and detectors, and solar cells. In spite of these successful applications there are very few universal properties to provide a general theoretical understanding of these materials. This research is focused on the investigation of how electrical and optical excitations of certain types of glasses decay over a long period of time. The goal of these studies is the development of an atomic/molecular level of understanding of these and related processes. This research will be performed with students and postdoctoral research associates who will thereby be prepared for employment in industry, government and academe.***