Localized Electronic States in Amorphous Semiconductors

University Of Utah, Salt Lake City UT

Investigators

Abstract

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. ***