Electron Spin Resonance Studies of Amorphous Silicon
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ELECTRON SPIN RESONANCE STUDIES OF AMORPHOUS SILICON DAVID K. BIEGELSEN Xerox Palo Alto Research Centers, Palo Alto, CA
94304
ABSTRACT Electron spin resonance and related spin dependent measurements have been used to make key contributions to the understanding of amorphous silicon, specifically as probes of the dominant states in the gap, recombination processes and doping. In this paper we give a cursory description of the techniques as they apply to this problem. We then review what has been learned in aSi:H usually from coupled results of spin resonance and other complementary experimental techniques. The results lead us to a surprisingly simple picture of the equilibrium and non-equilibrium behavior of defects and carriers in this prototypical amorphous semiconductor. INTRODUCTION Amorphous semiconductors have been an area of technological and fundamental interest for many years. It is only recently, however, that theoretical and experimental results have been achieved which make the field tractable for study. Theoretical notions of charge localization, negative effective correlation energies, chemical short-range order, and characteristic defects in bonding have provided a basis for a microscopic description. In this paper we will consider only the tetrahedrally-bonded material system known as amorphous silicon, a-Si. In early work researchers had tried to grow thin films of pure aSi by evaporation or sputtering. A large electron spin density [1] of order 1020 cm"3 and high conductivity showed that this material was far from an ideal "continuous random network" semiconductor. In 1975 Spear and Le Comber [2] demonstrated that thin films of a-Si could be grown (by glow discharge deposition from a silane plasma) with a relatively low density of states in the gap. This then allowed them to dope the material and form p-n junctions. The technological promise of radiation-hard thin film transistors, large-area, inexpensive solar photovoltaic cells, photoreceptors, etc. was thus indicated. The states in the gap of a semiconductor are dominant in determining the electronic properties, e.g. trapping and recombination. In a-Si the intrinsic gap states are a result of disorder of either the weak bonds in the band tails, or bonding defects. In a-Si it has been found that the density of states can vary by over four orders of magnitude, depending on the deposition conditions. To ascertain the microscopic nature of a-Si (or more properly, a-Si:H, because hydrogen, on the order of 10% is necessary to reduce the gap state density) and to supply feedback in the optimization of deposition conditions, a probe sensitive to the localized gap states is necessary. In this article we try to show how resonance techniques have been and will be useful in the study of this important material system. Electron spin resonance (ESR) of paramagnetic centers (impurities, defects, conduction electrons) in crystalline materials has been a rich and, in many cases, unique source of information about the localization and site symmetry of the el
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