Ion Irradiated Amorphous Silicon: A Model Approach to Dynamics of Defect Creation and Annihilation
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ION IRRADIATED AMORPHOUS SILICON: A MODEL APPROACH TO DYNAMICS OF
DEFECT CREATION AND ANNIHILATION
Jung H. Shin and Harry A. Atwater Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, CA 91125 ABSTRACT The dynamics of defect annihilation and creation in amorphous silicon (a-Si) are measured in detail using defect-related changes in the electrical conductivity of a-Si A model is proposed which for the first time can track the complete time evolution of defect population across the activation energy spectrum with explicit dependence on irradiation and annealing parameters. The model is based upon experimental activation energy spectrum, bimolecular recombination kinetics, and on the notion of a maximum density of defect states beyond which no additional defects can be supported. The new model predicts transient dynamics in defect population and describes structure of the defect population in detail. Its predictions are in good qualitative agreement, and in reasonable quantitative agreement with experimental data. Introduction The annihilation and injection of defects in a-Si, known as structural relation and unrelaxation, respectively, have been the subject of intensive studies recently. Relaxation is known to have profound effect on various properties of a-Si, e.g. stored enthalpy[1I, diffusivity of transition metals[2], lifetime of photocarriers[3], and electrical conductivity[4]. Relaxation has been associated with annihilation of point defects[5,6], and is generally thought to occur via bimolecular recombination of defects[U]. Relaxation is also a continuous process which continues until crystallization intervenes, and is characterized by a spectrum of activation energies. However, the total defect population is known to saturate approximately 1 at. %, at a dose of approximately 0.02 displacements per atom (dpa). Electron Transport in a-Si The Mott-Davis theory of variable range hopping[8] has been successful in explaining many aspects of electron transport in amorphous solids, and is the basis of analysis of experimental conductivity data described here. The theory postulates that the many defects inherent in an amorphous solid create localized electron states near the middle of the energy gap, which pin the Fermi level. Conduction at low and moderate temperatures then occurs through hopping (phonon-assisted tunneling) of electrons from one localized state to another, and follows the relation
=y
-2.
k
14(1)
where a4 is the decay constant of the electron wavefunction, and gFf is the density of electronic states near the Ferrmi level. If a reasonable estimate for a is made, then conductivity can be used as a very sensitive tool to study gEf. We assume a- I to be - lrnm[9]), Theory of activation energy spectrum As first treated by Narayanaswami[ 10] and further elaborated by Gibbs et. a.[ 11], the activation energy spectrum theory asserts that the activation energies of thermal processes in amorphous solids form a spectrum. In other words, there exists a density of
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