An Alternative Model for the Kinetics of Light-Induced Defects in A-Si:H
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AN ALTERNATIVE MODEL FOR THE KINETICS OF LIGHT-INDUCED DEFECTS IN A-Si:H Paulo V. Santos, W.B. Jackson and R.A. Street Xerox Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA 94304
ABSTRACT The kinetics of light-induced defect generation in a-Si:H was investigated over 6a wide range of illumination intensities and temperatures. The defect density around 101 cm-3 exhibits a power-law time dependence N, Gt' with e = 0.2 to 0.3, where G is the Gphoto-carrier generation rate. A model for the kinetics of defect generation is proposed based on the existence of an exponential distribution of defect formation energies in the amorphous network, associated with the valence band tail states. The model reproduces the observed time dependence of the defect density with an exponent c determined by the exponential width of the valence band tail. The temperature dependence of the defect generation rate is well-reproduced by the model, which provides a connection between the Stabler-Wronski effect and the weak-bond model.
INTRODUCTION Metastable defect generation induced by illumination (Stabler-Wronski effect[li ), current injection, or doping is a major obstacle for the application of hydrogenated amorphous silicon (a-Si:H) films in electronic devices. These defects have been associated with silicon dangling-bonds with electronic states deep in the gap of the material. Possible dangling-bond precursor sites are highly distorted Si-Si (or Si-H) bonds. These weak-bonds have a lower binding energy, and can be destabilized in the presence of excess carriers generated by illumination, current or doping. In a recent publication,[2] we demonstrated that the steady-state saturation density of light-induced defects can be explained in terms of a chemical equilibration process involving defects, photo-generated carriers, and weak-bonds with an exponential distribution of binding energies. The light intensity and the temperature dependence of the saturation density is well reproduced by assuming the weak-bonds to have the same distribution as that of the electronic states in the valence band tail. In this paper the model for the saturation density is extended in order to account for the kinetics of defect formation measured in the temperature range from 250K to 400K, and over a wide range of illumination intensities. The kinetics model has a dispersive character arising from the exponential distribution of weak-bond energies. A connection between this model, the np product kinetics proposed by Stutzmann et al. [3] and stretched-exponential type decay is also demonstrated. [4, 5]
RESULTS The samples used in this study are 1,um thick undoped-a-Si:H films grown by the glow discharge decomposition of pure silane. In the light-soaking experiments the light source was either a tungsten-halogen lamp with filters to cut wavelengths below 630 nm, or the 647.1 nm line of a Kr+ laser. Prior to each soaking experiment, the samples were annealed in the dark at 500 K for two hours, and then slowly cooled down (< 2K/min) to the soaking te
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