Defect Relaxation in a-Si:H Studied by Defect Absorption and Luminescence
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DEFECT RELAXATION IN a-Si:H STUDIED BY DEFECT ABSORPTION AND LUMINESCENCE
DAXING HAN, University of North Carolina at Chapel Hill, Department of Physics & Astronomy, Chapel Hill, NC 27599-3255; YANG XIAO, University of Colorado at Boulder, Department of ECE, Boulder, CO 803090425
ABSTRACT The experimental results about defect relaxation in a-Si:H have been reported recently 1 on several transient measurements, such as forward-bias-transient-current, optical bias 2 3 In this paper we report the effect on electron drift, and photo-capacitance transient. steady state results of the bias effect on sub-band-gap absorption and photon-emission spectra measurements in undoped samples. The optical bias effects the sub-band absorption. In additional to an enhancement of the sub-band gap absorption, the absorption threshold moved from 0.8 eV to 0.6 eV for undoped samples under optical bias at room temperature. The effects are larger in the lightsoaked-state than in the annealed-state. We also report on the generation rate dependence on photoluminescence (PL) spectra line shape. We found that the energy position of the defect PL band shifted from 1.1 eV to 0.9 eV when the excitation level increased 4,000 times. While motion of the quasi-Fermi level as a explanation cannot be eliminated, defect relaxation seems more plausible.
INTRODUCTION An important issue regarding the electron transport mechanism is the nature of the localized states in hydrogenated amorphous silicon. A common feature of the localized states is exponentially distributed tail states and continuously distributed gap states. For an undoped sample, the tail states originate from potential fluctuation, and the gap states are 0 the uncoordinated neutral silicon dangling bond states D as well as their charged states D-, D+. 4,5 However, in the inferred density of gap state distribution (DOS) experimental results have depended on the technique of the measurements, the environment of the 5 7 samples and the method used to analyze the data. - Consequently, the energy distribution and the nature of the gap states have remained unclear. Recent measurements on forward-bias-transient-current, 1 the optical bias effect on electron drift, 2 and on photo-capacitance-transient, 3 have shown that the results can not be simply explained by a motion of the quasi-Fermi level position in a one electron energy picture. Defect-relaxation takes place under non-equilibrium carrier excitation altering the simple picture. This means that the defect states are left in different metastable configurations upon bias, which then slowly relax to a equilibrium configuration after bias shut off. 1,2 Further, the results from transient capacitance indicates that the electron thermal emission time from a deep state is proportional to the electron residence time in the 3 defect. These new results suggest that the common picture of the defect states with a fixed energy position in the gap is invalid. In this paper we report the steady state results of the bias effect on sub-band-gap absorption
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