Recombination Process in the as-Deposited State of Hydrogenated Amorphous Silicon
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RECOMBINATION PROCESS IN THE AS-DEPOSITED STATE OF HYDROGENATED AMORPHOUS SILICON JONG-HWAN YOON Department of Physics, College of Natural Science, Kangwon National University,Chunchon, Kangwon-Do 200-701, Korea ABSTRACT Intrinsic deep defect-related recombination process has been studied in a series of undoped hydrogenated amorphous silicon(a-Si:H) films grown under different deposition conditions. Steady-state photoconductivity (oph) was measured as a function of deep defect density Nd, Urbach energy Eu, and dark Fermi energy Ef. It was found that 0 ph strongly depends on these parameters while Ef stays at the energy levels lower than 0.82 eV below Ec, but it is nearly independent of those while Ef stays at above 0.82 eV. These behaviors were found to be independent of the sample deposition conditions. These results indicates that subgap defect states enclosed by E=0.82 eV and Ef are the dominant recombination centers. INTRODUCTION Several works have shown that spin-active dangling bond defects with g=2.005 are the most likely recombination centers. Street[ 11and Voget-Grote et al.121 have studied the spin density dependence of aph in a-Si:H films with a wide range of spin density Ns, which is induced by varying the deposition conditions and the electron irradiation of different doses, respectively. They observed that for the high defect samples above Ns= IXl017 cm- 3 , 0 ph decreases with Ns. Dersch et al.[3] have suggested the recombination process controlled by spin-dependent tunneling and diffusion of localized band-tail electrons and holes in the samples with a high and low defect density, respectively. On the other hand, there are also many experimental and theoretical evidences that spin-active defects are not the unique recombination centers in determining the recombination process. Previously referred works [1,2] have also shown that Oph appears to be nearly independent of N. in the range below Ns= lx 1017 cm- 3 . This indicates that there must be a second type of recombination center, nonparamagnetic in nature. Guha et al.141 have suggested that recombination at a continuous distribution of states located between the trap-quasi-Fermi levels, caused by other defects together with spin-active dangling bonds, can give a better explanation of many experimental data. Qiu et al.[5] suggest that light-induced degradation of gt- product arises from the creation of new recombination centers near the dark Fermi level with a larger capture cross section than that of neutral dangling bonds. Therefore, the nature of the recombination centers and the recombination process in a-Si:H are still a matter of considerable uncertainty. In this work the effects of total intrinsic deep defect density, Urbach energy and dark Fermi energy on recombination have been studied in undoped a-Si:H. Mat. Res. Soc. Symp. Proc. Vol. 297. ©1993 Materials Research Society
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EXPERIMENTAL The a-Si:H films used in this study were deposited onto Coming 7059 glass substrates in a dc glow-discharge reactor. Two groups of samples were prepar
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