Photodegradation in a-Si:H Prepared by Hot-Wire CVD as a Function of Substrate and Filament Temperatures
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Photodegradation in a-Si:H Prepared by Hot-Wire CVD as a Function of Substrate and Filament Temperatures Daxing Han*, Guozhen Yue, Jing Lin, Hitoe Habuchi1, Eugene Iwaniczko2, and Qi Wang2 Dept of Phys & Astronomy, Univ of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3255, USA. 1Gifu National College of Technology, Sinsei-cho, Motosu-gun, Gifu, 501-04, Japan 2National Renewable Energy Laboratory, Golden, CO 80401, USA. ABSTRACT We have studied light-soaking effects, such as photoconductivity (PC) degradation kinetics, the changes of conductivity activation energy, Ea, and the defect density of states (DOS) in aSi:H films deposited by hot-wire CVD. Films were deposited in a substrate temperature range from 280 to 440 oC for filament temperatures of 1900 and 2100 oC. We find that (a) the photodegradation kinetics does not follow the stretched exponential rule for all of the samples; (b) the Fermi level position moves up after light-soaking for most samples; and (c) the metastable defect DOS deduced from sub-band gap absorption is not consistent with that deduced from the electron mobility-lifetime product. The results are discussed according to the possible mechanism in which charged defects exist in hot-wire a-Si:H films. INTRODUCTION Light-induced metastable defect creation (SWE) in a-Si:H has been studied for more than two decades[1-3]. A number of observations have shown that the kinetics for the light-induced PC degradation obeys a stretched exponential rule[2,4]. A weak-bond breaking model explained this degradation by bimolecular recombination of carriers[2], in which the PC is inversely proportional to the density of neutral dangling bonds (DB). The previous studies have been done in intrinsic a-Si:H films prepared by glow-discharge (GD) CVD. The material stability has been improved by using new deposition techniques such as hot-wire (HW) CVD [5]. The HW material is attractive because of the high deposition rate and some "magic" properties [5-7] as compared to the GD materials. For device-quality HW a-Si:H, only 2-3 at.% H is needed, compared to 810 at.% in GD samples. Furthermore, an NMR study suggests that most H atoms are highly clustered in relatively small volume fractions in HW films [6]. In these low H-content HW films, it was found that the internal friction is close to c-Si but much smaller than that in GD films [7]. These results along with the high mass density indicate that the HW a-Si:H network has better overall four-fold coordination. The improved structural ordering could be a key factor resulting in the improvement of electronic stability [8]. However, there are still some photodegradation effects in the HW films and their solar cells. We have searched for the origin of the metastability and tried to optimize deposition conditions. The purpose of studying this series of samples is to determine at what condition the materials are "magic". We would like the "magic" to bring us low defect density and high stability material. In this work, we report the PC, activation energy and density of
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