Light-Induced Metastable Changes in a-SiS x :H
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S.L. WANG AND P.C. TAYLOR Department of Physics, University of Utah, Salt Lake City UT 84112
ABSTRACT Metastable light-induced increases in the dark conductivities of a-SiS,:H alloys are explained as photo-activation of hydrogen-passivated sulfur donor sites. For a sulfur concentration (sulfur-to-silicon ratio) of 5.6 x 10-3 the excess dark conductivity as a function of illumination temperature is thermally activated with an activation energy of 0.72 eV. When the sulfur concentration is 3.3 x 10-2, the temperature dependence is very weak. This dramatic difference in the temperature dependence of the creation of increased dark conductivity is explained by a lowering of the annealing temperature for the metastable changes as the sulfur concentration increases. We discuss the influence of this new metastability on the possibility of obtaining more stable films.
INTRODUCTION 1 Those metastabilities associated with the Staebler-Wronski effect continue to be some of the most important impediments to the use of hydrogenated amorphous silicon (a-Si:H) in photovoltaic devices. In spite of intense investigations over many years, the microscopic mechanisms for these metastabilities are still not well understood, 2 ' 3 ' 4' 5 ' 6'7 ' 8 although it has been known for many years from electron spin resonance (ESR) experiments 9 that there is an increase in the neutral silicon danging bond density after optical excitation. It is thought that these defects act as recombination centers that decrease both the photo- and dark conductivities and shift the Fermi level toward midgap. One of the remaining controversies is whether these effects are intrinsic or extrinsic.3'10,11 It has been suggested8 ,12 that the light-induced defects are different from those residual defects that 3 exist in the material as prepared. Even though these differences cannot be detected by ESR,1 there appears to be a change in the effectiveness of the recombination process with the light-induced defects. Recently we have reported 14 that sulfur acts as an inefficient n-type dopant in a-Si:H. In addition, some of the sulfur-related donors are passivated by hydrogen in the annealed state but can be activated optically. 14 This optical activation results in an increase in both the photo- and dark conductivities and in a movement of the Fermi level toward the conduction band edge. 15 This effect is sometimes called persistent photoconductivity (PPC). We note that PPC has the opposite effect on the photo- and dark conductivities from the ordinary Staebler-Wronski effect. Therefore, the appropriate, small concentration of sulfur produces a material where the optically induced changes in the photo- and dark conductivities effectively disappear because of the counter-balancing influences of the PPC and StaeblerWronski effects. In this paper we report the dependences of the PPC effect in S-doped a-Si:H on illumination time and illumination temperature.
EXPERIMENTAL DETAILS The a-SiS,:H samples were prepared by plasma enhanced chemical vapor deposition (PECVD) by dec
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