Low Temperature Kinetics for the Growth and Decay of Band-Tallcarriers and Dangling Bonds in Hydrogenated Amorphous Sili
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ABSTRACT In hydrogenated amorphous silicon (a-Si:H), the kinetics of the light induced production of silicon dangling bonds and long-lived band-tail electrons and holes has been measured at temperatures between 65 and 340 K using light induced electron spin resonance (LESR). Below about 150 K the measurement of Si dangling bonds is masked by the accumulation of long-lived band-tail carriers. The kinetics of the growth and decay of these long-lived, trapped band-tail carriers consists of very fast components (- < ms) and very long components (- > h). Optical quenching of these long-lived carriers is not efficient at quenching energies of 0.6 eV. Afler removal of these long-lived band tail carriers by annealing at about 250 K we find that the total production of silicon dangling bonds at 65 K after 10 h of illumination is about a factor of five less than at 340 K. The dangling bond production resulting from 10 h of illumination is well fit to an underlying mechanism that, if thermally activated, exhibits an activation energy of approximately 10 meV. INTRODUCTION The most common metastability in the electronic properties of hydrogenated amorphous silicon (a-Si:H) is the Staebler-Wronski (S-W) effect [1], which is a decrease in both the photo- and dark conductivities after excitation with light of energy close to the optical bandgap. Most models that attempt to explain this effect attribute the decreases in conductivity to a metastable increase in the density of silicon dangling bonds. For this reason it is important to understand the kinetics for the production of silicon dangling bonds. Photoconductivity experiments [2,3] have recently shown that the S-W effect is only weakly dependent on temperature between 4 K and 300 K. To explain this very weak temperature dependence a photo-enhanced diffusion process for hydrogen in a-Si:H has been proposed [4]. In this paper we show that the production of silicon dangling bonds, as measured by electron spin resonance (ESR), is much less efficient at 65 K than at 300 K and that the ESR measurements below about 100 K are complicated by the presence of long-lived, optically excited carriers trapped in localized band-tail states. At temperatures below about 100 K, previous measurements have shown that the lifetimes of optically excited carriers vary by several orders of magnitude [5,6]. Some carriers recombine rapidly (
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