Are There Charged Dangling Bonds in Device Quality Amorphous Silicon?
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ARE THERE CHARGED DANGLING BONDS IN DEVICE QUALITY AMORPHOUS SILICON? M.S. BRANDT*, A. ASANO**, and M. STUTZMANN*** "*XeroxPalo Alto Research Center, Palo Alto, CA 94304, USA **Fuji Electric Corp., Yokosuka, 240-01 Japan ***Max-Planck-Institut ffir Festk6rperforschung, D-7000 Stuttgart 80, Germany.
ABSTRACT We discuss the possible existence of a considerable density of charged dangling bond defects in device-quality hydrogenated amorphous silicon, which for example has been postulated by recent thermal equilibrium models for the density-of-states distribution. Based on a quantitative analysis of spin resonance and light-induced spin resonance data at different temperatures as well as on subgap absorption measurements, we conclude that intrinsic a-Si:H only has a small density of charged defects caused by unintentional impurity doping. The same conclusion also holds for light-soaked a-Si:H and for samples which are dehydrogenated by annealing at high temperatures. INTRODUCTION A fundamental question which has persisted in the physics of amorphous silicon for many years now is whether the most important structural defect in this material, the silicon dangling bond, mainly occurs as a neutral or as a charged state in intrinsic, undoped samples (Fig.1). Although there can be little doubt that the characteristic electron spin resonance center with a g-value of 2.0055 generally observed in undoped a-Si or a-Si:H is due to dangling bond defects with a positive correlation energy in the neutral charge state (DO) [1, 2], the simultaneous presence of charged dangling bond-like states with a much higher density has been postulated by many authors based on different arguments. Originally, charged dangling bonds (T+ and T3) were proposed in analogy to chalcogenide glasses as a result of a negative effective correlation energy caused by a large chargeinduced lattice relaxation of the defects.[3, 4] More recently, charged dangling bond defects in device-quality hydrogenated amorphous silicon have been postulated on the basis of the thermal equilibrium approach applied to defect states in the gap of this material.[6, 5, 7, 8] In particular the equilibrium theories make quantitative predictions for the relative densities of charged defect states versus neutral dangling bonds which, in principle, can be tested experimentally. The charged to neutral density ratios postulated in recent publications range from 2 to 10 for device quality a-Si:H [7, 8], and similar ratios are also expected for not-optimized undoped material. In addition to charged defects due to lattice relaxation and/or thermal equilibration, high quality a-Si:H also contains some charged defects due to impurity doping. In particular oxygen and nitrogen in the bulk, but also surface adsorbates can give rise to a shift of the Fermi-energy at least in part of the sample.[9, 10] In this case, the existence of dangling bonds is closely related to the position of the Fermi-level, as shown in Fig. 1(b). In this paper, we will discuss the possible existence of charged defects
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