Light Intensity Exponents as Sensitive Tools for the Detection of Impurities in a-Si:H
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Light Intensity Exponents as Sensitive Tools for the Detection of Impurities in a-Si:H L. F. Fonseca, S.Z. Weisz, P. Alpuim1, V. Chu1, J.P. Conde2, R. Naides3 and I. Balberg3 Department of Physics, University of Puerto Rico, San Juan 00931, PR 1 Instituta de Engenharia (INESC), Rua Alves Redol 9, 1000 Lisbon, Portugal 2 Department of Materials Engineering, Instituto Superior Technico, 1048-001 Lisbon, Portugal 3 Racah Institute of Physics, The Hebrew University, Jerusalem 91904, Israel ABSTRACT We have shown recently that the temperature dependence of the phototransport properties can yield information regarding the state distribution in the forbidden gap of semiconductors. Of these properties the light intensity exponents of both, the majority carriers, γe, and the minority carriers, γh, were found to be very sensitive to the details of this distribution. In particular, noting that sub ½ values of the exponents are very unusual we have studied their origin in some a-Si:H materials. Finding experimentally such sub ½ values of γh and running computer simulations of the recombination processes in a-Si:H led us to the conclusion that these low values are due to acceptor-like centers which have a relatively high capture coefficient for the holes. We attribute these centers to the unintentional oxygen doping in a-Si:H. We will show that the oxygen presence, usually ignored in the discussions of the phototransport properties of a-Si:H, appears to be, in many cases, the dominant factor in the properties of “intrinsic” a-Si:H. INTRODUCTION Twenty years ago Paesler et al have pointed out the importance of the presence of oxygen in hydrogenated amorphous silicon, a-Si:H, attributing various properties of a-Si:H to this “story of O” [1]. While many studies were concerned with the effects of the presence of oxygen [2] in a-Si:H, very few of them were concerned with the effect of this presence on the transport and steady state phototransport properties [3]. Rather, the widly accepted relevant models suggested for a-Si:H, have interpreted the corresponding experimental data only in terms of the relative contributions of the well known “intrinsic” defect states of tetrahedraly bonded disordered semiconductors. These are the band-tail states and the dangling-bond states. In particular, we do not know of a work that pointed out the dominance of the oxygen unintentional doping on the phototransport properties of a-Si:H. In this paper we would like to show that ignoring the above role of oxygen is usually unjustified and that unintentional oxygen doping plays frequently a dominant role in the phototransport properties. In particular, we provide quite a convincing proof that the “safe hole traps”, found a decade ago by McMahon and Crandall [4], are oxygen related centers such as negatively charged oxygen ions [5] or oxygen induced acceptor-like centers [3]. The tool that we applied for the derivation of the above conclusions is the temperature dependence of the following four phototransport properties [6-9]: the mobility-lifetime produc
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