Sensitization of the Holes Lifetime by the Addition of Dangling Bonds in a-Si:H
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Sensitization of the Holes Lifetime by the Addition of Dangling Bonds in a-Si:H L.F. Fonseca, S. Z. Weisz, and I. Balberg1 Department of Physics, University of Puerto Rico, San Juan 00931, PR 1 The Racah Institute of Physics, The Hebrew University, Jerusalem 91904, Israel ABSTRACT This paper is concerned with the phenomenon of the increase of the holes lifetime with the increase of the dangling bond concentration in a-Si:H. This rather surprising phenomenon that was observed, but not discussed, previously is shown to be a non-trivial effect which is based on the charged nature of the dangling bonds and a special scenario of the concentrations of the various defect states in the material. The most important implication of our study is that the charged dangling bonds can sensitize the valence band tail states, in contrast with the accepted roles of these types of states. The present understanding suggests that many new interesting phototransport phenomena can be found in a-Si:H. INTRODUCTION It is generally accepted [1] that at room temperature the dangling bonds are the dominant recombination centers in intrinsic hydrogenated amorphous silicon (a-Si:H). Correspondingly, it was explicitly expected by many researchers [2-4] that the increase of the dangling bond concentration, Ndb, will yield a monotonic decrease of the mobility-lifetime (µτ) products of both, the majority (the electrons’ µeτe) and the minority (the holes’ µhτh) carriers. While this expectation is well known to be fulfilled by the electrons [1-8], our data [5,6], as well as data of others [7,8], have revealed an unexpected behavior of the holes. This behavior is manifested by a detactable maximum in the measured dependence of µhτh on Ndb in the range of 1016 ≤ Ndb ≤ 1017 cm-3. This maximum can be relatively large [8] or relatively small [5] or unobservable [24], suggesting that it has to do with the concentration and occupation of the various defect states. That this peak is not an experimental artifact, but rather a genuine effect can be concluded from its observation in the comprehensive computer simulations of Wang and Schwarz [7]. The model used for these simulations was the standard model of a-Si:H, in which well-accepted parameters for this material have been used. The behavior described above, for the two µτ products, as obtained in our simulations of the standard model (see below) is illustrated again in figure 1. The surprise arises from the fact that, a priori, the dangling bonds are expected [1,2,9,10] to result in a decrease of both µτ‘s with Ndb. Moreover, the basic physics of the sensitization effect implies that the increase of the lifetime of one charge carrier will be accompanied by the decrease of the lifetime of the other carrier [11,12]. However, in spite of this surprising behavior and its possible importance for photoelectronic devices based on a-Si:H [13], no discussion of this effect has been presented previously. In fact, this is the case even in the work in which it was found by both, experiments and simulations [7,8]. In this
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