Light-Intensity Dependence of the Staebler-Wronski Effect in a-Si:H with Various Densities of Defects
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A7.1.1
Light-Intensity Dependence of the Staebler-Wronski Effect in a-Si:H with Various Densities of Defects Minoru Kumeda1, Ryohei Sakai1, Akiharu Morimoto1, and Tatsuo Shimizu2, 1 Division of Electrical Engineering and Computer Science, Graduate School of Natural Science & Technology, Kanazawa University, Kanazawa 920-8667, Japan. 2 NTT Microsystem Integration Labs, Atsugi 243-0198 Japan. ABSTRACT The light-intensity dependence of the photocreation of dangling bonds (DBs) were investigated for a-Si:H films with increasing density of defects before light soaking. Samples in which the density of neutral DBs had been increased by annealing at 400 C for 1 h exhibited a weak light-intensity dependence of the photocreated DBs compared to that for the as-deposited sample. Furthermore, the sample which had been illuminated with a light intensity of 1 W/cm2 for 1 h also showed a weak dependence. The results can be qualitatively explained by using rate equations for the densities of DBs and floating bonds (FBs) based on the FB-mediated photocreation of DBs. When both the densities of DBs and FBs before illumination increase, the light-intensity dependence of the DB density for a moderate value of the illumination time becomes weaker, qualitatively consistent with the observed results. INTRODUCTION The mechanism of the Staebler-Wronski effect in a-Si:H has not yet been clear, despite many efforts for a long time[1]. According to Stutzmann et al.[2], the density of photocreated dangling bonds (DBs) obeys the following relation with r=2 in device quality a-Si:H films, Ns(t)3 - Ns(0)3 = AGrt.
(1)
Here, Ns(t) and Ns(0) are the DB densities for illumination-time t and before illumination, respectively, A is a constant, and G is the light intensity. They deduced this equation based on the model in which weak bonds are broken by phonons created by the nonradiative recombination of the photoexcited electrons and holes. Since this work, the illumination-time dependence has been reported by many authors for various a-Si:H films. However, the experimental studies on the light-intensity dependence are less than the illumination-time dependence. Recently, we reported that the value of r becomes smaller with increasing the N content in a-Si-N:H alloy films[3]. It is known that the density of charged DBs is increased prominently by adding N atoms in a-Si:H besides an increase in the density of neutral DBs[4]. The fact implies that the large change of r in a-Si-N:H alloy films might be due to the presence of the charged DBs. In order to clarify the role of the charged dangling bonds in the Staebler-Wronski effect, we investigated in detail the light-intensity dependence in a-Si:H films with increased densities of neutral DBs. Annealing above 350 C is known to make hydrogen atoms effuse out from
A7.1.2
a-Si:H and to increase the density of neutral DBs. Photodegradation is another way to increase the density of neutral DBs with no appreciable change in the density of charged DBs[5]. Hence the light-intensity dependence of the photocreat
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