Model of Hydrogen-Mediated Metastable Changes in a Two-Domain Amorphous Silicon Network
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Model of Hydrogen-Mediated Metastable Changes in a Two-Domain Amorphous Silicon Network Jonathan Baugh and Daxing Han Dept. of Physics and Astronomy, University of North Carolina at Chapel Hill Chapel Hill, NC 27599-3255 ABSTRACT A phenomenological model for the light-induced metastability of a-Si:H is proposed in which a two-domain model of the amorphous network plays a central role. Boundaries between high and low density domains are associated with a significant fraction of the clustered Si-H in aSi:H. Weakly bonded hydrogen at these boundaries catalyzes metastable local configuration changes in the Si network due to non-radiative carrier recombination, leading qualitatively to both the gross structural changes and the increase in electronic defect density that are observed experimentally. INTRODUCTION Device-quality hydrogenated amorphous silicon is known to be inhomogeneous on the nanometer scale. Silicon network inhomogeneity has been inferred from 29Si [1] and 1H nuclear magnetic resonance (NMR) [1, 2, 3] studies, atomic-force microscopy [4], observation of potential fluctuations [5] and recently by variable-coherence TEM [6]. On the other hand, 1H NMR [2, 3, 7] and small-angle neutron scattering (SANS) [8, 9] have clarified much about the scale and nature of the inhomogeneous hydrogen distribution. Recently, the electronic metastability (SWE) was found experimentally to correlate closely with global structural changes that also occur under the condition of high concentration of non-equilibrium carriers, namely a volume expansion that saturates on the order of 10-5–10-6 and is recovered by annealing [10]. In this work, we explore the possibility that the fundamental inhomogeneity of the Si network may act in concert with non-radiative carrier recombination and incorporated hydrogen to lead to a structural metastability that gives rise to a volume expansion. The key concept is that if there coexist relatively high- and low-order domains with sufficiently sharp boundaries that are host to weakly-bound hydrogens, then such boundaries will be unstable and may move slightly to favor higher entropy (when absorbing phonon energy due to carrier recombination) or oppositely to favor lower total configurational energy (in the case of annealing). In fact, the observed volume expansion would require exceedingly small average motion of such boundaries, whereas several reported experimental results should lead us to naturally expect such motion, given the existence of such domains and boundaries. MODEL 1
H NMR studies have concluded that there are significant volume fractions in a-Si:H that contain little or no hydrogen [2, 3], typically 10-20% for PE-CVD material. Recently, it was found that this H-free volume fraction is ~70% in at least some a-Si:H deposited by hot-wire (HW) CVD [3], which is about 2% more dense than the conventional PE-CVD a-Si:H with A19.1.1
negligible void concentration. If we assume that such H-free regions in both materials comprise high-order domains with density ρhigh, and the remainder is a l
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