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Determination of the Mobile-Hydrogen Charge State in Hydrogenated Amorphous Silicon Brent P. Nelson, Yueqin Xu, Robert C. Reedy, Richard S. Crandall, A. Harv Mahan, and Howard M. Branz National Renewable Energy Laboratory (NREL), 1617 Cole Blvd., Golden, CO 80401, USA ABSTRACT We find that hydrogen diffuses as H+, H0, or H- in hydrogenated amorphous silicon depending on its location within the i-layer of a p-i-n device. We annealed a set of five p-i-n devices, each with a thin deuterium-doped layer at a different location in the i-layer, and observed the D-diffusion using secondary ion mass spectrometry (SIMS). When H-diffuses in a charged state, electric fields in the device strongly influence the direction and distance of diffusion. When D is incorporated into a device near the p-layer, almost all of the D-diffusion occurs as D+, and when the D is incorporated near the n-layer, most of the D-diffusion occurs as D-. We correlate the preferential direction of D-motion at given depth within the i-layer, with the local Fermi level (as calculated by solar cell simulations), to empirically determine an effective correlation energy for mobile-H electronic transitions of 0.39 ± 0.1 eV. Using this procedure, the best fit to the data produces a disorder broadening of the transition levels of ~0.25 eV. The midpoint between the H0/+ and the H0/- transition levels is ~0.20 ± 0.05 eV above midgap. INTRODUCTION Hydrogen is needed to passivate dangling bonds and make hydrogenated amorphous silicon (a-Si:H) a viable thin-film semiconductor for use in photovoltaics, thin-film electronics, detectors and other devices. Typical H-contents of a-Si:H are between 5 and 15 at.%. Since a-Si:H is grown at elevated temperatures (typically over 180°C), there is considerable Hdiffusion during film growth. Further, recombination-induced H-motion at room temperature is often implicated as a contributing factor in the increase in defects in a-Si:H with illumination or carrier injection [1]. It is not entirely clear how H moves in a-Si:H after it is released from a tightly bound, neutral, Si-H configuration. In crystalline silicon (c-Si) it is proposed that, depending on the doping, H0 and H+ can diffuse by moving from one bond-center site to another, while H- will diffuse from one interstitial site to another [2]. Because devices made from a-Si:H have large electric fields (≥ 105 V/cm), these fields would have significant effects on H-motion if the H moves in a charged state (or with an accompanying charged dangling bond). In a-Si:H, Biswas et al. [3] proposed that hydrogen diffuses by breaking a Si-Si bond, forming a Si-H bond and a threefold-coordinated Si dangling bond (DB). The mobile H breaks successive Si-Si bonds as it migrates. The previously occupied Si-Si site rebonds while a new Si-H/DB complex forms on the next site. Thus, H moving through a-Si:H is accompanied by a DB. These authors did not consider the charge state of the mobile H/DB complex. There is long-standing experimental evidence for H+ [4,5] and H- [6,7] motion in doped c-Si