Diffusion of Hydrogen in Amorphous Silicon in the Low Concentration Regime
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DIFFUSION OF HYDROGEN IN AMORPHOUS SILICON IN THE LOW CONCENTRATION REGIME J. A. ROTH*, G. L. OLSON*, D. C. JACOBSON** AND J. M. POATE** Hughes Research Laboratories, Malibu, California 90265 **AT&T Bell Laboratories, Murray Hill, New Jersey 07974 ABSTRACT We report the first measurements of hydrogen diffusion kinetics in a-Si in the regime of low H concentration (< 2x10 19 cm-3 ). The results differ substantially from the diffusion behavior typically observed in hydrogenated a-Si:H at H concentrations >1020cm-3 . The activation energy and pre-exponential factor for low-concentration H diffusion are found to be 2.70 ± 0.02 eV and 2.2x10 4 cm 2 s- 1, respectively, and are shown to be independent of both annealing time and H concentration. It is difficult to reconcile the combination of high activation energy and large pre-exponential factor with a simple deep-trap-limited diffusion model. Consequently, an alternative mechanism for H diffusion involving the migration of dangling bonds coupled with a H bond-switching step is proposed. INTRODUCTION Studies of the diffusion of hydrogen in a-Si can provide valuable information on the nature and behavior of defects present in the material. Most measurements of H diffusion have been made in hydrogenated a-Si (a-Si:H), where the concentration of H is =1021 cm- 3 . The effect of primitive intrinsic defects such as isolated dangling bonds, which are present at much lower concentrations =1019cm-3 [11],is obscured in those measurements because the deep-lying intrinsic defects are totally occupied by H. In a-Si:H the defects that control diffusion have a H trapping energy of 1.5 eV [2], and are believed to be associated with multiple-H configurations on the surfaces of internal voids or on H platelets [3]. In order to gain information about the intrinsic defects in a-Si, it is therefore necessary to conduct experiments at low H concentrations comparable to or less than the relevant defect density. Current models of H diffusion in a-Si are based on the assumption that H diffuses readily through the interstices of the amorphous network until becoming trapped upon encountering a defect. The diffusion process can only continue after H is liberated from the trapping site and returned to an interstitial position. In this picture, the release is accomplished by thermal agitation, and the overall activation energy for diffusion is given by the energy to break the defect-H bond and place the H atom in a nearby interstitial site, plus the interstitial migration energy (0.45-0.5 eV [3]). Given the large de-trapping energy for H attached to an isolated dangling bond (e.g., 2.7-3.4 eV [4,5]), the diffusion coefficient predicted by the trap-limited diffusion model would be extremely small if the H concentration were low enough to ensure that only dangling bonds were occupied. In this letter we report on studies of H diffusion conducted at low H concentrations < 2x10 19 cm-3 , comparable to the dangling bond density, and we present the temperature dependence of the diffusion coefficient from 4
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