Diffusion and Solubility of Hydrogen in Amorphous and Microcrystalline Si:H Films
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Diffusion and Solubility of Hydrogen in Amorphous and Microcrystalline Si:H Films Wolfhard Beyer Institut für Photovoltaik, Forschungszentrum Jülich, D-52425 Jülich, Germany ABSTRACT Hydrogen diffusion and solubility effects in hydrogenated amorphous and microcrystalline silicon films are reviewed. Various diffusion-related effects have been observed which need to be considered in models of hydrogen diffusion. Hydrogen solubility is found to affect hydrogen incorporation and hydrogen transport. INTRODUCTION Since the discovery of hydrogen’s role of defect compensation in amorphous silicon and related materials in the early seventies, its incorporation and stability has been the topic of intensive research. Still, our present understanding of hydrogen incorporation and diffusion processes in hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon (µc-Si:H) is far from being complete. Reason is, in part, that a-Si:H and µc-Si:H, depending on preparation conditions, are materials of greatly varying structural properties. Moreover, various H incorporation and diffusion-related processes have been discovered in recent time, like H solubility effects and the dependence of hydrogen diffusion on time, H concentration, the diffusion source, dopants and illumination, enhancing the complexity of the problem. Intention of the present article is to review several of these effects aiming to provide a picture of our present state of knowledge. We focus on the experimental techniques of infrared (IR) absorption, H effusion and H diffusion measurements by secondary ion mass spectrometry (SIMS), which, besides nuclear magnetic resonance (NMR), have been some of the most successful characterization methods for hydrogen incorporation and stability. HYDROGEN BONDING As recognized as early as 1974, the presence of Si-H vibrations in IR spectra of a-Si:H demonstrates the presence of hydrogen in this material in bonds with silicon. While the incorporation of unbonded atomic H in Si:H materials is considered unlikely due to its strong reactivity, the incorporation of H2 is conceivable [1]. Early NMR [2] and calorimetric measurements [3, 4, 5] of H2 incorporation in plasma-grown a-Si:H suggested, however, that the amount of H2 present in device-grade a-Si:H of TS =250°C is less than 2 % of the total hydrogen content, decreasing to less than 1 % for material deposited at 25°C. The concentration of molecular H was found to increase with annealing up to T =500°C, pointing to a slow diffusion [5]. After annealing at 400°C (10 min), about 4 % of the total hydrogen concentration was reported to be H2 [3], while others reported 5 % after annealing at 500°C [4]. These H2 concentrations were considered to be contained in microvoids [3,4,5]. More recently, much higher amounts of molecular H (up to 40% of the total hydrogen content) have been reported from NMR experiments [6,7]. Independent measurements on such samples by other techniques like Raman or the calorimetric method would be useful. While IR absorption measurements are rather ins
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