High Quality Hydrogenated Amorphous Silicon Films with Significantly Improved Stability
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ABSTRACT High quality hydrogenated amorphous silicon (a-Si:H) films have been prepared by a simple "uninterrupted growth/annealing" plasma enhanced chemical vapor deposition (PECVD) technique, combined with a subtle boron-compensated doping. These a-Si:H films possess a high photosensitivity over 106, and exhibit no degradation in photoconductivity and a low light-induced defect density after prolonged illumination. The central idea is to control the growth conditions adjacent to the critical point of phase transition from amorphous to crystalline state, and yet to locate the Fermi level close to the midgap. Our results show that the improved stability and photosensitivity of a-Si:H films prepared by this method can be mainly attributed to the formation of a more robust network structure and reduction in the precursors density of light-induced metastable defects. INTRODUCTION The light-induced degradation of a-Si:H upon light-soaking makes a big obstacle toward a fiuitful application of the material. This effect was first discovered by Staebler and Wronski in 1977 (known as the S-W effect)[1], and since then, intensive research has been carried out to elucidate its mechanism. In the meantime, some of the progresses have also been made in the preparation of high quality and "stable" a-Si:H. Hajime Shiai et al [2]reported improved stability of a-Si:H with a rigid network, prepared by a "chemical annealing" technique. Vikram L. Dalal et al [3] also obtained improved stability for a-Si:H lightly compensated by boron (B) using a remote-plasma electron-cyclotron-resonance (ECR) apparatus. Y. Lee et al [4] and L. Yang et al [5] adopted hydrogen dilution to prepare a-Si:H films and solar cells, reaching a higher stabilized efficiency in less than 100 hours of one-sun illumination. J. Yang et al [6] obtained 14.6% initial and 13.0% stable efficiencies using a-Si alloy grown under heavy hydrogen dilution. A. H. Mahan et al used a hot-wire CVD technique[7] to deposit intrinsic a-Si:H, showing a significant reduction in the light-induced defects, compared to the usual device-quality PECVD a-Si:H. We also used a layer by layer deposition technique, similar to the chemical annealing, to prepare high quality a-Si:H films with markedly improved stability[8]. However, these techniques all have some shortcomings, such as complicated process or not high enough photosensitivity and stability, which are unsuitable for industrial applications. To overcome these difficulties, we have systematically investigated the effect of hydrogen dilution on the properties of a-Si:H films[9], and developed a simple "uninterrupted growth/annealing" PECVD technique, in which the growing process no longer needs to be interrupted, as does in the chemical annealing or layer by layer deposition, whereas the growing surface is continuously subjected to enhanced annealing treatment with atomic hydrogen produced in the highly hydrogen-diluted reactive gas. Meanwhile, very low levels of B-compensation was used to adjust the position of Fermi level (EF) close t
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