Increase of Hydrogen-Radical Density and Improvement of The Crystalline Volume Fraction of Microcrystalline Silicon Film
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INCREASE OF HYDROGEN-RADICAL DENSITY AND IMPROVEMENT OF THE CRYSTALLINE VOLUME FRACTION OF MICROCRYSTALLINE SILICON FILMS PREPARED BY HOT-WIRE ASSISTED PECVD METHOD Norimitsu Yoshida1, Takashi Itoh2, Hiroki Inouchi1 , Hidekuni Harada1, Katsuhiko Inagaki2, Noriyuki Yamana2, Kanta Yamamoto2, Shuichi Nonomura1, Shoji Nitta2 1. Environmental and Renewable Energy Systems Division, Graduate School of Engineering, 2. Department of Electrical and Electronic Engineering, Faculty of Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, JAPAN ABSTRACT Higher crystalline Si volume fractions in hydrogenated microcrystalline silicon ( µc-Si:H) films have been achieved by the hot-wire assisted plasma enhanced chemical vapor deposition (HWA-PECVD) method compared with those in films by conventional PECVD. µc-Si:H films can also be prepared by HWA-PECVD under typical conditions used for preparing hydrogenated amorphous silicon (a-Si:H) films by PECVD, in which the hydrogen-dilution ratio (H2 / SiH4) is ~ 10. The hot wire seems to produce hydrogen radicals. As a result, the HWAPECVD method can control hydrogen-radical densities in the RF plasma, and this method can also control the ratio of hydrogen coverage at the surface of the film.
INTRODUCTION The importance of the existence of excited hydrogen (hydrogen radicals) in the preparation process for µc-Si:H films has been emphasized in recent years [1]. For increasing the density of hydrogen radicals, VHF plasma CVD [1] is one of the key technologies. However, this technology requires complicated instrument action for preparing µc-Si:H films with a higher crystalline volume fraction at a lower hydrogen dilution ratio. We have previously suggested a new method for preparing µc-Si:H films called “hot-wire assisted PECVD (HWA-PECVD)” [2]. This method consists of two parts, PECVD and hot-wire (W filament) [2] for exciting only hydrogen. This technique has the advantage that the hydrogen-radical density is controllable without influencing the deposition conditions in PECVD. Using this method, the structure of µc-Si:H films was improved. That is, the crystalline volume fraction has been increased at a filament temperature of ~ 1650˚C and at a RF frequency of 13.56 MHz. In this paper, we have studied the filament-temperature dependence and the hydrogendilution-ratio dependence of the structure of µc-Si:H films prepared by HWA-PECVD. Comparing these results with those for films prepared by conventional PECVD, the role of the hydrogen radicals is discussed for enhanced crystallization of the film.
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EXPERIMENTAL DETAILS Figure 1 shows the set-up for HW A-PECVD [2]. The hydrogen inlet with the filament is attached ~ 10 cm away from the RF electrodes for PECVD. The silane inlet is placed near the substrate holder. This position of the silane inlet prevents deposition of µc-Si:H films by direct dissociation of silane at the filament. The filament was fabricated from W wire with a diameter of 0.25 mm coiled in a diameter of ~ 4 mm and a length of ~ 20 mm. The temperature of the fila
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