Growth Chemistry of Nanocrystalline Si:H Films
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Growth Chemistry of Nanocrystalline Si:H Films Vikram L. Dalal, Kamal Muthukrishnan, Daniel Stieler and Max Noack Dept. of Electrical and Computer Engr. and Microelectronics Research Center Ames, Iowa 50011, USA ABSTRACT We report on the growth of nanocrystalline Si:H films using both plasma CVD and remote hot wire deposition under systematically varied growth conditions. The films were grown from mixtures of silane and hydrogen. It was found that when the films were grown under low pressure VHF plasma growth conditions , the orientation of the film changed as the pressure increased. At the lowest pressures, the films were mainly oriented, but changed to orientation as the pressure increased. The grain size increased as the growth temperature increased. When the films were grown using remote hot wire deposition, the orientation depended upon both hydrogen dilution and growth temperature. As the hydrogen dilution increased, the grain size became smaller. Grain size as large as 36 nm was obtained by controlling the growth conditions in hot wire deposition. As the growth temperature increased, the size of grains increased. Growth rates also increased with increasing temperature. The data can be explained by invoking a growth model which recognizes that the natural growth direction for Si is , since the surface energy is highest for (220) plane. Random nucleation leads to grains. Bonded H is believed to inhibit the growth of grains. INTRODUCTION Nanocrystalline Si:H [nc-Si:H] is an important electronic and optical material, widely used for solar cells, image sensors and thin film transistors [1-5]. There are several methods for depositing nc-Si:H, among the most popular being PECVD, either at 13.5 MHz or at higher frequencies (40-100 MHz) [1], ECR plasma CVD [6,7], and hot wire deposition [8,9]. Most of the groups in the world use VHF plasma deposition, since it produces a plasma with lower energy ions. The films are typically grown under high hydrogen dilution (20:1 or greater), with lower dilutions leading to mixed phase or amorphous materials [1]. Most of the films deposited using VHF plasma have predominantly orientation, though under very high hydrogen dilution, the film structure changes to primarily orientation [1]. Typical grain sizes are in the 10-15 nm range. The films typically show a strong Raman peak at 520 cm-1, with an amorphous shoulder in the 480 cm-1 range. In recent work [10], we have shown that the crystallinity of the film is a strong function of pressure in a VHF plasma reactor operated at 45 MHz. We showed there that the films grown primarily at low pressures (50 mTorr) required much less hydrogen for crystallization than films grown under higher pressure conditions. In this paper, we extend that work to examine the crystallinity and grain orientation as functions of pressure in the reactor, growth temperature and hydrogen dilutions. We also examine films grown using a remote hot wire deposition technique, where the hot filament is 11 cm away from the substrate, and therefore, substrate h
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