Kinetics of Hydrogen Evolution and Crystallization in Hydrogenated Amorphous Silicon Films Studied by Thermal Analysis a
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+ Also with Department of Physics and Astronomy ABSTRACT We observed the processes of hydrogen evolution and crystallization in hydrogenated amorphous silicon 0.5-7 pm thick films (deposited by dc glow discharge on molybdenum) by differential scanning calorimetry (DSC), Raman scattering and thermogravimetric analysis (TGA). Investigation was made as a function of doping, deposition temperature and film thickness. For all the films, an endothermic DSC peak was observed at 694 oC (onset). That this peak was at least partly due to hydrogen evolution was shown by TGA, which showed weight loss beginning at 694 oC, and by evolved gas analysis, which showed hydrogen evolution at 694 °C. This temperature (658-704 OC) increased with increasing heating rate (5-30 OC/min). Doping reduced this temperature from 694 to 625 oC for boron doping and to 675 oC for phosphorous doping. Hydrogen evolution kinetics and FTIR results suggest that the siliconhydrogen bonding in the intrinsic film was a mixture of SiH and SiH 2 , and was predominantly SiH in the phosphorous doped films and SiH 2 in the boron doped films. Crystallization was independent of silicon-hydrogen bonding in the as-deposited amorphous silicon film. It was bulk (not interface) induced. No exothermic DSC peak accompanied the crystallization. The film
deposition temperature had little effect on the DSC result, but crystallization was enhanced by a higher deposition temperature. INTRODUCTION Amorphous silicon is a potential candidate for solar cell applications [1, 2]. It is well known that the presence of hydrogen in amorphous silicon, known as hydrogenated amorphous silicon (a-Si:H), is important to achieve a material with a low density of electron states within the energy band gap. This is due to the saturation of dangling bonds in the presence of hydrogen. However, a-Si:H solar cells suffer from degradation of solar cell efficiency due to light exposure [3]. On the other hand, such a stability problem does not occur inpolycrystalline silicon solar cells. It has been reported that polycrystalline silicon obtained by the crystallization of amorphous silicon has better structural and electrical properties than that produced by direct deposition [4, 5]. The structure of amorphous silicon is greatly dependent on the deposition technique and conditions [6-8]. Among these techniques, a-Si:H produced by the glow discharge technique shows better electrical characteristics and hence this is the technique which can produce device quality films. To our knowledge, there has been no systematic work done on the influence of hydrogen in glow discharge deposited a-Si:H films on the crystallization behavior. It is known that hydrogen evolution takes place on heating a-Si:H, as detected by mass spectrometry [9, 10]. However, this technique cannot detect physical and chemical processes taking place during the thermal treatment of a-Si:H, such as bond breaking and crystallization. Differential scanning 713 Mat. Res. Soc. Symp. Proc. Vol. 321. ©1994 Materials Research Society
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