Thermodynamics and Kinetics of Hydrogen Evolution in Hydrogenated Amorphous Silicon Films
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ABSTRACT The enthalpy (endothermic) of hydrogen evolution from p-type (boron doped) amorphous silicon with 17 at. % H was 4.8, 10.3, 15.8 and 17.3 kJ/g, the evolution temperature was 585, 606, 625 and 644 °C and the entropy of evolution was 5.6, 11.7, 17.5 and 18.9 J/g.K at heating rates of 5, 10, 20 and 30 OC/min respectively. That the enthalpy and entropy increased with heating rate means that the evolution involves not only Si-H bond breaking, but also Si-Si bond breaking and other defect formation. The Si-Si bond breaking and defect formation were enhanced at high heating rates, which caused high rates of hydrogen evolution. For n-type (phosphorous-doped) and intrinsic amorphous silicon with 25 and 23 at. % H respectively, the enthalpy and entropy of hydrogen evolution were higher than the p-type case, due to severe defect formation resulting from the higher hydrogen content. The activation energy of hydrogen evolution was 1.38, 2.5 and 4 kJ/g for the p-type, intrinsic and n-type materials respectively. Crystallization which occurred at temperatures higher than hydrogen evolution, was delayed for the amorphous silicon film in a higher disordered state after hydrogen evolution, suggesting that hydrogen evolution influenced the crystallization process. INTRODUCTION Amorphous silicon is a potential candidate for solar cell [1, 2] and thin film transistor applications [3]. 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. It is also known that hydrogen evolution takes place prior to solid phase crystallization on heating a-Si:-, as detected by mass spectrometry [4, 5]. Investigation of the hydrogen evolution process is of great scientific interest for the purpose of understanding the aSi:H structure (silicon-hydrogen bonding). Previous workers studied the kinetics of this process to determine the nature of silicon-hydrogen bonding in a-Si:H [6 - 8]. There had been little attention on the effect of hydrogen evolution on the thermodynamic state and on crystallization, although numerous studies had been conducted on the solid phase crystallization of amorphous silicon (with and without hydrogen) [9 - 11]. Furthermore, no previous report had been made on the thermodynamics of the hydrogen evolution process. In this work, by studying the thermodynamics (enthalpy and entropy changes) of the hydrogen evolution process, the thermodynamic state after hydrogen evolution was investigated. The enthalpy and entropy changes, in conjunction with the kinetics (activation energy) of the hydrogen evolution process further gave information on the thermodynamic state before hydrogen evolution. The effects of heating rate, hydrogen content and dopant type on these 319 Mat. Res. Soc. Symp. Proc. Vol. 377 0 1995 Materials Research Society
states were also addressed. The enthalpy and entropy chan
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