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The effect of stress, resulting from the presence of hydrogen, on the aluminum-induced crystallization of hydrogenated amorphous silicon films was studied. Layered thin films of hydrogenated and unhydrogenated amorphous silicon and aluminum, deposited by sputtering, were used to study this effect. The stress of the deposited films was determined by measuring the radius of curvature of c-Si substrates before and after deposition of the films. It was observed that unhydrogenated amorphous silicon films exhibit a high compressive stress compared with hydrogenated ones. The amount of stress is shown to decrease with increasing hydrogen content. It was also observed that aluminum always provides tensile stress. After the initial stress measurements, all the samples were annealed for 30 min at temperatures between 200 °C and 400 °C. X-ray diffraction was used to determine the crystallinity of the silicon films. The results of the study show that the temperature at which crystallization of amorphous silicon is initiated is lower for films with a lower initial stress. I. INTRODUCTION

Polycrystalline silicon (pc-Si) thin film solar cells and thin film transistors have been the focus of research and development for the past several years. Part of the intense interest in pc-Si is fueled by the fact that pc-Si is one of the most promising materials for economically viable thin film solar cells.1 To use pc-Si to fabricate low-cost solar cells, low processing temperatures are required because low processing temperatures allow the use of lowcost glass or plastic substrates. Pc-Si thin films can be fabricated by the crystallization of amorphous silicon by different methods. Solid phase crystallization of amorphous silicon (a-Si) is accomplished by annealing for 24 h at 600 °C. However, with the help of a suitable metal, the crystallization temperature can be reduced dramatically. When the crystallization process is aided by intimate contact with a metal, it is referred to as metal-induced crystallization of a-Si. When aluminum is the metal, it is referred to as aluminum-induced crystallization (AIC) of a-Si, and the crystallization process can be accomplished at temperatures as low as 150 °C.2 Recently, a significant amount of research has been

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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0318 2582 J. Mater. Res., Vol. 21, No. 10, Oct 2006 http://journals.cambridge.org Downloaded: 17 Mar 2015

focused on AIC using different a-Si deposition techniques. Among the techniques, plasma-enhanced chemical vapor deposition (PECVD) has been used extensively.2–7 Likewise, a number of researchers have studied sputter-deposited a-Si.7–10 However, little research has been reported on the effects of stress on the AIC of a-Si. Ching-Ming et al.11 studied the effect of stress on the AIC of a-Si by varying the thickness ratio of a-Si to Al in an a-Si/Al/glass structure. They reported that maximum crystallization occurs when the stress level is minimal. All of their films were annealed at a temp