Relaxation phenomena in evaporated amorphous silicon films
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Relaxation phenomena in evaporated amorphous silicon films R. R. De Avillez,a) L. A. Clevenger, and C. V. Thompson Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received 21 March 1989; accepted 7 June 1989) Power compensated differential scanning calorimetry, transmission electron microscopy, and density measurements were used to characterize relaxation phenomena associated with structural transitions in electron beam evaporated amorphous silicon thin films. Two transitions were observed in the amorphous films upon heating: a broad exothermic process below 600 K and an endothermic step beginning at 600 K. The broad exothermic process is due to the relief of bond distortions. The endothermic step is related to homogenization and densification of the structure of the amorphous silicon. Amorphous silicon (a-Si) films have been extensively investigated due to their important electronic properties.1 Nonetheless, the structure of the a-Si films and its effect on their physical and electronic properties are not yet fully understood. The density of a-Si films depends both on the preparation procedure and the previous thermal history,1 which implies that the atomic structure is not unique. Indeed, both short-range and intermediate-range ordering may affect the density and other properties. Short-range ordering is known to be present in a—Si films and is related to bond angle distortions,2'3 which have been recently measured by Raman spectroscopy.3 These distortions are capable of storing elastic energy, which may be observed as heat release in a calorimetric experiment. A broad exothermic process which started at about 570 K has already been observed in sputtered a—Si films through differential scanning calorimetry,4 but was not present in silicon layers amorphized by ion bombardment and studied using the same technique by Donovan et al.5 Nonetheless, Roorka and Sinke have also recently found a broad exothermic process in a-Si films6 amorphized by ion bombardment. These differences point to the importance of the preparation conditions. Intermediate-range ordering is also possible, but still lacks strong experimental evidence.7 In this communication we present data obtained by differential scanning calorimetry (DSC) for evaporated a-Si films. Our data seem to support the presence of both bond angle distortions and intermediate-range ordering. Free-standing a-Si thin films were used for calorimetry and transmission electron microscopy measurements. These films were prepared by first coating microscope slides with Shipley 1400-25 photoresist to a thickness of 1 Im. The slides were then post baked at 90 °C for 25 min and loaded into an electron beam evaporator. Silicon was "'Permanent address: Departmento de Ciencia dos materials e Metalurgia, Pontificia Universidade Catolica, 22452-Rio de Janeiro, RJ-Brazil. J. Mater. Res., Vol. 4, No. 5, Sep/Oct 1989
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then evaporated at either 0.2 nm/s or 0.3 nm/s
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