Dependence of Thermally Induced Metastability on Hydrogen Content in Amorphous Silicon
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DEPENDENCE OF THERMALLY INDUCED METASTABILITY ON HYDROGEN CONTENT IN AMORPHOUS SILICON.
STANISLAW M. PIETRUSZKO Warsaw University of Technology, Institute of Microelectronics and Optoelectronics, IMiO PW, Koszykowa 75, 00-662 Warsaw, Poland. ABSTRACT The thermally induced metastability was observed in unhydrogenated doped amorphous silicon prepred by Low Pressure Chemical Vapour Deposition (LPCVD). The equilibration temperature (T*) was found to be 3800± 200C. The preliminary study of the influence of hydrogen content on thermally induced metastability in amorphous silicon is reported. In the hydrogenated doped films the equilibration temperature changes from 3800 ± 200C to 1700 ± 100C, as H content increases to 12 at.%. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) and its alloys are used in many large area thin film semiconductors applications such as solar cells, thin film transistors, and other devices. However, the usefulness of these materials is limited by light degradation effect known also as Staebler-Wronski effect [1]. This reversible effect can also be introduced by carrier injection or thermal quenching and has been observed in doped amorphous silicon films [2]. It has been suggested that hydrogen plays central role in these metastable behavior [3, 4]. Most of hydrogenated amorphous silicon is deposited by glow discharge decomposition of silane and other hydrogen-bearing precursor gases. Therefore it is difficult to obtain a-Si:H films with low hydrogen content. However, low hydrogen content films can be deposited by physical vapor deposition techniques but such films often have more structural defects and are less suitable for careful studies. Above mentioned reasons make it difficult to observe metastable effects infilms with very low hydrogen content and only such changes in highly doped a-Si:H have been studied. To overcome this difficulty, I studied the thermally induced metastability in asgrown and doped by ion implantation unhydrogenated a-Si deposited at high temperatures by LPCVD technique. Such a high-temperature-deposited material may be well reconstructed and constituted a prototype of "perfect" a-Si. LPCVD a-Si has very low hydrogen content and post-hydrogenation by ion implantation allows to introduce low and high hydrogen doses with high accuracy to films after deposition. This makes possible the independent studies of the effect of hydrogenation that one can apply to investigations of metastability in a-Si. Branz and Iwaniczko [5] have demonstrated that percent level of H bonded to Si is not necessary to enable thermally induced metastability in P-doped a-Si, but they have not rule it out, either. They observed the quenched-in dark conductivity metastability in sputtered, P-implanted, a-Si with a total H content of only 0.1 at.% and found that T* in their films was 3550 ± 200C and suggested this result provides qualitative support for the glass-H model of metastability [14]. Also, Mahan and Mat. Res. Soc. Symp. Proc. Vol. 297. ©1993 Materials Research Society
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Vanecek [6]
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