Defect States in Hydrogenated Amorphous Silicon-Sulphur Alloys by ESR and PAS
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DEFECT STATES IN HYDROGENATED AMORPHOUS SILICON-SULPHUR ALLOYS BY ESR AND PAS Gaorong Han, Jianmin Oiao, Piyi Du, Zhonghua Jiang and Zishang Ding Dept. of Materials Science and Engineering, Zhejiang University, Hangzhou, China ABSTRACT We have presented ESR and PAS measurements for a series of a-SiS:H and a-Si: H films deposited by glow discharge at different parameters. The spin density in a-SiS:H alloys measured by ESR is essentially independent of the sulphur content, while the density of defects measured by PAS increases significantly with the increasing of sulphur content. The ESR signals in a-SiS:H alloys strongly depend on both annealing and illumination. The spin density increases up to 540 0 C and then decreases with raising annealing temperature for a-SiS:H and a-Si:H alloys. The results suggest that some new defects such as molecular hydrogen and microvoids are appeared when addition of sulphur to a-Si:H films. INTRODUCTION Silicon-based alloys have been extensively studied in recent years because their some properties are superior to those of a conventional a-Si:H film (1 ) . Hydrogenated amorphous silicon-sulphur alloys (a-SiS:H) were found to be a candidate for photoconductive materials in image devices and solar cells (2,3) . In general, new defect states are created in silicon-based alloys. Electron Spin Resonance (ESR) is one of the most useful techniques to study defect states with spin in the amorphous materials (4 ) . So far many ESR studies have been done for a-Si:H films, and only a few reports on ESR studies exist for siliconbased alloys. On the other hand, subgap absorption spectra determined by photoacoustic spectroscopy (PAS) involve an important information on the energy and number of defect states (5,6) . In this paper, we give the detailed information about defect states in aSiS:H films by ESR and PAS studies. For comparison, similar studies of sulphurfree a-Si:H film have been done. EXPERIMENTALS The a-SiS:H films were prepared by glow discharge decomposition of gasous mixture of SiH4 , H2 S and Hz in a capacitatively coupled system. The typical deposition conditions are listed as followings: deposition power, 20watts; reaction pressure, 2.Storr; substrate temperature, 235%; total flow rate, 3. 9SCCM; and reactive gas composition, HzS/(HzS+SiH 4+Hz), lOVol.%. The composition and structural properties of the samples were characterized by AES, IR and XPS. For the ESR studies the films were deposited on Al foils, peeled off in HCl and collected in glass tubes, this procedure led to ESR samples which contained about 3 mg material from one deposition. The samples were annealed in a vacuum of 10' torr for 45 minutes at required temperature to investigate the effect of annealing on defect states, and illuminated with 50mw/cm2 white light from a tungsten lamp to study the effect of illumination on defect states. The ESR measurements at X-band were performed with a JES-FX1XG spectrometer with 100KHz field modulation at room temperature. For the PAS studies the films were deposited on silica gla
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