Study on Structural, Electrical and Optical Properties of Microcrystalline Si:H and SiC:H Films
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STUDY ON STRUCTURAL, ELECTRICAL AND OPTICAL PROPERTIES OF MICROCRYSTALLINE Si:H AND SiC:H FILMS F.Demichelis,G.Crovini,C.F.Pirri,E.Tresso, Dip. Fisica Politecnico Torino (ITALY) A.Rubino, G.Nobile, E.Terzini, G.Conte, ENEA - Portici, Napoli (ITALY) ABSTRACT We report results on a study on jic-Si:H and gtc-SiC:H films deposited by PECVD. The crystallinity fraction and the crystal sizes have been evaluated by X-ray diffractometry, Raman spectroscopy and Transmission Electron Microscopy(TEM). Infrared vibrational spectra of both jW-Si:H and jic-SiC:H samples have been studied to obtain information on their structure. A comparison between the structure of the amorphous and diphasic amorphous-microcrystalline samples has been performed. Optical properties were obtained by transmission-reflectance and PDS measurements. Electronic transport mechanisms through the conductivity measurements in a wide range of temperatures (50-500 K) have been defined. Structural and electron density models have been discussed and used to interpret the experimental results.
INTRODUCTION Like amorphous hydrogenated silicon, a-Si:H, microcrystalline hydrogenated silicon, gic-Si:H, appears to be a promising material for use in electronic devices requiring large sizes such as solar cells, display devices and sensors (1-3). At the same time there has been much interest on amorphous silicon carbon, a-SiC:H, and microclystalline silicon carbon, lic-SiC:H, alloys: since their optical gap can be increased till to 3 eV, they can be used as window material in many optoelectronic applications (4-9). Both types of materials can be deposited at relatively low temperature with the same deposition techniques: high dilution of silane and methane in hydrogen along with a high discharge power are considered to be the conditions to produce microcrystalline films using Plasma Enhanced Chemical Vapour Deposition (PECVD) technique.
EXPERIMENTAL We deposited a-Si:H, g.c-Si:H, a-SiC:H and gtc-SiC:H films by Plasma Enhanced Chemical Vapour Deposition (PECVD) technique by diluting the silane or silane-methane mixtures with hydrogen. Typical deposition conditions were: substrate temperature = 200°C, power density = 0.3 W/cm-, total pressure = 40 Pa, H, flow rate = 200 sccm, SiH 4 flow rate = 3-8 sccm and CH4 flow rate = 0 - 2 sccm. Elemental composition, Raman spectroscopy, X-ray diffractometry, TEM and high resolution TEM (HRTEM) have been performed to define the degree of crystallinity (7). Infrared (IR) transmission measurements over the region 400 to 4000 cm' were made relative to uncoated silicon substrate as reference on a Perkin-Elmer FT-IR 2000. The optical absorption coefficients in the range 1-3 eV were obtained from photothermal deflection spectroscopy (PDS) combined with transmittance and reflectance measurements. The temperature dependence of dark conductivity was performed in the range 50-500 K under a vacuum < 104 Pa by means of a Keithley 617 Electrometer. The ohmicity of contacts in the range ±1000 V/cm was verified. The temperature of the films was
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