Plasma Deposition and Characterization of Stable a-Si:H (Cl) From a Silane-Dichlorosilane Mixture
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reduction in the metastability observed when it is subjected to an excess carrier concentration are required properties of the material. Recent work has shown that the growth of a-Si:H(CI) films from gas mixtures of dichlorosilane (SiH 2CI 2 ) and SiH 4 by dc-triode plasma CVD at higher growth rate can be achieved, 1 without incurring the problems of powder formation usually associated with other methods employed to increase the growth rate. In addition, light soaking of films grown by an electron cyclotron generated 2 , 3 and PECVD 4, 5 with such gas mixtures have reported that the films are more stable against the creation of light induced defects. Therefore with the addition of SiH 2C12 to the SiH 4 the two requirements stated above are satisfied. It has also been pointed out that SiH 2 C12 is cheaper than SiH 4 which is therefore, an additional boon for utilizing such a source gas. In this paper, we report on the film properties in the as-deposited state and after light soaking by pulsed laser illumination and AMI illumination for 900 hours for films grown by PECVD with SiH 2 CI 2 concentrations up to 60 % in the SiH 4 . The films were characterized by optical reflection/transmission 113
Mat. Res. Soc. Symp. Proc. Vol. 377 01995 Materials Research Society
spectroscopy, infra-red transmission spectroscopy, and photo-conductivity, Raman spectroscopy, and sub-bandgap optical darkabsorption by the constant photocurrent method (CPM). EXPERIMENT The films were grown by PECVD in a conventional capacitively coupled deposition chamber equipped with two parallel electrodes 10 cm in diameter with a separation of 40 mm. Prior to deposition the chamber was degassed and evacuated to a base pressure less than 3x10-7 Torr by a turbomolecular-rotary pump combination. The source gases were mixed in the gas lines upstream from the chamber ensuring uniform mixing of the gases before entering the reaction region. The films reported here were deposited on Corning 7059 glass substrates and a highly polished, intrinsic crystalline silicon wafer to a thickness of 2 mm. The latter being used to determine the hydrogen content and hydrogen bonding configuration by infra-red (IR) spectroscopy. The substrates were placed on the grounded bottom electrode (anode) and an rf power of 2 W, and frequency of 13.56 MHz applied to the top electrode (cathode). The films were deposited at a constant pressure of 30 mTorr, a substrate temperature of 250 oc, and a constant total flow rate of 5 sccm. The flow rate of the SiH2C12 was varied between 0 and 3 sccm to give a concentration range between 0 and 60 % of the total flow rate. The glass substrates were sectioned and had aluminum electrodes thermally evaporated in a coplanar configuration (gap width = 0.2 mm, length -- 3 mm). Two such coplanar samples were prepared and the defect density determined by CPM in this as-prepared state by integration of the subbandgap absorption spectrum after subtracting the contribution of the exponential Urbach tail slope and using the conversion factor 1.9 x 1016 cm
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