Structural and optoelectronic properties of amorphous and microcrystalline silicon deposited at low substrate temperatur
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EXPERIMENT
A. Film Preparation The films were deposited by HW-CVD and RF-PECVD in a UHV-quality system with a base pressure < 5 x 108 Torr. The sample was clamped to the grounded upper electrode which was, except for the room temperature depositions, heated to the temperature of deposition, Tsub. For the HW deposition, a single tungsten filament of 0.5 mm diameter and approximately 7 cm length was placed 5 cm from the substrate and was resistively heated with a DC power supply. The filament temperature was measured with an optical pyrometer (T5, - 2500 TC) and the pressure was kept constant at 20 mTorr. The details of the HW deposition system are described elsewhere [3]. For the RF deposition, the inter-electrode distance was 3 cm, the density of RF power used was 50 mW/cm 2 (in all but the room temperature depositions where it was 100 mW/cm2 ), and the pressure was 100 mTorr. Both in the HW and RF depositions the total gas flow was kept at around 10 seem, except for the higher dilutions where it was necessary to increase the fluxes so that the SiH 4 flux was not less than 0.5 sccm, which was the lower limit for the silane mass flow controller. B. Film Characterization. The dark conductivity ord was measured between 110 'C and room temperature on coplanar Cr contacts. The activation energy Ea was calculated from ad = co exp[- Ea /(k 5 T)]. The steadystate photoconductivity ujph was measured as a function of generation rate using bandpass-filtered illumination to give an approximately uniform carrier generation throughout the thickness of the film. In this work, up, refers to the photoconductivity at a carrier generation rate of 1021 cm-3 s-'. The constant photocurrent method, CPM, was used to measure the subgap absorption [8]. The deep defect density N, of the amorphous films was calculated from NS=CcPM x a (1.2 eV), with CCPM =1016 Cm-2 [9]. The hydrogen content CH and the microstructure factor R were determined using infrared spectroscopy. CH was calculated from the density of silicon atoms, Nsi= 5 x 1022 cm-', and the density of bonded hydrogen atoms, NH (CH = NH/ NH+NSj). NH was calculated from the integrated absorption coefficient of the Si-H wagging modes located around 630 cm-' [10]. R was calculated from the deconvolution of the stretching band into two peaks, one centered at approximately 2000 cm-' (I2000) and the other centered around 2100 cm- (I2,00), R =2oo( 12000+ 2,100)[10]. Raman spectra were measured in the backscattering geometry using a Raman microprobe. The power of the incident beam was set below 50 mW to avoid thermally induced crystallization. For microcrystalline films, the Raman spectrum around the crystalline silicon transverse optical (TO) peak was deconvoluted into their integrated crystalline Gaussian peak, 1, (. 520 cm 1) amorphous Gaussian peak, I. (- 480 cm -'), and intermediate Gaussian peak, Im ( 510 cm [11,12]. The crystalline fraction, Xc, was calculated from Xc = (I, + Im)/(I, + Im+ Ia)[13]. The crystalline size, dRanan, was calculated from dRaman = 27rjBl Aw, where Ac is the sh
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