Development of p -type microcrystalline silicon carbon alloy films by the very high frequency plasma-enhanced chemical v
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We developed p-type c-silicon carbon alloy thin films by the very high frequency plasma-enhanced chemical vapour deposition technique using a SiH4, H2, CH4, and B2H6 gas mixture at low power (55 mW/cm2) and low substrate temperatures (150–250 °C). Effects of substrate temperature and plasma excitation frequency on the optoelectronic and structural properties of the films were studied. A film with conductivity 5.75 Scm−1 and 1.93 eV optical gap (E04) was obtained at a low substrate temperature of 200 °C using 63.75 MHz plasma frequency. The crystalline volume fractions of the films were estimated from the Raman spectra. We observed that crystallinity in silicon carbon alloy films depends critically on plasma excitation frequency. When higher power (117 mW/cm2) at 180 °C with 66 MHz frequency was applied, the deposition rate of the film increased to 50.7 Å/min without any significant change in optoelectronic properties.
I. INTRODUCTION
II. EXPERIMENTAL DETAILS
Very thin films of p-type c–Si:H have proven to be excellent window and barrier layers of solar cells because of their high conductivity and transparency.1 The deposition rate of c–Si:H prepared by the usual 13.56-MHz plasma-enhanced chemical vapor deposition (PECVD) technique is very low (5–10 Å/min). Recently the very high frequency (VHF) PECVD technique has become very important for the growth of good-quality microcrystalline material at high deposition rate.2 The p-type c– SiC:H films show higher optical gap than c–Si:H due to incorporation of carbon. So c–SiC:H films having higher optical transparency than c–Si:H material with high conductivity should be very useful materials as the window layer of p-i-n solar cells to improve the conversion efficiency. This material is also used in one or more internal tunnel junctions of multijunction solar cells.3 However, to date, only limited work on c–SiC:H films by the VHF-PECVD technique has been reported.4 Here an investigation on silicon–carbon alloy films prepared with PECVD in the frequency range 13.56–120 MHz (VHF) was made. Effect of substrate temperature on the nucleation process was also studied.
The samples were prepared in a capacitively coupled multichamber VHF-PECVD system decomposing silane (SiH4), methane (CH4), diborane (B2H6; 1% in hydrogen) and hydrogen (H2) gas mixture. Substrate temperature was varied from 150 to 250 °C and the plasma frequency from 13.56 to 120 MHz. The silane dilution in gas mixture [(SiH4 × 100)/(SiH4 + H2)%] was varied from 0.5% to 3%. The methane flow ratio[CH4/(SiH4 + CH4 + H2 + B2H6)] and diborane doping ratio [B2H6/ (SiH4 + CH4 + H2 + B2H6)] were fixed at 5 × 10−3 and 5 × 10−5, respectively. Most of the films were deposited at 55 mW/cm2. The chamber pressure was fixed at 300 mtorr. Dark and photoconductivity measurements were carried out in a gap cell configuration after annealing the sample in vacuum (approximately 10−6 torr) at 150 °C for about an hour. The photoconductivity was measured under white light intensity of 60 mW/cm2 using a tungsten halogen lamp.
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