Stability Improvement of a-Si:H Films Deposited in Sqwm-55 kHz Glow Discharge Plasma
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modulation of low frequency (55kHz) glow discharge and the increase of a-Si:H films stability observed. To understand the origin of stability increase of SQWM 55 kHz a-Si:H films the analysis of growth mechanism, films microstructure and electronic properties were carried out. EXPERIMENT Samples of a-Si:H films were deposited by LF 55 kHz PECVD method [5, 6] from the pure silane (100%SiH 4) on quartz and p-type monocrystalline (c-Si) substrates. The gas pressure, the substrate temperature and the gas flow rate were maintained at 70 Pa, 225 0C and 200 sccm, respectively. Two regimes of discharge power were used: (i) continuous mode (cw) and (ii) Square-Wave Modulated (SQWM) plasma mode. In the case of SQWM the times of LF plasma switch on, To,, and switch off, Ton.,were monitored. The LF discharge power of SQWM plasma was varied from 200 to 600 W. The growth rates were determined from thickness measurements of deposited films which were carried out by means of interference microscope MII-4. The effective deposition rate, VDCf, which is the growth rate in the SQWM regime for an effective time, teff, during which the plasma was switched on, was calculated according to the equations: d (1) -Doff tde
tff
.T Ton td + Togf
n,
(2)
where d and td are the film thickness and the deposition time, respectively. The technological parameters of a-Si:H film deposition at continuos and modulated plasma regimes are presented in Table 1. For dark and photoconductivity measurements Al coplanar contacts separated by 0.6 mm interelectrode distance were deposited on the top of a-Si:H films on quartz substrates. The measurements were carried out in the temperature range of 20-2500 C and at applied electric field of -1OV/cm. For photoconductivity measurements He-Ne laser (k=0.633 jtm) with the illumination intensity of 1017 photon-cm 2 .sec1 was used. The simulation of photoconductivity data according to models [4] was carried out to estimate the density of defect states, ND, in aSi:H. The Staebler-Wronski effect was investigated by measurements of time dependence of photoconductivity under He-Ne laser illumination. The light-induced defect generation kinetics was measured at elevated temperatures of 40 to 80 °C during the illumination time of 8 hours. DSC measurements were performed on a "Du Pont" DSC 910 microcalorimeter in the temperature range from 20 0C to 5700 C at the heating rate of 10 °C/min [9]. Samples were prepared in the form of powder by mechanical scrapping the films off the substrates and then were located in a hermetically sealed aluminum ampoule. The surface morphology of a-Si:H films was investigated by P4-NT-MDT atomic force microscope using different AFM techniques [5, 6]. RESULTS The deposition rates, density of defect states, ND, and rl~tt product (where rl, 4 and -t are quantum efficiency, carrier drift mobility and life time, respectively) for a-Si:H films fabricated
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Table 1. The deposition rate, VD, the effective deposition rate, VD ff, the density of defect states, ND, and il-it product for a-Si:H fil
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