Effect of Temperature and Temperature Uniformity on Plasma and Device Stability
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A10.4.1
EFFECT OF TEMPERATURE AND TEMPERATURE UNIFORMITY ON PLASMA AND DEVICE STABILITY G. Ganguly, M.S. Bennett, D.E. Carlson and R.R. Arya BP Solar, Toano, Virginia ABSTRACT We have investigated the changes in the cathode potential in a dc discharge of silane and hydrogen used to deposit the intrinsic layer of p-i-n type solar cells at deposition rates from 1 to 10Å/s with the superstrate temperature at 200°C and 250°C. Under plasma conditions that lead to higher deposition rates (5-10Å/s), fluctuations of the cathode potential which are suggestive of the formation and de-trapping of particulates in/from the plasma, are observed at 200°C but disappear at 250°C. Improvement of the temperature uniformity over the plasma region from 1.7°C/cm to 0.7°C/cm removes the fluctuations of the cathode potential even at 200°C, indicating that the particulates are formed predominantly at the plasma boundary. Consequently, the stability of solar cells with i-layers deposited at ~10Å/s in the center of the plasma region at the same superstrate temperature improved by 26% suggesting that multiple silicon containing molecules diffuse from the edge to the center of the plasma region. INTRODUCTION The deposition rate of the amorphous silicon (a-Si:H) film continues to limit the throughput of photovoltaic module production plants [1]. It is well known that the stability of a-Si:H films and solar cells deposited at higher growth rates improve with increasing temperature [2-4]. Initially, it was assumed that increasing the temperature of the growth surface enhanced surface mobility of growth precursors, which resulted in an improved structure and hence more stable material [2,3]. However, it has been shown that the gas phase temperature can have a significant effect on the electronic properties of a-Si:H films, which was understood in terms of the thermal activation of the growth precursors [5,6]. Recently, the electron temperature in a silane plasma was observed to decrease with both increasing substrate temperature and hydrogen dilution of silane [7]. This reduction in electron temperature appears to reduce the incidence of multiple silicon containing molecules, which have been associated with light induced degradation [8]. Complementarily, these molecules are believed to evolve into particulates, when the number of silicon atoms increases to five or more [9], and their formation rate has been observed to decrease at higher gas temperatures [10]. We have found that the potential on the cathode in a dc silane gas discharge can be understood in terms of negative ion formation that alters the plasma impedance due to the lower mobility of ions relative to electrons [11]. Here, we report that the cathode potential variation suggests particulate formation under plasma conditions that lead to higher deposition rates. We show that increasing the superstrate temperature from 200 to 250°C suppresses this behavior. Further, we find that improvement of the temperature uniformity over the plasma region alleviates the instability even at 200°C, an
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