Effects of Excitation Frequency and H 2 Dilution on Cluster Generation in Silane High-Frequency Discharges
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A9.5.1
Effects of Excitation Frequency and H2 Dilution on Cluster Generation in Silane High-Frequency Discharges Masaharu Shiratani, Kazunori Koga, Atsushi Harikai, Takanori Ogata, and Yukio Watanabe Department of Electronics, Kyushu University, Fukuoka 812-8581, Japan
ABSTRACT Reduction of cluster amount in silane discharges is the key to decreasing microstructure parameter Rα of a-Si:H films deposited with the discharges. The cluster amount is found to be reduced more than one order of magnitude using 60 MHz discharges instead of 28 MHz ones or using H2 dilution of an H2/ SiH4 ratio of 5. The cluster-suppressed plasma CVD using 60 MHz discharges realizes deposition of a-Si:H films of Rα~ 0 at a fairly high rate of 0.55 nm/s. Moreover, a downstream cluster collection method of high sensitivity has been developed for detecting a small amount of clusters formed under deposition conditions of Rα< 0.01. INTRODUCTION Particles below a few nanometers in size (clusters) generated in silane high frequency discharges have been pointed out to be closely related to the light-induced defects in the hydrogenated amorphous silicon (a-Si:H) solar cell [1-4]. Previously, the growth kinetics of clusters has been studied by using two in-situ methods for detecting them: the double pulse discharge (DPD) method [4,5] and the photon counting laser light scattering (PCLLS) method [3, 5-10]. Based on the results obtained in those studies, we have developed a cluster-suppressed plasma CVD method utilizing gas viscous and thermophoretic forces exerted on clusters and have demonstrated deposition of a-Si:H films of Rα~ 0 [11], while a deposition rate of 0.2 nm/s is low compared to a goal value of 2.0 nm/s. The VHF discharge and H2 dilution have been conventionally employed to deposit films of device quality at a high rate. These facts motivate us to study effects of VHF discharge and H2 dilution on cluster generation in the discharge. Moreover, by using the cluster suppression method together with the PCLLS method, we have found that a reduction in cluster amount in the discharge brings about significant decrease of microstructure parameter Rα [11], which is related to light-induced degradation of a-Si:H films [1, 2]. Since clusters under deposition conditions of Rα< 0.01 are hardly detected by the PCLLS method, development of an alternative method of high sensitivity for detecting them is necessary for studying correlation between cluster amount and film quality in the regime of interest. In this paper, we will report on experimental results regarding the effects of VHF discharge and H2 dilution on cluster generation, which are obtained by using the DPD method, and describe a downstream cluster collection (DCC) method by which clusters can be detected even in a range of small cluster amount for Rα< 0.01 is satisfied. EXPERIMENTAL METHODS Experiments were carried out using two capacitively-coupled high frequency discharge
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