High rate deposition of cluster-suppressed amorphous silicon films deposited using a multi-hollow discharge plasma CVD
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1210-Q07-10
High rate deposition of cluster-suppressed amorphous silicon films deposited using a multi-hollow discharge plasma CVD
Kazunori Koga, Hiroshi Sato, Yuuki Kawashima, William M. Nakamura, and Masaharu Shiratani Department of Electronics, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, JAPAN
ABSTRACT We have examined effects of gas velocity and gas pressure on a deposition rate of hydrogenated amorphous silicon (a-Si:H) films and on a volume fraction of clusters in the films using a multi-hollow discharge plasma CVD method. The maximum deposition rate realized for each pressure exponentially increases with decreasing the pressure from 1.0 Torr to 0.1 Torr, whereas the volume fraction of clusters very slightly increases with increasing the deposition rate. Based on the results, we have succeeded in depositing highly stable a-Si:H films of 4.9x1015cm-3 in a stabilized defect density at a rate of 3.0nm/s using the method.
INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) is most widely employed as a top cell material of thin film Si tandem solar cells. Because light induced defects in a-Si:H films limits performance of a-Si:H devices, light induced degradation represents a key issue for their applications such as the tandem solar cells [1,2]. In SiH4 discharges which are commonly used for a-Si:H film depositions, there coexist three deposition precursors: SiH3 radicals, higher-order silane (HOS) related radicals in a size range below 0.5 nm, and amorphous nanoparticles (clusters) in a size range between 0.5nm and 10 nm [3-6]. SiH3 radicals are the main deposition precursor for high quality films. HOS related radicals and clusters are incorporated into the films during the deposition and may degrade film qualities. In our recent studies, a-Si:H films with a less volume fraction of clusters in the films have been found to show less light induced degradation [5,7]. We have succeeded in depositing highly stable a-Si:H films using a multihollow discharge plasma CVD method by which the volume fraction of clusters in films deposited in the upstream region is significantly reduced by driving clusters towards the downstream region. One important remaining issue is that the deposition rate is a rather low rate of 0.12 nm/s compared to a goal value of 2.0 nm/s [8]. For the conventional parallel plate discharges, films deposited at a higher rate shows severe light induced degradation. There is a trade-off between the deposition rate and their light-induced degradation. In this paper, we report dependence of deposition rate and the volume fraction on gas velocity as a parameter of gas pressure using the multi-hollow discharge plasma CVD method. We also report deposition of highly stable a-Si:H films of 4.9x1015cm-3 at a rate of 3.0nm/s using the method.
EXPERIMENTAL Experiments were carried out using a multi-hollow discharge plasma CVD reactor as shown in Fig. 1. Three electrodes, each of which had 24 holes of 5 mm in diameter, were placed 2 mm apart in a stainless steel tube of 60 mm in inner diameter
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