The Properties of a-SiC:H and a-SiGe:H Films Deposited by 55 kHz PECVD
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ABSTRACT The deposition processes and the properties of a-SiC:H and a-SiGe:H films in 55 kHz glow discharge were investigated. The analysis of deposition rate and RBS measurements showed that the chemical reactions between SiHn spices and CH 4 control the incorporation of C in a-SiC:H films. High deposition rates of a-SiC:H and a-SiGe:H films fabricated by 55 kHz PECVD is caused by the increase of radical fluxes to the growth surface. The specific features of a-SiC:H and a-SiGe:H microstructure were revealed by IR and AFM analysis. In a-SiC:H films the islands of low size were distinguished on the surfaces of large islands. The large variation of the total hydrogen content in a-SiGe:H did not affect the optical bandgap, while the hydrogen related microstructure controlled the electronic properties such as dark conductivity, 1lp.r product, defect density and Urbach slope. The results of optoelectronic properties and SW effect measurements of 55 kHz a-SiC:H and a-SiGe:H films demonstrated the increased stability in comparison with a-Si:H. INTRODUCTION The a-SiC:H and a-SiGe:H films are currently widely used as a high and a low bandgap optoelectronic materials for a tandem and a triplet solar cell fabrication. The use of these materials increases the utilization of solar spectrum and stabilized efficiency of solar cells, which is the key problem of these devices. However, the properties of these materials are highly sensitive to the fabrication technology. The main drawbacks of a-SiC:H alloys are a low deposition rate (-3A/c) and a complex microstructure highly sensitive to deposition condition. For a-SiGe:H alloys the deterioration of electronic properties with the addition of Ge to a-Si:H is observed. It is not now understood yet whether this deterioration of electronic properties is due to the inherent property of this material, or it is the consequence of imperfect technology. In this work we investigated the deposition and the properties of a-SiC:H and a-SiGe:H alloys fabricated by 55 kHz PECVD at the different amount of the methane and germane content in a gas mixture. EXPERIMENT The a-SiC:H and a-SiGe:H films were fabricated by 55 kHz PECVD [1,2]. For deposition of a-SiC:H and a-SiGe:H with various composition the gas mixtures with different contents of methane Rc=100.[CH4]/[SiH 4+CH4]%, and germane Rc•=100.[GeH4]/[SiH 4+GeH 4]%, were used (see Table 1). The substrate temperature, the LF power and the total gas pressure were kept at constant values of 320 0 C, 200W and 90 Pa, and 2250 C, 150W and 70 Pa for the deposition of a-SiC:H and a-SiGe:H films, respectively. The films were deposited on c-Si and Coming 7059 glass substrates. 43 Mat. Res. Soc. Symp. Proc. Vol. 557 © 1999 Materials Research Society
Table 1. The deposition rate of a-SiC:H and a-SiGe:H films fabricated by 55 kHz PECVD at different gas mixture compositions. Sample, N YC5 17 YC5,23
YC5 31 YC5 41
....
Rc, %
Deposition
Sample,
Ro , %
Deposition
. 0 20
rate, A/s. 14.4 11.1
N YG6 12 YG6 23
0 9.1
rate, A/see 11.1 8.9
10.0 8.9
YG6 33
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