Very Wide-Gap and Device-Quality a -Si:H from Highly H 2 Diluted SiH 4 Plasma Decomposed by High RF Power
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ABSTRACT The H2 dilution technique at a high deposition rate (R,) was investigated by depositing hydrogenated 2 amorphous silicon (a-Si:H) under a high if power density of 750 mW/cm , which is 20 times as large as that
of conventional conditions. It was found that the H2 dilution ratio 7 ( = [H2 gas flow rate] / [SiHl4 gas flow rate]) tendency of the film properties, such as the H content (CH), optical gap (EoQ), SiH 2 /SiH and photocon2 ductivity (ayph)of a-Si:H is different for the high rf power (750 mW/cm ) and the medium rf power (75 mW/ 2 cm ) conditions. Under medium rf power, the C H' E0 ptand SiH 2/SiH decrease as y increases. Under the high
if power, on the contrary, the CH and Eop, monotonously increase while maintaining a low SiHJSiH and a high 0ph of 10-6 S/cm as y increases. These results suggest that increasing the rf power enhances the H incorporation reactions due to H 2 dilution. It is thought that a high rf power causes the depletion of SiH 4 and hence the extinction of H radicals, expressed by SiHl4 + H* -> SiHl3* + H2, is suppressed. A high H radical density enhances the incorporation of H into a-Si:H, resulting in very wide-gap a-Si:H with a high CH 1 3 Consequently, very wide-gap a-Si:H with device-quality (EP of 1.82 eV with an (cthv) plot, corresponding to > 2.1 eV with Tauc's plot, and aph of 10-6 S/cm) can be obtained at a high RD of 12 A/s without carbon
alloying. INTRODUCTION 2 A stabilized conversion efficiency of 9.5% for a submodule (1200 cm ) with an a-Si/a-SiGe tandem cell
structure has been achieved [1]. To achieve further improvement in the conversion efficiency of solar cells, wide-gap, high-quality a-Si:H, which is suitable for the top i-layer of stacked cells and/or the p-layer, is eagerly awaited [2]. Hydrogenated amorphous silicon carbide (a-SiC:H) is a well-known wide-gap material, but increasing the C content of a-SiC:H to enhance the optical gap causes a decrease in the film quality of a-SiC:H, such as its conductivity, because the incorporation of carbon atoms distorts the silicon network [3]. On the other hand, it was reported that the H plasma treatment and H2 dilution are useful for preparing wide-gap, high-quality a-Si:H [4], except that the deposition rate decreases (< I A/s). In this study, a HI2dilution technique at a high deposition rate was investigated by preparing a-Si:H 2 under the high rf power density of 750 mW/cm , which is 20 times as large as that of conventional condi-
tions [4]. EXPERIMENTAL The a-Si:H films were prepared by using a capacitively coupled rf plasma CVD reactor on Coming #1737 glass substrates and crystalline silicon substrates. The H2 dilution ratio (7 =[H 2] / [SiHlj) was changed 145 Mal. Res. Soc. Symp. Proc. Vol. 557 © 1999 Materials Research Society
Table I. Deposition conditions for a-Si:H rf power2 Pressure (Pa)
Ts (°C)
(mW/cm )
High rf power 180 Medium rf power 180 Low rf power (Conventional) 80-180
750 75 10-30
H2 dilution ratio
1.7 x 102 1.7 x 102 1.3-2.7 x 10
0-25 0-25 0-50
2 from 0 to 25 at the rf power density o
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