Low Hydrogen Content, High Quality Hydrogenated Amorphous Silicon Grown by Hot-Wire CVD
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Mat. Res. Soc. Symp. Proc. Vol. 557 © 1999 Materials Research Society
Resistive Heater Filament
Substrates +kubstrate
Shutter
Figure 1: The original HWCVD reaction chamber at NREL consisting of a standard 6-way, vacuum cross with 11.4 cm diameter flanges. In this figure the silane flow is into the page, perpendicular to the filament.
Heating Elements -Substrates
Silane Flow
-
Filament
Side View Figure 2:
Shutterament Power to Filament
End View
The isothermal HWCVD reaction chamber at NREL consisting of a 10 cm
diameter stainless steel tube, 28 cm long. The silane flow is left to right in side view and into the page-parallel to the filament-in the end view. In both reactor designs we power 0.5 mm diameter, tungsten filaments with currents ranging from 13.5 to 16.5 amps AC. This provides filament temperatures from 19500 to 2250'C, respectively. The powered ends of the filaments receive the voltage necessary to obtain the desired operating current, and the other ends of the filaments are grounded. The HWCVD a-Si:H materials containing - 4 at. % H grown in both chambers are quite similar. The initial material properties are of essentially the same quality and degrade with similar kinetics to comparable stabilized states. However, whenever we grow materials in the tube with H below 4 at. %, we find that the material properties become worse with decreasing H content. This is not true for our a-Si:H grown in the cross reactor, in which the material properties remain exceedingly good down to the detection limit of H by infrared techniques; as well as becoming more stable with decreasing H [1]. The primary way we control the H content of our films is by controlling the substrate temperature. We find that the H content in the a-Si:H we make in our cross reactor decreases monotonically from 20 at. % at substrate temperatures of 180 0C, down to • 1 at. % at substrate temperatures a 400'C [4]. The H content of the a-Si:H we make in our tube reactor also decreases monotonically with substrate temperature, but with a significantly different shape, having about 10 at. % H at substrate temperatures of 200'C, down to ! 1 at. % H at substrate temperatures Ž 450'C [2]. The influence of substrate temperature on H incorporation of a-Si:H grown in the tube reactor is more similar to our PECVD processes than the HWCVD a-Si:H grown in the cross reactor. The dark conductivity of the films we
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grow in the tube reactor are very good (< 10.0' S/cm) up to substrate temperatures of 400'C. Above 400'C, the dark conductivity of the films rapidly increases with increasing substrate temperature [2] and even begin to develop microcrystalline silicon (pc-Si) inclusions above 550'C. Films made in the cross reactor maintain good (low) dark conductivity with substrate temperatures above 400TC, and do not show similar trends of growing inferior material with increasing substrate temperature. In this paper, we report on what we have learned about the difficulty of growing low-H (. 4 at. %) a-Si:H in our tube reactor that is of the same e
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