Effect of Gas Phase Hydrogen-Dilution on the Nucleation, Growth, and Interfaces of a-Si 1-x C x :H
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EFFECT OF GAS PHASE HYDROGEN-DILUTION ON THE NUCLEATION, GROWTH, AND INTERFACES OF a-SilxCx:H Yiwei Lu, Ilsin An, M. Gunes*, M. Wakagi*,#, C.R. Wronski*, and R.W. Collins Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802. * Also with the Department of Electrical and Computer Engineering. # Permanent address: Hitachi Research Laboratory, Hitachi, Ltd., Hitachi-shi, 319-12 Japan. ABSTRACT The microstructural evolution of a-Sil-,Cx:H with an optical gap of 1.95 eV, prepared by plasma-enhanced chemical vapor deposition (PECVD), has been studied versus gas phase H2 dilution by real time spectroscopic ellipsometry. As the H2/(CH 4 +SiH 4 ) flow ratio is increased to 24, the monolayer-scale features of the growth process suggest an enhancement in precursor diffusion on substrate and film surfaces. Such features include a reduction in nucleation density, extensive surface smoothening during coalescence, and an increase the structural stability and density of the final film. We suggest a causal connection between these characteristics, and the photoelectronic properties of the film, which also improve with H2 -dilution. Potentially detrimental effects of H2-dilution when a-Sil-xCx:H is deposited on TCO's, including metal contamination at interfaces with Sn0 2 and H-diffusion into ZnO, are also characterized. INTRODUCTION AND SUMMARY The ability of H2 -dilution of reactive gases in PECVD to improve the photoelectronic properties of a-SilCx:H has been recognized for some time [1]. Recently, high performance wide gap solar cells have been reported, utilizing the improved alloys for the intrinsic layer [2]. Initially, it was proposed that improved alloys result when conditions at the growth surface are favorable [1]. For example, with increased H2-dilution more rapid exchange of C-containing species and H may occur between the surface and gas phase, enhancing C-atom crosslinking and reducing the concentration of Si-CH 3 bonding units [3,4] in the film. More recently, Street has stressed the beneficial role of sub-surface mobile H in equilibrating the network [5]. There have been few direct measurements of the surface of a-Sil-,C,:H and how it is affected by gas phase H during growth. Thus, we have applied spectroscopic ellipsometry (SE) in real time to characterize the microstructural evolution on the monolayer scale for PECVD a-Sil-xCx:H with H12 -dilution as the variable parameter. We report evidence of the increasing dominance of favorable surface processes as the flow ratio H2 /(CH 4 +SiH 4 ) is increased from 0 to 24, and this trend correlates well with improvements in electrical properties. The favorable processes include a low nucleation density, extensive smoothening of the surface in the thin film coalescence process, and longer term smoothening during bulk growth. These are indicators of a high precursor diffusion length and more effective network cross-linking during coalescence. There is also a general interest in preparing p-type SilxCx:H on TCO surfaces using H2 dilution.
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