Influence of Doping on the Structural Properties of Micro-Crystalline Silicon Prepared with the VHF-GD Technique at Low
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INFLUENCE OF DOPING ON THE STRUCTURAL PROPERTIES OF MICROCRYSTALLINE SILICON PREPARED WITH THE VHF-GD TECHNIQUE AT LOW DEPOSITION TEMPERATURES. F. Finger', K. Prasad 1 , S. Dubail1, A. Shah 1 , X.-M. Tang 2 , J. Weber 2 and W. Beyer 3 I Institut de Microtechnique, Universit6 de Neuchatel, CH-2000 Neuchatel, Switzerland 2 Institut de Physique, Universit6 de Neuchatel, CH-2000 Neuchatel, Switzerland 3 Institut f&r Schicht- und Ionentechnik, Forschungszentrum Jiilich, D-5170 Julich, Germany. ABSTRACT In doped and undoped microcrystalline silicon prepared with Very High Frequency Glow Discharge, hydrogen is found to be mainly located at the grain boundaries from where it desorbs easily at low annealing temperatures. In undoped material hydrogen evolution peaks are between 400'C and 500'C. Upon doping, a new major peak appears at 300'C and a strong reduction of the typical Si-H infrared absorption bands are found for doped samples when annealed up to 300'C. This is accompanied by an increase of the conductivity due to either de-passivation of dopants in the crystallites or a favourable reconstruction at the grain boundaries. Hydrogen profiles show a hydrogen depletion at the film/air interface that is more profound in doped material, thus correlating with the appearance of the low temperature evolution peak. The high free carrier density in the crystallites of the doped material gives rise to strong optical absorption. Although correlating nicely with conductivity, the free carrier absorption cannot be evaluated simply in terms of the Drude theory. In view of the high conductivities and the dominance of Si-H surface bonds we argue that our material does not contain a large amount of amorphous tissue. INTRODUCTION Recently we have shown that with glow discharge at high excitation frequencies in the VHF band (VHF-GD) highly conductive doped microcrystalline silicon (gc-Si:H) layers can be deposited at lower levels of RF input power and lower substrate temperatures as compared with GD at 13.56 MHz [1]. We have attributed this result to the influence of the high excitation frequency on the process plasma and the film growth. It was argued that in silane plasmas at high excitation frequency (70 MHz) there is a better dissociation and ionization of the process gas while keeping the electrode potentials at low levels. This leads to a high radical density and a high ion flux (with low ion energy) towards the substrate. Both a high radical density, especially atomic H, and a high ion flux can be beneficial for the growth of gc-Si:H through their influence on the surface processes [1,2]. In the present study, further investigations of the structural and electronic properties of intrinsic and doped gc-Si:H films grown with VHF-GD are presented. To assess how far the high conductivities can be related to structural differences (e.g. volume fraction of amorphous tissue, structure at the grain boundaries, hydrogen bonding) in the material investigated here as compared with other gc-Si:H films, such as usually prepared at 13.56 MHz, w
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