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Effects of the presence/absence of HCl during gate oxidation on the electrical and structural properties of polycrystalline silicon thin‐film transistors

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2 Author(s)
Proano, R.E. ; Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853 ; Ast, D.G.

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The influence of the presence or absence of HCl during gate oxidation at 1100 and 1150 °C on the electrical and structural properties of polycrystalline silicon thin‐film transistors was investigated. Devices processed without HCl exhibited a lower leakage current, a larger current switching ratio, a 25%–55% increase in carrier mobility, and a 21%–30% lower grain‐boundary trap density. Materials investigation showed that the improvement was not primarily due to an increase in grain size, which was about 1000 Å irrespective of processing conditions, but to a change in crystallite orientation. X‐ray diffraction measurements showed an increased {110} texture in devices processed without HCl. In addition, in the latter devices, dopant diffusion under the gate from the source and drain contacts was reduced by 7.6% to 12%. A linear relationship was observed between the average grain‐boundary trap density and the average lateral dopant diffusion length. These findings are explained by considering the point defect chemistry and its relationship to grain‐boundary mobility. Removal of HCl from the oxidation ambient increases the concentration of silicon self‐interstitials. The increased migration of these point defects to grain boundaries enhances their nonconservative motion and therefore increases grain‐boundary mobility. This in turn, allows the boundary to more completely acquire a lower‐energy configuration. This lower‐energy configuration of the boundary contains a minimum of broken bonds and therefore minimizes both the grain‐boundary electrical activity and the diffusion along grain boundaries.

Published in:

Journal of Applied Physics  (Volume:66 ,  Issue: 5 )