A real time ellipsometry study of the growth of amorphous silicon on transparent conducting oxides

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Abstract
I n situ fast kinetic ellipsometry is used to investigate the early stage of the growth of hydrogenated amorphous silicon (a-Si:H) on tin-doped indium oxide (ITO), tin oxide (TO), and zinc oxide substrates. Transparent conducting oxide (TCO) substrates obtained from various sources and deposited by different techniques are studied. A wide range of deposition parameters of a-Si:H films is considered. For a comparison with growth of amorphous silicon from silane, the ellipsometry data on pure hydrogen plasma degradation of TCO surfaces are also presented. In case of ITO and TO films, a two-step deposition is observed: the first step is dominated by the reduction of TCO and formation of metallic In or Sn (≊10 s time scale) and the second step is the subsequent growth of a-Si:H on the reduced substrates. In contrast, ZnO does not show any reduction. The substrate temperature during a-Si:H deposition is found to have the most important influence on the chemical reduction of the substrates. Variations of other deposition parameters like rf power, total gas pressure, ion bombardment, and p- or n-type doping do not lead to a reduction-free deposition on ITO and TO films. The substrate surface roughness plays an important role during the long-term growth of a-Si:H. In particular, a memory of surface roughness of the TCO substrates is retained in the growing a-Si:H films. The technological consequences of this study are outlined.