The effect of the underlayer on the Al-induced crystallized (AIC) Ge thin film is investigated to achieve a high-quality Ge layer on a conducting-layer-coated glass substrate. We found that the crystal orientation and the grain size of the AIC-Ge layer strongly depend on the underlayer material. We explain that this phenomenon is related to the interfacial energy between Ge and the underlayer material and/or the crystal property of the underlayer material, since the Ge nucleation likely occurs at the interface under the growth condition employed in this study. Among the samples with Al-doped ZnO, ITO, and TiN conducting underlayers, the TiN sample yields the highest crystal quality: the (111) orientation fraction of 96% and the average grain size of approximately 100 µm. Therefore, the selection of the conducting underlayer material is significantly important to design advanced photovoltaic devices based on Ge thin films on glass.
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