Tetsuya Yamamoto
We have been developing a unique deposition method together with a growth process to achieve tailor-made properties, such as carrier concentration (Ne ) and Hall mobility (µH), of degenerate n- type-doped wide-bandgap oxide films prepared on amorphous glass substrates. We, very recently, reported that 500 nm thick ZnO-based textured polycrystalline films consisting of 490-nm-thick Al- doped ZnO (AZO) polycrystalline films deposited on 10 nm thick Ga-doped ZnO (GZO) polycrystalline films exhibited a high µH of 50.1 cm2 /Vs with a Ne of 2.55×1020 cm−3. The film growth process was a substrate temperature as low as 200 ºC with no post heat annealing process. Firstly, the very thin GZO films were prepared on glass substrates by ion plating with dc arc discharge, which has been developed by our group and the AZO films were then deposited on the GZO films by Direct Current Magnetron Sputtering (DC-MS). The GZO interface layers with a preferential c-axis orientation play a critical role in producing AZO films with a well-defined (0001) orientation and a flat surface, whereas AZO films deposited by only DC-MS showed a mixture of the c-plane and the other plane orientation, resulting in very rough surfaces, to exhibit a low µH of 38.7 cm2 /Vs with a Ne of 2.22×1020 cm−3. The key point is to reduce a contribution of grain boundary scattering to the carrier transport due to the drastically improved crystallographic orientation and alignment between columns. Our results indicate that high µH polycrystalline oxide films possess rather unique equiaxed columnar grain structure, which enriches our current knowledge of ultimate carrier transport.
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