ZnO has become attractive as a new candidate for short wavelength optical devices and transparent conducive oxides because of its wide bandgap of 3.37 eV and a large exciton binding energy of 60 meV. The growth technique of atomic layer deposition (ALD) is effective to realize precise control of crystal growth and large area uniformity at low growth temperature. In our previous work, we have achieved the atomic layer growth of ZnO thin films on sapphire substrates. However, the crystal quality of those films was insufficient for the application to optical devices because of the large lattice mismatch of 18 percent between ZnO and c-plane sapphire substrate. In this work, in order to improve the crystal quality, we tried to grow ZnO thin films on ZnO substrate by ALD and crystal quality was evaluated by X-ray diffraction (XRD) measurements and transmission electron microscopy (TEM) observations.
ZnO thin films were grown on glass, c-plane sapphire and ZnO substrates by ALD. Diethylzinc (DEZ) and H₂O were used as Zn and O precursors, respectively, and these precursors were alternately introduced into the growth chamber. It was found that the atomic layer growth was achieved at the growth temperatures ranging from 175 to 250 ^oC, which was independent of the types of the substrates. By XRD measurements, both the (0 0 0 2) and (1 0 -1 1) orientations were observed for ZnO thin films grown on glass and sapphire substrates, while only the (0 0 0 2) orientation was observed for ZnO thin films grown on ZnO substrates. The defect configuration of ZnO thin films was evaluated by TEM. It was found that the defect density was reduced by increasing the growth temperature from 200 to 250 ^oC for all types of the substrates. In addition, it was clearly observed that the crystal quality of ZnO thin films was greatly improved by using ZnO substrate instead of glass or sapphire substrates.

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