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The polarization and surface reactivity of the ZnO nanorods (NRs) grown on the (0001̅) N-polar GaN substrate and the (0001) Ga-polar GaN substrate by hydrothermal process were studied. We found that the polarization direction of the ZnO NRs grown on the GaN substrates depends on the surface polarity of the GaN growth substrates. The surface planes of ZnO NRs on the Ga-polar GaN substrate and the N-polar GaN substrate are the Zn-polar (positive-charged surface) and the O-polar (negative-charged surface) ZnO surface plane, respectively. The O-polar ZnO plane is more chemically stable than the Zn-polar ZnO plane. After the water dissolution process, the Zn-polar ZnO NRs transformed to the tube-like nanostructure and the O-polar ZnO NRs formed hexagonal facets on the top of ZnO NRs. X-ray absorption spectroscopy (XAS) was used to analyze the electronic structures of the growth surface of the ZnO NRs on the two GaN growth substrates. The absorption peaks in the XAS spectral reveal the density of the dangling bonds on the terminal surface of ZnO NRs and the occupancy probability of the hybridized orbitals of the terminal surface of ZnO NRs. With the information on the electronic structure of the surface planes of the ZnO NRs, the surface plane of the ZnO NRs on the different GaN substrates can be verified.

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In this study, the Al3+–Sn4+ substitution reaction in the AlN-doped SnO2 thin films is confirmed by photoluminescence and X-ray photoelectron spectrum analysis. Also, both Al3+–Sn4+ and N3––O2– substitution reactions are verified by computational simulation, Vienna ab initio simulation package (VASP). The computational simulation shows that both Al and N impurity dopants generate an unoccupied band at the upper valence band maximum, which produces holes within the upper valence band region. Both Al3+–Sn4+ and N3––O2– substitution reactions contribute to the p-type conversion of AlN-doped SnO2 thin films. Annealing AlN-doped SnO2 (Al content is 14.65%) thin films at high-temperature (larger than 350 °C), N outgassing would occur and cause the p-type conduction of the annealed AlN-doped SnO2 thin films back to n-type conduction. Yet, in this work, we found that the Al3+–Sn4+ substitution reaction in the high Al-doping concentration of Al-doped and AlN-doped SnO2 (the Al content is between 29% and 33.2%) thin films would be activated considerably, as they are annealed at a temperature over 500 °C. With a higher Al-doping concentration (Al concentration is 33.2%) in the Al-doped SnO2 thin films, we found that the critical annealing temperature for the n-to-p conduction transition decreases to 500 °C. The Al dopants in the AlN-doped SnO2 thin films annealed at high annealing temperature not only stabilize the N3––O2– substitution reactions but also produce hole carriers by the Al3+–Sn4+ substitution reactions. The Al3+–Sn4+ substitution makes the AlN-doped SnO2 retain the p-type conduction in the high-temperature annealing.

The Gd-doped Al-doped Zn oxide (AZO) thin films are prepared and characterized in this study. The findings show that when the Gd doping concentration exceeded a threshold of approximately 3-5 wt%, the resistivity of the Gd:AZO thin film was reduced to a point that was lower than the resistivity of the pure AZO thin film. The reduction in resistivity was caused by the increase of the carrier concentration. This study proposes that the increase in carrier concentration was caused by the additional Gd 3þ -Zn 2þ substitution reaction. After performing 10 wt% Gd doping, the transmittance of the Gd:AZO thin film in the near UV region was increased. Following an annealing process at 200 8C, the transmittance of the annealed 100-nm Gd:AZO (10 wt%) thin films was over 80% at the 375 nm wavelength, which was approximately 40% higher than that of the annealed pure AZO thin film. The 600-nm Gd:AZO (10 wt%) annealed at 200 8C is found to have the best figure of merit value (0.24 ohm À1 ) at 375 nm (near UV regime) among all studied Gd:AZO (10 wt%) thin films in this study.