Undoped and Ga-and Al-doped ZnO films were synthesized using sol-gel and spin coating methods and characterized by X-ray diffraction, high-resolution scanning electron microscopy (SEM), optical spectroscopy and Hall-effect measurements. SEM measurements reveal an average grain size of 20 nm and distinct individual layer structure. Measurable conductivity was not detected in the unprocessed films; however , annealing in hydrogen or zinc environment induced significant conductivity (∼10 −2 Ω.cm) in most films. Positron annihilation spectroscopy measurements provided strong evidence that the significant enhancement in conductivity was due to hydrogen passivation of Zn vacancy related defects or elimination of Zn vacancies by Zn interstitials which suppress their role as deep acceptors. Hydrogen passivation of cation vacancies is shown to play an important role in tuning the electrical conductivity of ZnO, similar to its role in passivation of defects at the Si/SiO2 interface that has been essential for the successful development of complementary metal-oxide-semiconductor (CMOS) devices. By comparison with hydrogen effect on other oxides, we suggest that hydrogen may play a universal role in oxides passivating cation vacancies and modifying their electronic properties. © 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license

The optical properties of a high quality bulk ZnO, thermally post treated in a forming gas environment are investigated by temperature dependent continuous wave and time-resolved photoluminescence (PL) measurements. Several bound and free exciton transitions along with their first excited states have been observed at low temperatures, with the main neutral-donor-bound exciton peak at 3.3605 eV having a linewidth of 0.7 meV and dominating the PL spectrum at 10 K. This bound exciton transition was visible only below 150 K, whereas the A-free exciton transition at 3.3771 eV persisted up to room temperature. A-free exciton binding energy of 60 meV is obtained from the position of the excited states of the free excitons. Additional intrinsic and extrinsic fine structures such as polariton, two-electron satellites, donor-acceptor pair transitions, and longitudinal optical-phonon replicas have also been observed and investigated in detail. Time-resolved PL measurements at room temperature reveal a biexponential decay behavior with typical decay constants of ϳ170 and ϳ864 ps for the as-grown sample. Thermal treatment is observed to increase the carrier lifetimes when performed in a forming gas environment.

High quality n-ZnO films on commercial p-type 6H–SiC substrates have been grown by plasma-assisted molecular-beam epitaxy, and n-ZnO∕p-SiC heterojunction mesa structures have been fabricated. Current-voltage characteristics of the structures had a very good rectifying diode-like behavior with a leakage current less than 2×10−4A∕cm2 at −10V, a breakdown voltage greater than 20V, a forward turn on voltage of ∼5V, and a forward current of ∼2A∕cm2 at 8V. Photosensitivity of the diodes was studied at room temperature and a photoresponsivity of as high as 0.045A∕W at −7.5V reverse bias was observed for photon energies higher than 3.0eV.

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The surface band bending, as well as the effect of plasma-induced damage on band bending, on GaN surfaces, was investigated. The upward band bending, measured by surface potential electric force microscopy (a variant of atomic force microscopy), for the as-grown n-type GaN was about 1.0 eV which increased to ∼1.4 eV after reactive ion etching (RIE). UV illumination decreased the band bending by 0.3 eV with time constants on the order of seconds and hundreds of seconds for the as-grown and RIE treated GaN, respectively. This implies that there is a higher density of the surface states in the samples subjected to the RIE process. After the RIE treatment, the shape of the photoluminescence spectrum remained unchanged, but the intensity dropped by a factor of 3. This effect can be attributed to nonradiative defects created near the surface by the RIE treatment.

We report on the reduction of threading dislocations in GaN overlayers grown by organometallic vapor phase epitaxy on micro-porous TiN networks. These networks were obtained by in situ annealing of thin Ti layers deposited in a metalization chamber, on the ͑0001͒ face of GaN templates. Observations by transmission electron microscopy indicate dislocation reduction by factors of up to 10 in GaN layers grown on TiN networks compared with the control GaN. X-ray diffraction shows that GaN grown on the TiN network has a smaller ͑102͒ plane peak width ͑4.6 arcmin͒ than the control GaN ͑7.8 arcmin͒. In low temperature photoluminescence spectra, a narrow excitonic full-width-at-half-maximum of 2.4 meV was obtained, as compared to 3.0 meV for the control GaN, confirming the improved crystalline quality of the overgrown GaN layers.