The crystal structure of a-plane GaN/ZnO heterostructures on r-plane sapphire was investigated by using the XRD and TEM measurment. It was found the formation of (220) ZnGa2O4 and crystal orientation of semipolar (10$\bar 1$3) GaN at GaN/ZnO interface. The epitaxial relation of normal surface direction are the sapphire (1$\bar 1$02) // a-GaN (11$\bar 2$0) and ZnGa2O4 (220) // semi-polar GaN (10$\bar 1$$\bar 3$). Beside, the emission peak energy of ZnO appears shift about 60 meV in the GaN/ZnO/GaN heterostructures due to the re-crystallization of ZnO layer with Ga or N atom and the formation of the localized state.

We studied the polarization-dependent photoluminescence (PL) of a-plane GaN /AlGaN multiple quantum wells (MQWs) grown on r-plane sapphire substrate with the various well width from 1.5 to 7.33 nm. To clarify the reasons of light emission polarization properties, we applied the 6×6 k‧p model to simulate the E-k dispersion relation and the wave functions to obtain the polarization optical transitions. According to the experimental result, the PL emission peak position exhibits a red-shifted with increasing well width, due to the reduction of the quantum confinement effect. The polarization ratio of a-plane GaN/AlGaN MQWs increased from 0.236 to 0.274 with increasing the quantum well width. This phenomenon is believed to be that y-polarized light emission gradually dominates the PL spectrum and thus enhances the polarization ratio.

We report that the high crystalline and high efficiency green emission semipolar {101̅1} InGaN/GaN multiple quantum wells (MQWs) grown on the {101̅1} facets of GaN nanopyramid arrays by selective area epitaxy. Clear and sharp interfaces of the semipolar {101̅1} InGaN/GaN MQWs was observed by transmission electron microscopy images. As comparing with (0001) MQWs, the internal electric field of {101̅1} MQWs was remarkably reduced from 1.7 MV/cm to 0.5 MV/cm, and the room temperature (RT) internal quantum efficiency (IQE) at green emission was enhanced by about 80%. This greatly enhancement of IQE is due to suppress the polarization effect in the {101̅1} MQWs which shorten the radiative recombination to compete with nonradiative recombination at RT. These results evince that the {101̅1} planes are promising for solving the efficiency green gap of III-nitride light emitters.

We reported the systematical study of optical properties of hexagonal AlxGa1-xN epitaxial films grown on c-sapphire substrate using metal-organic chemical vapor deposition. By performing Fourier transform infrared spectroscopy measurements, the high-frequency dielectric constants and phonon frequencies can be obtained by theoretically fitting the experimental infrared reflectance spectra using a four-phase layered model. The high-frequency dielectric constant of AlxGa1-xN varies between 4.98 and 4.52 for ε∝,│(polarization perpendicular to the optical axis) and between 4.95 and 4.50 for ε∝,//(polarization parallel to the optical axis) respectively when the aluminum composition changes from 0.15 to 0.24. Furthermore, from experimental infrared reflectance spectra of AlxGa1-xN films, a specific absorption dip at 785 cm-1 was observed when the aluminum composition is larger than 0.24. The dip intensity increases and the dip frequency shifts from 785 to 812 cm-1 as aluminum composition increases from 0.24 to 0.58. According to the reciprocal space map of x-ray diffraction measurements, the emergence of this dip could be resulted from the effects of strain relaxation in AlGaN epitaxial layers due to the large lattice mismatch between GaN and AlGaN epitaxial film.

We report the experimental observation of a very strong cavity polariton dispersion in a multi-axial mode GaN microcavity. The linewidth of photoluminescent (PL) spectrum covers a few cavity axial modes. The resonant photoluminescent peaks have a strong dispersion. The frequency spacing between adjacent peaks decreases by almost a factor of five from 470nm to 370nm. The strong dispersion can be well described by cavity polariton dispersion, but not by the dispersion of the refractive index of GaN. The measured exciton-photon interaction constant is 260 meV. It is an order of magnitude higher than the typically reported values for GaN microcavities

The microstructure of Ba[Mg1/3(NbxTa(4−x)/4)2/3]O3 ceramics (x = 1, 2, 3, and 4) was investigated. Ordered structure and domain boundary were observed by high-resolution transmission electron microscopy. The disordered structure of 0.41 nm lattice modulation and the 1:2 ordering structure of 0.71 nm lattice modulation in the [111] direction are both formed in Ba[Mg1/3(NbxTa(4−x)/4)2/3]O3 ceramics. Except for the incoherent interface between disordered and 1:2 ordering domain, the coherent interface between two 1:2 ordering domains can also be observed in the Ba[Mg1/3(NbxTa(4−x)/4)2/3]O3 ceramics. In the Ba[Mg1/3(Nb1/4Ta3/4)2/3]O3 ceramic, the formation of an extra ordering structure (lattice modulation of 1.24 nm in the [111] direction) on the antiphase domain boundary with proper Nb substitution contributes to lower dielectric loss due to the stabilization of the domain boundary.