Band Edge Emission Improvement by Energy Transfer in HybridⅢ-Nitride/Organic Semiconductor Nanostructure

GaN nanorods are fabricated using inductively coupled plasma etching with Ni nano-island masks. The poly [2- methoxy-5-（2-ethyl）hexoxy-l,4-phenylenevinylene] （MEH-PPV）/GaN-nanorod hybrid structure is fabricated by depositing the MEH-PPV film on the GaN nanorods by using the spin-coating process. In the hybrid structure, the spatial separation is minimized to achieve high-emciency non-radiative resonant energy transfer. Optical properties of a novel device consisting of MEH-PPV/GaN-nanorod hybrid structure is studied by analyzing photoluminescenee （PL） spectra. Compared with the pure GaN nanorods, the PL intensity of the band edge emission of GaN in the MEH-PPV/GaN-nanorods is enhanced as much as three times, and the intensity of the yellow band is suppressed slightly. The obtained results are analyzed by energy transfer between the GaN nanorods and the MEH-PPV. An energy transfer model is proposed to explain the phenomenon.

Spectra Analysis of a Novel Ti-Doped LiAlO2 Single Crystal

LiAlO2 single crvstals doped with Ti at concentration 0.2at.% are grown by the Czochralskl technique with dimensions φ42×55mm. Ti ions in the crystal are quadrivalence proven by comparing the absorption and fluorescence spectra of pure LiAlO2 and Ti： LiAlO2. After air and Li-rich atmosphere annealing, the absorption peaks in the range of 600-800nm disappear. We conclude that 682 and 756nm absorption peaks are attributed to the VLi and Vo absorptions, respectively： The peaks at 716nm and 798nm may stem from the VLi^＋ and absorptions. The colour-centre model can be applied to explain the experimental phenomena. Ti^4＋-doping produces more lithium vacancies in the LiAlO2 crystal. The intensities of [LiO4] and the associated bonds remain unchanged, which improves the anti-hydrolyzation and thermal stability of LiAlO2 crystals.

Studies on Linear and Nonlinear Intersubband Optical Absorptions in a Wurtzite AlGaN/GaN Coupling Quantum Well

Based on the density matrix approach and iterative procedure, a detailed procedure for the calculation of the linear and nonlinear intersubband optical absorption coefficients is given in wurtzite GaN-based coupling quantum wells （CQWs）. The simple analytical formulas for electronic eigenstates and the linear and nonlinear optical absorption coefficients in the systems are also deduced. Numerical result on a typical A1GaN/GaN CQW shows that, the linear and nonlinear optical absorption coefficients sensitively depend on the structural parameters of the CQW system as well as the incident optics beam intensity.

Progress in bulk GaN growth

Three main technologies for bulk GaN growth, i.e., hydride vapor phase epitaxy （HVPE）, Na-flux method, and am- monothermal method, are discussed. We report our recent work in HVPE growth of GaN substrate, including dislocation reduction, strain control, separation, and doping of GaN film. The growth mechanisms of GaN by Na-flux and ammonother- mal methods are compared with those of HVPE. The mechanical behaviors of dislocation in bulk GaN are investigated through nano-indentation and high-space resolution surface photo-voltage spectroscopy. In the last part, the progress in growing some devices on GaN substrate by homo-epitaxy is introduced.

Piezoelectric polarization and quantum size effects on the vertical transport in AlGaN/GaN resonant tunneling diodes

In this work,the electronic properties of resonant tunneling diodes（RTDs） based on GaN-AlxGa（1-x）N double barriers are investigated by using the non-equilibrium Green functions formalism（NEG）.These materials each present a wide conduction band discontinuity and a strong internal piezoelectric field,which greatly affect the electronic transport properties.The electronic density,the transmission coefficient,and the current–voltage characteristics are computed with considering the spontaneous and piezoelectric polarizations.The influence of the quantum size on the transmission coefficient is analyzed by varying GaN quantum well thickness,AlxGa1-xN width,and the aluminum concentration xAl.The results show that the transmission coefficient more strongly depends on the thickness of the quantum well than the barrier;it exhibits a series of resonant peaks and valleys as the quantum well width increases.In addition,it is found that the negative differential resistance（NDR） in the current–voltage（I–V） characteristic strongly depends on aluminum concentration xAl.It is shown that the peak-to-valley ratio（PVR） increases with xAlvalue decreasing.These findings open the door for developing vertical transport nitrides-based ISB devices such as THz lasers and detectors.

Fabrication of porous graphitic carbon nitride-titanium dioxide heterojunctions with enhanced photo-energy conversion activity

Porous graphite-phase polymeric carbon nitride（GPPCN）/TiO2 donor-acceptor heterojunction was facilely fabricated through the combination of a template technique with a co-calcination process,which exhibited much higher photoelectric activity compared to pristine carbon nitride and TiO2.The precursor of porous GPPCN（pGPPCN）,porous melem,was prepared by using a green template,calcium carbonate,which could be easily removed by diluted hydrochloride.The pGPPCN/TiO2 heterojunction was then obtained by the assembly and subsequent co-calcination of TiO2 nanoparticles with porous melem.The formation of pGPPCN/TiO2 donor-acceptor heterojunction prepared by this method showed improved surface area and light absorption.Moreover,the composite presented much higher photo-energy conversion activity than those of GPPCN,pGPPCN and TiO2,which could be mainly ascribed to the high charge carrier separation efficiency.This study provides a new approach for the design and development of various photocatalysts with high efficiency for applications in energy fields.

Effect of Annealing on the Optical Properties of GaN Films Grown by Pulsed Laser Deposition

In the present study, gallium nitride thin films were grown by using pulsed laser deposition. After the growth samples were annealed at 400 and 600 ℃ in the nitrogen atmosphere. Surface morphology of the as-grown and annealed samples was observed by atomic force microscopy. Post-growth annealing results in an improved surface roughness of the films. Chemical analysis of the samples was performed by X-ray photoelectron spectroscopy. Stoichiometric gallium nitride thin films were obtained for the samples annealed at 600 ℃. Optical measurements of the samples were performed to measure the band gap and optical constants of the films. Effect of annealing on the band gap and optical constants of the films was studied.