In this thesis, two series of self-assembled GaN and AlxGa1-xN nanorods are grown by plasma-assisted molecular beam epitaxy (PAMBE) on Si(111) wafer. The Al contents in AlxGa1-xN nanorods is varied from 6% to 75% by changing the Al cell beam flux (BFM). Second, the GaN/AlGaN multiple quantum wells (MQWs) with variation thickness are grown on the GaN nanorods with a p-GaN layer on the top. Al concentration is determined by electron probe x-ray micro-analysis (EPMA) and x-ray diffraction (XRD). The reflection high-energy electron diffraction (RHEED) and scanning electron microscopy (SEM) images show that the height, density and morphology of nanorods depend on the Al content. The (micro-)PL, CL and Raman spectra also show the variation of the characterization from those of GaN to AlN. The transmission electron microscopy (TEM) images show that the GaN/AlGaN MQWs structures with well widths of 1, 2, 3, 4, 6, 8 and 16 c-LC (Lattice constant on c-direction) were successful grown on the nanorods. The (micro-)PL and CL spectra show red-shift of the peak position with the decrease of Mg-doped concentration. When the well thickness is less then 4 c-LC, the CL spectra show blue-shift of the peak position with the decrease of the well thickness due to the Quantum-confined effect and the polarization effect in MQWS.

In recent years, even though the light output of GaN-based LED continues to increase, the brightness (~20 lm/W) is still low compared to conventional lighting systems and it is necessary to further improve the light extraction of LEDs. In this study, we utilize flip-chip technique, photoresist microlenses, reflectors and thermoelectric cooler to increase the light extraction of GaN MQW LED. Electroluminescence (EL) and power angular distribution are used to measure the light output intensity of LED. From temperature dependent current-voltage (I-V-T) characteristics, the charge carrier transport mechanisms at different biased regions are also investigated. In the results, back emission of LED with SiO2/Al reflector has maximum light intensity ( 3.28£gW ) , which is higher than front emission one ( 2.73£gW ) in vertical emitting area ( at 90 angles). LED with P.R. microlenses (refractive index, n=1.62) on backside could improve the light extraction of LED (about 1.2 times) as well. The enhancement of light output is duo to the reduction of light absorption from the metal contact and Fresnel¡¦s transmission losses at GaN (n=2.4)/air (n=1) interface. Finally, we fabricate a high brightness LED with above light enhancement design. EL intensity of LED is increased about 1.25 times than conventional one. Therefore, we can manufacture a LEDs array with above designs to obtain high light output for future solid-state illumination.

The atomic arrangements and electronic structures of [0001] oriented GaN nanowires with different side surfaces have been studied by the first-principles molecular dynamics (MD) method and the conventional first-principles electronic structure calculation method. It is found that due to the dangling bond effects, the Ga-N bonds on the side surfaces of the nanowire tilt with Ga surface atoms moving inward. The radius of the nanowire is found to be reduced with respect to the wire truncated from a bulk GaN solid, which can be attributed to the surface tension effect. Due to the large ratio between the numbers of surface atoms and bulk atoms, the electronic structures of these nanowires are very different from those of bulk and films due to the large number of surface atoms or dangling-bond states, so that a bulk-like energy gap can not be clearly defined.

In this study, SiO2 and TiO2 films were deposited on GaN, their physical and chemical properties were measured. An Al/SiO2/GaN and Al/TiO2/GaN MOS structures were used for the electrical measurements. To improve the electrical properties, we investigated the characteristics of SiO2 and TiO2 films after annealing in nitrogen, oxygen, and nitrous oxide ambient. The highest dielectric constant of 3.91 and 28.68, and lowest leakage current density of 8.97¡Ñ10-5 A/cm2 at 2 MV/cm and 2¡Ñ10-2 A/cm2 at 1 MV/cm for the N2O-annealed SiO2 film and TiO2 film can be obtained.

In this study, metal-oxide-semiconductor (MOS) capacitances were prepared with rf magnetron sputtering of Ta2O5 on both n-type GaN and p-type GaN. And metal-insulator-semiconductor (MIS) capacitances were prepared with conjugated rigid-rod polymers PBT on n-type GaN. The processes of fabrication the diodes were shown, and the structures of MOS/MIS diodes were represented. Hysteresis was observed in high-frequency capacitance-voltage (C-V) measurements. And the hysteresis was changed with different scanning delay time on scanning step. They were ascribed to mobile charges and interface charges. The carrier concentration were calculated and compared with the Hall results. The flatband voltage and threshold voltage were calculated and compared with C-V curves which were measured.

In this study, we discuss AlGaN/GaN high electron-mobility heterostructs grown by metal organic chemical vapor deposition technique. We analyzed the samples by optical and magnetoelectrical experiments to probe the dependence of the piezoelectric effect on the structural difference. We hope our results may be useful for the design of nitride heterostructures. The E-beam evaporator operation manual given in this thesis may be useful for future users.

Veeco Applied EPI 930 molecular beam epitaxy system equipped with a radio frequency plasma assisted nitrogen source has been introduced and the growth procedure and some specialized measurements are also described. The GaN thin films grown by RF-MBE have been talked about nitridation, low temperature GaN buffer layer, and GaN epilayer. The nitridation is a necessary for grown GaN on c-sapphire. From the result of the HRXRD symmetric (002) rocking curve, the magnitude of the FWHM of the GaN films without nitridation was larger than the films with nitridation. The growth temperature of the LT GaN buffer layer was the major factor on the quality of GaN epilayer which grown on the almost without nitridated sapphire substrate. The growth condition of high growth temperature, thin, low growth rate, and low N/Ga ratio of the LT GaN buffer layer can improve the sequent GaN epilayer quality. On the other hand, in the N/Ga flux ratio of GaN epitaxy layer experiment, we have found three interesting results. First, the narrowest peak width of PL spectrum appeared in a slight Ga-rich condition. Second, the smallest of HRXRD FWHM appeared in the nearly stoichiometry condition. Third, the highest electron mobility and less overall dislocations appeared in a slight N-rich condition. Finally, we report the results about AlGaN/GaN heterostructure grown by metalorganic chemical vapor deposition. The piezoelectric effect on the Alx-£_In£_Ga1-xN/GaN heterostructures was investigated and we found that a little In atom in the spacer (£_< 0.01 %) will substantially reduce the strain at interface due to the much larger size of the In atom in comparison to Al and Ga atoms. The electric field at the interface is reduced one order of magnitude smaller than that of the heterostructure without In atom. Two SdH oscillations beat each other due to the population of the lowest two subbands was been seen. Another two SdH oscillations beating have been observed in modulation-doped AlxGa1-xN/GaN heterostructures caused by the finite zero-filed spin splitting due to the inversion-asymmetry-induced bulk k3 term.

We study the optical properties of InGaN/GaN multiple quantum wells grown on silicon (111) substrate with different buffer layers. Because of the lattice mismatch and mismatch in thermal expansion coefficient, there exists stresses in the nitride sample grown on silicon substrates, which influence the growth properties and optical properties. A set of buffer layers was proposed in order to reduce the stress in our samples. The influence on optical properties is investigated in our work. In Raman spectra, we observed the characteristic phonon mode of GaN. According to the variation of E2 mode, the stress can be estimated. From our results, growing buffer layers can effectively reduce the stress in the sample. From temperature dependent and power dependent photoluminescence¡]PL) measurement, we found that appropriate buffer layers bring about less stress and better efficiency of luminescence. There are absorption of GaN and some vibrational behaviors in PLE spectra. According to the stokes shift calculated from temperature dependence PL and PLE spectra, we infer that the mechanism of recombination is not only carrier localization. The recombination is involved with the interaction of carriers and longitudinal optical phonons, and the stokes shift is independence on temperature.

In recent years, even though the light output of GaN-related LED continues to increase, the brightness is still low compared to conventional lighting systems and it is necessary to further improve the light extraction of LEDs. In this study, the characteristics of LPD-SiO2 film and Al/SiO2/GaN MOS diode were investigated in advance of the formation of SiO2 micro structure for improving the oxide quality and controlling the deposition parameters. Temperature-difference method, post-annealing treatment, photochemical treatment, sulfurated treatment and etc. were used for the purposes of better properties of the MOS structure and the LED. To obtain higher light extraction efficiency of GaN LED, hemispherical SiO2 microlens was formed on the conventional and the flip-chip LEDs. The deposition mechanism had been developed to obtain the further improvements on the electrical and optical properties. The influences of epoxy encapsulation on the LEDs without and with microlens were also studied. Considering the refractive index of SiO2 is close to that of the epoxy, the enhancements of light extraction efficiency and angular optical distribution of GaN LED by using SiO2 microlens will be degraded after encapsulating. Therefore, we also tried to deposit ZnO film and rod on GaN LED by LPD method to maintain or further enhance the light extraction efficiency of GaN LEDs by the combining the micro structure and the epoxy encapsulation.

In order to develop high speed photo-electronic device, first, we grew one layer of GaN by MOPVE to decline lattice mismatch. Then we used PA-MBE to grow AlxGa1-xN/GaN heterostructure III-V semiconductor. Via changing the content of aluminum, we can confer the characteristic of these samples. In these samples, we controlled the content of aluminum by changing the vapor of aluminum. And then we used X-ray diffraction, SEM, AFM, low temperature Hall measurement and SdH to study the characteristic of these samples. Throughout X-ray diffraction, the aluminum content x are 1.76%, 2.3%, 14.33%, 22.03% and 37.26%. Due to (004) AlGaN Rocking Curve F.W.H.M. are only 300 arcsec, the quality of the five samples are extraordinary. In addition, SEM and AFM measurement indicate that this series samples¡¦ interface are very smooth, and the roughest sample is only 2nm. It can make sure that samples were grown in mode of two-dimensional (2D). With low temperature Hall measurement, we can find out the Coulomb scattering which is from the defect are very small in the sample A, B, C, D. And the mobility of this series samples are very high, the highest mobility is sample A at 8K which is 19593 cm2/Vs. We can observe the oscillation of the sample C, D obviously in SdH measurement indicate that the 2DEG is confined in the potential well.