Deep levels in thin GaN epilayers grown by metal-organic chemical vapor deposition on different templates were studied by photocapacitance spectroscopy and deep-level transient spectroscopy (DLTS) using Schottky barrier diodes. We observed the reduction of electrically and optically active traps in GaN grown with in situ SiN_x nanonetwork and SiO₂ striped mask or conventional epitaxial lateral overgrowth technique (ELO) as compared to a typical control layer on a sapphire substrate. All samples measured by DLTS in the temperature range from 80 K to 400 K exhibited traps with activation energies 0.55-0.58 eV and 0.21-0.28 eV. The lowest concentration of both traps was achieved for the sample with 6 min deposition of SiN_x nanonetwork, which was lower than that for the sample prepared by conventional ELO, and much lower than that in the control. The steady-state photocapacitance spectra of all samples taken at 80 K over the spectral range 0.75-3.50 eV demonstrated a similar trend for all the layers. The photocapacitance spectra exhibited defect levels with optical threshold energies of 1.2-1.3, 1.6, 2.2 and 3.1 eV. The determined concentrations of traps were compared and the results were consistent with DLTS measurements. The layer with SiN_x nanonetwork has the lowest concentrations of optically active traps with the standard GaN control layer being the worst in terms of trap concentrations. The consistent trend among the photocapacitance spectroscopy and DLTS results suggests that SiN_x network can effectively reduce deep levels in GaN, which otherwise can deteriorate both optical and electrical performance of GaN-based devices.

The surface morphologies of unintentionally doped epitaxial laterally overgrown c-plane and a-plane GaN samples subjected to photoelectrochemical (PEC) etching in aqueous KOH is reported. By maintaining the etch in the carrier-limited regime, elucidation of the optically and electrically active defects can be achieved. Results correlating surface morphologies after PEC etching with TEM results verify the reduction of threading dislocations in the overgrown "wing" regions, as compared with the "windows" (seed regions) for both a- and c-plane GaN ELO samples. Also, within and near the window regions of the a-GaN ELO samples, PEC etching reveals a significant amount of basal stacking faults that propagate to the surface. This work represents a systematic evaluation of the effects of PEC etching on polar and nonpolar surfaces of the GaN layers grown by the conventional ex situ ELO method. The surface morphology and the whisker densities after PEC etching of c-plane GaN samples grown using SiN_x nanonetwork mask layers, a method referred to as in situ nano-ELO, also indicates significant improvement of the material quality. The identification of variations in surface morphology at different times during PEC etching of GaN may have utility in that the assessment of the material quality can be made and assorted nanopatterns of GaN surfaces can be intentionally achieved in a controllable, large-scale, and inexpensive manner.

We report a comparison of various gate dielectrics including SiO₂, Si₃N₄, ZrO₂ and Pb(Zr, Ti)O₃ (PZT) on AlGaN/GaN heterojunction field-effect transistors, deposited by PECVD, MBE, and sputtering respectively. In terms of I-V characteristics, maximum drain-source current could be enhanced under positive gate voltage and the reverse leakage current level decreases by orders of magnitude. In terms of DC measurements, very thin SiO₂ layers can improve performance, which may be due to the passivation effect to remove surface states. No significant difference exists between control and the Si₃N₄ and ZrO₂ samples. Slightly reduction in transconduction is observed on the sample with PZT probably because of the much thicker layer was utilized. The thickness of insulator layers examined from C-V measurements reveals a better crystal quality can be obtained by PECVD deposition. While the RF S-parameters measurements shows the PZT gate dielectric brings the highest cut-off frequency or the lowest gate capacitance confirmed also by C-V data, which makes it a better candidate for microwave applications.

The polarization fields in the c-axis-oriented hexagonal GaN system cause spatial separation of electrons and holes in quantum wells, reducing the quantum efficiency, and resulting in a red shift of the emission as well as a blue shift with increasing injected carrier density. In this paper, we report on the growth and optical characterization of InGaN/GaN multiple quantum wells (MQWs) on nonpolar (112¯0) a- and polar (0001) c-planes, as well as two semipolar planes, (112¯2) and (11¯01) of GaN. There are two kinds of a-plane used in this study. One of the (112¯0) a-planes was obtained on (11¯00) m-plane sapphire substrates during the epitaxial lateral overgrowth (ELO) of (112¯2) oriented semipolar GaN films, while the other one was planar a-plane GaN which was grown on (11¯01) r-plane sapphire substrates. The semipolar (112¯2) and (11¯01) planes were obtained as sidewall facets during the ELO of c-plane GaN with the mask stripes aligned along the GaN m-axis and a-axis, respectively. InGaN/GaN multiple quantum wells (MQWs) with a nominal well thickness of 4 nm and a barrier thickness of 8 nm were grown on these five GaN samples by metalorganic chemical vapor deposition. Excitation power dependent photoluminescence (PL) measurements were carried out on these quantum well structures to study the effect of polarization-induced electric field on the band-edge emission. The quantum-well emission energy from the two a-plane MQWs showed zero shift, compared to a 74 meV blue shift for the c-plane MQWs when the excitation power was increased from 1.3 mW to 37.0 mW. The semipolar (112¯2) showed a blue shift of 35 meV with increased excitation power, suggesting reduced polarization compared to that of c-plane. No quantum-well emission could be observed for the MQWs on (11¯01) semipolar planes. The shift in the quantum-well emission energy was attributed to the change of the screening effect of photon-generated carriers in the quantum wells at different excitation powers. For exact notation please see manuscript

In this work, we prepared an organic phosphor and investigated its thermal as well as fluorescent properties. The experimental results reveal that lab-made organic phosphor exhibits excellent thermal stability (Decomposition temperature (T_d) = 374 °C) and good fluorescent quantum yield (Φ = 0.88). The organic phosphor was then coated onto the blue LED chip to form a white light-emitting diode. The size of blue LED die used throughout this work was 15 mil square and the dominated wavelength was 460 nm. For the package process, the organic phosphor was firstly mixed with the epoxy and dipped it into the LED bowl of lead frame. Secondly, the pure epoxy resin was full-filled within the lamp model. For the measurement of spectrum and the C.I.E. value, it was found that the near white weight proportional of the epoxy resin A, B and the organic phosphor were 1 : 1 : 0.1.