Hailiang Dong

Surface Morphology Evolution Mechanisms of InGaN/GaN Multiple Quantum Wells with Mixture N2/H2-Grown GaN Barrier

Surface morphology evolution mechanisms of InGaN/GaN multiple quantum wells (MQWs) during GaN barrier growth with different hydrogen (H2) percentages have been systematically studied. Ga surface-diffusion rate, stress relaxation, and H2 etching effect are found to be the main affecting factors of the surface evolution. As the percentage of H2 increases from 0 to 6.25%, Ga surface-diffusion rate and the etch effect are gradually enhanced, which is beneficial to obtaining a smooth surface with low pits density. As the H2 proportion further increases, stress relaxation and H2 over- etching effect begin to be the dominant factors, which degrade surface quality. Furthermore, the effects of surface evolution on the interface and optical properties of InGaN/GaN MQWs are also profoundly discussed. The comprehensive study on the surface evolution mechanisms herein provides both technical and theoretical support for the fabrication of high-quality InGaN/GaN heterostructures.

Effects of GaxZn1−xO nanorods on the photoelectric properties of n-ZnO nanorods/p-GaN heterojunction light-emitting diodes

A Ga-doped ZnO nanorod array has been synthesized on a pGaN/Al2O3 substrate by a hydrothermal method at low temperature. The structures and morphologies of the samples were measured by XRD and FE-SEM. The results show that the Ga-doped ZnO nanorods have excellent crystallinity and good epitaxial relationships with the substrate. With increasing Ga doping, the ZnO nanorods will grow along the [001] direction rapidly resulting in a decreasing average diameter. At the same time, the incorporation of Ga also signicantly affects the optical and electrical properties of the n-ZnO nanorods/p-GaN heterojunction light-emitting diode. From the photoluminescence spectrum, it was found that Ga doping can be effectively regulated by the UV and visible emission peak intensity ratio of the ZnO nanorods. The I–V characteristics curve indicates that the n-ZnO nanorods/p-GaN heterojunction light-emitting diode will have better conductivity with increasing Ga doping concentration. Finally, the heterojunction light-emitting diode achieves green light emission under forward bias.

Understanding the Growth Mechanism of GaN Epitaxial Layers on Mechanically Exfoliated Graphite

The growth mechanism of GaN epitaxial layers on mechanically exfoliated graphite is explained in detail based on classic nucleation theory. The number of defects on the graphite surface can be increased via O-plasma treatment, leading to increased nucleation density on the graphite surface. The addition of elemental Al can effectively improve the nucleation rate, which can promote the formation of dense nucleation layers and the lateral growth of GaN epitaxial layers. The surface morphologies of the nucleation layers, annealed layers and epitaxial layers were characterized by field-emission scanning electron microscopy, where the evolution of the surface morphology coincided with a 3D-to-2D growth mechanism. High-resolution transmission electron microscopy was used to characterize the microstructure of GaN. Fast Fourier transform diffraction patterns showed that cubic phase (zinc-blend structure) GaN grains were obtained using conventional GaN nucleation layers, while the hexagonal phase (wurtzite structure) GaN films were formed using AlGaN nucleation layers. Our work opens new avenues for using highly oriented pyrolytic graphite as a substrate to fabricate transferable optoelectronic devices.

GaN epitaxial layers grown on multilayer graphene by MOCVD

In this study, GaN epitaxial layers were successfully deposited on a multilayer graphene (MLG) by using metal-organic chemical vapor deposition (MOCVD). Highly crystalline orientations of the GaN films were confirmed through electron backscatter diffraction (EBSD). An epitaxial relationship between GaN films and MLG is unambiguously established by transmission electron microscope (TEM) analysis. The Raman spectra was used to analyze the internal stress of GaN films, and the spectrum shows residual tensile stress in the GaN films. Moreover, the results of the TEM analysis and Raman spectra indicate that the high quality of the MLG substrate is maintained even after the growth of the GaN film. This high-quality MLG makes it possible to easily remove epitaxial layers from the supporting substrate by micro-mechanical exfoliation technology. This work can aid in the development of transferable devices using GaN films.

Enhanced light extraction efficiency of a InGaN/GaN micro-square array light-emitting diode chip

A InGaN/GaN micro-square array light-emitting diode (LED) chip (micro-chip) has been successfully fabricated by the focused ion beam (FIB) etching technique, which can reduce ohmic contact degradation in the fabrication process of three-dimensional (3D) structure devices. Our results show that the micro-chip exhibits a similar current–voltage performance compared to the corresponding InGaN/GaN planar LED chip (planar-chip). At the driving current of 20 mA, the output power of the micro-chip is improved by 17.8% in comparison to that of the planar-chip. A relatively broad emission and enhanced emission intensity in the perpendicular direction are obtained in angular-resolved EL (AREL) measurements for the micro-chip. Three-dimensional finite difference time domain (FDTD) simulations have also proven enhanced emitted optical energy distribution. The enhancement mechanism is correlated to the increased light extraction efficiency (LEE) of the micro-chip, mainly owing to more photons from the exposed MQWs surfaces that can be efficiently extracted by the micro-square array.