Wei Tongbo

Advances and prospects in nitrides based light-emitting-diodes

Due to their low power consumption, long lifetime and high efficiency, nitrides based white light-emitting-diodes (LEDs) have long been considered to be a promising technology for next generation illumination. In this work, we provide a brief review of the development of GaN based LEDs. Some pioneering and significant experiment results of our group and the overview of the recent progress in this field are presented. We hope it can provide some meaningful information for the development of high efficiency GaN based LEDs and solid-state-lighting.

Characterization of Thick GaN Films Directly Grown on Wet-Etching Patterned Sapphire by HVPE

Thick GaN films of high quality are directly grown on wet-etching patterned sapphire in a vertical hydride vapour phase epitaxy reactor. The optical and structural properties of GaN films are studied using scanning electronic microscopy and cathodoluminescence. Test results show that initial growth of hydride vapour phase epitaxy GaN occurs not only on the mesas but also on the two asymmetric sidewalls of the V-shaped grooves without selectivity. After the two-step coalescence near the interface, the GaN films near the surface keep on growing along the direction perpendicular to the long sidewall. Based on Raman results, GaN of the coalescence region in the grooves has the maximum residual stress and poor crystalline quality over the whole GaN film, and the coalescence process can release the stress. Therefore, stress-free thick GaN films are prepared with smooth and crack-free surfaces by this particular growth mode on wet-etching patterned sapphire substrates.

Light-extraction enhancement of freestanding GaN-based flip-chip light-emitting diodes using two-step roughening methods

The light extraction enhancement of freestanding GaN-based flip-chip light-emitting diodes (FS-FCLEDs) using two-step roughening methods is investigated. The output power of LEDs fabricated by using one-step and two-step roughening methods are compared. The results indicate that two-step roughening methods show more potential for light extraction. Compared with flat FS-FCLEDs, the output power of FS-FCLEDs with a nanotextured hemisphere surface shows an enhancement of 90.7%.

Columnar Structures and Stress Relaxation in Thick GaN Films Grown on Sapphire by HVPE

Thick GaN films with high quality are directly grown on sapphire in a home-built vertical hydride vapour phase epitaxy (HVPE) reactor. The optical and structural properties of large scale columnar do-mains near the interface are studied using cathodoluminescence and micro-Raman scattering. These columnar do-mains Show a strong emission intensity due to extremely high free carrier concentration up to 2 x 10(19) cm(-3), which are related with impurities trapped in structural defects. The compressive stress in GaN Elm clearly decreases with increasing distance from interface. The quasi-continuous columnar domains play an important role in the stress relaxation for the upper high quality layer.

Effect of Stress in GaN/AlInGaN Grown on GaN Templates with Different Stress States

We clarify the effect of the stress in GaN templates on the subsequent AlInGaN deposition by simply growing 150nm AlInGaN on a 30 μm GaN template (sample 1) prepared by hydride vapor phase epitaxy and a 2.3 μm thin control GaN template (sample 2) prepared by metalorganic chemical vapor deposition. X-ray diffraction and secondary iron mass spectroscopy measurements reveal the stress states (tensile stress and full relaxed for samples 1 and 2, respectively) and compositions (Al0.169In0.01 Ga0.821N, Al0.171In0.006Ga0.823N for samples 1 and 2, respectively) of AlInGaN. By carefully eliminating other possible factor, as template surface roughness, it is concluded that different stress states of AlInGaN should stem from different stress states of GaN templates.

Improvement of carrier distribution in dual wavelength light-emitting diodes

The effect of different barriers between green and blue light regions in dual wavelength light emitting diodes was studied. Compared with a traditional sample, electroluminescence and photoluminescence spectra of the newly designed samples showed peak intensity improvements and smaller blue-shifts with increasing injection current level, and the bottom quantum-wells light emitting is enhanced. All these phenomena can be ascribed to reduced barrier thickness and indium doping in the quantum-barrier influencing electric fields and more holes injecting into the bottom QWs.