Yi Xiaoyan

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.

Enhancement of blue InGaN light-emitting diodes by using AlGaN increased composition-graded barriers*

The characteristics of nitride-based blue light-emitting diodes (LEDs) with AlGaN composition-graded barriers are analyzed numerically. The carrier concentrations in the quantum wells (QWs), the energy band diagrams, the electrostatic fields, and the light output power are investigated by APSYS software. The simulation results show that the LED with AlGaN composition-graded barriers has a better performance than its AlGaN/InGaN counterpart owing to the increase of hole injection and the enhancement of electron confinement. The simulation results also suggest that the output power is enhanced significantly and the efficiency droop is markedly improved when the AlGaN barriers are replaced by AlGaN composition-graded barriers.

Electrical characteristics of a vertical light emitting diode with n-type contacts on a selectively wet-etching roughened surface

Low resistance and thermally stable n-type contacts to N-polar GaN are essentially important for vertical light emitting diodes (VLEDs). The electrical characteristics of VLEDs with n-type contacts on a roughened and flat N-polar surface have been compared. VLEDs with contacts deposited on a roughened surface exhibit lower leakage currents yet a higher operating voltage. Based on this, a new scheme by depositing metallization contacts on a selectively wet-etching roughened surface has been developed. Excellent electrical and optical characteristics have been achieved with this method. An aging test further confirmed its stability.

Light extraction efficiency enhancement of GaN-based light emitting diodes by a ZnO current spreading layer

Gallium nitride (GaN) based light emitting diodes (LEDs) with a thick and high quality ZnO film as a current spreading layer grown by metal-source vapor phase epitaxy (MVPE) are fabricated successfully. Compared with GaN-based LEDs employing a Ni/Au or an indium tin oxide transparent current spreading layer, these LEDs show an enhancement of the external quantum efficiency of 93% and 35% at a forward current of 20 mA, respectively. The full width at half maximum of the ZnO (002) ω-scan rocking curve is 93 arcsec, which corresponds to a high crystal quality of the ZnO film. Optical microscopy and atomic force microscopy are used to observe the surface morphology of the ZnO film, and many regular hexagonal features are found. A spectrophotometer is used to study the different absorption properties between the ZnO film and the indium tin oxide film of the GaN LED. The mechanisms of the extraction quantum efficiency increase and the series resistance change of the GaN-based LEDs with ZnO transparent current spreading layers are analyzed.