K.K. Leung

Physical mechanisms for hot-electron degradation in GaN light-emitting diodes

We report investigations on the degradation of GaN-based light-emitting diodes due to high dc current stress by examining two types of devices with the same fabrication procedures except for the growth conditions for the InGaN quantum wells (QWs). Higher trimethylindium and triethylgallium fluxes are used for type A devices resulting in a threefold increase in the InGaN QWs growth rate compared to type B devices. Detailed structural and optoelectronic properties of the devices are investigated by transmission electron microscopy, atomic force microscopy, thermal imaging, I-V measurements, and the low-frequency noise properties of the devices as a function of the stress time, tS. The experimental data show that the QWs in type B devices are dominated by spiral growth and they have substantially higher strain nonuniformity than type A devices. The highly strained GaN/InGaN interfaces in device B are also responsible for the faster increase in the defect density due to hot-electron injection. The defects enhance the trap-assisted tunneling in the multiple quantum wells (MQWs) resulting in the development of hot spots among type B devices after high current stressing of the MQWs. This in turn leads to an increase in the defect generation rate resulting in a thermal run-away condition that ultimately resulted in the failure of the device. The data show that an increase in the growth rate in the InGaN layer led to the domination by the step flow growth mode over the spiral growth mode in the MQWs. This is the main reason for the reduction in the dislocation density in type A devices and hence their increase in device reliability.We report investigations on the degradation of GaN-based light-emitting diodes due to high dc current stress by examining two types of devices with the same fabrication procedures except for the growth conditions for the InGaN quantum wells (QWs). Higher trimethylindium and triethylgallium fluxes are used for type A devices resulting in a threefold increase in the InGaN QWs growth rate compared to type B devices. Detailed structural and optoelectronic properties of the devices are investigated by transmission electron microscopy, atomic force microscopy, thermal imaging, I-V measurements, and the low-frequency noise properties of the devices as a function of the stress time, tS. The experimental data show that the QWs in type B devices are dominated by spiral growth and they have substantially higher strain nonuniformity than type A devices. The highly strained GaN/InGaN interfaces in device B are also responsible for the faster increase in the defect density due to hot-electron injection. The defects enh...

Application of a graphene buffer layer for the growth of high quality SnS films on GaAs(100) substrate

Tin mono-sulfide (SnS) thin films have been grown by molecular beam epitaxy (MBE) on two different substrates, GaAs (100) and soda lime glass at 400°C. High resolution X-ray Diffraction (HXRD) and Scanning Electron Microscopy (SEM) are used to characterize the structural properties of the as grown SnS films. By introducing a graphene buffer layer between the SnS thin film and the substrate, the XRD rocking curves full width at half maximum (FWHM) of the SnS film grown on GaAs (100) and soda lime glass decrease from 2.92° to 0.37° and from 6.58° to 2.04° respectively, indicating a significant improvement of SnS thin films.

Low-frequency noise in GaN diodes

In this paper we will investigate the origin of low-frequency excess noise in GaN-based LEDs. We will present experimental results on the impact of material growth processes on the properties of the low-frequency noise. Based on the results we will present a model on the underlying process for low-frequency excess noise in GaN-based LED structures and their relationship with the degradation of the device due to hot-electron stressing. We also demonstrate the use of low-frequency noise measurement as a characterization tool for the optimization of the growth parameters for the multiple quantum wells.

Synthetics of ZnO Nanowires on GaN/Sapphire Substrate by Gold Catalyst

ZnO nanowires were grown on Au-coated GaN layer on c-plane sapphire by chemical vapor deposition (CVD). As-prepared ZnO oxides were characterized by a scanning electron microscope (SEM) and a transmission electron microscope (TEM). The results show that the growth of ZnO nanowires strongly depends on the location of GaN/sapphire substrates. The diameters of the resulting nanowires were in the range 60 nm with typical length about 10μm. The formation of ZnO nanowires with different morphologies at various positions of the substrate is explained by the mechanisms of vapor-solid and vapor-liquid-solid, respectively.

Low‐frequency Noise Characterizations of GaN‐based LEDs With Different Growth Parameters

We investigated the effects of the material growth parameters on the properties of 1/f noise in GaN‐based LEDs under high current stress. Two sets of growth parameters were used in which the fluxes for trimethyl indium (TMI) and triethyl gallium (TEG) have been varied. TEM results show that type A devices (TMI/[TMI+TEG]u2009=u200964.7%) produces much sharper GaN/InGaN interfaces than type B devices (TMI/[TMI+TEG]u2009=u200979.9%). Detailed characterizations of the optoelectronic and low‐frequency noise properties of the devices were conducted. It is noted that both type A and B devices exhibited degradations in both optoelectronic and low‐frequency noise when subjected to current stress. We performed systematic investigations on the degradations of the properties of the devices as a function of the stress time. Experimental results indicate type B devices exhibit much higher rate of degradations than type A devices. The experimental results show that the interface quality of the devices has strong impact on the optoelect...

Characterizations of InGaN/GaN MQWs with different growth parameters

We investigated the effects of the growth parameters on the microstructural, optoelectronic and low-frequency noise properties of InGaN/GaN multiple quantum well (MQW). Dc current stress was applied to the devices and their degradations were investigated as a function of the stress time.

Electrical and Structural Studies of Laser-debonded GaN Light Emitting Diodes

We report experimental investigation of laser-assisted debonding of GaN-based light emitting diodes grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrates. Transmission electron microscopy is employed to characterize the laser affected zone of the debonded LEDs. The findings from TEM studies are in good agreement with our thermal analysis predictions. The as-debonded surface is then roughened by photo-electro chemical (PEC) wet etching to form nano-scaled hexagonal pyramid structures and this morphology is found to be useful for light extraction on LEDs.