Xuecheng Wei

Effect of the graded electron blocking layer on the emission properties of GaN-based green light-emitting diodes

We report on the effect of a graded AlGaN electron blocking layer (GEBL) on the emission properties of InGaN/GaN multiple quantum wells light-emitting diode (LED). The adoption of GEBL in the LED enhances the electroluminescence intensity and reduces the wavelength blue-shift with increasing injection current. The light output power of the GEBL LED is enhanced by 163% and 415% at 20 and 350 mA, respectively. Moreover, the forward voltage of the GEBL LED is reduced by 0.38 V at the forward current of 20 mA.

Fabrication and optical characteristics of phosphor-free InGaN nanopyramid white light emitting diodes by nanospherical-lens photolithography

A novel nanopattern technique of nanospherical-lens photolithography is introduced to fabricate the InGaN nanopyramid white (NPW) light-emitting diodes (LEDs) by selective area growth. Highly ordered NPW LED arrays are achieved after optimizing the growth conditions. It is found that the NPW LEDs vary from warm white light to cool with the increase in growth temperature. For the cool white NPW LEDs, the spectrum is similar to the conventional white LEDs obtained from the blue LEDs combined with yellow phosphors. The blue emission originates from the upper sidewalls of nanopyramids, and yellow light is mainly emitted from the lower ridges with respect to the base of nanopyramids. Furthermore, simulation shows that the light extraction efficiency of NPW LEDs is about 4 times higher compared with conventional ones, and the escape cone is as much as 85 degrees due to their three-dimensional nanopyramid structures. These observations suggest that the proposed phosphor-free NPW LEDs may have great potential for highly efficient white lighting

High-indium-content InxGa1−xAs/GaAs quantum wells with emission wavelengths above 1.25 μm at room temperature

High-indium-content InxGa1-xAs/GaAs single/multi-quantum well (SQW/MQW) structures have been systematically investigated. By optimizing the molecular-beam epitaxy growth conditions, the critical thickness of the strained In0.475Ga0.525As/GaAs QWs is raised to 7 nm, which is much higher than the value given by the Matthews and Blakeslee model. The good crystalline quality of the strained InGaAs/GaAs MQWs is proved by x-ray rocking curves. Photoluminescence measurements show that an emission wavelength of 1.25 mum at room temperatures with narrower full width at half maximum less than 30 meV can be obtained. The strain relaxation mechanism is discussed using the Matthews-Blakeslee model

Optical properties of nanopillar AlGaN/GaN MQWs for ultraviolet light-emitting diodes

Nanopillar AlGaN/GaN multiple quantum wells ultraviolet light-emitting diodes (LEDs) were fabricated by nanosphere lithography and dry-etching. The optical properties of the nanopillar LEDs were characterized by both temperature-dependent and time-resolved photoluminescence measurements. Compared to an as-grown sample, the nanopillar sample has a PL emission peak blue-shift of 7 meV, a 42% enhanced internal quantum efficiency at room temperature and a reduced radiative recombination lifetime from 870 picosecond to 621 picosecond at 7K. These results are directly from the suppressed quantum confined stark effect that is due to the strain relaxation in the nanopillar MQWs, further revealed by micro-Raman measurement. Additionally, finite-difference time domain simulation also proves better light extraction efficiency in the nanopillar LEDs.

Phosphor-free nanopyramid white light-emitting diodes grown on { 10 1 ¯ 1 } planes using nanospherical-lens photolithography

We reported a high-efficiency and low-cost nano-pattern method, the nanospherical-lens photolithography technique, to fabricate a SiO2 mask for selective area growth. By controlling the selective growth, we got a highly ordered hexagonal nanopyramid light emitting diodes with InGaN/GaN quantum wells grown on nanofacets, demonstrating an electrically driven phosphor-free white light emission. We found that both the quantum well width and indium incorporation increased linearly along the {10 (1) over bar1} planes towards the substrate and the perpendicular direction to the {10 (1) over bar1} planes as well. Such spatial distribution was responsible for the broadband emission. Moreover, using cathodoluminescence techniques, it was found that the blue emission originated from nanopyramid top, resembling the quantum dots, green emission from the InGaN quantum wells layer at the middle of sidewalls, and yellow emission mainly from the bottom of nanopyramid ridges, similar to the quantum wires.

Anomalous temperature dependence of photoluminescence in self-assembled InGaN quantum dots

Self-assembled InGaN quantum dots (QDs) were fabricated by metal-organic chemical vapor deposition. Abnormal temperature dependence of photoluminescence (PL) was observed. The integrated PL intensity of QDs sample shows a dramatic increase in a temperature range from 160 K to 215 K and reaches the maximum value at 215 K instead of 10 K as usual. To interpret this phenomenon, a theoretic model of temperature induced carrier redistribution mechanism is designed using rate equation, which fits closely with the experimental result. It is concluded that carriers’ redistribution from shallow QDs or wetting layer to deep QDs gives rise to the unique behavior for InGaN QDs structure.

Stimulated emission at 288 nm from silicon-doped AlGaN-based multiple-quantum-well laser.

We demonstrated stimulated emission at 288 nm from a silicon-doped AlGaN-based multiple-quantum-well (MQW) ultraviolet (UV) laser grown on sapphire. The optical pumping threshold energy density of the UV laser was 64 mJ/cm2, while lasing behavior was not observed in undoped AlGaN MQWs. This means silicon doping could effectively reduce the lasing threshold of UV lasers, and the mechanism was studied showing that the silicon-doped AlGaN MQWs had a 41% higher internal quantum efficiency (IQE) compared with the undoped one. The transmission electron microscopy characterization showed that silicon doping explicitly improved the crystallographic quality of MQWs. Calculation of the polarization charge in the MQWs further revealed that the advantage of better structure quality outweighed the reduction of internal polarization field by Si doping for the IQE enhancement and successful stimulated emission.

Surface plasmon-enhanced nanoporous GaN-based green light-emitting diodes with Al2O3 passivation layer.

A surface plasmon (SP)-enhanced nanoporous GaN-based green LED based on top-down processing technology has been successfully fabricated. This SP-enhanced LED consists of nanopores passing through the multiple quantum wells (MQWs) region, with Ag nanorod array filled in the nanopores for SP-MQWs coupling and thin Al(2)O(3) passivation layer for electrical protection. Compared with nanoporous LED without Ag nanorods, the electroluminescence (EL) peak intensity for the SP-enhanced LED was greatly enhanced by 380% and 220% at an injection current density of 1 and 20A/cm(2), respectively. Our results show that the increased EL intensity is mainly attributed to the improved internal quantum efficiency of LED due to the SP coupling between Ag nanorods and MQWs.

Investigation of Efficiency and Droop Behavior Comparison for InGaN/GaN Super Wide-Well Light Emitting Diodes Grown on Different Substrates

In this work, efficiency droop of InGaN/GaN multiple-quantum-well LEDs with super wide well (WW) is discussed by comparing the external quantum efficiency (EQE) of GaN grown on sapphire and FS-GaN substrates. The luminescence and electrical characteristics of these WW LEDs are also experimentally and theoretically analyzed. With the increase of well width from 3 nm to 6 nm, high V-pits density and more strain relaxation are found in WW LED on sapphire, which exhibits greatly reduced peak efficiency but almost negligible droop behavior. In contrast, despite a larger polarization field, WW LED on FS-GaN shows obviously enhanced peak efficiency and comparable droop compared to the counterpart with 3-nm well. The Auger recombination probably dominates the mechanism of efficiency droop rather than defect-related nonradiative recombination or polarization effect in the WW LED on both sapphire and FS-GaN, especially at high current density.

The Effect of Growth Pressure and Growth Rate on the Properties of Mg-Doped GaN

In this work, the effects of growth pressure and growth rate on electrical and structural properties of Mg-doped GaN were investigated. It has been shown that enhanced growth rates induced by higher growth pressures may lead to decreased structural and electrical properties of p-type GaN layers. If the growth rate is kept unchanged, higher growth pressures will be beneficial for the quality of Mg-doped GaN due to the enhanced NH3 overpressure.