Hongxi Lu

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 0.38u2009V 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

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 160u2009K to 215u2009K and reaches the maximum value at 215u2009K instead of 10u2009K 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.

The improvement of GaN-based light-emitting diodes using nanopatterned sapphire substrate with small pattern spacing

Self-assembly SiO2 nanosphere monolayer template is utilized to fabricate nanopatterned sapphire substrates (NPSSs) with 0-nm, 50-nm, and 120-nm spacing, receptively. The GaN growth on top of NPSS with 0-nm spacing has the best crystal quality because of laterally epitaxial overgrowth. However, GaN growth from pattern top is more difficult to get smooth surface than from pattern bottom. The rougher surface may result in a higher work voltage. The stimulation results of finite-difference time-domain (FDTD) display that too large or too small spacing lead to the reduced light extracted efficiency (LEE) of LEDs. Under a driving current 350 mA, the external quantum efficiencies (EQE) of GaN-based LEDs grown on NPSSs with 0-nm, 50-nm, and 120-nm spacing increase by 43.3%, 50.6%, and 39.1%, respectively, compared to that on flat sapphire substrate (FSS). The optimized pattern spacing is 50 nm for the NPSS with 600-nm pattern period.

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.

Interfacial contribution to magnetocapacitance in La0.05Tb0.95MnO3/La0.67Sr0.33MnO3/SrTiO3 heterojunctions

La0.05Tb0.95MnO3/La0.67Sr0.33MnO3/SrTiO3 heterojunctions were fabricated by laser molecular-beam epitaxy. Dielectric properties as functions of temperature (77-320 K) and frequency (10(2)-10(6) Hz) of these junctions were investigated in detail. The sample showed a notable magnetoresistive effect due to the contribution of the La0.67Sr0.33MnO3 layer and a diffuse dielectric anomaly which was found to arise from Maxwell-Wagner (MW) relaxation. Both positive and negative magnetocapacitive behaviours were observed. These behaviours were found to be intimately linked to the dielectric anomaly and can be well explained based on the combined role of the MW effect and magnetoresistance.

Clustering effect in the crystallization process of a CuZr amorphous alloy

The structural relaxation and crystallization processes of the Cu50Zr50 amorphous alloy have been studied by field ion microscopy (FIM) on an atomic scale. An interesting phenomenon which we call the clustering effect was observed for the first time as far as we know. In the temperature range 673–723 K, clusters consisting of 3, 4, or 5 atoms formed and migrated towards certain crystalline centers. They then combined with one another and rearranged to produce an ordered atomic array. This clustering process including the formation, migration, combination and rearrangement of clusters is considered as a structural relaxation process.

Catalytic Activation of Mg-Doped GaN by Hydrogen Desorption Using Different Metal Thin Layers

The annealing of Mg-doped GaN with Pt and Mo layers has been found to effectively improve the hole concentration of such material by more than 2 times as high as those in the same material without metal. Compared with the Ni and Mo catalysts, Pt showed good activation effect for hydrogen desorption and ohmic contact to the Ni/Au electrode. Despite the weak hydrogen desorption, Mo did not diffuse into the GaN epilayer in the annealing process, thus suppressing the carrier compensation phenomenon with respect to Ni and Pt depositions, which resulted in the high activation of Mg acceptors. For the GaN activated with the Ni, Pt, and Mo layers, the blue emission became dominant, followed by a clear peak redshift and the degradation of photoluminescence signal when compared with that of GaN without metal.

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.