Naixin Liu

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.

Enhanced performance of InGaN-based light emitting diodes through a special etch and regrown process in n-GaN layer

We reported that the peak efficiency together with the efficiency droop in InGaN-based light emitting diodes could be effectively modified through a simple and low-cost etch-regrown process in n-GaN layer. The etched n-GaN template contained pyramid arrays with inclined side planes. The following lateral overgrowth process from the etched n-GaN template substantially reduced the edge dislocation density and residential compressive strain in epilayers. The efficiency droop of LED samples thus could be modified due to the reduced polarization field, resulting from the strain relaxation in epilayers. What is more, the peak efficiency and reverse current leakage were also modified due to the reduction of dislocations.

Elimination of defects in In–Mg codoped GaN layers probed by strain analysis

The effect of In codoping effect in GaN:Mg layers were investigated through strain analysis. A hydrostatic lattice expansion is induced by In doping which cannot be simply explained by the size effect of In and Mg dopants. Together with the photoluminescence spectrums, this lattice expansion is a strong evidence for the elimination of nitrogen vacancies. The biaxial strain and edge dislocations are reduced with In doping. A theoretic model of interaction between In atoms and edge dislocations are established to explain this phenomenon.

Improvement of the efficiency droop of GaN-LEDs using an AlGaN/GaN superlattice insertion layer

With an n-AlGaN (4 nm)/GaN (4 nm) superlattice (SL) inserted between an n-GaN and an InGaN/GaN multiquantum well active layer, the efficiency droop of GaN-based LEDs has been improved. When the injection current is lower than 100 mA, the lumen efficiency of the LED with an n-AlGaN/GaN SL is relatively small compared to that without an n-AlGaN/GaN SL. However, as the injection current increases more than 100 mA, the lumen efficiency of the LED with an n-AlGaN/GaN SL surpasses that of an LED without an n-AlGaN/GaN SL. The wall plug efficiency of an LED has the same trend as lumen efficiency. The improvement of the efficiency droop of LEDs with n-AlGaN/GaN SLs can be attributed to a decrease in electron leakage due to the enhanced current spreading ability and electron blocking effect at high current densities. The reverse current of LEDs at −5 V reverse voltage decreases from 0.2568029 to 0.0070543 μA, and the electro-static discharge (ESD) pass yield of an LED at human body mode (HBM)-ESD impulses of 2000 V increases from 60% to 90%.

Study of C60-TDCN Nanometer Scale Thin Films

A novel polymer, toluylene 2, 4-dicarbamidonitrile (TDCN) with carbon-nitrogen conjugated backbones, has been synthesised in our Polymer Laboratory. The C60-TDCN thin films have been prepared by the ICB-TOFMS deposition system. The C60-TDCN thin films are very good crystalline and possess electric bistability.

The Effects of Mg Back Diffusion Capping Layers on the Performance Enhancement of Blue Light Emitting Diodes With a p-InGaN Last Barrier

The effects of thin Mg back diffusion capping layers on the performance enhancement of InGaN/GaN light emitting diodes (LEDs) with p-InGaN last barrier are investigated experimentally and numerically. By inserting a thin capping layer before the p-InGaN last barrier, the Mg back diffusion effect could be effectively suppressed and the optical performance of LEDs could be enhanced due to the improved hole injection and reduced electron leakage. What is more, the p-InGaN LED with GaN capping layer showed further reduced efficiency droop compared with that with InGaN capping layer as a result of the more uniform hole distribution in active layers.

Persistent photoconductivity in neutron irradiated GaN

Unintentionally doped GaN films grown by MOCVD were irradiated with neutrons at room temperature. In order to investigate the influence of neutron irradiation on the optical properties of GaN films, persistent photoconductivity (PPC) and low temperature photoluminescence (PL) measurements were carried out. Pronounced PPC was observed in the samples before and after neutron irradiation without the appearance of a yellow luminescence (YL) band in the PL spectrum, suggesting that the origin of PPC and YL are not related. Moreover, PPC phenomenon was enhanced by neutron irradiation and quenched by the followed annealing process at 900 °C. The possible origin of PPC is discussed.

High quality AlGaN grown on GaN template with HT-AlN interlayer

When AlGaN is grown on GaN template, crack networks invariably generate when the thickness of the AlGaN layers over GaN exceeds the critical value. We used thin high temperature deposited AlN layer (HT-AlN) as the interlayer between GaN template and AlGaN epilayer which was very effective in eliminating the cracks in AlGaN epilayer. AlGaN layers with high Al mole fractions were also grown. Characterization showed that the crystalline quality of AlGaN epilayer was fairly good even when the Al mole fraction was high.

Characterization of AlGaN on GaN template grown by MOCVD

AlxGa1-xN layer was grown on sapphire substrate with GaN template by Metal Organic Chemical Vapor Deposition system (MOCVD). High temperature AlN (HT-AlN) interlayer was inserted between AlxGa1-xN layer and GaN template to solve the cracking problem that often appears on AlxGa1-xN surface when directly grown on high temperature GaN template. Optical microscope, scanning electron microscopy (SEM), atomic force microscope (AFM), high resolution x-ray diffraction (HRXRD) and cathodoluminescence (CL) were used for characterization. It was found that the cracking was successfully eliminated. Furthermore, the crystalline quality of AlxGa1-xN layer with HT-AlN interlayer was much improved. Interference fringes were found in the HRXRD images. CL test showed that yellow emission was much reduced for AlGaN layer with HT-AlN interlayer.