Wang Yu-Qi

Theoretical Analysis of Current Crowding Effect in Metal/AlGaN/GaN Schottky Diodes and Its Reduction by Using Polysilicon in Anode

There exists a current crowding effect in the anode of AlGaN/GaN heterojunction Schottky diodes, causing local overheating when working at high power density, and undermining their performance. The seriousness of this effect is illustrated by theoretical analysis. A method of reducing this effect is proposed by depositing a polysilicon layer on the Schottky barrier metal. The effectiveness of this method is provided through computer simulation. Power consumption of the polysilicon layer is also calculated and compared to that of the Schottky junction to ensure the applicability of this method.

Determination of rare earth elements in plant protoplasts by MAA

A preliminary study on the speciation of rare earth elements in plant cells has been carried out by molecular activation analysis (MAA). Mesophyll protoplasts ofBrassica napus were isolated by enzymatic digestion. After being washed with isosmotic solution containing EDTA for several times, the protoplasts were purified by gradient centrifugation. Then the concentration of rare earth elements (REEs) in the protoplasts was determined by neutron activation analysis. The result shows that REEs can enter the cells of the plant.

Growth and properties of GaAs nanowires on fused quartz substrate

The growth of GaAs nanowires directly on fused quartz substrates using molecular beam epitaxy via a vapor-liquid-solid mechanism with gold as catalyst is reported. Unlike conventional Au-catalyst MBE growth of nanowires (NWs) on GaAs substrates, zinc blende is found to be the dominant crystal structure for NWs grown on fused-quartz substrates by MBE. Further transmission electron microscopy measurements show that the prepared ZB NWs have the growth direction of [112] and lamellar {111} twins extend through the length of NWs. Although there are longitudinal planar defects that extend through NWs, the narrow full width at half maximum of PL implies high crystal quality of NWs grown on fused-quartz substrates.

Effect of III/V Ratio of HT-AlN Buffer Layer on Polarity Selection and Electrical Quality of GaN Films Grown by Radio Frequency Molecular Beam Epitaxy

We investigate the effect of Al/N ratio of the high temperature (HT) AlN buffer layer on polarity selection and electrical quality of GaN films grown by radio frequency molecular beam epitaxy. The results show that low Al/N ratio results in N-polarity GaN films and intermediate Al/N ratio leads to mixed-polarity GaN films with poor electrical quality. GaN films tend to grow with Ga polarity on Al-rich AlN buffer layers. GaN films with different polarities are confirmed by in-situ reflection high-energy electron diffraction during the growth process. Wet chemical etching, together with atomic force microscopy, also proves the polarity assignments. The optimum value for room-temperature Hall mobility of the Ga-polarity GaN film is 703 cm2/V.s, which is superior to the N-polarity and mixed-polarity GaN films.

Growth of Strain Free GaN Layers on (0001) Oriented Sapphire by Using Quasi-Porous GaN Template

We report the reduced-strain gallium-nitride (GaN) epitaxial growth on (0001) oriented sapphire by using quasi-porous GaN template. A GaN film in thickness of about 1 μm was initially grown on a (0001) sapphire substrate by molecular beam epitaxy. Then it was dealt by putting into 45% NaOH solution at 100°C for 10 min. By this process a quasi-porous GaN film was formed. An epitaxial GaN layer was grown on the porous GaN layer at 1050°C in the hydride vapour phase epitaxy reactor. The epitaxial layer grown on the porous GaN is found to have no cracks on the surface. That is much improved from many cracks on the surface of the GaN epitaxial layer grown on the sapphire as the same as on GaN buffer directly.

Properties of GaN on different polarity buffer layers by hydride vapour phase epitaxy

This paper reports on N-, mixed-, and Ga-polarity buffer layers are grown by molecular beam epitaxy (MBE) on sapphire (0001) substrates, with the GaN thicker films grown on the buffer layer with different polarity by hydride vapour epitaxy technique (HVPE). The surface morphology, structural and optical properties of these HVPE-GaN epilayers are characterized by wet chemical etching, scanning electron microscope, x-ray diffraction, and photoluminescence spectrum respectively. It finds that the N-polarity film is unstable against the higher growth temperature and wet chemical etching, while that of GaN polarity one is stable. The results indicate that the crystalline quality of HVPE-GaN epilayers depends on the polarity of buffer layers.

The Near Surface Morphology of P3HT:PCBM Blend Films Studied by Near-Edge X-Ray Absorption Fine Structure

The microstructure of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) blends with various annealing temperatures are investigated in detail by the near-edge x-ray absorption fine structure. Under higher annealing temperature (Tanneal ≥ 160°C), P3HT shows an unusual fluidity and aggregation in the surface. After annealing, the enrichment polymer component recrystallizes and forms a single P3HT phase layer in the surface of blend films. Moreover, it gives direct evidence of the PCBM content diffusing to the near surface of blend films during annealing treatment. These findings are beneficial to improving the morphology of polymer/fullerene blend films.

A Novel Sandwich Needlelike Structure in Annealed P3HT:PCBM Blend Films

A unique needlelike sandwich structure is observed in the P3HT:PCBM blend films after annealing at 220°C. The real-time observation of a growing needle indicates that the needle exhibits 7 μm/min longitudinal growth rate and no lateral growth. Both confocal fluorescence and Raman microscopic mapping measurements reveal that these needles have a PCBM core sandwiched between P3HT edges. According to the eutectic nature of P3HT:PCBM nature, when annealing at high temperature (~220°C), the aggregation of PCBM results in recrystallization of P3HT in the PCBM-depleted regions. These results will give clearer understanding of the melting, diffusion, and recrystallization behavior of the organic eutectic system.

Orientation and Structure of Controllable GaAs Nanowires Grown on GaAs (311)B Substrates by Molecular Beam Epitaxiy

GaAs nanowires (NWs) are grown on GaAs (311)B substrates by gold assisted molecular beam epitaxy technology. Combined scanning and transmission electron microscopy analyses, the crystallographic orientations of NWs are studied. It is found that crystallographic orientations of NWs are closely related to their crystal structures: NWs of zinc blende structure grow along 〈001〉 directions and NWs of wurtzite structure grow along 〈0001〉 directions. The influence of impinging Ga flux on morphology and crystal structure of the NWs is also discussed. It is observed that NWs prefer to grow along zinc blende 〈001〉 directions at lower Ga flux, while NWs tend to grow along the wurtzite 〈0001〉 directions with only a small portion along the zinc blende 〈001〉 direction at a higher Ga flux. The control of crystal structure and orientation of NWs can be achieved effectively by changing the Ga flux.

Quantum dots-templated growth of strain-relaxed GaN on a c-plane sapphire by radio-frequency molecular beam epitaxy

We investigated the quantum dots-templated growth of a (0001) GaN film on a c-plane sapphire substrate. The growth was carried out in a radio-frequency molecular beam epitaxy system. The enlargement and coalescence of grains on the GaN quantum dots template was observed in the atom force microscopy images, as well as the more ideal surface morphology of the GaN epitaxial film on the quantum dots template compared with the one on the AlN buffer. The Ga polarity was confirmed by the reflected high energy electron diffraction patterns and the Raman spectra. The significant strain relaxation in the quantum dots-templated GaN film was calculated based on the Raman spectra and the X-ray rocking curves. Meanwhile, the threading dislocation density in the quantum dots-templated film was estimated to be 7:1 10 7 cm −2 , which was significantly suppressed compared with that of the AlN-buffered GaN film. The roomtemperature Hall measurement showed an electron mobility of up to 1860 cm 2 /V s in the two-dimensional electron gas at the interface of the Al0:25Ga0:75N/GaN heterojunction.