Yin Zhi-Jun

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.

Effect of double AlN buffer layer on the qualities of GaN films grown by radio-frequency molecular beam epitaxy

This paper reports that the GaN thin films with Ga-polarity and high quality were grown by radio-frequency molecular beam epitaxy on sapphire (0001) substrate with a double AlN buffer layer. The buffer layer consists of a high-temperature (HT) AlN layer and a low-temperature (LT) AlN layer grown at 800°C and 600°C, respectively. It is demonstrated that the HT-AlN layer can result in the growth of GaN epilayer in Ga-polarity and the LT-AlN layer is helpful for the improvement of the epilayer quality. It is observed that the carrier mobility of the GaN epilayer increases from 458 to 858cm2/Vs at room temperature when the thickness of LT-AlN layer varies from 0 to 20 nm. The full width at half maximum of x-ray rocking curves also demonstrates a substantial improvement in the quality of GaN epilayers by the utilization of LT-AlN layer.

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.

Theoretical and Experimental Optimization of InGaAs Channels in GaAs PHEMT Structure

The ground-state energy level (GEL) and electron distribution of GaAs pseudomorphic high-electron-mobility transistors (PHEMTs) are analyzed by a self-consistent solution to the Schrodinger—Poisson equations. The indium composition and thickness of the InGaAs channel are optimized according to the GEL position. The GEL position is not in direct proportion to 1/d2 (d is the channel thickness) by considering the influence of electron distribution in the InGaAs channel. Indium composition 0.22 and channel thickness 9nm are obtained by considering the mismatch between InGaAs and AlGaAs. Several PHEMT samples are grown according to the theoretical results and mobility 6300 cm2/Vs is achieved.

Effect of Reactor Pressure on Qualities of GaN Layers Grown by Hydride Vapour Phase Epitaxy

The influence of reactor pressure on GaN layers grown by hydride vapour phase epitaxy (HVPE) is investigated. By decreasing the reactor pressure from 0.7 to 0.5 atm, the GaN layer growth mode changes from the island-like one to the step flow. The improvements in structural and optical properties and surface morphology of GaN layers are observed in the step flow growth mode. The results clearly indicate that the reactor pressure, similarly to the growth temperature, is one of the important parameters to influence the qualities of GaN epilayers grown by HVPE, due to the change of growth mode.

Effect of Crucibles on Qualities of Self-Seeded Aluminium Nitride Crystals Grown by Sublimation

Self-seeded aluminium nitride (AIN) crystals are grown in tungsten and hot pressed boron nitride (HPBN) crucibles with different shapes by a sublimation method. The qualities of the AIN crystals are characterized by high-resolution transmission electronic microscopy (HRTEM), scanning electron microscopy (SEM) and Micro-Raman spectroscopy. The results indicate that the better quality crystals can be collected in conical tungsten crucible.

GaN layers with different polarities prepared by radio frequency molecular beam epitaxy and characterized by Raman scattering

GaN layers with different polarities have been prepared by radio-frequency molecular beam epitaxy (RF-MBE) and characterized by Raman scattering. Polarity control are realized by controlling Al/N flux ratio during high temperature AlN buffer growth. The Raman results illustrate that the N-polarity GaN films have frequency shifts at A1(LO) mode because of their high carrier density; the forbidden A1(TO) mode occurs for mixed-polarity GaN films due to the destroyed translation symmetry by inversion domain boundaries (IDBS); Raman spectra for Ga-polarity GaN films show that they have neither frequency shifts mode nor forbidden mode. These results indicate that Ga-polarity GaN films have a better quality, and they are in good agreement with the results obtained from the room temperature Hall mobility. The best values of Ga-polarity GaN films are 1042 cm2/Vs with a carrier density of 1.0×1017 cm−3.