Kang Jun-yong

AlGaN-Based Deep-Ultraviolet Light Emitting Diodes Fabricated on AlN/sapphire Template

We report on the growth and fabrication of deep ultraviolet (DUV) light emitting diodes (LEDs) on an AlN template which was grown on a pulsed atomic-layer epitaxial buffer layer. Threading dislocation densities in the AlN layer are greatly decreased with the introduction of this buffer layer. The crystalline quality of the AlGaN epilayer is further improved by using a low-temperature GaN interlayer between AlGaN and AlN. Electroluminescences of different DUV-LED devices at a wavelength of between 262 and 317 nm are demonstrated. To improve the hole concentration of p-type AlGaN, Mg-doping with trimethylindium assistance approach is performed. It is found that the serial resistance of DUV-LED decreases and the performance of DUV-LED such as EL properties is improved.

X-ray reflectivity and atomic force microscopy studies of MOCVD grown AlxGa1-xN/GaN superlattice structures

The grazing incidence X-ray reflectivity (GIXR) technique and atomic force microscopy (AFM) were exploited to obtain an accurate evaluation of the surfaces and interfaces for metalorganic chemical vapor deposition grown AlxGa1−xN/GaN superlattice structures. The X-ray diffraction results have been combined with reflectivity data to evaluate the layer thickness and Al mole fraction in the AlGaN layer. The presence of a smooth interface is responsible for the observation of intensity oscillation in GIXR, which is well correlated to step flow observation in AFM images of the surface. The structure with a low Al mole fraction (x = 0.25) and thin well width has a rather smooth surface for the Rrms of AFM data value is 0.45 nm.

Green Emission from a Strain-Modulated InGaN Active Layer

National Natural Science Foundation of China [60876008, 61076091]; Program for New Century Excellent Talents in Fujian Province University

AlGaN-Based Solar-Blind Schottky Photodetectors Fabricated on AlN/Sapphire Template

We report AlGaN-based back-illuminated solar-blind Schottky-type ultraviolet photodetectors with the cutoff-wavelength from 280nm to 292nm without bias. The devices show low dark current of 2.1 × 10−6A/cm2 at the reverse bias of 5 V. The specific detectivity D* is estimated to be 3.3 × 1012cmHz1/2 W−1. To guarantee the performance of the photodetectors, the optimization of AlGaN growth and annealing condition for Schottky contacts were performed. The results show that high-temperature annealing method for Ni/Pt Schottky contacts is effective for the reduction of leakage current.

DEEP DONOR LEVELS IN TE-DOPED GAASP ALLOY

Deep donor levels in Te-doped GaAs1-xPx for a large range of compositions have been studied by deep level transient spectroscopy (DLTS). Three kinds of deep levels A, B, and C were observed. Only level A appears in all the samples; it is considered that level A is originated from DX centers. No any regularity of presentation for levels B and C was able to find. Their properties are probably more complicated.

Fine structure in the electron emission process for two DX-like centers in Sn-doped AlGaAs

Fine structure in the electron emission process for DX(Sn) centers in AlGaAs has been studied with high resolution Laplace defect spectroscopy. The influence of the different local configuration of Al and Ga atoms around the centers on the electron thermal emissions was observed. An experimental evidence for the microscopic structure of two DX-like centers in Sn-doped AlGaAs is provided.

Identifications of photoluminescence and deep level transient spectra for Te-doped AlxGa1-xAs

A bound-exciton and a donor-acceptor (D-A) pair emission in photoluminescence are identified for Te-doped AlxGa1-xAs. The D-A pair emission near band edge is assigned as Te-related donor to carbon acceptor transition and the donor binding energy is determined as a function of composition x up to 0.75. Except the deep DX level, a new shallower deep state is found in deep level transient spectra under light illumination and is associated with Te-related donor state in PL.

Au and Ti induced charge redistributions on monolayer WS2

By using the first-principles calculations, structural and electronic properties of Au and Ti adsorbed WS2 monolayers are studied systematically. For Au-adsorbed WS2, metallic interface states are induced in the middle of the band gap across the Fermi level. These interface states origin mainly from the Au-6s states. As to the Ti adsorbed WS2, some delocalized interface states appear and follow the bottom of conduction band. The Fermi level arises into the conduction band and leads to the n-type conducting behavior. The n-type interface states are found mainly come from the Ti-3d and W-5d states due to the strong Ti–S hybridization. The related partial charge densities between Ti and S atoms are much higher and increased by an order of magnitude as compared with that of Au-adsorbed WS2. Therefore, the electron transport across the Ti-adsorbed WS2 system is mainly by the resonant transport, which would further enhances the electronic transparency when monolayer WS2 contacts with metal Ti. These investigations are of significant importance in understanding the electronic properties of metal atom adsorption on monolayer WS2 and offer valuable references for the design and fabrication of 2D nanodevices.

Cubic ZnO films obtained at low pressure by molecular beam epitaxy

A zinc oxide thin film in cubic crystalline phase, which is usually prepared under high pressure, has been grown on the MgO (001) substrate by a three-step growth using plasma-assisted molecular beam epitaxy. The cubic structure is confirmed by in-situ reflection high energy electron diffraction measurements and simulations. The x-ray photoelectron spectroscopy reveals that the outer-layer surface of the film (less than 5 nm thick) is of ZnO phase while the buffer layer above the substrate is of ZnMgO phase, which is further confirmed by the band edge transmissions at the wavelengths of about 390 nm and 280 nm, respectively. The x-ray diffraction exhibits no peaks related to wurtzite ZnO phase in the film. The cubic ZnO film is presumably considered to be of the rock-salt phase. This work suggests that the metastable cubic ZnO films, which are of applicational interest for p-type doping, can be epitaxially grown on the rock-salt substrates without the usually needed high pressure conditions.

Evidence of the Precipitates in the V-Shaped Defects of Undoped GaN

Precipitates in undoped GaN epilayers with specular surfaces grown on sapphire substrates were imaged by super high-resolution electron microscopy (HREM) and analyzed by energy-dispersive x-ray spectrometry (EDXS). The HREM images of the precipitates appeared more or less hexagonal intersection with several ten-nanometer large, which is the same as that of the V-shape defects. The EDXS spectra of the precipitates were mainly composed of the characteristic x-rays of oxygen, carbon, and gallium elements. The results suggest that the precipitates nucleate at the pinholes of the V-shape cavities and grow as the GaN growth.