Zhihua Xiong

Behavior of aluminum adsorption and incorporation at GaN(0001) surface: First- principles study

First-principles calculations are performed to study the energetics and atomic structures of aluminum adsorption and incorporation at clean and Ga-bilayer GaN(0001) surfaces. We find the favorable adsorption site changes from T4 to T1 as Al coverage increased to 1 monolayer on the clean GaN(0001) surface, and a two-dimensional hexagonal structure of Al overlayer appears. It is interesting the Al atoms both prefer to concentrate in one deeper Ga layer of clean and Ga-bilayer GaN(0001) surface, respectively, while different structures could be achieved in above surfaces. For the case of clean GaN(0001) surface, corresponding to N-rich and moderately Ga-rich conditions, a highly regular superlattice structure composed of wurtzite GaN and AlN becomes favorable. For the case of Ga-bilayer GaN(0001) surface, corresponding to extremely Ga-rich conditions, the Ga bilayer is found to be sustained stable in Al incorporating process, leading to an incommensurate structure directly. Furthermore, our calculations provide an explanation for the spontaneous formation of ordered structure and incommensurate structure observed in growing AlGaN films. The calculated results are attractive for further development of growth techniques and excellent AlGaN/GaN heterostructure electronic devices.

Energetics and magnetism of Co-doped GaN(0001) surfaces: A first-principles study

A comprehensive first-principles study of the energetics, electronic, and magnetic properties of Co-doped GaN(0001) thin films are presented and the effect of surface structure on the magnetic coupling between Co atoms is demonstrated. It is found that Co atoms prefer to substitute the surface Ga sites in different growth conditions. In particular, a CoN/GaN interface structure with Co atoms replacing the first Ga layer is preferred under N-rich and moderately Ga-rich conditions, while CoGax/GaN interface is found to be energetically stable under extremely Ga-rich conditions. It is worth noted that the antiferromagnetic coupling between Co atoms is favorable in clean GaN(0001) surface, but the existence of ferromagnetism would be expected to occur as Co concentration increased in Ga-bilayer GaN(0001) surface. Our study provides the theoretical understanding for experimental research on Co-doped GaN films and might promise the Co:GaN system potential applications in spin injection devices.

Structure, Electrical and Optical Properties of the Polar ZnO(0001) Surfaces

Based on the density functional theory, using the first-principles calculations method, the geometrical structure, electronic structure and optical properties of the ZnO(0001) surface were investigated. The calculated results show that a great change appears in the structure after the surface relaxation. The new energy levels were found in the band structure. The conductivity of the ZnO (0001) surface becomes stronger, showing the electrical properties of metal. On the other hand, imaginary part of the dielectric function of the ZnO(0001) surface also changes certainly. A new peak is observed in the low energy region. The results provide a theoretical basis for photoelectric device manufacturing and further development on the ZnO surface.

Effect of interfacial composition on Ag-based Ohmic contact of GaN-based vertical light emitting diodes

By investigating the effect of a defective interface structure on Ag-based Ohmic contact of GaN-based vertical light-emitting diodes, we found a direct relationship between the interfacial composition and the Schottky barrier height of the Ag(111)/GaN(0001) interface. It was demonstrated that the Schottky barrier height of a defect-free Ag(111)/GaN(0001) interface was 2.221 eV, and it would be dramatically decreased to 0.375 eV with the introduction of one Ni atom and one Ga vacancy at the interface structure. It was found that the tunability of the Schottky barrier height can be attributed to charge accumulations around the interfacial defective regions and an unpinning of the Fermi level, which explains the experimental phenomenon of Ni-assisted annealing improving the p-type Ohmic contact characteristic. Lastly, we propose a new method of using Cu as an assisted metal to realize a novel Ag-based Ohmic contact. These results provide a guideline for the fabrication of high-quality Ag-based Ohmic contact ...

Effect of the Electrode Shape on Properties of GaN-Based LED Chips

Before cutting into a single chip, we have tested LED chip with different electrode that is produced under the same condition. With drive current changes, comparing the intensity of light emission of LED chip and operating voltage about the two samples. The conclusion shows that the shape of electrode makes influence on the performance of LED, and the performance of circular electrode with angle is better. The result demonstrates that the better electrode structure of LED chip can improve its performance.

Structure and Optical Properties of Superlattices ZnO Doped with Mg

Based on the first-principles calculations, we present a study for Mg-doped superlattices ZnO. We find that the volume of superlattices ZnO is reduced, while the band gap is extended. The dielectric function peaks of superlattices ZnO become smaller in the low-energy region. Interestingly, we further find Superlattices ZnO has great geometrical stability due to the electrostatic potential of the doped layer is smaller than that of pure ZnO. The results provide a theoretical basis for photoelectric device manufacturing.

Effect of Er doping on the electronic structure and optical properties of ZnO

ZnO has a significant advantage for applications in optical devices. Especially ZnO doped with the rare earths (RE) shows great electronic and optical properties. Based on the density functional theory, using the first-principles calculations method, the crystal structure, electronic structure and optical properties of ZnO doped with various concentrations of Er were investigated. The calculated results show that with the increase of concentrations of Er, the volume of ZnO system is expanded. Simultaneously, the band gap of ZnO with dopant system becomes broad. However, the conductivity of system is enhanced with the decrease of Er concentration. On the other hand, imaginary part of the dielectric function of ZnO doped with Er also changes certainly. A new peak is observed in the low energy region. The results are helpful to gain a systematic understanding of geometrical structures, electrical structures and optical properties of Er-doped ZnO.