Z. Q. Zeng

Controlled growth of Zn-polar ZnO epitaxial film by nitridation of sapphire substrate

Surface nitridation is used to eliminate O-polar inversion domains and control the growth of single-domain Zn-polar ZnO film on sapphire (0001) substrate by rf-plasma-assisted molecular-beam epitaxy. It is found that the nitridation temperature is crucial for achieving quality AlN buffer layers and ZnO films with cation polarity, as demonstrated by ex situ transmission electron microscopy. Under optimal growth conditions, a 4×4 surface reconstruction was observed, which is confirmed to be a characteristic surface structure of the Zn-polar films, and can be used as a fingerprint to optimize the ZnO growth.

Low-temperature interface engineering for high-quality ZnO epitaxy on Si(111) substrate

ZnO(0001)∕Si(111) interface is engineered by using a three-step technique, involving low-temperature Mg deposition, oxidation, and MgO homoepitaxy. The double heterostructure of MgO(111)∕Mg(0001)∕Si(111) formed at −10°C prevents the Si surface from oxidation and serves as an excellent template for single-domain ZnO epitaxy, which is confirmed with in situ reflection high-energy electron diffraction observation and ex situ characterization by transmission electron microscopy, x-ray diffraction, and photoluminescence. The low-temperature interface engineering method can also be applied to control other reactive metal/Si interfaces and obtain high-quality oxide templates accordingly.

Cubic nitridation layers on sapphire substrate and their role in polarity selection of ZnO films

Well-defined cubic AlN ultrathin layers formed by nitridation of Al2O3 (0001) substrate at various temperatures were observed by high-resolution transmission electron microscopy. The polarity of the AlN layers strongly depends on the substrate pretreatment and nitridation temperature. The structure of the AlN layers plays a key role in polarity selection of subsequent ZnO films, and both Zn-polar and O-polar ZnO films could be steadily obtained by control of the cubic AlN layers.

Controlled growth of O-polar ZnO epitaxial film by oxygen radical preconditioning of sapphire substrate

Oxygen radicals pregrowth treatment and surface nitridation were used to eliminate Zn-polar inversion domains and control the growth of single-domain O-polar ZnO film on sapphire (0001) substrate by rf plasma-assisted molecular beam epitaxy. We found that the formation of oxygen-terminated sapphire surface prior to nitridation is crucial for achieving the anion polarity in subsequent AlN and ZnO layers, as demonstrated by formation of the 3×3 surface reconstruction during ZnO growth and ex situ polarity determination. This method, in general, can be applied to growth of other polar films, such as II-VI oxides and III-V nitrides, on sapphire (0001) substrates.

Microstructure and crystal defects in epitaxial ZnO film grown on Ga modified (0001) sapphire surface

Surface modification of sapphire (0001) by Ga can eliminate multiple rotation domains in ZnO films. The existence of Ga at ZnO∕sapphire interface was confirmed by x-ray energy dispersive spectroscopy in a transmission electron microscope. Atomic detail of mismatch dislocations at interface was imaged by high resolution transmission electron microscopy. Inside the ZnO film, there is a high density of stacking fault. Both pure gliding of ZnO(0001) plane and condensation of vacancies or interstatials are possible mechanisms to generate the stacking fault.

Surface modification of MgAl2O4 (111) for growth of high-quality ZnO epitaxial films

A magnesium wetting layer was used to modify the surface structure of MgAl2O4 (111) substrate to achieve growth of high-quality ZnO film by radio frequency plasma-assisted molecular beam epitaxy. It is found that this magnesium layer plays a crucial role in 30° rotation domain elimination, defect density reduction, and polarity control of ZnO film, as demonstrated by in situ reflection high-energy electron diffraction and ex situ transmission electron microscopy. Atomic force microscopy observation shows smooth ZnO surfaces with clearly resolved atomic steps of the films.

Interface engineering for lattice-matched epitaxy of ZnO on (La,Sr)(Al,Ta)O3(111) substrate

ZnO∕(La,Sr)(Al,Ta)O3(LSAT) heterointerface is engineered to control the crystallographic orientation of ZnO films grown by plasmas-assisted molecular beam epitaxy. Lattice-matched in-plane alignment of [112¯0]ZnO‖[112¯]LSAT has been realized using Mg modification of the substrate surface, which is confirmed with in situ reflection high-energy electron diffraction observation, and ex situ characterization of x-ray diffraction and transmission electron microscopy. The low-temperature deposition and high-temperature treatment of the Mg layer on the oxygen-terminated LSAT(111) surface results in selective nucleation of a MgO interface layer which serves as a template for single-domain epitaxy of ZnO. Oxygen-polar ZnO film with an atomically smooth surface has been obtained, which is favorable for metal-ZnO Schottky contact with high barrier height.

Electronic and magnetic structures of 4f in Ga1-xGdxN

Electronic structures and magnetic properties of Gd-doped GaN have been investigated within the framework of density functional theory. First, the density of states and band structure of GdN (terminal compound of Ga1−xGdxN) are presented. Second, the wurtzite type GaN:Gd magnetic semiconductors are studied by changing the Gd content. Magnetic stability, net spin exchange splitting and correlation of 4f electrons are analyzed, comparing them to the GaN:TM cases. Finally, the magnetic moment in Gd-doped GaN is calculated. Changing the Gd concentration hardly influences the magnetic moment of the system. Analyzing the x = 0.03 125 case, we find that the polarized magnetic moment of N by Gd atoms is very small, only about 0.01 μB and the polarization of N away from Gd can be ignored. Both long range spin polarization of the GaN matrix by the Gd atoms and the obvious magnetic moment change with Gd content are not found in our work. This study provides a further understanding of electronic and magnetic structures for Ga1−xGdxN.

Enhanced thermoelectric power factor with impurity-induced resonant level

Based on the resonant level concept, we presented an analytic calculation for the enhancement of the thermoelectric properties of semiconductor materials with element doping. We show that the power factor can be significantly enhanced due to a resonant energy level introduced by some element lies in the host bands and near the Fermi level and band edge. Our calculations reveal that by choosing optimal parameters for the element doping, such as impurity level, and doping concentration, one can obtain an optimum power factor for improved thermoelectric performance.

Ionic transport properties in doped δ-Bi2O3

We have performed the first principles calculations on doped δ-Bi2O3 to investigate ionic conductivity. The crystal structure and the ionic conductivity are discussed in total energy and density of states (DOS) from the calculations. The stabilized δ-phase Bi2O3 doped rare-earth metal was explained from DOS data. By doping Ca, Sr, La, Gd or Sm, the ion conductivity monotonically decreases, while doping with impurity Y, Tb, Dy, Er or Tm, the ion conductivity firstly increases and then decrease. Our results support the effective oxygen vacancies mechanism.