Huaping Song

Dislocation scattering in AlxGa1-xN/GaN heterostructures

The theoretical electron mobility limited by dislocation scattering of a two-dimensional electron gas confined near the interface of AlxGa1-xN/GaN heterostructures was calculated. Based on the model of treating dislocation as a charged line, an exponentially varied potential was adopted to calculate the mobility. The estimated mobility suggests that such a choice can simplify the calculation without introducing significant deviation from experimental data, and we obtained a good fitting between the calculated and experimental results. It was found that the measured mobility is dominated by interface roughness and dislocation scattering at low temperatures if dislocation density is relatively high (>10(9) cm(-2)), and accounts for the nearly flattening-out behavior with increasing temperature.

Influence of band bending and polarization on the valence band offset measured by x-ray photoelectron spectroscopy

The influence of band bending and polarization on the valence band offset measured by x-ray photoelectron spectroscopy (XPS) is discussed, and a modification method based on a modified self-consistent calculation is proposed to eliminate the influence and thus increasing the precision of XPS. Considering the spontaneous polarization at the surfaces and interfaces and the different positions of Fermi levels at the surfaces, we compare the energy band structures of Al/Ga-polar AlN/GaN and N-polar GaN/AlN heterojunctions, and give corrections to the XPS-measured valence band offsets. Other AlN/GaN heterojunctions and the piezoelectric polarization are also introduced and discussed in this paper.

Effect of interface barrier between carbon nanotube film and substrate on field emission

The influence of interface barrier on field emission of carbon nanotubes (CNTs) was investigated theoretically and experimentally. A double-potential barrier model was proposed to calculate the electron tunneling probability through the interface and surface barriers. The calculation result reveals that the difference of the electron tunneling probability through the two barriers is responsible for the nonlinearity of the Fowler–Nordheim (FN) plots for the field emission of the CNTs. To verify this model, a series of the CNTs were synthesized on the Si substrates covered with different thicknesses of SiO2 layers as the interface barrier. Based on their field emission properties, it was found that the FN plots of the field emission of these CNTs deviated from the FN law when the applied electric fields were over a critical value, which was strongly dependent on the thicknesses of the SiO2 layer. Therefore, the interface barrier has an important role in determining the field emission property of the CNTs. T...

Influence of stress on the magnetic domain structure in Fe81Ga19 alloys

Grains, grain boundaries, magnetic domain distributions, and their correlations in Fe81Ga19 alloys as-cast, annealed, and under compressive and grinding stresses were investigated using scanning electron acoustic microscopy (SEAM) and magnetic force microscopy (MFM). For as-cast and annealed samples, the main domain structure obtained using SEAM is not clear. For samples under stresses, the main domains are obvious and regular. For the sample under compressive stress, its stripe domains array orderly and perpendicularly to the stress direction. The subdomains of as-cast sample observed using MFM show a clear dendritic structure. However, for the sample under compressive stress, the subdomains are irregular. These results may be helpful to understand the magnetostrictive behavior of Fe–Ga alloys under stress.

Band alignment of InN∕GaAs heterojunction determined by x-ray photoelectron spectroscopy

The valence band offset (VBO) of the InN/GaAs heterojunction is directly determined by x-ray photoelectron spectroscopy to be 0.94 +/- 0.23 eV. The conduction band offset is deduced from the known VBO value to be 1.66 +/- 0.23 eV, and a type-II band alignment forms at the InN/GaAs heterojunction

Investigation of cracks in GaN films grown by combined hydride and metal organic vapor-phase epitaxial method

Cracks appeared in GaN epitaxial layers which were grown by a novel method combining metal organic vapor-phase epitaxy (MOCVD) and hydride vapor-phase epitaxy (HVPE) in one chamber. The origin of cracks in a 22-μm thick GaN film was fully investigated by high-resolution X-ray diffraction (XRD), micro-Raman spectra, and scanning electron microscopy (SEM). Many cracks under the surface were first observed by SEM after etching for 10 min. By investigating the cross section of the sample with high-resolution micro-Raman spectra, the distribution of the stress along the depth was determined. From the interface of the film/substrate to the top surface of the film, several turnings were found. A large compressive stress existed at the interface. The stress went down as the detecting area was moved up from the interface to the overlayer, and it was maintained at a large value for a long depth area. Then it went down again, and it finally increased near the top surface. The cross-section of the film was observed after cleaving and etching for 2 min. It was found that the crystal quality of the healed part was nearly the same as the uncracked region. This indicated that cracking occurred in the growth, when the tensile stress accumulated and reached the critical value. Moreover, the cracks would heal because of high lateral growth rate.

Well-aligned Zn-doped tilted InN nanorods grown on r-plane sapphire by MOCVD

Well-aligned tilted Zn-doped InN nanorods have been grown successfully on r-plane sapphire in a horizontal metal-organic chemical vapor deposition system. All of the nanorods are symmetrically tilted in two opposite directions. X-ray diffraction and transmission electron microscopy measurements show that the nanorods are single-crystalline and have exactly the same epitaxial orientation as the a-plane InN film. The nanorod has a new cross sectional shape of an axial symmetry pentagon and the axis of symmetry is the c-axis of the crystal. Zn dopant plays a crucial role in the growth progress, being an important factor in controlling the morphology of the InN nanomaterials and their properties.

The role of zinc dopant and the temperature effect on the controlled growth of InN nanorods in metal–organic chemical vapor deposition system

Vertically well-aligned InN nanorods have been synthesized successfully by introducing diethylzinc (DEZn) into metal–organic chemical vapor deposition system. X-Ray diffraction and transmission electron microscopy measurements show that InN nanorods are single-crystalline and Zn-doped. In-depth studies indicate that DEZn inhibits the InN growth along m-plane and further leads to the self-formation indium droplets acting as catalyst. The whole growth process of InN nanorods is a combined result of the restriction effect from DEZn and catalytic effect from indium droplets. Also, the temperature effect on the growth has been studied intensively. Our research on the controlled growth of high-quality InN nanorods is very important for InN-based optoelectronic and electronic nano-devices.

GaN grown with InGaN as a weakly bonded layer

GaN thin films were grown with InGaN as an interlayer. The GaN films show different stress states with and without an InGaN interlayer. The influence of the In composition in InGaN on the properties of the high temperature (HT) GaN overlayer was discussed and potential stress-free points were extrapolated. The effect of H2 on the growth of HT GaN was found to be that it assisted the decomposition of InGaN. A nearly stress free GaN single-crystalline film with mirror-like morphology and single polarity was obtained by inserting an appropriate InGaN interlayer and the growth mechanism of GaN, with a regular network as the template, was discussed. Our research on the controllable growth of high-quality GaN thin film is very important for GaN-based optoelectronic and electronic devices.

Contrast mechanism of magnetic domains in electron acoustic imaging

Contrast origin of magnetic domain structure imaged by scanning electron acoustic microscopy (SEAM) is studied by analyzing qualitatively the interaction between an incident electron beam and a sample. Some parameters related to the acoustic signal detected by a piezoelectric transducer are obtained through solving a vibration equation. Contrasted with magnetic domain structures of Terfenol-D and Fe81Ga19 alloys obtained by SEAM in linear and nonlinear (second harmonic) modes, imaging mechanism of magnetic domains is attributed to piezomagnetic coupling mechanism, magnetostrictive coupling mechanism, and thermal-wave coupling mechanism.