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. (C) 2008 American Institute of Physics.

Elasticity, band structure, and piezoelectricity of BexZn1-xO alloys

Lattice constants, elasticity, band structure and piezoelectricity of hexagonal wideband gap BexZn1-xO ternary alloys are calculatedusing firstprinciples methods. The alloys' lattice constants obey Vegard's law well. As Be concentration increases, the bulk modulus and Young's modulus of the alloys increase, whereas the piezoelectricity decreases. We predict that BexZn1-xO/GaN/substrate (x = 0.022) multilayer structure can be suitable for high-frequency surface acoustic wave device applications. Our calculated results are in good agreement with experimental data and other theoretical calculations. (c) 2008 Elsevier B.V. All rights reserved.

Effect on nitrogen acceptor as Mg is alloyed into ZnO

Our Raman measurement indicates that the intensity of the peaks (510 and 645 cm(-1)) related to nitrogen concentration is enhanced in MgZnO compared with that in ZnO. Using first-principles band structure methods, we calculated the formation energy and transition energy level for nitrogen acceptor in ZnO and random MgxZn1-xO (with x=0.25) alloy. Our calculations show that the incorporation of nitrogen can be enhanced as Mg is alloyed into ZnO, which agrees with our experiments. The acceptor energy level deeper in the alloy ascribes to the downward shift of the valence-band maximum edge in the presence of magnesium. (c) 2008 American Institute of Physics.

Valence band offset of MgO/InN heterojunction measured by x-ray photoelectron spectroscopy

MgO may be a promising gate dielectric and surface passivation film for InN based devices and the valence band offset of MgO/InN heterojunction has been measured by x-ray photoelectron spectroscopy. The valence band offset is determined to be 1.59 +/- 0.23 eV. Given the experimental band gap of 7.83 for the MgO, a type-I heterojunction with a conduction band offset of 5.54 +/- 0.23 eV is found. The accurate determination of the valence and conduction band offsets is important for use of MgO/InN electronic devices. (c) 2008 American Institute of Physics.

Valence band offset of ZnO/GaAs heterojunction measured by x-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy has been used to measure the valence band offset at the ZnO/GaAs heterojunction interface. The valence band offset is determined to be 2.39 +/- 0.23 eV. As a consequence, a type-II heterojunction with a conduction band offset of -0.44 +/- 0.23 eV is found. The directly obtained value is in good agreement with the result of theoretical calculations based on the interface-induced gap states and the chemical electronegativity theory. (c) 2008 American Institute of Physics.

Energy band alignment of SiO2/ZnO interface determined by x-ray photoelectron spectroscopy

Thin SiO2 interlayer is the key to improving the electroluminescence characteristics of light emitting diodes based on ZnO heterojunctions, but little is known of the band offsets of SiO2/ZnO. In this letter, energy band alignment of SiO2/ZnO interface was determined by x-ray photoelectron spectroscopy. The valence band offset Delta E-V of SiO2/ZnO interface is determined to be 0.93 +/- 0.15 eV. According to the relationship between the conduction band offset Delta E-C and the valence band offset Delta E-V Delta E-C=E-g(SiO2)-E-g(ZnO)-Delta E-V, and taking the room-temperature band-gaps of 9.0 and 3.37 eV for SiO2 and ZnO, respectively, a type-I band-energy alignment of SiO2/ZnO interface with a conduction band offset of 4.70 +/- 0.15 eV is found. The accurate determination of energy band alignment of SiO2/ZnO is helpful for designing of SiO2/ZnO hybrid devices and is also important for understanding their carrier transport properties. (C) 2009 American Institute of Physics. [DOI 10.1063/1.3204028]

Localized-Surface-Plasmon Enhanced the 357 nm Forward Emission from ZnMgO Films Capped by Pt Nanoparticles

The Pt nanoparticles (NPs), which posses the wider tunable localized-surface-plasmon (LSP) energy varying from deep ultraviolet to visible region depending on their morphology, were prepared by annealing Pt thin films with different initial mass-thicknesses. A sixfold enhancement of the 357 nm forward emission of ZnMgO was observed after capping with Pt NPs, which is due to the resonance coupling between the LSP of Pt NPs and the band-gap emission of ZnMgO. The other factors affecting the ultraviolet emission of ZnMgO, such as emission from Pt itself and light multi-scattering at the interface, were also discussed. These results indicate that Pt NPs can be used to enhance the ultraviolet emission through the LSP coupling for various wide band-gap semiconductors.

Gap states and microstructure of microcrystalline silicon thin films

The density of states (DOS) above Fermi level of hydrogenated microcrystalline silicon (mu c-Si H) films is correlated to the material microstructure. We use Raman scattering and infrared absorption spectra to characterize the structure of the films made with different hydrogen dilution ratios. The DOS of the films is examined by modulated photocurrent measurement. The results have been accounted for in the framework of a three-phase model comprised of amorphous and crystalline components, with the grain boundary as the third phase. We observed that the DOS increases monotonically as the grain boundary volume fractions f(gb) is increased, which indicates a positive correlation between the DOS and the grain boundary volume fraction.

Lattice polarity detection of InN by circular photogalvanic effect

We report an effective and nondestructive method based on circular photogalvanic effect (CPGE) to detect the lattice polarity of InN. Because of the lattice inversion between In- and N-polar InN, the energy band spin splitting is opposite for InN films with different polarities. Consequently under light irradiation with the same helicity, CPGE photocurrents in In- and N-polar layers will have opposite directions, thus the polarity can be detected. This method is demonstrated by our CPGE measurements in both n- and p-type InN films.

Optical properties of aluminum-, gallium-, and indium-doped Bi4Ti3O12 thin films

Undoped and Al-, Ga-, and In-doped Bi4Ti3O12 thin films were prepared on fused quartz substrates by chemical solution deposition. Their microstructures and optical properties were investigated by x-ray diffraction and UV-visible-NIR spectrophotometer, respectively. The optical band-gap energies, Urbach energies, and linear refractive indices of all the films are derived from the transmittance spectrum. Following the single oscillator model, the dispersion parameters such as the average oscillator energy (E-0) and dispersion energy (E-d) are achieved. The energy band gap and refractive indices are found to decrease with introducing the dopants of Al, Ga, and In, which is useful for the band-gap engineering and optical waveguide devices. The refractive index dispersion parameter (E-0/S-0) increases and the chemical bonding quantity (beta) decreases in all the films compared with those of bulk. It is supposed to be caused by the nanosize grains in films. (c) 2009 American Institute of Physics. [DOI 10.1063/1.3138813]

Substantial photo-response of InGaN p-i-n homojunction solar cells

InGaN p-i-n homojunction structures were grown by metal-organic chemical vapor deposition, and solar cells with different p-contact schemes were fabricated. X-ray diffraction measurements demonstrated that the epitaxial layers have a high crystalline quality. Solar cells with semitransparent p-contact exhibited a fill factor (FF) of 69.4%, an open-circuit voltage (V-oc) of 2.24 V and an external quantum efficiency (EQE) of 41.0%. On the other hand, devices with grid p-contact showed the corresponding values of 57.6%, 2.36 V, 47.9% and a higher power density. These results indicate that significant photo-responses can be achieved in InGaN p-i-n solar cells.

Valence band offset of ZnO/SrTiO3 heterojunction measured by x-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy has been used to measure the valence band offset (VBO) of the ZnO/SrTiO3 heterojunction. It is found that a type-II band alignment forms at the interface. The VBO and conduction band offset (CBO) are determined to be 0.62 +/- 0.23 and 0.79 +/- 0.23 eV, respectively. The directly obtained VBO value is in good agreement with the result of theoretical calculations based on the interface-induced gap states and the chemical electronegativity theory. Furthermore, the CBO value is also consistent with the electrical transport investigations.

Nitrogen defects and ferromagnetism in Cr-doped dilute magnetic semiconductor AlN from first principles

High Curie temperature of 900 K has been reported in Cr-doped AlN diluted magnetic semiconductors prepared by various methods, which is exciting for spintronic applications. It is believed that N defects play important roles in achieving the high-temperature ferromagnetism in good samples. Motivated by these experimental advances, we use a full-potential density-functional-theory method and supercell approach to investigate N defects and their effects on ferromagnetism of (Al,Cr)N with N vacancies (V-N). We investigate the structural and electronic properties of V-N, single Cr atom, Cr-Cr atom pairs, Cr-V-N pairs, and so on. In each case, the most stable structure is obtained by comparing different atomic configurations optimized in terms of the total energy and the force on every atom, and then it is used to calculate the defect formation energy and study the electronic structures. Our total-energy calculations show that the nearest substitutional Cr-Cr pair with the two spins in parallel is the most favorable and the nearest Cr-V-N pair makes a stable complex. Our formation energies indicate that V-N regions can be formed spontaneously under N-poor condition because the minimal V-N formation energy equals -0.23 eV or Cr-doped regions with high enough concentrations can be formed under N-rich condition because the Cr formation energy equals 0.04 eV, and hence real Cr-doped AlN samples are formed by forming some Cr-doped regions and separated V-N regions and through subsequent atomic relaxation during annealing. Both of the single Cr atom and the N vacancy create filled electronic states in the semiconductor gap of AlN. N vacancies enhance the ferromagnetism by adding mu(B) to the Cr moment each but reduce the ferromagnetic exchange constants between the spins in the nearest Cr-Cr pairs. These calculated results are in agreement with experimental observations and facts of real Cr-doped AlN samples and their synthesis. Our first-principles results are useful to elucidate the mechanism for the ferromagnetism and to explore high-performance Cr-doped AlN diluted magnetic semiconductors.

Quantum Confinement and Electronic Properties of Rutile TiO2 Nanowires

The quantum confinement effect, electronic properties, and optical properties of TiO2 nanowires in rutile structure are investigated via first-principles calculations. We calculate the size- and shape-dependent band gap of the nanowires and fit the results with the function E-g = E-g(bulk) + beta/d(alpha). We find that the quantum confinement effect becomes significant for d < 25 angstrom, and a notable anisotropy exists that arises from the anisotropy of the effective masses. We also evaluate the imaginary part of the frequency-dependent dielectric function [epsilon(2)(omega)] within the electric-dipole approximation, for both the polarization parallel [epsilon(parallel to)(2)(omega)] and the perpendicular [epsilon 1/2(omega)] to the axial (c) direction. The band structure of the nanowires is calculated, with which the fine structure of epsilon(parallel to)(2)(omega) has been analyzed.

Valence band offset of InN/4H-SiC heterojunction measured by x-ray photoelectron spectroscopy

The valence band offset (VBO) of InN/4H-SiC heterojunction has been directly measured by x-ray photoelectron spectroscopy. The VBO is determined to be 0.55 +/- 0.23 eV and the conduction band offset is deduced to be -2.01 +/- 0.23 eV, indicating that the heterojunction has a type-I band alignment. The accurate determination of the valence and conduction band offsets is important for applications of InN/SiC optoelectronic devices.