Proton irradiation-induced defects in undoped GaSb studied by positron lifetime spectroscopy and photoluminescence

Undoped GaSb was irradiated by 2.6 MeV protons. The irradiation-induced defects were studied by positron lifetime spectroscopy (PLS) and photoluminescence (PL). Positron lifetime measurements showed that vacancy-type defects were introduced after irradiation, and divacancies were formed at higher irradiation dose. Annealing experiments revealed there were different annealing steps between the as grown and proton-irradiated samples, the reason for which was tentatively attributed to the formation of divacancies in the proton-irradiated samples during annealing. All the vacancy defects could be annealed out at around 500 degrees C. The PL intensity quickly fell down after proton irradiation and decreased with increasing irradiation dose, indicating that irradiation induced non-irradiative recombination centers, whose candidates were assigned to the vacancy defects induced by proton irradiation.

Determination of wurtzite InN/cubic In2O3 heterojunction band offset by x-ray photoelectron spectroscopy

In2O3 is a promising partner of InN to form InN/In2O3 heterosystems. The valence band offset (VBO) of wurtzite InN/cubic In2O3 heterojunction is determined by x-ray photoemission spectroscopy. The valence band of In2O3 is found to be 1.47 +/- 0.11 eV below that of InN, and a type-I heterojunction with a conduction band offset (CBO) of 0.49-0.99 eV is found. The accurate determination of the VBO and CBO is important for use of InN/In2O3 based electronic devices.

Measuring spin diffusion of electrons in bulk n-GaAs using circularly dichromatic absorption difference spectroscopy of spin gratings

Circular dichromatic absorption difference spectroscopy is developed to measure the spin diffusion dynamics of electrons in bulk n-GaAs. This spectroscopy has higher detection sensitivity over homodyne detection of spin-grating-diffracted signal. A model to describe circular dichromatic absorption difference signal is derived and used to fit experimental signal to retrieve decaying rate of spin gratings. A spin diffusion constant of D-s=201 +/- 25 cm(2)/s for bulk n-GaAs has been measured at room temperature using this technique and is close to electron diffusion constant (D-c), which is much different from the case in GaAs quantum wells where D-s is markedly less than D-c.

Measurement of polar C-plane and nonpolar A-plane InN/ZnO heterojunctions band offsets by x-ray photoelectron spectroscopy

The valence band offsets of the wurtzite polar C-plane and nonpolar A-plane InN/ZnO heterojunctions are directly determined by x-ray photoelectron spectroscopy to be 1.76 +/- 0.2 eV and 2.20 +/- 0.2 eV. The heterojunctions form in the type-I straddling configuration with a conduction band offsets of 0.84 +/- 0.2 eV and 0.40 +/- 0.2 eV. The difference of valence band offsets of them mainly attributes to the spontaneous polarization effect. Our results show important face dependence for InN/ZnO heterojunctions, and the valence band offset of A-plane heterojunction is more close to the "intrinsic" valence band offset.

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.

Determination of MgO/AlN heterojunction band offsets by x-ray photoelectron spectroscopy

MgO is a promising gate dielectric and surface passivation film for GaN/AlGaN transistors, but little is known of the band offsets in the MgO/AlN system. X-ray photoelectron spectroscopy was used to measure the energy discontinuity in the valence band (Delta E-v) of MgO/AlN heterostructures. A value of Delta E-v=0.22 +/- 0.08 eV was obtained. Given the experimental band gap of 7.83 eV for MgO, a type-I heterojunction with a conduction band offset of similar to 1.45 eV is found. The accurate determination of the valence and conduction band offsets is important for use of III-N alloys based electronic devices.

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.

Investigation of GaAs/AlGaAs interfaces by reflectance-difference spectroscopy

The in-plane optical anisotropy of several GaAs/AlGaAs quantum well samples with different well widths has been measured at room temperature by reflectance-difference spectroscopy (RDS). The RDS line shapes are found to be similar in all the samples examined here, which dominantly consist of two peak-like signals corresponding to 1HH-->1E and 1LH-->1E transition. As the well width is decreased, or the 1 ML InAs layer is inserted at one interface, the intensity of the anisotropy increases quickly. Our detail analysis shows that the anisotropy mainly arises from the anisotropic interface roughness. The results demonstrate that the RDS technique is sensitive to the interface structures.

Quantum-confined Stark effect and built-in dipole moment in self-assembled InAs/GaAs quantum dots

Quantum-confined Stark effect and built-in dipole moment in self-assembled InAs/GaAs quantum dots (QDs), which are grown at relative low temperature (460degreesC) and embedded in GaAs p-i-n structure, have been studied by dc-biased electroreflectance. Franz-Keldysh oscillations from the undoped GaAs layer are used to determine the electric field under various bias voltages. Stark shift of -34 meV for the ground-state interband transition of the QDs is observed when the electric field increases from 105 to 308 kV/cm. The separation of the electron and hole states in the growth direction of 0.4 nm, corresponding to the built-in dipole moment of 6.4x10(-29) C m, is determined. It is found that the electron state lies above that of the hole, which is the same as that predicted by theoretical calculations for ideal pyramidal InAs QDs. (C) 2004 American Institute of Physics.

Nonradiative recombination centers in Ga(As,N) and their annealing behavior studied by Raman spectroscopy

Nitrogen-related defects in diluted Ga(As,N) have been detected by Raman scattering in resonance with the localized E+ transition. These defects are attributed to local vibrational modes of nitrogen dimers on Ga- and As-lattice sites. Rapid thermal annealing under appropriate conditions is found to be able to remove the nitrogen dimers. The required minimum annealing temperature coincides with the threshold-like onset of strong, near-band-gap photoluminescence. This finding suggests that the nitrogen dimers are connected with nonradiative recombination centers. (C) 2004 American Institute of Physics.

Asymmetric quantum-confined Stark effects of hierarchical self-assembly of GaAs/AlxGa1-xAs quantum dots

Quantum-confined Stark effects in GaAs/AlxGa1-xAs self-assembled quantum dots are investigated theoretically in the framework of effective-mass envelope function theory. The electron and hole energy levels and optical transition energies are calculated in the presence of an electric field in different directions. In our calculation, the effect of finite offset, valence-band mixing, the effects due to the different effective masses of electrons and holes in different regions, and the real quantum dot structures are all taken into account. The results show that the electron and hole energy levels and the optical transition energies can cause blueshifts when the electric field is applied along the opposite to the growth direction. Our calculated results are useful for the application of hierarchical self-assembly of GaAs/AlxGa1-xAs quantum dots to photoelectric devices. (c) 2005 American Institute of Physics.

Characterization of stress induced in SOS and Si/gamma-Al2O3/Si heteroepitaxial thin films by Raman spectroscopy

Raman spectroscopy technique has been performed to investigate the stress induced in as-grown silicon-on-sapphire (SOS), solid-phase-epitaxy (SPE) re-grown SOS, and Si/gamma-Al2O3/Si double-heteroepitaxial thin films. It was demonstrated that the residual stress in SOS film, arising from mismatch and difference of thermal expansion coefficient between silicon and sapphire, was reduced efficiently by SPE process, and that the stress in Si/gamma-Al2O3/Si thin film is much smaller than that of as-grown SOS and SPE upgraded SOS films. The stress decrease for double heteroepitaxial film Si/gamma-Al2O3/Si mainly arises from the smaller lattice mismatching of 2.4% between silicon top layer and the gamma-Al2O3/Si epitaxiial composite substrate, comparing with the large lattice mismatch of 13% for SOS films. It indicated that gamma-Al2O3/Si as a silicon-based epitaxial substrate benefits for reducing the residual stress for further growth of silicon layer, compared with on bulk sapphire substrate. (c) 2005 Elsevier B.V. All rights reserved.

Surface-enhanced resonant Raman spectroscopy (SERRS) of single-walled carbon nanotubes absorbed on the Ag-coated anodic aluminum oxide (AAO) surface

In this paper, we developed a new kind of substrate, the silver-coated anodic aluminum oxide (AAO), to investigate the characters of surface-enhanced resonant Raman scattering (SERRS) of the dilute single-walled carbon nanotubes. Homogeneous Ag-coated AAO substrate was obtained by decomposing the AgNO3 on the surface of AAO. single-walled carbon nanotubes (SWNTs) were directly grown onto this substrate through floating catalyst chemical vapor deposition method (CVD). SERRS of SWNTs was carried out using several different wavelength lasers. The bands coming from metallic SWNTs were significantly enhanced. The two SERRS mechanisms, the "electromagnetic" and "chemical" mechanism, were mainly responsible for the experiment results. (c) 2005 Elsevier B.V. All rights reserved.

Interface-related in-plane optical anisotropy of quantum wells studied by reflectance-difference spectroscopy

The in-plane optical anisotropy of three groups of GaAs/AlGaAs quantum well structures has been studied by reflectance-difference spectroscopy (RDS). For GaAs/Al0.36Ga0.64As single QW structures, it is found that the optical anisotropy increases quickly as the well width is decreased. For an Al0.02Ga0.98As/AlAs multiple QW with a well width of 20nm, the optical anisotropy is observed not only for the transitions between ground states but also for those between the excited states with transition index n up to 5. An increase of the anisotropy with the transition energy, or equivalently the transition index n, is clearly observed. The detailed analysis shows that the observed anisotropy arises from the interface asymmetry of QWs, which is introduced by atomic segregation or anisotropic interface roughness formed during the growth of the structures. More, when the 1 ML InAs is inserted at one interface of GaAs/AlGaAs QW, the optical anisotropy of the QW can be increased by a factor of 8 due to the enhanced asymmetry of the QW. These results demonstrate clearly that the RDS is a sensitive and powerful tool for the characterization of semiconductor interfaces.