Anisotropic exchange splitting of excitons in (001)GaAs/Al0.3Ga0.7As superlattice studied by reflectance difference spectroscopy

Anisotropic exchange splitting (AES) is induced by the joint effects of the electron-hole exchange interaction and the symmetry reduction in quantum wells and quantum dots. A model has been developed to quantitatively obtain the electron-hole exchange energy and the hole-mixing energy of quantum wells and superlattices. In this model, the AES and the degree of polarization can both be obtained from the reflectance difference spectroscopy. Thus the electron-hole exchange energy and the hole-mixing energy can be completely separated and quantitatively deduced. By using this model, a (001)5 nm GaAs/7 nm Al0.3Ga0.7As superlattice sample subjected to [110] uniaxial strains has been investigated in detail. The n=1 heavy-hole (1H1E) exciton can be analyzed by this model. We find that the AES of quantum wells can be linearly tuned by the [110] uniaxial strains. The small uniaxial strains can only influence the hole-mixing interaction of quantum wells, but have almost no contribution to the electron-hole exchange interaction. (c) 2008 American Institute of Physics.

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

The valence band offset (VBO) of the wurtzite ZnO/4H-SiC heterojunction is directly determined to be 1.61 +/- 0.23 eV by x-ray photoelectron spectroscopy. The conduction band offset is deduced to be 1.50 +/- 0.23 eV from the known VBO value, which indicates a type-II band alignment for this heterojunction. The experimental VBO value is confirmed and in good agreement with the calculated value based on the transitive property of heterojunctions between ZnO, SiC, and GaN. (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]

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.

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.