Defects around self-organized InAs quantum dots measured by slow positron beam

Self-organized InAs quantum dots (QDs) have been fabricated by molecular beam epitaxy. The authors try to use a slow positron beam to detect defects in and around self-organized QDs, and point defects are observed in GaAs cap layer above QDs. For the self-organized InAs QDs without strain-reducing layer, it is free of defects. However, by introducing a strain-reducing layer, the density of point defects around larger sized InAs QDs increased. The above results suggest that low energy positron beam measurements may be a good approach to detect depth profiles of defects in QD materials. (c) 2007 American Institute of Physics.

Enhanced Pockels effect in GaN/AlxGa1-xN superlattice measured by polarization-maintaining fiber Mach-Zehnder interferometer

Six-period 4 nm GaN/10 nm AlxGa1-xN superlattices with different Al mole fractions x were prepared on (0001) sapphire substrates by low-temperature metal-organic chemical vapor deposition. The linear electro-optic (Pockels) effect was studied by a polarization-maintaining fiber-optical Mach-Zehnder interferometer system with an incident light wavelength of 1.55 mu m. The measured electro-optic coefficients, gamma(13)=5.60 +/- 0.18 pm/V, gamma(33)=19.24 +/- 1.21 pm/V (for sample 1, x=0.3), and gamma(13)=3.09 +/- 0.48 pm/V, gamma(33)=8.94 +/- 0.36 pm/V (for sample 2, x=0.1), respectively, are about ten times larger than those of GaN bulk material. The enhancement effect in GaN/AlxGa1-xN superlattice can be attributed to the large built-in field at the interfaces, depending on the mole fraction of Al. (C) 2007 American Institute of Physics.

Electro-optic coefficients of Si0.75Ge0.25/Si/Si0.5Ge0.5 asymmetric superlattice measured by polarization-maintaining fiber-optic Mach-Zehnder interferometer

Ten-period 5.5 nm Si0.75Ge0.25/10.3 nm Si/2.5 nm Si0.5Ge0.5 trilayer asymmetric superlattice was prepared on Si (001) substrate by ultrahigh vacuum chemical vapor deposition at 500 degrees C. The stability of Mach-Zehnder interferometer was improved by utilizing polarization-maintaining fibers. According to the electro-optic responses of the superlattice with the light polarization along [110] and [-110], respectively, both electro-optic coefficients gamma(13) and gamma(63) of such asymmetric superlattice were measured. gamma(13) and gamma(63) are 2.4x10(-11) and 1.3x10(-11) cm/V, respectively, with the incident light wavelength at 1.55 mu m. (c) 2006 American Institute of Physics.

Polarity dependence of hexagonal inclusions and cubic twins in GaN/GaAs(001) epilayers measured by conventional X-ray pole figure and grazing incident diffraction pole figure

The distribution of mixed phases and its dependence on the polarity of cubic GaN epilayers are investigated by conventional X-ray pole figure and grazing incident diffraction (GID) pole figure. The hexagonal inclusions and cubic twins can be classified into two portions: one is formed with strict crystalline orientations, the other with crystalline misorientations. The former can be measured by conventional pole figures which reveal that the density of lamellate hexagonal grains and cubic twins located on (1 1 1)(Ga) and ((1) over bar (1) over bar1)(Ga) along [1 (1) over bar 0] direction are higher than those on ((1) over bar 1 1), and (1 (1) over bar 1)(N) along [110] direction. However, the low signals from tiny mixed phases with crystalline misorientations, detected by GID pole figures, distribute in a larger phi region near the [1 1 0] and [(1) over bar (1) over bar 0] directions with much weaker intensity, and in a smaller phi region near the [1 (1) over bar 0] and [(1) over bar 1 0] directions with slightly stronger intensity. (C) 2001 Elsevier Science B.V. All rights reserved.

Triclinic deformation and anisotropic strain relaxation of an InAs film on a GaAs(001) substrate measured by a series of symmetric double crystal X-ray diffraction

We presented a series of symmetric double crystal X-ray diffraction (DCXD) measurements, (0 0 4), (2 2 0) and (2 - 2 0) diffraction, to investigate the strain relaxation in an InAs film grown on a GaAs(0 0 1) substrate. The strain tensor and rotation tensor were calculated according to the DCXD results. It is found that the misfit strain is relaxed nearly completely and the strain relaxation caused a triclinic deformation in the epilayer. The lattice parameter along the [1 1 0] direction is a little longer than that along the [1 - 1 0] direction. Furthermore, a significant tilt, 0.2 degrees, towards the [1 1 0] direction while a very slight one: 0.002 degrees, towards [1 - 1 0] direction were discussed. This anisotropic strain relaxation is attributed to the asymmetric distribution of misfit dislocations, which is also indicated by the variation of the full-width at half-maximum (FWHM) of (0 0 4) diffraction along four azimuth angles. (C) 1998 Elsevier Science B.V. All rights reserved.

Valence band offset of ZnO/BaTiO3 heterojunction measured by X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy has been used to measure the valence band offset of the ZnO/BaTiO3 heterojunction grown by metal-organic chemical vapor deposition. The valence band offset (VBO) is determined to be 0.48 +/- 0.09 eV, and the conduction band offset (CBO) is deduced to be about 0.75 eV using the band gap of 3.1 eV for bulk BaTiO3. It indicates that a type-II band alignment forms at the interface, in which the valence and conduction bands of ZnO are concomitantly higher than those of BaTiO3. The accurate determination of VBO and CBO is important for use of semiconductor/ferroelectric heterojunction multifunctional devices.

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.

Valence band offset of MgO/TiO2 (rutile) heterojunction measured by X-ray photoelectron spectroscopy

The valence band offset (VBO) of MgO/TiO2 (rutile) heterojunction has been directly measured by Xray photoelectron spectroscopy. The VBO of the heterojunction is determined to be 1.6 +/- 0.3 eV and the conduction band offset (CBO) is deduced to be 3.2 +/- 0.3 eV, indicating that the heterojunction exhibits a type-I band alignment. These large values are sufficient for MgO to act as tunneling barriers in TiO2 based devices. The accurate determination of the valence and conduction band offsets is important for use of MgO as a buffer layer in TiO2 based field-effect transistors and dye-sensitized solar cells.

Stoichiometry in GaAs grown in outer space measured nondestructively

A semi-insulating (SI) GaAs single crystal ingot was successfully grown in a recoverable satellite. The two-dimensional distribution of stoichiometry in space-grown SI-GaAs single crystal wafer was studied nondestructively based upon x-ray Band diffraction. The avenge stoichiometry in the space-grown crystal is 0.50007 with mean square deviation of 6 x 10(-6), and shows a better stoichiametric property than the ground-grown SI-GaAs. The average etch pit density (EPD) of dislocations in the crystal revealed by molten KOH is 2.0 x 10(4) cm(-2), and the highest EPD is 3.1 x 10(4) cm(-2). This result indicates that the structural properly of the crystal is quite good.