F. Iikawa

Absence of ferromagnetic order in high quality bulk Co-doped ZnO samples.

Bulk Zn1�xCoxO samples were synthesized via standard solid-state reaction route with different Co molar concentrations up to 21%. A detailed microstructural analysis was carried out to investigate alternative sources of ferromagnetism, such as secondary phases and nanocrystals embedded in the bulk material. Conjugating different techniques we confirmed the Zn replacement by Co ions in the wurtzite ZnO structure, which retains, however, a high crystalline quality. No segregated secondary phases neither Co-rich nanocrystals were detected. Superconducting quantum interference device magnetometry demonstrates a paramagnetic Curie–Weiss behavior with antiferromagnetic interactions. We discuss the observed room temperature paramagnetism of our samples considering the current models for the magnetic properties of diluted magnetic semiconductors.

Aharonov-Bohm interference in neutral excitons: effects of built-in electric fields

We report a comprehensive discussion of quantum interference effects due to the finite structure of neutral excitons in quantum rings and their first experimental corroboration observed in the optical recombinations. The signatures of built-in electric fields and temperature on quantum interference are demonstrated by theoretical models that describe the modulation of the interference pattern and confirmed by complementary experimental procedures.

Ferromagnetic nanoclusters formed by Mn implantation in GaAs

Ferromagnetic clusters were incorporated into GaAs samples by Mn implantation and subsequent annealing. The composition and structural properties of the Mn-based nanoclusters formed at the surface and buried into the GaAs sample were analyzed by x-ray and microscopic techniques. Our measurements indicate the presence of buried MnAs nanoclusters with a structural phase transition around 40 °C, in accord with the first-order magneto-structural phase transition of bulk MnAs. We discuss the structural behavior of these nanoclusters during their formation and phase transition, which is an important point for technological applications.

Optical emission of InAs nanowires

Wurtzite InAs nanowire samples grown by chemical beam epitaxy have been analyzed by photoluminescence spectroscopy. The nanowires exhibit two main optical emission bands at low temperatures. They are attributed to the recombination of carriers in quantum well structures, formed by zincblende-wurtzite alternating layers, and to the donor-acceptor pair. The blue-shift observed in the former emission band when the excitation power is increased is in good agreement with the type-II band alignment between the wurtzite and zincblende sections predicted by previous theoretical works. When increasing the temperature and the excitation power successively, an additional band attributed to the band-to-band recombination from wurtzite InAs appears. We estimated a lower bound for the wurtzite band gap energy of approximately 0.46 eV at low temperature.

Raman scattering from InGaAs/GaAs strained-layer superlattices

Abstract We report the results of room temperature Raman scattering experiments on a series of In0.12Ga0.88As/GaAs strained-layer superlattices. From these measurements the amount of strain present in each type of layer is determined quantitatively. This information is used to discuss the question of critical thicknesses for this type of superlattice.

X-ray method to study temperature-dependent stripe domains in MnAs∕GaAs(001)

MnAs films grown on GaAs (001) exhibit a progressive transition between hexagonal (ferromagnetic) and orthorhombic (paramagnetic) phases at wide temperature range instead of abrupt transition during the first-order phase transition. The coexistence of two phases is favored by the anisotropic strain arising from the constraint on the MnAs films imposed by the substrate. This phase coexistence occurs in ordered arrangement alternating periodic terrace steps. We present here a method to study the surface morphology throughout this transition by means of specular and diffuse scattering of soft x rays, tuning the photon energy at the Mn2p resonance. The results show the long-range arrangement of the periodic stripe-like structure during the phase coexistence and its period remains constant, in agreement with previous results using other techniques.

On the origin of the blueshift from type-II quantum dots emission using microphotoluminescence

We have studied type-II InP/GaAs self-assembled quantum dots by microphotoluminescence spectroscopy. Sharp spectral features were observed on top of a broad emission band. They are associated to statistical fluctuations from the ensemble of dots. Photoluminescence measurements as a function of the excitation intensity revealed markedly distinct behaviors: the broadband contour shows a large blueshift while the energy positions of the sharp features remain basically constant. We show that the large blueshift of the broad emission band in type-II quantum dots is not due to the barrier interface potential variation, but to the state filling of higher-energy states.

Photoreflectance measurements on Si δ‐doped GaAs samples grown by molecular‐beam epitaxy

We investigate δ‐doped GaAs samples grown by molecular‐beam epitaxy with different silicon areal concentration and cap layer thickness, using photoreflectance spectroscopy. The features observed on the high‐energy side of the fundamental gap are attributed to transitions involving electronic sub‐bands in the δ‐doped potential well that take into account the diffusion of the dopants.

Thermally induced In/Ga interdiffusion in InxGa1−xAs/GaAs strained single quantum well grown by LPMOVPE

Abstract A strained single quantum well of GaAs/In x Ga 1− x Ga/GaAs ( x =0.23) has been grown by low pressure MOVPE on a (100) GaAs substrate at 625° C. Samples were annealed under AsH 3 /H 2 at different temperatures in the range 750 to 900° C. Since the quantum well thickness (∼80 A) is below the critical value for this lattice mismatched system, we assume that the InGaAs layer is commensurate with the GaAs substrate. We analyse the low temperature (2 K) photoluminescence of the electron to heavy hole transition in the quantum well of these samples to study the In/Ga interdiffusion at the InGaAs/GaAs interfaces. The position in energy of the photoluminescence peaks shift to higher energies when annealing. The shifts are quantitatively interpreted in terms of changes in the quantum well profile due to the In and Ga interdiffusion. We determined the interdiffusion coefficient at 850° C to be 3×10 −17 cm 2 /s, with an activation energy 2.07 eV. The values obtained for the In/Ga interdiffusion coefficient are larger than those published for the Al and Ga interdiffusion in AlGaAs/GaAs heterojunctions.

Defect spectroscopy of single ZnO microwires

The point defects of single ZnO microwires grown by carbothermal reduction were studied by microphotoluminescence, photoresistance excitation spectra, and resistance as a function of the temperature. We found the deep level defect density profile along the microwire showing that the concentration of defects decreases from the base to the tip of the microwires and this effect correlates with a band gap narrowing. The results show a characteristic deep defect levels inside the gap at 0.88 eV from the top of the VB. The resistance as a function of the temperature shows defect levels next to the bottom of the CB at 110 meV and a mean defect concentration of 4 × 1018 cm−3. This combination of techniques allows us to study the band gap values and defects states inside the gap in single ZnO microwires and opens the possibility to be used as a defect spectroscopy method.