OPTICAL CHARACTERISTICS OF GAAS/ALGAAS RIDGE-QUANTUM-WELL-WIRES GROWN BY MBE ON NONPLANAR SUBSTRATES

With conventional photolithography and wet chemical etching, we have realized GaAs/AlGaAs buried ridge-quantum-well-wires (RQWWs) with vertically stacked wires in lateral arrays promising for device application, which were grown in situ by a single-step molecular beam epitaxy growth and formed at the ridge tops of mesas on nonplanar substrates. Confocal photoluminescence (CPL) and polarization-dependent photoreflectance (PR) are applied to study optical characteristics of RQWWs. Lateral bandgap modulation due to lateral variation of QW layer thickness is demonstrated not only by CPL but also by PR. As one evidence for RQWWs, a large blue shift is observed at the energy level positions for electronic transitions corresponding to quantum wells (QWs) at the ridge tops of mesas compared with those corresponding to QWs on nonpatterned areas of the same sample. The blue shift is in contradiction with the fact that the GaAs QW layers at the tops of the mesas are thicker than those on nonpatterned areas. The other evidence for RQWWs, optical anisotropy is provided by the polarization-dependent PR, which results from lateral quantum size effect existing at the tops of the mesas.

Photoluminescence study on coarsening of self-assembled InAlAs quantum dots on GaAs (001)

Red-emission at similar to 640 nm from self-assembled In0.55Al0.45As/Al0.5Ga0.5As quantum dots grown on GaAs substrate by molecular beam epitaxy (MBE) has been demonstrated. We obtained a double-peak structure of photoluminescence (PL) spectra from quantum dots. An atomic force micrograph (AFM) image for uncapped sample also shows a bimodal distribution of dot sizes. From the temperature and excitation intensity dependence of PL spectra, we found that the double-peak structure of PL spectra from quantum dots was strongly correlated to the two predominant quantum dot families. Taking into account quantum-size effect on the peak energy, we propose that the high (low) energy peak results from a smaller (larger) dot family, and this result is identical with the statistical distribution of dot lateral size from the AFM image.

Photon-photon scattering in a plasma channel

The Heisenberg-Euler correction due to photon-photon scattering, a still unverified quantum electrodynamics effect, on electromagnetic wave interaction inside a plasma channel is investigated theoretically. From a signal laser beam in the relativistic overdense plasma channel, photon-photon scattering can produce a detectable output beam of different frequency and polarization. (C) 2003 American Institute of Physics.

Enhancement of near-band-edge photoluminescence of ZnO thin films in sandwich configuration at room temperature

ZnO/ITO/ZnO sandwich structure films were fabricated. The effects of buffer layer on the structure and optical properties of ZnO films were investigated by x-ray diffraction (XRD), photoluminescence, optical transmittance, and absorption measurements. XRD spectra indicate that a buffer layer has the effects of lowering the grain orientation of ZnO films and increasing the residual stresses in the films. The near-band-edge emissions of ZnO films deposited on both single indium tin oxide (ITO) buffer and ITO/ZnO double buffers are significantly enhanced compared with that deposited on a bare substrate due to the quantum confinement effect. (C) 2006 American Institute of Physics.

Structure and UV emission of nanocrystal ZnO films by thermal oxidation of ZnS films

ZnO films prepared by the thermal oxidation of the ZnS films through thermal evaporation are reported. The as-deposited ZnS films have transformed to ZnO films completely at 400 degrees C. The 400-700 degrees C annealed films with a preferential c-axis (002) orientation have a hexagonal wurtzite structure. The band gap of ZnO films shifts towards longer wavelength with the increase of the annealing temperature. The relationship between the band gap energy of ZnO films and the grain size is discussed. The shift of the band gap energy can be ascribed to the quantum confinement effect in nanocrystal ZnO films. The photoluminescence spectra of ZnO films show a dominant ultraviolet emission and no deep level or trap state defect emission in the green region. It confirms the absence of interstitial zinc or oxygen vacancies in ZnO films. These results indicate that ZnO film prepared by this simple thermal oxidation method is a promising candidate for optoelectronic devices and UV laser. (c) 2005 Elsevier BN. All rights reserved.

Effects of GaN Capping on the Structural and the Optical Properties of InN Nanostructures Grown by Using MOCVD

InN nanostructures with and without GaN capping layers were grown by using metal-organic chemical vapor deposition. Morphological, structural, and optical properties were systematically studied by using atomic force microscopy, X-ray diffraction (XRD) and temperature-dependent photoluminescence (PL). XRD results show that an InGaN structure is formed for the sample with a GaN capping layer, which will reduce the quality and the IR PL emission of the InN. The lower emission peak at similar to 0.7 eV was theoretically fitted and assigned as the band edge emission of InN. Temperature-dependent PL shows a good quantum efficiency for the sample without a GaN capping layers; this corresponds to a lower density of dislocations and a small activation energy.

Investigation on the structural origin of n-type conductivity in InN films

This work presents a study of the correlation between the electrical properties and the structural defects in nominally undoped InN films. It is found that the density of edge-type threading dislocations (TDs) considerably affects the electron concentration and mobility in InN films. The Hall-effect measured electron concentration increases, while the Hall mobility decreases with the increase in the edge-type TD density. With the combination of secondary ion mass spectrometry and positron annihilation analysis, we suggest that donor-type point defects at the edge-type TD lines may serve as dominant donors in InN films and affect the carrier mobility.

Investigation of native defects and property of bulk ZnO single crystal grown by a closed chemical vapor transport method

Hall effect, Raman scattering, photoluminescence spectroscopy (PL), optical absorption (OA), mass spectroscopy, and X-ray diffraction have been used to study bulk ZnO single crystal grown by a closed chemical vapor transport method. The results indicate that shallow donor impurities (Ga and Al) are the dominant native defects responsible for n-type conduction of the ZnO single crystal. PL and OA results suggest that the as-grown and annealed ZnO samples with poor lattice perfection exhibit strong deep level green photoluminescence and weak ultraviolet luminescence. The deep level defect in as-grown ZnO is identified to be oxygen vacancy. After high-temperature annealing, the deep level photoluminescence is suppressed in ZnO crystal with good lattice perfection. In contrast, the photoluminescence is nearly unchanged or even enhanced in ZnO crystal with grain boundary or mosaic structure. This result indicates that a trapping effect of the defect exists at the grain boundary in ZnO single crystal. (C) 2007 Elsevier B.V. All rights reserved.

Thickness dependent dislocation, electrical and optical properties in InN films grown by MOCVD

InN thin films with different thicknesses are grown by metal organic chemical vapor deposition, and the dislocations, electrical and optical properties are investigated. Based on the model of mosaic crystal, by means of X-ray diffraction skew geometry scan, the edge dislocation densities of 4.2 x 10(10) cm(-2) and 6.3 x 10(10) cm(-2) are fitted, and the decrease of twist angle and dislocation density in thicker films are observed. The carrier concentrations of 9 x 10(18) cm(-3) and 1.2 x 10(18) cm(-3) are obtained by room temperature Hall effect measurement. V-N is shown to be the origin of background carriers, and the dependence of concentration and mobility on film thickness is explained. By the analysis of S-shape temperature dependence of photoluminescence peak, the defects induced carrier localization is suggested be involved in the photoluminescence. Taking both the localization and energy band shrinkage effect into account, the localization energies of 5.05 meV and 5.58 meV for samples of different thicknesses are calculated, and the decrease of the carrier localization effect in the thicker sample can be attributed to the reduction of defects.

Origin and Enhancement of Hole-Induced Ferromagnetism in First-Row d(0) Semiconductors

The origin of ferromagnetism in d(0) semiconductors is studied using first-principles methods with ZnO as a prototype material. We show that the presence of spontaneous magnetization in nitrides and oxides with sufficient holes is an intrinsic property of these first-row d(0) semiconductors and can be attributed to the localized nature of the 2p states of O and N. We find that acceptor doping, especially doping at the anion site, can enhance the ferromagnetism with much smaller threshold hole concentrations. The quantum confinement effect also reduces the critical hole concentration to induce ferromagnetism in ZnO nanowires. The characteristic nonmonotonic spin couplings in these systems are explained in terms of the band coupling model.

Effects of polarization field on formation of two-dimensional electron gas in (0001) and (11(2)over-bar0) plane AlGaN/GaN heterostructures

Photoluminescence (PL) and temperature-dependent Hall effect measurements were carried out in (0001) and (11 (2) over bar0) AlGaN/GaN heterostructures grown on sapphire substrates by metalorganic chemical vapor deposition. There are strong spontaneous and piezoelectric electric fields (SPF) along the growth orientation of the (0001) AlGaN/GaN heterostructures. At the same time there are no corresponding SPF along that of the (1120) AlGaN/GaN. A strong PL peak related to the recombination between two-dimensional electron gas (2DEG) and photoexcited holes was observed at 3.258 eV at room temperature in (0001) AlGaN/GaN heterointerfaces while no corresponding PL peak was observed in (11 (2) over bar0). The existence of a 2DEG was observed in (0001) AlGaN/GaN multi-layers with a mobility saturated at 6000 cm(2)/V s below 80 K, whereas a much lower mobility was measured in (11 (2) over bar0). These results indicated that the SPF was the main element to cause the high mobility and high sheet-electron-density 2DEG in AlGaN/GaN heterostructures. (C) 2004 Elsevier B.V. All rights reserved.

Origin and evolution of photoluminescence from Si nanocrystals embedded in a SiO2 matrix

A detailed analysis of the photoluminescence (PL) from Si nanocrystals (NCs) embedded in a silicon-rich SiO2 matrix is reported. The PL spectra consist of three Gaussian bands (peaks A,B, and C), originated from the quantum confinement effect of Si NCs, the interface state effect between a Si NC and a SiO2 matrix, and the localized state transitions of amorphous Si clusters, respectively. The size and the surface chemistry of Si NCs are two major factors affecting the transition of the dominant PL origin from the quantum confinement effect to the interface state recombination. The larger the size of Si NCs and the higher the interface state density (in particular, Si = O bonds), the more beneficial for the interface state recombination process to surpass the quantum confinement process, in good agreement with Qin's prediction in Qin and Li [Phys. Rev. B 68, 85309 (2003)]. The realistic model of Si NCs embedded in a SiO2 matrix provides a firm theoretical support to explain the transition trend.

Photoluminescence and transmission spectra of nanocrystalline GaAs1-xSbx embedded in silica films

The composite films of the nanocrystalline GaAs1-xSbx-SiO2 have been successfully deposited on glass and GaSb substrates by radio frequency magnetron co-sputtering. The 10K photoluminescence (PL) properties of the nanocrystalline GaAs1-xSbx indicated that the PL peaks of the GaAs1-xSbx nanocrystals follow the quantum confinement model very closely. Optical transmittance spectra showed that there is a large blue shift of optical absorption edge in nanocrystalline GaAs1-xSbx-SiO2 composite films, as compared with that of the corresponding bulk semiconductor, which is due to the quantum confinement effect.

Formation of GaAs/AlGaAs and InGaAs/GaAs nanorings by droplet molecular-beam epitaxy

GaAs/AlGaAs lattice-matched nanorings are formed on GaAs (100) substrates by droplet epitaxy. The crucial step in the formation of nanorings is annealing Ga droplets under As flux for proper time. The observed morphologic evolution of Ga droplets during annealing does not support the hypothesis that As atoms preferentially react with Ga around the periphery of the droplets, but somehow relates to a dewetting process similar to that of unstable films. Photoluminescene (PL) test results confirm the quantum-confinement effect of these GaAs nanorings. Using similar methods, we have fabricated InGaAs/GaAs lattice-mismatched rings. (c) 2005 American Institute of Physics.

NIP a-Si : H solar cells on stanless steel with p-type nc-Si : H window layer

The successful application of boron-doped hydrogenated nanocrystalline silicon as window layer in a-Si: H nip solar cells on stainless steel foil with a thickness of 0.05 mm is reported. Open circuit voltage and fill factor of the fabricated solar cell were 0.90V and 0.70 respectively. The optical and structural properties of the p-layers have been investigated by using UV-VIS and Raman spectroscopy. It is confirmed that the p-layer is hydrogenated nanocrystalline silicon with a wide optical gap due to quantum size effect.