Characterization of deep centers in AlGaAs/InGaAs/GaAs pseudomorphic HEMT structures grown by molecular beam epitaxy and hydrogen treatment

In AlGaAs/InGaAs/GaAs PM-HEMT structures, the characterization of deep centers, the degradation in electrical and optical properties and their effects on electrical performance of the PM-HEMTs have been investigated by DLTS, SIMS, PL and conventional van der Pauw techniques. The experimental results confirm that the deep level centers correlate strongly with the oxygen content in the AlGaAs layer, the PL response of PM-HEMTs, and the electrical performance of the PM-HEMTs. Hydrogen plasma treatment was used to passivate/annihilate these centers, and the effects of hydrogenation were examined.

Deep centers in AlGaAs/GaAs GRIN-SCH SQW laser structures grown by MBE and MOCVD

The deep centers in AlGaAs/GaAs graded index-separate confinement heterostructure single quantum well (GRIN-SCHSQW) laser structures grown by MBE and MOCVD have been investigated using deep level transient spectroscopy (DLTS) technique, The majority and minority carrier DLTS spectra show that the deep (hole and electron) traps (Hi and E3), having large capture cross sections and concentrations, are observed in the graded n-AlxGa1-xAs layer of laser structures in addition to the well-known DX centers. For laser structures grown by MBE, the deep hole trap H1 and the deep electron trap E3 may be spatially localized in the interface regions of discontinuous variation Al mole fraction of the n-AlxGa1-xAs layer with x = 0.20-0.43. For laser structures grown by MOCVD, the deep electron trap E3 may be spatially localized in the n-AlxGa1-xAs layer with x = 0.18-0.30, and the DX center may be spatially localized in the interface regions of discontinuous variation Al mole fraction of the AlxGa1-xAs layer with x = 0.22-0.30.

Annealing ambient controlled deep defect formation in InP

Deep defects in annealed InP have been investigated by deep level transient capacitance spectroscopy (DLTS), photo induced current transient spectroscopy (PICTS) and thermally stimulated current spectroscopy (TSC). Both DLTS results of annealed semiconducting InP and PICTS and TSC results of annealed semi-insulating InP indicate that InP annealed in phosphorus ambient has five defects, while lid? annealed in iron phospbide ambient has two defects. Such a defect formation phenomenon is explained in terms of defect suppression by the iron atom diffusion process. The correlation of the defects and the nature of the defects in annealed InP are discussed based on the results.

Observation of deep electron states in n-type Al-doped ZnS1-xTex grown by molecular beam epitaxy

Deep level transient spectroscopy (DLTS) technique was used to investigate deep electron states in n-type Al-doped ZnS1-xTex epilayers grown by molecular fiction epitaxy (MBE), Deep level transient Fourier spectroscopy (DLTFS) spectra of the Al-doped ZnS1-xTex (x = 0. 0.017, 0.04 and 0.046. respectively) epilayers reveal that At doping leads to the formation of two electron traps at 0.21 and 0.39 eV below the conduction hand. 1)DLTFS results suggest that in addition to the rules of Te as a component of [lie alloy as well as isoelectronic centers, Te is also involved in the formation of all electron trip, whose energy level relative to the conduction hand decreases a, Te composition increases.

Nonvolatile holograms in LiNbO3 : Fe : Cu by use of the bleaching effect

We report our observation of a bleaching effect under an ultraviolet exposure in LiNbO3:Fe:Cu crystals. Two three-step recording-transferring-fixing schemes are proposed to record nonvolatile photorefractive holograms in such crystals. In the schemes two red laser beams and an ultraviolet illumination are used selectively to write the charge grating in the shallow-level Fe centers, to develop the charge grating in the deep-level Cu centers by transferring the charge grating in the Fe centers, and to fix only the charge grating in the Cu centers for unerasable read-out. Experimental results, verifications, and an optimal recording scheme are given. A comparison of the lithium niobate crystals of the same double-doping system of Fe:Mn, Ce:Mn, Ce:Cu, and Fe:Cu is outlined. (C) 2002 Optical Society of America.

Epitaxial growth of ZnO thin films on LiGaO2 substrates by pulsed-laser deposition

Lattice-matched (Delta(a/a) = 1.8-3.4%) (001) LiGaO2 substrates have been employed for the first time to grow ZnO thin films by pulsed-laser deposition at 350-650 degrees C with oxygen partial pressure of 20Pa. XRD shows that a highly c-axis-oriented ZnO film can be deposited on (001) LiGaO2 substrate at 500 degrees C. AFM images reveal the surfaces of as-deposited ZnO films are smooth and root-mean-square values are 6.662, 5.765 and 6.834 nm at 350, 500 and 650 degrees C, respectively. PL spectra indicate only near-band-edge UV emission appears in the curve of ZnO film deposited at 500 degrees C. The deep-level emission of ZnO film deposited at 650 degrees C probably results from Li diffusion into the film. All the results illustrate substrate temperature plays a pretty important role in obtaining ZnO film with a high quality on LiGaO2 substrate by pulsed-laser deposition. (c) 2006 Elsevier B.V. All rights reserved.

Annealing effect on properties of ZnO thin films grown on LiNbO3 substrates by MOCVD

ZnO films were grown on (0 0 0 1) LiNbO3 substrates by metal organic chemical vapor deposition (MOCVD). Annealing of ZnO films was performed in air for I h at 800 degrees C. The effects of annealing on the structural and optical properties of ZnO thin films on LiNbO3 substrates were investigated using X-ray diffraction (XRD), atomic force microscopy (AFM) and photoluminescence (PL) measurements. XRD patterns and AFM showed that the as-grown and the annealed ZnO films grown on LiNbO3 substrates had c-axis preferential orientation, the crystallinity of the ZnO films grown on LiNbO3 Substrates was improved, and the grain size increased by annealing. The PL spectra showed that the intensity of the UV near-band-edge peak was increased after annealing, while the intensity of visible peak (deep-level emission) decreased. (c) 2005 Elsevier B.V. All rights reserved.

Growth and properties of ZnO thin film on beta-Ga2O3(100) substrate by pulsed laser deposition

Zinc oxide (ZnO) thin films were grown on the beta-Ga2O3 (100) substrate by pulsed laser deposition (PLD). X-ray diffraction (XRD) indicated that the ZnO films are c-axis oriented. The optical and electrical properties of the films were investigated. The room temperature Photoluminescence (PL) spectrum showed a near band emission at 3.28 eV with two deep level emissions. Optical absorption indicated a visible exciton absorption at room temperature. The as-grown films had good electrical properties with the resistivities as low as 0.02 Omega cm at room temperature. Thus, beta-Ga2O3 (100) substrate is shown to be a suitable substrate for fabricating ZnO film. (c) 2006 Elsevier B.V. All rights reserved.

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.

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.

Fabrication and transport properties of ZnO/Nb-1 wt %-doped SrTiO3 epitaxial heterojunctions

(110) ZnO/(001) Nb-1 wt %-doped SrTiO3 n-n type heteroepitaxial junctions were fabricated using the pulse laser deposition method. A diodelike current behavior was observed. Different from conventional p-n junctions or Schottky diodes, the diffusion voltage was found to increase with temperature. At all temperatures, the forward current was perfectly fitted on the thermionic emission model. The band bending at the interface can qualitatively explain our results, and the extracted high ideality factor at low temperatures, as well as large saturation currents, is ascribed to the deep-level-assisted tunneling current through the junction. (C) 2008 American Institute of Physics.

Narrowing of band gap and low-temperature spin glass behavior of FeNi co-doped ZnO nanowires

This letter reports on the Raman, optical and magnetic properties of FeNi co-doped ZnO nanowires prepared via a soft chemical solution method. The microstructural investigations show that the NiFe co-dopants are substituted into wurtzite ZnO nanostructure without forming any secondary phase. The co-doped nanowires show a remarkable reduction of 34 nm (267.9 meV) in the optical band gap, while suppression in the deep-level defect transition in visible luminescence. Furthermore, these nanowires exhibit ferromagnetism and an interesting low-temperature spin glass behavior, which may arise due to the presence of disorder and strong interactions of frustrated spin moments of Ni and Fe co-dopants on the ZnO lattice sites. Copyright (C) EPLA, 2009

Structural and optical properties of ZnO films on SrTiO3 substrates by MOCVD

ZnO films have been fabricated on (0 0 1), (0 1 1) and (1 1 1) SrTiO3 (STO) substrates by metal-organic chemical vapour deposition (MOCVD). It is interesting that the ZnO films on (0 0 1) and (0 1 1) STO substrates show polar and semipolar orientations, which are different from previous reports, while the same growing direction of polar ZnO with previous results is found on (1 1 1) STO. For the atomic arrangements, two orthogonal domains and a single domain are observed on (0 0 1) and (1 1 1) STO, respectively. Photoluminescence spectra show that every sample has a sharp near-band-edge emission peak at about 3.28 eV without any deep-level emission band between 1.5 and 2.8 eV, implying a high optical quality. A violet emission around 3.0 eV is observed only in ZnO films on (0 0 1) and (0 1 1) STO substrates grown at 600 degrees C, which is discussed briefly. Additionally, the semipolar ZnO does not weaken the emission efficiency along with the reduction in the polarization effect compared with polar ZnO. These results show that high-quality polar and semipolar ZnO films can be grown on STO substrates by MOCVD.

Photoluminescence characterization of nanocrystalline ZnO array

High-density and uniform well-aligned ZnO sub-micron rods are synthesized on the silicon substrate over a large area. The morphology, and structure of the ZnO sub-micron rods are investigated by x-ray diffraction, transmission electron microscopy and Raman spectra. It is found that the ZnO sub-micron rods are of high crystal quality with the diameter in the range of 400-600 nm and the length of several micrometres long. The optical properties were studied bill photoluminescence spectra. The results show that the intensity of the ultraviolet emission at 3.3 eV is rather high, meanwhile the deep level transition centred at about 2.38 eV is weak. The free exciton emission could also be observed at low, temperature, which implies the high optical quality of the ZnO sub-micron rods. This growth technique provides one effective way to fabricate the high crystal quality ZnO nanowires array, which is very important for potential applications in the new-type optoelectronic nanodevices.

Effect of In0.2Ga0.8As and In0.2Al0.8As combination layer on band offsets of InAs quantum dots

We demonstrate the self-organized InAs quantum dots capped with thin and In0.2Al0.8As and In0.2Ga0.8As combination layers with a large ground and first excited energy separation emission at 1.35 mum at room temperature. Deep level transient spectroscopy is used to obtain quantitative information on emission activation energies and capture barriers for electrons and holes. For this system, the emission activation energies are larger than those for InAs/GaAs quantum dots. With the properties of wide energy separation and deep emission activation energies, self-organized InAs quantum dots capped with In0.2Al0.8As and In0.2Ga0.8As combination layers are one of the promising epitaxial structures of 1.3 mum quantum dot devices. (C) 2004 American Institute of Physics.