Structural and optical properties of ZnO films on Si substrates using a gamma-Al2O3 buffer layer

High quality ZnO films have been successfully grown on a Si (100) substrate by metal organic chemical vapour deposition with a gamma-Al2O3 buffer. The crystal structure, surface morphology and optical properties of the ZnO films were characterized by x-ray diffraction, Raman spectroscopy, atomic force microscopy and photoluminescence (PL) spectroscopy. The propel-ties of the films with the Al2O3 buffer were improved in comparison with those of as-grown ZnO films. It is shown that the ZnO films with the gamma-Al2O3 buffer grown on Si (100) substrates have a highly-preferential c-axis (0002) orientation, a narrow (0002) peak, smooth surface morphology and better PL spectral properties. This demonstrates that the use of gamma-Al2O3/Si as a ZnO substrate is beneficial for reducing the residual stress for further growth of ZnO films, compared with the growth on bulk Si substrates.

Optical properties of self-assembled InAs/InAlAs/InP quantum wires with different InAs deposited thickness

We report on the photoluminescence (PL) properties of InAs/InAlAs/InP quantum wires (QWRs) with various InAs deposited thickness. The PL linewidth of the QWRs decreases with increasing InAs deposited thickness due to the different thicknesses of the QWRs and defects in the samples. The defects and lateral composition modulation of the InAlAs layers play an important role in the temperature-dependent PL properties of the samples. (c) 2005 Elsevier B.V. All rights reserved.

Effects of electric field on the electronic structure and optical properties of quantum rods with wurtzite structure

The Hamiltonian of wurtzite quantum rods with an ellipsoidal boundary under electric field is given after a coordinate transformation. The electronic structure and optical properties are studied in the framework of the effective-mass envelope-function theory. The quantum-confined Stark effect is illustrated by studying the change of the electronic structures under electric field. The transition probabilities between the electron and hole states decrease sharply with the increase of the electric field. The polarization factor increases with the increase of the electric field. Effects of the electric field and the shape of the rods on the exciton effect are also investigated. The exciton binding energy decreases with the increase of both the electric field and the aspect ratio. In the end, considering the exciton binding energy, we calculated the band gap variation of size- and shape-controlled colloidal CdSe quantum rods, which is in good agreement with experimental results.

Structural and optical properties of strain-compensated GaAsSb/GaAs quantum wells with high Sb composition

The structural and optical properties of GaAsSb/GaAs quantum wells (QWs) and strain-compensated GaAsP/GaAs/GaAsSb/GaAs/GaAsP QWs grown on a GaAs substrate by molecular beam epitaxy are investigated using high-resolution x-ray diffraction and photoluminescence (PL) measurements. We demonstrated that the insertion of tensile GaAsP layers into the active region of GaAsSb/GaAs QWs effectively improves the structural and optical quality. Even the Sb composition is as high as 0.39. The PL spectra at 11 K and room temperature indicate that the PL peak of strain-compensated QWs has a narrower linewidth and higher intensity in comparison to the sample without strain compensation. The results of PL peak blueshift with increasing excitation show the strain-compensated GaAsSb/GaAs interface characteristic of type-I band alignment. (C) 2003 American Institute of Physics.

A study of the growth and optical properties of AlInGaN alloys

We have studied the growth and optical properties of AlInGaN alloys in this article. By the measurement of three samples, we found that the incorporation of In decreases with the increase of temperature, while there is nearly no change for the incorporation of Al. The sample grown at the lowest temperature had the best material and optical properties, which owes to the high In component, because the In component can reduce defects and improve the material quality. We also used the time-resolved photoluminescence(PL) to study the mechanism of recombination of carriers, and found that the time dependence of PL intensity was not in exponential decay, but in stretched-exponential decay. Through the study of the character of this decay, we come to the conclusion that the emission comes from the recombination of localized excitons. Once more, this localization exhibites the character of quantum dots, and the stretched, exponential decay results from the hopping of carriers between different localized states. In addition, we have used the relation of emission energy dependence of carrier's lifetime and the character of radiative recombination and non-radiative combination to confirm our conclusion.

Size evolution and optical properties of self-assembled InAs quantum dots on different matrix

InAs quantum dots have been grown by solid source molecular beam epitaxy on different matrix to investigate the effect on the structure and optical properties. High density of 1.02 x 10(11) cm(-2) of InAs islands on In0.15Ga0.85As and In0.15Al0.85As underlying layer has been achieved. Atomic force microscopy and photoluminescence spectra show the size evolution of InAs islands on In0.15Ga0.85As underlying layer. A strong 1.3 mum photoluminescence from InAs islands on In0.15Ga0.85As underlying layer and with InGaAs strain-reduced layer has been obtained. Single-mirror light emitting diode structures with InAs quantum dots capped by InGaAs grown on InGaAs layer as active layer were fabricated and the corresponding radiative efficiency was deduced to be as high as 20.5%. Our results provide important information for optimizing the epitaxial structures of 1.3 mum wavelength quantum dots devices. (C) 2003 Elsevier B.V. All rights reserved.

Optical properties of InGaAs self-assembled quantum dots with InAlAs wetting layer

A new self-assembled quantum dots system where InGaAs dots are formed on InAlAs wetting layer and embedded in GaAs matrix has been fabricated. The photoluminescence linewidth increases with increasing temperature, which is very different from normal In(Ga)As/GaAs quantum dots. The results are attributed to a higher energy of the wetting layer which breaks the carrier transfer channel between dots and keeps the dots more isolated from each other.

Stress and its effect on optical properties of GaN epilayers grown on Si(111), 6H-SiC(0001), and c-plane sapphire

The stress states in unintentionally doped GaN epilayers grown on Si(111), 6H-SiC(0001), and c-plane sapphire, and their effects on optical properties of GaN films were investigated by means of room-temperature confocal micro-Raman scattering and photoluminescence techniques. Relatively large tensile stress exists in GaN epilayers grown on Si and 6H-SiC while a small compressive stress appears in the film grown on sapphire. The latter indicates effective strain relaxation in the GaN buffer layer inserted in the GaN/sapphire sample, while the 50-nm-thick AlN buffer adopted in the GaN/Si sample remains highly strained. The analysis shows that the thermal mismatch between the epilayers and the substrates plays a major role in determining the residual strain in the films. Finally, a linear coefficient of 21.1+/-3.2 meV/GPa characterizing the relationship between the luminescent bandgap and the biaxial stress of the GaN films is obtained. (C) 2003 American Institute of Physics.

Study of optical properties in GaAs1-xSbx/GaAs single quantum wells

GaAsSb/GaAs single quantum wells (SQWs) grown by molecular beam epitaxy are studied by selectively-excited photoluminescence (SEPL) measurement. For the first time, we have simultaneously observed the PL, from both type I and type II transitions in GaAsSb/GaAs heterostructure in the SEPL. The two transitions exhibit different PL, behaviours under different excitation energy. As expected, the peak energy of type I emission remains constant in the whole excitation energy range we used, while type U transition shows a significant blue shift with increasing excitation energy. The observed blue shift is well explained in terms of electron-hole charge separation model at the interface. Time-resolved(TR) PL exhibits more type 11 characteristic of GaAsSb/GaAs QW. Moreover, the results of the excitation-power-dependent PL and TRPL provide more direct information on the type-II nature of the band alignment in GaAsSb/GaAs quantum-well structures. By combining the experimental results with some simple calculations, we have obtained the strained and unstrained valence band offsets of Q(v) = 1.145 and Q(v)(0) = 0. 76 in our samples, respectively.

Effects of the crystal structure on electrical and optical properties of pyrite FeS2 films prepared by thermally sulfurizing iron films

Based on experimental results and theoretical analysis effects of the crystal structure on the optical and electrical properties of pyrite FeS2 films produced by thermally sulfurizing iron films at various temperatures have been systematically studied. The results indicate that the crystal structure and some related factors, such as the crystallization and the stoichiometry, remarkably influence the optical and electrical performances of the pyrite films. It is also shown that the preferred orientation of the crystal grain plays a major role in determining the crystal structure and the optical and electrical properties of the pyrite FeS2 films. Also we find that it is the crystal grains, rather than the particles that exercise a decisive influence on the electrical performance of pyrite films. (C) 2003 Elsevier Science B.V. All rights reserved.

Effects of seed dot layer and thin GaAs spacer layer on the structure and optical properties of upper In(Ga)As quantum dots

The structure and optical properties of In(Ga)As with the introduction of InGaAlAs or InAlAs seed dot layers are investigated. The area density and size homogeneity of the upper InGaAs dots are efficiently improved by the introduction of a buried layer of high-density dots. Our explanation for the realization of high density and size homogeneity dots is presented. When the GaAs spacer layer is too thin to cover the seed dots, the upper dots exhibit some optical properties like those of a quantum well. By analyzing the growth dynamics, we refer to this kind of dot as an empty-core dot. (C) 2003 American Institute of Physics.

Nonlinear optical properties and thermal stability of the poled polymer NTAB/PEK-c

Films of high glass' transition temperature polymer polyetherketone doped with chromophore 2,2'[4-[(5-nitro-2-thiazolyl)azophenyl]-amino]-bisethanol NTAB) were prepared, poled by the corona-onset poling setup which includes a grid voltage making the surface-charge distribution uniform at elevated temperature. The thickness of the films was measured by the Model 2010 Prism Coupler system. Second harmonic generation d(33) was measured by the second harmonic generation method, and the d33 is 38.12 pm/V at 1064 nm under the absorption correction. The nonlinear optical activity maintains is 80% of its initial value. (C) 2002 Elsevier Science B.V. All rights reserved.

The effects of rapid thermal annealing on the optical properties of Zn1-xMnxSe epilayer grown by MOCVD on GaAs substrate

Zn1-xMnxSe thin films with different Mn compositions are grown by metal-organic chemical vapor deposition on GaAs substrate. Good crystallinity of sample is evidenced by X-ray diffraction and rocking-curve measurements. Photoluminescence (PL) properties were carefully studied. A dominant PL peak close to the band edge is observed at low temperature for samples with higher Mn concentration. The temperature-dependent PL and time-resolved photoluminescence show that this emission peak is associated with the recombination of exciton bound to Mn-induced impurity bound states. It is found that rapid thermal annealing can induce reorganization of Mn composition in alloys and significantly reduce the density of impurity induced by Mn incorporation and improve the intrinsic interband transition. (C) 2002 Elsevier Science B.V. All rights reserved.

Magneto-optical properties of diluted magnetic semiconductor quantum dots

The magnetoexciton polaron (MP) is investigated theoretically in a diluted magnetic semiconductor quantum dot (QD), with the Coulomb interaction and the sp-d exchange interaction included. The MP energy decreases rapidly with increasing magnetic field at low magnetic field and saturates at high magnetic field for small QDs, and the dependences of the MP energy on magnetic field are quite different for different QD radii due to the different carrier-induced magnetic fields B-MP. The competition between the sp-d exchange interaction and the band gap shrinkage results in there being a maximum exhibited by the MP energy With increasing temperature. Our numerical results are in good agreement with experiment (Maksimov A A, Bacher G, MacDonald A, Kulakovskii V D, Forchel A, Becker C R, Landwehr G and Molenkamp L W 2000 Phys. Rev. B 62 R7767).

Structural and optical properties of InAlGaN films grown directly on low-temperature buffer layer with (0001)sapphire substrate

Quaternary InAlGaN film has been grown directly on top of low-temperature-deposited GaN buffer layer by low-pressure metalorganic vapor phase epitaxy. High-resolution X-ray diffraction and photoluminescence (PL) results show that the film has good crystal quality and optical property. Temperature-dependent PL and time-resolved PL (TRPL) have been employed to study the carriers recombination dynamics in the film. The TRPL signals can be well fitted as a stretched exponential function exp[-(t/tau)(beta)] from 14 to 250 K, indicating that the emission is attributed to the radiative recombination of excitons localized in disorder quantum nanostructures such as quantum disks originating from indium (In) clusters or In composition fluctuation. The cross-sectional high-resolution electron microscopy measurement further proves that there exist the disorder quantum nanostructures in the quaternary. By investigating the dependence of the exponential parameter beta on the temperature, it is shown that the multiple trapping-detrapping mechanism dominates the diffusion among the localized states. The localized states are considered to have two-dimensional density of states (DOS) at 250 K, since radiative recombination lifetime tau(r) increases linearly with increasing temperature. (C) 2002 Elsevier Science B.V. All rights reserved.