Dose rate dependence of electrical characteristics of lead zirconate titanate capacitors

The influence of gamma-radiation dose rate on the electrical properties of lead zirconate titanate capacitors was investigated. More severe degradations in dielectric constant, coercive field, remanent polarization and capacitance-voltage (C-V) curves occurred with increasing radiation dose at lower dose rates. The electrical properties exhibited distinct radiation dose rate dependence and the worst-case degradation occurred at the lowest dose rate. The radiation-induced degradation of parameters such as the coercive field drift and distortion of the C-V curve can be recovered partly through post-irradiation annealing.

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.

Pressure dependence of Mn2+ luminescence in differently sized ZnS : Mn nanoparticles

The pressure behavior of Mn2+ emission in the 10-, 4.5-, 3.5-, 3-, and 1-nm-sized ZnS:Mn2+ nanoparticles is investigated. The emission shifts to lower energies with increasing pressure, and the shift rate (the absolute value of the pressure coefficient) is larger in the ZnS:Mn2+ nanoparticles than in bulk. The pressure coefficient increases with the decrease in particle size with the 1-nm-sized particles as an exception. Pressure coefficient calculations based on the crystal field theory are in agreement with the experimental results. The pressure dependence of the emission intensity is also size dependent. For nanoparticles 1 and 3 nm in size, the luminescence intensity of Mn2+ decreases dramatically with increasing pressure, while, for bulk and particles with average sizes of 3.5, 4.5, and 10 nm, the luminescence intensity of Mn2+ is virtually unchanged at different pressures. The bandwidth increases faster with increasing pressure for smaller particles. This is perhaps due to the fact that there are more Mn2+ ions at the near-surface sites and because the phonon frequency is greater for smaller particles. These new phenomena provide some insight into the luminescence behavior of Mn2+ in ZnS:Mn2+ nanoparticles.

Abnormal temperature dependence of photoluminescence from self-assembled InAs quantum dots covered by an InAlAs/InGaAs combination layer

In this report we have investigated the temperature dependence of photoluminescence (PL) from self-assembled InAs quantum dots (QDs) covered by an InAlAs/InGaAs combination layer. The ground state experiences an abnormal variation of PL linewidth from 15 K up to room temperature. Meanwhile, the PL integrated intensity ratio of the first excited state to the ground state for InAs QDs unexpectedly decreases with increasing temperature, which we attribute to the phonon bottleneck effect. We believe that these experimental results are closely related to the partially coupled quantum dots system and the large energy separation between the ground and the first excited states. (C) 2003 Elsevier Science Ltd. All rights reserved.

Large excitation-power dependence of pressure coefficients of InxGa1-xN/InyGa1-yN quantum wells

Excitation-power dependence of hydrostatic pressure coefficients (dE/dP) of InxGa1-xN/InyGa1-yN multiple quantum wells is reported. When the excitation power increases from 1.0 to 33 mW, dE/dP increases from 26.9 to 33.8 meV/GPa, which is an increase by 25%. A saturation behavior of dE/dP with the excitation power is observed. The increment of dE/dP with increasing carrier density is explained by an reduction of the internal piezoelectric field due to an efficient screening effect of the free carriers on the field.

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.

Temperature dependence of photoluminescence of flat and undulated SiGe/Si multiple quantum wells

Photoluminescence (PL) of strained SiGe/Si multiple quantum wells (MQW) with flat and undulated SiGe well layers was studied at different temperature. With elevated temperature from 10K, the no-phonon (NP) peak of the SiGe layers in the flat sample has firstly a blue shift due to the dominant transition converting from bound excitons (BE) to free excitons (FE), and then has a red shift when the temperature is higher than 30K because of the narrowing of the band gap. In the undulated sample, however, monotonous blue shift was observed as the temperature was elevated from 10 K to 287 K. The thermally activated electrons, confined in Si due to type-II band alignment, leak into the SiGe crest regions, and the leakage is enhanced with the elevated temperature. It results in a blue shift of the SiGe luminescence spectra.

Theory and experiment of two-photon 'direct' interference for type-II spontaneous parametric down-conversion with an ultrafast pulse laser pump

Fourth-order spatial interference of entangled photon pairs generated in the process of spontaneous parametric down-conversion pumped by a femtosecond pulse laser has been performed for the first time. In theory, it takes into account the transverse correlation between the two photons and is used to calculate the dependence of the visibility of the interference pattern obtained in Young's double-slit experiment. In this experiment, a short focal length tens and two narrow band interference filters were adopted to eliminate the effects of the broadband pump laser and improve the visibility of the interference pattern under the condition of nearly collinear light and degenerate phase matching.

Dependence of optical properties on the structure of multi-layer self-organized InAs quantum dots emitting near 1.3 mu m

Self-organized InAs quantum dots (QDs) have been fabricated by molecular beam epitaxy and characterized by photoluminescence (PL). For both single- and multi-layer QDs, PL intensity of the first excited state is larger than that of the ground state at 15 K. Conversely, at room temperature (RT), PL intensity of the first excited state is smaller than that of the ground state. This result is explained by the phonon bottleneck effect. To the ground state, the PL intensities of the multi-layer QDs are larger than that of the single-layer QDs at 15 K, while the intensities are smaller than that of the single-layer QDs at RT. This is due to the defects in the multi-layer QD samples acting as the nonradiative recombination centers. The inter-diffusion of Ga and In atoms in the growth process of multi-layer QDs results in the PL blueshift of the ground state and broadening of the full-width at half-maximum (FWHM), which can be avoided by decreasing the spacers' growth temperature. At the spacers' growth temperature of 520degreesC, we have prepared the 5-layer QDs which emit near 1.3 mum with a FWHM of 31.7 meV at RT, and 27.9 meV at 77 K. (C) 2002 Published by Elsevier Science B.V.

Effects of annealing on properties of spin-valve pinned with FeMn alloy

The magnetic properties of spin-valve pinned by FeMn layer were investigated after it was annealed at different temperatures. Its property was dependent on the vacuum annealing temperature. The pinning field could be increased through annealing at a temporature lower than 200degreesC;the pinning field would reduce and other properties be deteriorated as the annealing temperature was higher than 200degreesC; the pinning effect lost and giantmagnetic resistance disappeared at 300degreesC. Based on the results of AES analysis it was concluded that the diffusion in spin-valve multilayer was along grain boundary.

Structure and optical properties of self-assembled InAs/GaAs quantum dots with In0.15Ga0.85As underlying layer

A high density of 1.02 x 10(11) cm(-2) of InAs islands with In(0.15)Gao(0.85)As underlying layer has been achieved on GaAs (10 0) substrate by solid source molecular beam epitaxy. Atomic force microscopy and PL spectra show the size evolution of InAs islands. A 1.3 mum photoluminescence (PL) from InAs islands with In(0.15)Gao(0.85)As underlying layer and InGaAs strain-reduced layer has been obtained. Our results provide important information for optimizing the epitaxial structures of 1.3 mum wavelength quantum dots devices. (C) 2002 Elsevier Science B.V. All rights reserved.

Depth dependence of the tetragonal distortion of a GaN layer on Si(111) studied by Rutherford backscattering/channeling

Rutherford backscattering and channeling have been used to characterize the structure of a GaN layer grown on a Si(111) substrate. The results show that a 1.26 mum GaN epitaxial layer with a rather abrupt interface and a good crystalline quality (chi(min)=3.4%) can be grown on a Si(111) substrate. Using the channeling angular scan around an off-normal <1 (2) over bar 13> axis in the {10 (1) over bar0} plane of the GaN layer, the tetragonal distortion e(T), which is caused by the elastic strain in the epilayer, can be determined. Moreover, the depth dependence of the e(T) can be obtained using this technique. A fully relaxed (e(T)=0) GaN layer for a thickness <2.8 mum is expected. (C) 2002 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.

Substrate dependence of InGaAs quantum dots grown by molecular beam epitaxy

A systematic study of self-organized In0.5Ga0.5As quantum dots (QDs) and islands grown by molecular beam epitaxy on (100) and (n11) A/B GaAs substrates is given, where n varies from 1 to 5. Low-temperature photoluminescence results show that the properties of the dots have a strong dependence on the substrate orientation as revealed by atomic force microscopy, consistent with the differences in size, shape, and distribution of QDs on different substrates. From (100) to (111) surface, the photoluminescence peak position of dots on B surfaces is found to blueshift more than that on A surfaces. QDs are also formed on (511) A surface. The positional distribution of these dots exhibits a wavy shape, which is related to the corrugated structure of this surface. Two kinds of islands are formed on (111) A surface, but further work is needed to explain the mechanism of these islands. (C) 2001 American Vacuum Society.

Strong temperature dependence of photoluminescence properties in visible InAlAs quantum dots

Strong temperature dependence of optical properties has been studied in visible InAlAs/AlGaAs quantum dots, by employing photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements. The fast redshift of the exciton emission peak was observed at much lower temperature range compared to that observed in the InAs/GaAs QDs. In TRPL we did not observe the constant decay time even at low temperature. Instead, the observed decay time increases quickly with increasing temperature, showing 2D properties in the transient dynamic process. We attributed our results to the strong lateral coupling effect, which results in the formation of the local minibands or extended states from the discrete energy levels. (C) 2000 Elsevier Science B.V. All rights reserved.