Optical and electrical characterizations of ZnSe self-organized quantum dots grown by molecular beam epitaxy

Optical and electrical properties of ZnSe self-organized quantum dots were investigated using photoluminescence, capacitance-voltage, and deep level transient Fourier spectroscopy techniques. The temperature dependence of photoluminescence was employed to clarify the mechanism of photoluminescence thermal quenching processes in ZnSe quantum dots. A theoretic fit on considering a two-step quenching processes well explained the experimental data. The apparent carrier concentration profile obtained from capacitance-voltage measurements exhibits an accumulation peak at the depth of about 100nm below the sample surface, which is in good agreement with the location of the quantum dot layer. The electronic ground state of ZnSe quantum dots is determined to be about 0.11 eV below the conduction band of ZnS, which is similar to that obtained by simulating the thermal quenching of ZnSe photoluminescence.

Optical properties of InGaAs quantum dots formed on InAlAs wetting layer

We have fabricated a new self-assembled quantum dot system where InGaAs dots are formed on InAlAs wetting layer and embedded in GaAs matrix. The low-temperature photoluminescence and atomic force microscopy measurements confirm the realization of the structure. In contrast to traditional InAs/Ga(Al)As quantum dots, the temperature dependence of the photoluminescence of the dots in such a structure exhibits an electronically decoupled feature due to a higher energy level of the wetting layer which keeps the dots more isolated from each other. (C) 2001 Published by Elsevier Science B.V.

Investigation of defects in low-temperature-grown GaAs using optical transient spectroscopy

Optical transient current spectroscopy (OTCS) has been used to investigate defects in the low-temperature-grown GaAs after postgrowth rapid thermal annealing (RTA). Two samples A and B were grown at 220 degreesC and 360 degreesC on (001) GaAs substrates, respectively. After growth, samples were subjected to 30s RTA in the range of 500-800 degreesC. Before annealing, X-ray diffraction measurements show that the concentrations of the excess arsenic for samples A and B are 2.5 x 10(19) and 1 x 10(19) cm(-3), respectively. It is found that there are strong negative decay signals in the optical transient current (OTC) for the annealed sample A. Due to the influence of OTC strong negative decay signals, it is impossible to identify deep levels clearly from OTCS. For a comparison, three deep levels can be identified for sample B before annealing. They are two shallower deep levels and the so-called As-Ga antisite defect. At the annealing temperature of 600 degreesC, there are still three deep levels. However, their structures are different from those in the as-grown sample. OTC strong negative decay signals are also observed for the annealed sample B. It is argued that OTC negative decay signals are related to arsenic clusters. (C) 2000 Elsevier Science B.V. All rights reserved.

Influence of InxGa1-xAs (0 <= x <= 0.3) cap layer on structural and optical properties of self-assembled InAs/GaAs quantum dots

Optical and structural properties of self-organized InAs/GaAs quantum dots (QDs) with InxGa1-xAs or GaAs cover layers grown by molecular beam epitaxy (MBE) have been characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM) and photoluminescence (PL) measurements. The TEM and AFM images show that the surface stress of the InAs QDs was suppressed by overgrowth of a InxGa1-xAs covering layer on the top of the QDs and the uniformity of the QDs preserved. PL measurements reveal that red shifts of the PL emission due to the reduction of the surface strain of the InAs islands was observed and the temperature sensitivity of the PL emission energy was suppressed by overgrowth of InxGa1-xAs layers compared to that by overgrowth of GaAs layers.

Optical transitions in cubic GaN grown by metalorganic chemical vapor deposition on GaAs (100) substrate

Photoluminescence measurements have been performed on cubic GaN films with carrier concentration as low as 3 x 10(13) cm(-3). From the temperature and excitation intensity dependence, the emission lines at 3.268, 3.150 and 3.081 eV were assigned to the excitonic, donor-acceptor pair, and free-to-acceptor transitions, respectively Additionally, we observed two additional emission lines at 2.926 and 2.821 eV, and suggested that they belong to donor-acceptor pair transitions. Furthermore, from the temperature dependence of integral intensities, we confirmed that three donor-acceptor pair transitions (3.150, 2.926, and 2.821 eV) are from a common shallow donor to three different accepters. The excitonic emission at 3.216 eV has a full-width-at-half-maximum value of 41 meV at room temperature, which indicates a good optical quality of our sample.

Photoluminescence and optical quenching of photoconductivity studies on undoped GaN grown by molecular beam epitaxy

Deep levels in undoped GaN materials grown by modified molecular beam epitaxy (MBE) are investigated by photoluminescence (PL) and optical quenching of photoconductivity measurements. A broad band which extends from 2.1 to 3.0 eV with a maximum at about 2.7 eV is observed, and four prominent quenching bands were found located at 2.18, 2.40, 2.71, and 2.78 eV above the valence band, respectively. These levels are attributed to four holes trap levels existence in the material. The defects cannot be firmly identified at present. (C) 2000 Elsevier Science B.V, All rights reserved.

Correlated structural and optical investigation of terbium-doped zinc oxide nanocrystals

Tb3+-doped zinc oxide nanocrystals with a hexagonal wurzite structure were successfully prepared by reaction between Zn-O-Tb precursors and LiOH in ethanol. Good incorporation of Tb3+ in ZnO nanocrystals is proved by XRD, FTIR, PL and PLE measurements. The presence of acetate complexes to zinc atoms on particle surfaces is disclosed by FTIR results. Emission from both Tb3+ ions and surface states in ZnO matrix, as well as their correlation were observed. The luminescence mechanism is discussed. (C) 2000 Elsevier Science B.V. All rights reserved.

Influences of initial buffer layer deposition on electrical and optical properties in cubic GaN grown on GaAs(100) by metalorganic chemical vapor deposition

We present some results on the effect of initial buffer layer on the crystalline quality of Cubic GaN epitaxial layers grown on GaAs(100) substrates by metalorganic chemical vapor deposition. Photoluminescence and Hall measurements were performed to characterize the electrical and optical properties of cubic GaN. The crystalline quality subsequently grown high-temperature (HT) cubic GaN layers strongly depended on thermal effects during the temperature ramping process after low temperature (LT) growth of the buffer layers. Atomic force microscope (AFM) and reflection high-energy electron diffraction (RHEED) were employed to investigate this temperature ramping process. Furthermore, the role of thermal treatment during the temperature ramping process was identified. Using the optimum buffer layer, the full width at half maxim (FWHM) at room temperature photoluminescence 5.6 nm was achieved. To our knowledge, this is the best FWHM value for cubic GaN to date. The background carrier concentration was as low as 3 x 10(13) cm(-3). (C) 2000 Published by Elsevier Science S.A. All rights reserved.

Optical properties of nanometer silicon prepared by silicon ion implanted into SiO2 layers

The samples of silicon nanocrystals (nc-Si) were prepared by Si ion implanted into SiO2 layers. Photoluminescence spectra were measured at room temperature and their dependence on thermal annealing was investigated. The experimental results show that PL peaks originate from the defects in SiO2 layers caused by ion implantation when the thermal annealing temperature is lower than 800 C. The PL peak from nc-Si was observed when the thermal annealing temperature was higher than 900 C, and PL intensity reached its maximum at the thermal annealing temperature of 1100 C. As the annealing temperature increases the red shift of PL peak from nc-Si shows the quantum size effect. The characterized Raman scattering peak of nc-Si was observed at the right angle scattering configuration for the first time. It provides further support for the PL measurements.

Effects of rapid thermal annealing on the optical properties of GaNxAs1-x/GaAs single quantum well structure grown by molecular beam epitaxy

The effect of rapid thermal annealing (RTA) on the optical properties of GaNxAs1-x/GaAs strained single quantum well (SQW) was studied by low-temperature photoluminescence (PL). The GaNxAs1-x/GaAs SQW structures were prepared by dc active nitrogen plasma assisted molecular beam epitaxy. PL measurements on a series of samples with different well widths and nitrogen compositions were used to evaluate the effects of RTA. The annealing temperature and time were varied from 650 to 850 degrees C and 30 s to 15 min, respectively. Remarkable improvements of the optical properties of the samples were observed after RTA under optimum conditions. The interdiffusion constants have been calculated by taking into account error function diffusion and solving the Schrodinger equation. The estimated interdiffusion constants D are 10(-17)-10(-16) cm(2)/s for the earlier annealing conditions. Activation energies of 6-7 eV are obtained by fitting the temperature dependence of the interdiffusion constants. (C) 2000 American Institute of Physics. [S0021-8979(00)10401-3].

High energy ion implantation enhanced intermixed quantum well structures and their absorption characteristics in passive optical waveguides

We have shown that high energy ion implantation enhanced intermixing (HE-IIEI) technology for quantum well (QW) structures is a powerful technique which can be used to blue shift the band gap energy of a QW structure and therefore decrease its band gap absorption. Room temperature (RT) photoluminescence (PL) and guided-wave transmission measurements have been employed to investigate the amount of blue shift of the band gap energy of an intermixed QW structure and the reduction of band gap absorption, Record large blue shifts in PL peaks of 132 nm for a 4-QW InGaAs/InGaAsP/InP structure have been demonstrated in the intermixed regions of the QW wafers, on whose non-intermixed regions, a shift as small as 5 nm is observed. This feature makes this technology very attractive for selective intermixing in selected areas of an MQW structure. The dramatical reduction in band gap absorption for the InP based MQW structure has been investigated experimentally. It is found that the intensity attenuation for the blue shifted structure is decreased by 242.8 dB/cm for the TE mode and 119 dB/cm for the TM mode with respect to the control samples. Electro-absorption characteristics have also been clearly observed in the intermixed structure. Current-Voltage characteristics were employed to investigate the degradation of the p-n junction in the intermixed region. We have achieved a successful fabrication and operation of Y-junction optical switches (JOS) based on MQW semiconductor optical amplifiers using HE-IIEI technology to fabricate the low loss passive waveguide. (C) 1997 Published by Elsevier Science B.V.

Effect of noncoherent islands on the optical properties of the 1.3 mu m InAs/GaAs quantum dots during rapid thermal annealing

The effect of thermal annealing of InAs/GaAs quantum dots (QDs) with emission wavelength at 1.3 mu m have been investigated by photoluminescence (PL) and transmission electron microscopy (TEM measurements. There is a dramatic change in the A spectra when the annealing temperature is raised up to 800 degrees C: an accelerated blushifit of the main emission peak of QDs together with an inhomogeneous broadening of the linewidth. The TEM images shows that the lateral size of normal QDs decreases as the annealing temperature is increased, while the noncoherent islands increase their size and densit. A small fraction of the relative large QDs contain dislocations when the annealing temperature increases up to 800 degrees C. The latter leads to the strong decrease of the PL intensity.

Radar interferometer measurements of space debris using the Evpatoria RT-70 transmitter

The ability of the Evpatoria RT-70 radar complex to perform research on space debris was investigated in four trial experiments during 2001-2003. The echo-signals of 25 objects at geostationary, highly elliptical and medium-altitude orbits were recorded on magnetic tapes at radio telescopes in Russia, Italy, China and Poland. The multi-antenna system configuration gives potential to supplement the classic radar data with precise angular observations using the technique of Very Long Baseline Interferometry. The first stage of such processing was fulfilled by the correlator in N. Novgorod, Russia. The cross-correlation of transmitted and received signals was obtained for the 11 objects on the Evpatoria-Bear Lakes, Evpatoria-Urumqi and Evpatoria-Noto baselines. This activity also promoted developing the optical observations of geostationary objects, conducted for the improvement of the radar target ephemerides. (C) 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.

The exciton-longitudinal-optical-phonon coupling in InGaN/GaN single quantum wells with various cap layer thicknesses

This paper studies the exciton-longitudinal-optical-phonon coupling in InGaN/GaN single quantum wells with various cap layer thicknesses by low temperature photoluminescence (PL) measurements With increasing cap layer thickness, the PL peak energy shifts to lower energy and the coupling strength between the exciton and longitudinal-optical (LO) phonon, described by Huang-Rhys factor, increases remarkably due to an enhancement of the internal electric field With increasing excitation intensity, the zero-phonon peak shows a blueshift and the Huang-Rhys factor decreases These results reveal that there is a large built-in electric field in the well layer and the exciton-LO phonon coupling is strongly affected by the thickness of the cap layer

Optical characterization of InAs monolayer quantum structures grown on (311)A, (311)B, and (100)GaAs substrates

Low-temperature photoluminescence and excitation spectra from InAs monolayer quantum structures, grown on (311)A, (311)B, and (100) GaAs substrates, are investigated, The structures were grown simultaneously by conventional molecular-beam epitaxy (MBE), The experimental results show that the quality of InAs monolayer on (311)B GaAs substrate is obviously better in crystal quality than those on the two other oriented GaAs substrates. In addition, the transition peaks of the InAs layer grown on (311) GaAs substrates shift to higher energy with respect to that from the InAs layer grown on (100) GaAs substrate.