W. C. Chou

Photoluminescence properties of self-assembled InN dots embedded in GaN grown by metal organic vapor phase epitaxy

Photoluminescence (PL) properties of InN dots embedded in GaN were investigated. We observed a systematic blueshift in the emission energy as the average dot height was reduced. The widely size-tunable emission energy can be ascribed to the size quantization effect. Temperature-dependent PL measurements show that the emission peak energies of the dots are insensitive to temperature, as compared with that of bulk film, indicating the localization of carriers in the dots. A reduced quenching of the PL from the InN dots was also observed, implying superior emission properties for the embedded InN dot structures.

Impacts of ammonia background flows on structural and photoluminescence properties of InN dots grown on GaN by flow-rate modulation epitaxy

Structural and photoluminescence (PL) properties of InN dots grown on GaN by metal organic vapor phase epitaxy using the flow-rate modulation technique, and their dependence on growth conditions, were investigated. An ammonia (NH3) background flow was intentionally supplied during indium deposition periods to control the kinetics of adatoms and hence the morphology of InN dots. Samples prepared under lower NH3 background flows generally exhibit narrower and more intense PL signals peaked at lower emission energies. The authors point out that the NH3 background flow is an important parameter that controls not only the nucleation process but also the emission property of InN dots.

Optical properties of self-assembled ZnTe quantum dots grown by molecular-beam epitaxy

The morphology and the size-dependent photoluminescence (PL) spectra of the type-II ZnTe quantum dots (QDs) grown in a ZnSe matrix were obtained. The coverage of ZnTe varied from 2.5 to 3.5 monolayers (MLs). The PL peak energy decreased as the dot size increased. Excitation power and temperature-dependent PL spectra are used to characterize the optical properties of the ZnTe quantum dots. For 2.5- and 3.0-ML samples, the PL peak energy decreased monotonically as the temperature increased. However, for the 3.5-ML sample, the PL peak energy was initially blueshifted and then redshifted as the temperature increased above 40K. Carrier thermalization and carrier transfer between QDs are used to explain the experimental data. A model of temperature-dependent linewidth broadening is employed to fit the high-temperature data. The activation energy, which was found by the simple PL intensity quenching model, of the 2.5, 3.0, and 3.5 MLs were determined to be 6.35, 9.40, and 18.87meV, respectively.

Formation of a precursor layer in self-assembled CdTe quantum dots grown on ZnSe by molecular beam epitaxy

The growth mode of CdTe quantum dots (QDs) grown on highly lattice-mismatched ZnSe buffer was investigated. CdTe QDs (0.6 to 5.0 mono-layers (MLs)) were deposited on the Se-stabilized ZnSe buffer layers using an alternating supply of Cd and Te atomic sources. Cross-sectional transmission electron microscopy and photoluminescence (PL) measurements revealed the existence of a CdSe-like two-dimensional precursor layer (PCL). The prominent difference in the temperature-dependent PL peak shift was associated with the emissions from the respective CdSe PCL and CdTe QDs. In addition, the PL excitation measurement demonstrated the existence of the first QD excited excitonic state.

Optical characterization of ZnSe epilayers and ZnCdSe∕ZnSe quantum wells grown on Ge∕Ge0.95Si0.05∕Ge0.9Si0.1∕Si virtual substrates

Several approaches have been employed to grow high-quality ZnSe epilayers on Ge∕Ge0.95Si0.05∕Ge0.9Si0.1∕Si virtual substrates. The ZnSe epilayers were characterized by photoluminescence spectroscopy. Migration enhanced epitaxy and inserting an in situ thermal annealing ZnSe buffer layer effectively reduced the intensity of deep level emissions from the ZnSe epilayer grown on a 6°-tilted Ge∕Ge0.95Si0.05∕Ge0.9Si0.1∕Si virtual substrate. Optimized conditions for growing high-quality ZnSe were used to deposit ZnCdSe∕ZnSe multiple quantum wells on Ge∕Ge0.95Si0.05∕Ge0.9Si0.1∕Si virtual substrates. Photoluminescence spectroscopy revealed quantum-confinement effect in the ZnCdSe multiple quantum wells. The evolution of the exciton emission peak energy and the linewidth as a function of temperature indicate a low density of localized sites in the sample with a well width of 1nm. In the high-temperature regime, the thermal quenching of the excitonic emission intensity from ZnCdSe quantum well structures was governe...

The photoluminescence decay time of self-assembled InAs quantum dots covered by InGaAs layers

The temperature dependence of the time-resolved photoluminescence (PL) of self-assembled InAs quantum dots (QDs) with InGaAs covering layers was investigated. The PL decay time increases with temperature from 50 to 170 K, and then decreases as the temperature increases further above 170 K. A model based on the phonon-assisted transition between the QD ground state and the continuum state is used to explain the temperature dependence of the PL decay time. This result suggests that the continuum states are important in the carrier capture in self-assembled InAs QDs.

Studies on the electronic and vibrational states of colloidal CdSe/ZnS quantum dots under high pressures

The electronic and vibrational states of colloidal core/shell CdSe/ZnS quantum dots are studied at room temperature by using high pressure optical measurements. Pressure-induced quadratic variations of lattice constants can be observed clearly from both photoluminescence (PL) and Raman spectra up to ~7?GPa. This quadratic relationship is consistent with the theoretical prediction. The pressure coefficients of linear and quadratic terms are 32?meV?GPa?1, ?1?meV?GPa?2 for PL and 4.2?cm?1?GPa?1, ?0.1?cm?1?GPa?2 for Raman measurements, respectively.

Optical studies of InN epilayers on Si substrates with different buffer layers

We have investigated the photoluminescence (PL) and time-resolved PL from the InN epilayers grown on Si substrates with different buffer layers. The narrowest value of the full width at half maximum of the PL peak is 52 meV with the AlN/AlGaN/GaN triple buffer layer, which is better than previous reports on similar InN epilayers on Si substrates. Based on the emission-energy dependence of the PL decays, the localization energy of carriers is also the least for the InN with a triple buffer layer. According to the x-ray diffraction measurements, we suggest that the reduced lattice mismatch between the InN epilayer and the top buffer layer is responsible for improvement of sample quality using the buffer-layer technique.

Current Properties of GaN V-Defect Using Conductive Atomic Force Microscopy

Current conduction behavior on GaN V-defect was studied comprehensively using conductive atomic force microscopy. Experimental results indicate that the forward current in the V-defect region is at least three-order higher than that at surrounding area. On the other hand, a snowflake-like leakage current pattern was observed in the V-defect owing to the ease of current breakdown at the crest lines and perimeters. Further static current–voltage measurement suggests that the current flow is governed by Schottky emission and Fowler–Nordheim tunneling for V-defect region and surrounding area, respectively.

Photoluminescence Studies of In-Doped GaN:Mg Films

Photoluminescence (PL) studies of In-doped GaN:Mg films revealed that the Mg-related emission at 3.1 eV is enhanced by more than one order of magnitude on the shoulder of the broad band centered at 2.8 eV for GaN:Mg after an optimal In concentration was added into the films. This enhancement of the 3.1 eV band is believed to be associated with the reduction in the number of self-compensation centers. A slow decay in PL intensity evolution was also observed, which may be ascribed to a local energy barrier that impedes carriers that relax into the valence band. The temperature dependences of the decay time constants were measured and a barrier energy as high as ~103±7 meV was obtained for In-doped GaN:Mg as compared with 69±8 meV for GaN:Mg.