Photocurrent derivative spectra of ZnCdSe-ZnSe double multi-quantum wells

Photocurrent (PC) spectra of ZnCdSe-ZnSe double multi-quantum wells are measured at different temperature. Its corresponding photocurrent derivative (PCD) spectra are obtained by computing, and the PCD spectra have greatly enhanced the sensitivity of the relative weak PC signals. The polarization dependence of the PC spectra shows that the transitions observed in the PC spectra are heavy-hole related, and the transition energy coincide well with the results obtained by envelope function approximation including strain. The temperature dependence of the photocurrent curves indicates that the thermal activation is the dominant transport mechanism of the carriers in our samples. The concept of saturation temperature region is introduced to explain why the PC spectra have different temperature dependence in the samples with different structure parameters. It is found to be very useful in designing photovoltaic devices.

Selective double quantum resolved correlation experiment for the complete separation of entire proton NMR spectra of enantiomers

The present study reports a two dimensional NMR experiment which separates single quantum spectra of enantiomers from that of a racemic mixture. This is a blend of selective double quantum refocusing, for resolving couplings and chemical shift interactions along two dimensions followed by correlation of the selectively excited protons to the entire coupled spin network. The concept is solely based on the presence of distinct intra methyl dipolar couplings of different enantiomers when dissolved in chiral orienting media. The analysis of single enantiomer spectrum obtained from respective F-2 cross sections yield all the spectral information. (C) 2011 Elsevier Inc. All rights reserved.

Transmission spectrum of a double quantum-dot-nanocavity system in photonic crystals

We investigate the optical transmission properties of a combined system which consists of two quantum-dot-nanocavity subsystems indirectly coupled to a waveguide in a planar photonic crystal. A Mollow-like triplet and the growth of sidebands are found, reflecting intrinsic optical responses in the complex microstructure.

Generation and behavior of pure-edge threading misfit dislocations in InxGa1-xN/GaN multiple quantum wells

The relaxation of the misfit strain by the formation of misfit dislocations in InxGa1-xN/GaN multiple quantum wells grown by metal-organic chemical-vapor deposition was investigated by the cross-sectional transmission electron microscopy, double crystal x-ray diffraction, and temperature-dependent photoluminescence. It is found that the misfit dislocations generated from strain relaxation are all pure-edge threading dislocations with burgers vectors of b=1/3<11 (2) over bar0>. The misfit dislocations arise from the strain relaxation due to the thickness of strained layer greater than the critical thickness. The relaxation of strained layer was mainly achieved by the formation of dislocations and localization of In, while the dislocations changed their slip planes from {0001} to {10 (1) over bar0}. With the increasing temperature, the efficiency of photoluminescence decrease sharply. It indicates that the relaxation of the misfit strain has a strong effect on optical efficiency of film. (C) 2004 American Institute of Physics.

Structural and optical properties of GaAsSb/GaAs heterostructure quantum wells

The structural and optical properties of MBE-grown GaAsSb/GaAs multiple quantum wells (MQWs) as well as strain-compensated GaAsSb/GaAs/GaAsP MQWs are investigated. The results of double crystal X-ray diffraction and reciprocal space mapping show that when strain-compensated layers are introduced, the interface quality of QW structure is remarkably improved, and the MQW structure containing GaAsSb layers with a high Sb composition can be coherently grown. Due to the influence of inserted GaAsP layers on the energy band and carrier distribution of QWs, the optical properties of GaAsSb/GaAs/GaAsP MQWs display a lot of features mainly characteristic of type-I QWs despite the type-II GaAsSb/GaAs interfaces exist in the structure. (C) 2004 Elsevier B.V. All rights reserved.

Effect of critical thickness on structural and optical properties of InxGa1-xN/GaN multiple quantum wells

InGaN/GaN multiquantum-well (MQW) structures grown by metalorganic chemical-vapor deposition on n-type GaN and capped by p-type GaN were investigated by cross-sectional transmission electron microscopy, double crystal x-ray diffraction, and temperature-dependent photoluminescence. For the sample with strained-layer thicknesses greater than the critical thicknesses, a high density of pure edge type threading dislocations generated from MQW layers and extended to the cap layer was observed. These dislocations result from a relaxation of the strained layers when their thicknesses are beyond the critical thicknesses. Because of indium outdiffusion from the well layers due to the anneal effect of Mg-doped cap layer growth and defects generated from strain relaxation, the PL emission peak was almost depressed by the broad yellow band with an intensity maximum at 2.28 eV. But for the sample with strained-layer thicknesses less than the critical thicknesses, it has no such phenomenon. The measured critical thicknesses are consistent with the calculated values using the model proposed by Fischer, Kuhne, and Richter. (C) 2004 American Institute of Physics.

Effect of misfit dislocation originated from strained layer on photoluminescence properties of InxGa1-xN/GaN multiple quantum wells

In-x Ga1-xN/GaN multiple quantum well (MQW) samples with strain-layer thickness lager/less than the critical one are investigated by temperature-dependent photoluminescence and transmission electron microscopy, and double crystal x-ray diffraction. For the sample with the strained-layer thickness greater than the critical thickness, we observe a high density of threading dislocations generated at the MQW layers and extended to the cap layer. These dislocations result from relaxation of the strain layer when its thickness is beyond the critical thickness. For the sample with the strained-layer thickness greater than the critical thickness, temperature-dependent photoluminescence measurements give evidence that dislocations generated from the MQW layers due to strain relaxation are main reason of the poor photoluminescence property, and the dominating status change of the main peak with increasing temperature is attributed to the change of the radiative recombination from the areas including dislocations to the ones excluding dislocations.

Interdot interaction induced zero-bias maximum of the differential conductance in parallel double quantum dots

We have studied the equilibrium and nonequilibrium electronic transports through a double quantum dot coupled to leads in a symmetrical parallel configuration in the presence of both the inter- and the intradot Coulomb interactions. The influences of the interdot interaction and the difference between dot levels on the local density of states (LDOS) and the differential conductance are paid special attention. We find an interesting zero-bias maximum of the differential conductance induced by the interdot interaction, which can be interpreted in terms of the LDOS of the two dots. Due to the presence of the interdot interaction, the LDOS peaks around the dot levels epsilon(i) are split, and as a result, the most active energy level which supports the transport is shifted near to the Fermi level of the leads in the equilibrium situation. (c) 2006 American Institute of Physics.

Capacitance-voltage characteristic as a trace of the exciton evolvement from spatially direct to indirect in quantum wells

We have investigated the photo-excited capacitance-voltage (C-V) characteristics as well as the photoluminescence spectra under different biases of a wide quantum well (QW) embedded in an n(+)-i-n(+) double-barrier structure. The pronounced peak feature at zero bias in the C-V spectrum observed upon illumination is regarded as a kind of quantum capacitance related to the quantum confined Stark effect, originating from the spatial separation of the photo-generated electron and hole gas in the QW. This fact is further demonstrated through the comparison between the C-V curve with the PL intensity versus applied voltage relationship under the same excitation. The results may provide us with a more direct and sensitive means in the detection of the separation and accumulation of both types of free carriers-electrons and holes-in low-dimensional semiconductor structures, especially in a new type of optical memory cell.

Effect of rapid thermal annealing on InGaAs/GaAs quantum wells

We have studied the effect of rapid thermal annealing (RTA) on highly strained InGaAs/GaAs quantum wells by using photoluminescence (PL) and double-crystal X-ray diffraction (DCXRD) measurements. It is found that a distinct additional PL emission peak can be observed for the annealed samples. This PL emission possesses features similar to the PL emission from InGaAs/GaAs quantum dots (QDs) with the same indium content. It is proposed that this emission stems from QDs, which were formed during the annealing process. This formation is attributed to the favorable diffusion due to the inhomogeneous strain distribution in the InGaAs layer intersurface. The DCXRD measurements also confirm that the dominant relaxation is strain enhanced diffusion under the low annealing temperatures. (C) 2000 Elsevier Science B.V. All rights reserved.

LO-PHONON-ASSISTED TUNNELING IN ASYMMETRIC DOUBLE-WELL STRUCTURES WITH THICK BARRIERS

Nonresonant electron tunneling between asymmetric double quantum wells in AlxGa1-xAs/GaAs systems has been investigated by using steady-state and time-resolved photoluminescence spectra. Experimental evidence of LO-phonon-assisted tunneling through thick barriers has been obtained by enhancing excitation power densities or applying electric fields perpendicular to the well plane. LO-phonon-assisted tunneling times have also been estimated from the variation of the decay time of the narrow-well photoluminescence with applied electric fields. Our findings suggest that LO phonons in the barriers play an important role in the tunneling transfer.

PRESSURE-DEPENDENCE OF PHOTOLUMINESCENCE FROM ZNSE-TE-(CDSE)(1) (ZNSE)(3) SUPERLATTICE QUANTUM-WELLS

Pressure dependence of photoluminescence from ZnSe:Te-(CdSe)(1)(ZnSe)(3) short period superlattice quantum wells is reported. In addition to the exciton band from the superlattice layers, strong bands for localized excitons self-trapped al single Te (Te-1) atom, double Te atoms (Te-2) and Te clusters (Te-n, n greater than or equal to 3) as well as for the free excitons in isoelectronic Te incorporated ZnSe layers are observed. Significant differences in the pressure and temperature dependencies of the observed exciton transitions are presented and discussed.

Structural and optical properties of GaAsSb/GaAs heterostructure quantum wells

The structural and optical properties of MBE-grown GaAsSb/GaAs multiple quantum wells (MQWs) as well as strain-compensated GaAsSb/GaAs/GaAsP MQWs are investigated. The results of double crystal X-ray diffraction and reciprocal space mapping show that when strain-compensated layers are introduced, the interface quality of QW structure is remarkably improved, and the MQW structure containing GaAsSb layers with a high Sb composition can be coherently grown. Due to the influence of inserted GaAsP layers on the energy band and carrier distribution of QWs, the optical properties of GaAsSb/GaAs/GaAsP MQWs display a lot of features mainly characteristic of type-I QWs despite the type-II GaAsSb/GaAs interfaces exist in the structure. (C) 2004 Elsevier B.V. All rights reserved.

Magnetoresistance and transistor-like behavior of a double quantum-dot via crossed Andreev reflections

The electric current and the magnetoresistance effect are studied in a double quantum-dot system, where one of the dots QD(a) is coupled to two ferromagnetic electrodes (F-1; F-2), while the second QD(b) is connected to a superconductor S. For energy scales within the superconductor gap, electric conduction is allowed by Andreev reflection processes. Due to the presence of two ferromagnetic leads, non-local crossed Andreev reflections are possible. We found that the magnetoresistance sign can be changed by tuning the external potential applied to the ferromagnets. In addition, it is possible to control the current of the first ferromagnet (F-1) through the potential applied to the second one (F-2). We have also included intradot interaction and gate voltages at each quantum dot and analyzed their influence through a mean field approximation. The interaction reduces the current amplitudes with respect to the non-interacting case, but the switching effect still remains as a manifestation of quantum coherence, in scales of the order of the superconductor coherence length. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4723000]

Tunable molecular resonances of a double quantum dot Aharonov-Bohm interferometer

We investigate resonant tunnelling through molecular states of an Aharonov-Bohm (AB) interferometer composed of two coupled quantum dots. The conductance of the system shows two resonances associated with the bonding and the antibonding quantum states. We predict that the two resonances are composed of a Breit-Wigner resonance and a Fano resonance, of which the widths and Fano factor depend on the AB phase very sensitively. Further, we point out that the bonding properties, such as the covalent and ionic bonding, can be identified by the AB oscillations.