Laterally defined freely suspended quantum dots in GaAs/AlGaAs heterostructures

Free-standing beams containing a two-dimensional electron system are shaped from a GaAs/AlGaAs heterostructure. Quantum point contacts and (double) quantum dots are laterally defined using metal top gates. We investigate the electronic properties of these nanostructures by transport spectroscopy. Tunable localized electron states in freely suspended nanostructures are a promising tool to investigate the electron-phonon interaction.     

Fermi-edge singularity in photoluminescence spectra of modulation-doped AlGaAs/InGaAs/GaAs quantum wells

The photoluminescence study of Fermi-edge singularity (FES) in modulation-doped pseudomorphic Al _x Ga_1?x As/In _y Ga_1?y As/GaAs quantum well (QW) heterostructures is presented. In the above QW structures the optical transitions between n = 1 and n = 2 electronic subband to the n = 1 heavy hole subband (E ₁₁ and E ₂₁ transitions, respectively) are observed with FES appearing as a lower energy shoulder to the E ₂₁ transition. The observed FES is attributed to the Fermi wave vector in the first electronic subband under the conditions of population of the second electronic subband. The FES appears at about 10 meV below E ₂₁ transition around 4·2 K. Initially it gets stronger with increasing temperature and becomes a distinct peak at about 20 K. Further increase in temperature quenches FES and reaches the base line at around 40 K.     

Optical control of energy-level structure of few electrons in AlGaAs/GaAs quantum dots

Optical control of the lateral quantum confinement and number of electrons confined in nanofabricated GaAs/AlGaAs quantum dots is achieved by illumination with a weak laser beam that is absorbed in the AlGaAs barrier. Precise tuning of energy-level structure and electron population is demonstrated by monitoring the low-lying transitions of the electrons from the lowest quantum-dot energy shells by resonant inelastic light scattering. These findings open the way to the manipulation of single electrons in these quantum dots without the need of external metallic gates.     

Emission wavelength trimming of self-assembled InGaAs/GaAs quantum dots with GaAs/AlGaAs superlattices by rapid thermal annealing

The effects of rapid thermal annealing on InGaAs quantum dots grown by atomic layer molecular beam epitaxy to the structural transformation and optical properties are investigated. No misfit dislocation was observed from either the as-grown or annealed dots. The size and composition of the quantum dots become more uniform upon annealing mainly from the height fluctuation as predicted by the theoretical model. Large bandgap blue shifts, resulted from the In and Ga interdiffusion, were observed with the preservation of three-dimensional carrier confinement. The GaAs/AlGaAs superlattice was found to minimize the defect diffusion and dot interdiffusion during the high-temperature epitaxial overgrowth.     

Mechanism of photoluminescence quenching of InGaAs/GaAs quantum dots resulting from nanoprobe indentation

The low-temperature (10 K) photoluminescence (PL) of self-assembled InGaAs/GaAs quantum dots (QDs) was measured under the elastic indentation of a flat cylindrical nanoprobe that generates localized strain fields around itself. As the indentation force increases, the intensity of the PL fine peak from a single QD firstly increases, followed by a decrease, and is finally quenched. The observed force at which a PL peak disappears, i.e., the quenching force varies from QD to QD. This variation is ascribed to the diversely distributed strain fields in and around each QD and therefore can be related to the QD location with respect to the nanoprobe center. In order to clarify the mechanism of PL quenching, a numerical simulation of the strain distribution is carried out by a 3-dimensional finite element method. The modification of the energy band structure resulting from strain is then calculated based on the deformation potential theory. We concluded that the PL quenching observed experimentally can be attributed to the electron-repulsion resulting from the strain-induced potential gradient. Based on this mechanism, an indentation-induced shift of the electron-potential in bulk GaAs, at which the PL from QDs is quenched, was deduced to be 43.5-133.5 meV.     

Temperature dependent photoluminescence investigation of the effect of growth pause induced ripening in InAs/GaAs quantum dot heterostructures

Self-assembled InAs/GaAs quantum dot (QD) heterostructures grown by solid state molecular beam epitaxy (MBE) were subjected to growth ripening pause of comparatively shorter durations (0–50 s) at the growth temperature (520 °C). The islands are found to increase in size with the growth pause and correspondingly their density decreases. Though the photoluminescence spectra of the islands subjected to growth pause is found to follow conventional QD systems, a contradiction is noticed in the calculated values of the activation energy of the dots. We ascribed this contradiction due to the poor crystalline quality of the ripened QDs as a result of desorption and sublimation of indium during the pause at high growth temperature.     

尺寸分布和界面混合对InAs/GaAs量子点光学性质的影响

研究了自组织生长模式(S-K modes)下量子点尺寸的不均匀分布对量子点发光性质的影响,对其光致发光峰进行了拟合计算.研究发现,量子点尺寸的不均匀分布导致了量子点发光峰的展宽以及发光峰位的红移.另一方面,后处理工艺中的退火及质子注入引起的界面混合导致了量子点PL谱发光峰的蓝移及半高宽的减小.     

Longitudinal photoconductivity of GaAs/AlGaAs quantum wires

The photoconductivity (PC) of undoped GaAs/AlGaAs quantum wires (QWRs) along the wire direction is studied for the first time. The PC spectrum reveals a strong substrate-related background as well as several small structures, some of which could be connected with the QWRs. This suggestion is confirmed by the observed polarization dependence of the PC and by photoluminescence (PL) and photoluminescence excitation (PLE) measurements on a similar sample. Prolonged pre-illumination of the sample with infrared light (hν=1.18 eV) considerably reduces the background in the PC spectrum, and makes the QWR structures better resolved.     

Temperature dependent optical properties of stacked InGaAs/GaAs quantum rings

In this paper we describe the results of temperature dependent photoluminescence intensity and decay time measurements of In(Ga)As/GaAs quantum rings where the depth of barrier is varied from sample to sample. The activation energy found for the reduction of the exciton decay time as a function of the temperature is approximately half the value of the thermionic escape energy of excitons. The temperature dependant behaviour is ascribed to the carriers lost via the excited state to the WL.

The time resolved PL study indicates that thermal escape mechanisms is not so affected by reducing the spacer thickness, but it's influenced essentially by the excited state recombination.     

Temperature dependent luminescence of azobenzene capped CdS quantum dots

Cadmium sulfide (CdS) quantum dots (QDs) are prepared at room temperature by "form-fill-seal" method, while the azobenzene is used as surfactant to control the particle size and to prevent agglomeration. The typical size of CdS nanoparticles is estimated as 2 nm by X-ray diffraction. The absorption spectra of CdS QDs are measured at room temperature and a new absorption peak associated with the surface excited state is found. The luminescence property of the CdS QDs is studied at room temperature and low temperature. Two photoluminescence peaks exist in the temperature range of 8-300 K. One peak at 460 nm is attributed to CdS QDs, while the other one at 667 nm comes from the transition of surface excited state and its intensity decreases with temperature increasing.     

Time-resolved photoluminescence of type-II Ga(As)Sb/GaAs quantum dots embedded in an InGaAs quantum well

Optical properties and carrier dynamics in type-II Ga(As)Sb/GaAs quantum dots (QDs) embedded in an InGaAs quantum well (QW) are reported. A large blueshift of the photoluminescence (PL) peak is observed with increased excitation densities. This blueshift is due to the Coulomb interaction between physically separated electrons and holes characteristic of the type-II band alignment, along with a band-filling effect of electrons in the QW. Low-temperature (4?K) time-resolved PL measurements show a decay time of [Formula: see text]?ns from the transition between Ga(As)Sb QDs and InGaAs QW which is longer than that of the transition between Ga(As)Sb QDs and GaAs two-dimensional electron gas ([Formula: see text]?ns).     

Morphology and composition of InAs/GaAs quantum dots

Surface compositional maps of self-organized InAs/GaAs quantum dots were obtained with laterally resolved photoemission spectroscopy. We found a surface In concentration of about 0.85 at the center of the islands which decreases to 0.75 on the wetting layer. Comparison with concentration values found in the core of similar dots suggests a strong In segregation on the topmost surface layers of the dots and on the surrounding wetting layer. Furthermore, the morphological properties of the dots such as size and density have been measured with plan-view transmission electron microscopy and low energy electron microscopy.     

Synthesis and photoluminescence of ZnS quantum dots

Single-phase zinc sulphide (ZnS) quantum dots were synthesized by a chemical method. The influence of the pH value of the Zn(CH3COO)2 solution on the size and photoluminescence properties of the ZnS quantum dots was evaluated. X-ray power diffraction, transmission electron microscopy, and ultraviolet-visible spectroscopy were used to characterize the structure, size, surface states, and photoluminescence properties of ZnS quantum dots. The results showed that the crystal structure of ZnS quantum dots was a cubic zinc blende structure, and their average diameter was about 3.0 nm. ZnS quantum dots with good distribution and high purity were obtained. A strong broad band centered at about 320 nm was observed in the excitation spectrum of ZnS quantum dots. Their emission spectrum peaking at about 408 nm, was due mostly to the trap-state emission. The relative integrated emission intensity of ZnS quantum dots decreased as the pH value of the Zn(CH3COO)2 solution increased, which could be ascribed to the increase in average diameter of the ZnS quantum dots as the pH value of Zn(CH3COO)2 solution increased.     

Temperature stability of photoluminescence in heterostructures with InGaAs/GaAs quantum well and Mn-delta-doped acceptor layer in GaAs barrier

The temperature dependence of the electro- and photoluminescence of heterostructures with InGaAs/GaAs quantum well and a closely spaced manganese delta (δ)-doped layer in the GaAs barrier was investigated. It is found that the proposed heterostructures exhibit increased temperature stability and decreased temperature quenching as compared to the control structures containing no δ-doped layers. An increase in the operating temperature is explained by the appearance of an additional potential barrier for electrons due to the δ-doped acceptor layer formation in the near-surface barrier.     

Room-temperature photoluminescence at 1.55 μm from heterostructures with InAs/InGaAsN quantum dots on GaAs substrates

Room-temperature photoluminescence (PL) at 1.55 μm from heterostructures with InAs/InGaAsN quantum dots (QDs) grown by MBE on GaAs substrates is demonstrated for the first time. The effect of nitrogen incorporated into InAs/InGaAsN QDs on the PL wavelength and intensity was studied. The integral intensity of PL from the new structure with InAs/(In)GaAsN QDs is comparable to that from a structure with InGaAsN quantum wells emitting at 1.3 μm.     

Peculiar temperature-power dependence of AlGaAs/GaAs multi quantum well

The dependence of the integrated photoluminescence on the excitation power intensity in Al(0.3)Ga(0.7)As/GaAs multi quantum well is studied. Four peaks are found in the photoluminescence spectra, which are corresponding to the four quantum wells in the sample. The temperature dependence of the exponent alpha of the power law shows peculiar behavior for the quantum well of width 11.2 nm (peak C). The value of the exponent alpha exceeds the quadratic value predicted by the steady state model near room temperature. All other peaks shows linear dependence in the low temperature range which switches to super linear in the high temperature range with values of alpha less than 2. Carriers thermal capture and re-trapping is discussed. The presented results are a sign of thermal dissociation of exciton in quantum well near room temperature. The peculiar behavior is attributed to the excess flow of the charge carriers to this QW by thermal escape from other QWs, and also due to excess free carriers because of exciton dissociation.     

Temperature and pump power dependent photoluminescence characterization of MBE grown GaAsBi on GaAs

Bulk and quantum well GaAs_1?xBi_x/GaAs layers with Bi mole fractions from 0.02 to 0.10 are grown by molecular-beam epitaxy at temperatures ranging from 280 to 320?°C. The samples are characterized using temperature and pump-power dependent photoluminescence measurements covering 8–300?K and 1–250?mW (7–1,800?W/cm²), respectively. The results indicate that there is strong reduction in bandgap energy with the incorporation of small amounts of Bi and that GaAsBi most likely forms a weak type-I band alignment with GaAs.     

单光子源低密度长波长InAs／GaAs量子点的制备

通过优化分子束外延生长条件,得到室温发光在1300nm低密度的自组织InAs/GaAs量子点.使用极低的InAs生长速率(0.001单层/秒)可以把量子点的密度降低到4×106cm-2.这些结果使得InAs/GaAs量子点可以作为单光子源应用在未来的光纤基量子密码、量子通信中.     

Photoluminescence study of strain-induced GaInNAs/GaAs quantum dots

Strain-induced Ga_0.8In_0.2N _y As_1–y quantum dots on GaAs were fabricated by metal organic vapor phase epitaxy. Nitrogen concentration y was varied between 0% and 1.3%. The effect of nitrogen concentration on the optical properties of the quantum dots was investigated by continuous-wave and time-resolved photoluminescence measurements. Carrier localization in the states below the band edge of the nitrogen-containing quantum well has been observed. These states are thought to originate from the variation of the quantum well width or from the fluctuation of the composition. Such variations have been identified in GaInNAs quantum wells on GaAs without stressor islands. The measured energy difference of the quantum well and quantum dot ground-state peak energies increase with increasing temperature.     

Effect of size non-uniformity on photoluminescence from ensembles of InAs quantum dots embedded in GaAs

The photoluminescence (PL) spectrum from ensembles of InAs/GaAs quantum dots (QDs) is calculated. The effect of the dot size distribution and the variation of the associated confining potentials on the PL spectra are estimated. It is found that the intermixing of the interfaces causes an increase of the PL spectra energy. The size distribution determines the spectrum width and makes the PL line shape asymmetric with a high-energy tail. Moreover, the non-uniform size distribution also results in a redshift of the PL peak. The experimental PL spectrum is well explained by the size distribution and intermixing effect within the effective mass approximation.