Emission from neutral and charged excitons in a single quantum dot in a magnetic field

We report on optical spectroscopy of self-assembled InAs quantum dots in a magnetic field. We describe how we measure the emission characteristics of a single quantum dot (QD) in high magnetic fields at low temperature using a miniature, fiber-based confocal microscope. Example results are presented on a QD whose charge can be controlled using a field-effect device. For the uncharged, singly and doubly charged excitons we find a diamagnetism and the spin Zeeman effect. In contrast, for the triply-charged exciton we find a fundamentally different behavior. Anti-crossings in magnetic field imply that confined states of the QD are hybridized with Landau-like levels associated with the two-dimensional continuum.     

MBE自组织生长多层竖直自对准InAs量子点结构的研究

利用MBE方法在(001)GaAs衬底上生长了多层竖直自对准InAs量子点结构。透射电子显微镜的观察表明,多层量子点成一系列柱状分布。同单层量子点相比,多层量子点的光荧光谱线发生红移。这表明由于量子点中载流子波函数的扩展和交迭,柱中量子点之间有耦合现象发生。光荧光谱线半高宽随温度的反常变化说明载流子还会在邻近柱中隧穿.     

Time-resolved photoluminescence spectroscopy of InAs quantum dots on InP with various InAlGaAs barrier thicknesses

The Optical characteristics of InAs quantum dots (QDs) embeded in InAlGaAs on InP have been investigated by photoluminescence (PL) spectroscopy and time-resolved PL. Four different QD samples are grown by using molecular beam epitaxy, and all the QD samples have five-stacked InAs quantum dot layers with a different InAlGaAs barrier thickness. The PL yield from InAs QDs was increased with an increase in the thickness of the InAlGaAs barrier, and the emission peak positions of all InAs QD samples were measured around 1.5 μm at room temperature. The decay time of the carrier in InAs QDs is decreased abruptly in the QD sample with the 5 nm InAlGaAs barrier. This feature is explained by the tunneling and coupling effect in the vertical direction and probably defect generation.     

Effect of elastic anisotropy on the strain fields and band edges in stacked InAs/GaAs quantum dot nanostructures

The effect of elastic anisotropy on the strain fields and confinement potentials in InAs/GaAs quantum dot (QD) nanostructures was investigated for an isolated dot and a stacked multi-layer dots using finite element analysis and model solid theory. The assumption of isotropy tends to underestimate especially hydrostatic strain that is known to modify confinement potentials in conduction band. Consideration of anisotropy results in a wider band gap and shallower potential well as compared with the isotropic model. Since the band gap and potential well depth would be related to opto-electronic properties of quantum dot systems via quantum mechanical effects, it is suggested that consideration of elastic anisotropy in the calculation of strains and band structures is necessary for the design of QD-based opto-electronic devices.     

Barrier height and tunneling current in Schottky diodes with embedded layers of quantum dots

Electrical characteristics of silicon Schottky diodes containing Ge quantum dot (QD) arrays are investigated. It has been found that the potential barrier height at the metal-semiconductor contact can be controlled by introducing dense QD layers, which is a consequence of the formation of a planar electrostatic potential of charged QDs. When the applied voltage is varied, the ideality factors of Schottky barriers exhibit oscillations due to the tunneling of holes through discrete levels in quantum dots.     

含InAs自组装量子点肖特基二极管的关联放电效应

制作了含自组织量子点的金属半导体金属双肖特基势垒器件,研究了器件的电流输运特性.在量子点充放电造成的电流迟滞回路的基础上,观察到了电压扫描过程中的电流由低态到高态的跳跃现象.这种电流跳跃来源于充电量子点的关联放电效应.根据量子点系统的哈密顿量,分析了充电量子点关联放电的原因.这种关联放电效应起源于量子点与2DEG的相互作用,当一个量子点放电时通过量子点和2DEG电流的变化会影响其他的量子点,从而促使其放电,这种过程在整个系统中放大导致所有的量子点放电 关键词： 关联效应 自组装量子点     

Surface integral determination of built-in electric fields and analysis of exciton binding energies in nitride-based quantum dots

We present a simple analytical approach to calculate the built-in strain-induced and spontaneous piezoelectric fields in nitride-based quantum dots (QDs) and then apply the method to describe the variation of exciton, biexciton and charged exciton energy with dot size in GaN/AlN QDs. We first present the piezoelectric potential in terms of a surface integral over the QD surface, and confirm that, due to the strong built-in electric field, the electrons are localised near the QD top and the holes are localised in the wetting layer just below the dot. The strong localisation and smaller dielectric constant results in much larger Coulomb interactions in GaN/AlN QDs than in typical InAs/GaAs QDs, with the interaction between two electrons, J_ee, or two holes, J_hh, being about a factor of three larger. The electron–hole recombination energy is always blue shifted in the charged excitons, X⁻ and X⁺, and the biexciton, and the blue shift increases with increasing dot height. We conclude that spectroscopic studies of the excitonic complexes should provide a useful probe of the structural and piezoelectric properties of GaN-based QDs.     

Effects of a longitudinal magnetic field on spin injection and detection in InAs/GaAs quantum dot structures

Effects of a longitudinal magnetic field on optical spin injection and detection in InAs/GaAs quantum dot (QD) structures are investigated by optical orientation spectroscopy. An increase in the optical and spin polarization of the QDs is observed with increasing magnetic field in the range 0-2?T, and is attributed to suppression of exciton spin depolarization within the QDs that is promoted by the hyperfine interaction and anisotropic electron-hole exchange interaction. This leads to a corresponding enhancement in spin detection efficiency of the QDs by a factor of up to 2.5. At higher magnetic fields, when these spin depolarization processes are quenched, the electron spin polarization in anisotropic QD structures (such as double QDs that are preferably aligned along a specific crystallographic axis) still exhibits a rather strong field dependence under non-resonant excitation. In contrast, such a field dependence is practically absent in more 'isotropic' QD structures (e.g.?single QDs). We attribute the observed effect to stronger electron spin relaxation in the spin injectors (i.e.?wetting layer and GaAs barriers) of the lower-symmetry QD structures, which also explains the lower spin injection efficiency observed in these structures.     

Strain distribution and electronic states in stacked InAs/GaAs quantum dots with dot spacing 0–

We have calculated the strain distribution and electronic structures in stacked InAs/GaAs quantum dots (QDs) with the dot spacing 6–. We used the elastic continuum theory for the strain distribution, and the 8-band k·p theory for the electronic structures. For the triply stacked QDs, the light-hole (LH) component of the hole ground state increases with decreasing the dot spacing. The LH component in the columnar QD (dot spacing ) reaches 21.1% which is 4.8 times larger than that in the single QD due to the reduction of the biaxial strain. Further increase of the LH component (up to 28.6%) is obtained in the fivefold-stacked columnar QD. This result suggests a possibility of increase in the TM-mode transition in the columnar QDs.     

Resonant tunneling of electrons through single self-assembled InAs quantum dot studied by conductive atomic force microscopy

Resonant tunneling of electrons through a quantum level in single self-assembled InAs quantum dot (QD) embedded in thin AlAs barriers has been studied. The embedded InAs QDs are sandwiched by 1.7-nm-thick AlAs barriers, and surface InAs QDs, which are deposited on 8.3 nm-thick GaAs cap layer, are used as nano-scale electrodes. Since the surface InAs QD should be vertically aligned with a buried one, a current flowing via the buried QD can be measured with a conductive tip of an atomic force microscope (AFM) brought in contact with the surface QD-electrode. Negative differential resistance attributed to electron resonant tunneling through a quantized energy level in the buried QD is observed in the current–voltage characteristics at room temperature. The effect of Fermi level pinning around nano-scale QD-electrode on resonance voltage and the dependence of resonance voltage on the size of QD-electrodes are investigated, and it has been demonstrated that the distribution of the resonance voltages reflects the size variation of the embedded QDs.     

Photoluminescence of InAs quantum dots embedded in graded InGaAs barriers

The effects of the top barrier and the dot density on photoluminescence (PL) of the InAs quantum dots (QDs) sandwiched by the graded In_xGa_1−xAs barriers grown by metal-organic vapor phase epitaxy (MOVPE) have been studied. Two emission peaks corresponding to the ground state and the 1st excited state transitions of the QD structures have been observed, which matches well to the theoretical calculation. The PL emission linewidth and intensity of the InAs QDs structure are improved by reducing the Indium/Gallium composition variation of the graded In_xGa_1−xAs top barrier layer of the structure. The QDs’ ground states filling excitation power depends on the crystal quality of the InGaAs barrier layer and the QD density. The extracted thermal activation energy for the QDs’ PL emission is sensitive to the QD size.     

Charged and neutral biexciton–exciton cascade in a single quantum dot within a photonic bandgap

We investigate the recombination dynamics of positively charged and neutral biexcitons and excitons in a single InAs/GaAs quantum dot (QD) within a two-dimensional (2D) photonic bandgap (PBG). The 2D PBG makes the exciton lifetime four times longer and enhances photon-extraction efficiency compared to those without the PBG. Photon cross-correlation measurements demonstrate the cascade emissions of both charged and neutral biexcitons–excitons from the same QD. In the charged case, a hole in the p-shell relaxes into the s-shell between the cascade, and the corresponding transition is confirmed based on the spin configuration. The long exciton lifetime with the PBG helps us to reveal the spin dynamics that did not clearly appear in intrinsic QDs.     

Anomalous temperature dependence of photoluminescence spectra from InAs/GaAs quantum dots grown by formation–dissolution–regrowth method

Two kinds of InAs/GaAs quantum dot(QD) structures are grown by molecular beam epitaxy in formation–dissolution–regrowth method with different in-situ annealing and regrowth processes. The densities and sizes of quantum dots are different for the two samples. The variation tendencies of PL peak energy, integrated intensity, and full width at half maximum versus temperature for the two samples are analyzed, respectively. We find the anomalous temperature dependence of the InAs/GaAs quantum dots and compare it with other previous reports. We propose a new energy band model to explain the phenomenon. We obtain the activation energy of the carrier through the linear fitting of the Arrhenius curve in a high temperature range. It is found that the Ga As barrier layer is the major quenching channel if there is no defect in the material. Otherwise, the defects become the major quenching channel when some defects exist around the QDs.     

自组织InAs/GaAs量子点垂直排列生长研究

利用自组织生长InAs/GaAs量子点的垂直相关排列机制，生长了上下两层用6.5nm GaAs间隔的InAs结构.下层InAs已经成岛，由于应力传递效应，上层InAs由二维生长向三维成岛生长的转变提前发生，临界厚度从1.7ML变成小于1.5ML.透射电子显微镜截面象显示形成上下两层高度差别很大的InAs量子点，但是由于两层量子点之间存在强烈的电子耦合，光致发光谱中只有与包含大量子点的InAs层相对应的一个发光峰.     

Comparison of single-layer and bilayer InAs/GaAs quantum dots with a higher InAs coverage

Epitaxially grown self-assembled InAs quantum dots (QDs) have found applications in optoelectronics. Efforts are being made to obtain efficient quantum-dot lasers operating at longer telecommunication wavelengths, specifically 1.3 μm and 1.55 μm. This requires narrow emission linewidth from the quantum dots at these wavelengths. In InAs/GaAs single layer quantum dot (SQD) structure, higher InAs monolayer coverage for the QDs gives rise to larger dots emitting at longer wavelengths but results in inhomogeneous dot-size distribution. The bilayer quantum dot (BQD) can be used as an alternative to SQDs, which can emit at longer wavelengths (1.229 μm at 8 K) with significantly narrow linewidth (∼16.7 meV). Here, we compare the properties of single layer and bilayer quantum dots grown with higher InAs monolayer coverage. In the BQD structure, only the top QD layer is covered with increased (3.2 ML) InAs monolayer coverage. The emission line width of our BQD sample is found to be insensitive towards post growth treatments.     

Formation of nanoscale heterointerfaces by selective area metalorganic vapor-phase epitaxy and their applications

We describe fabrication methods of GaAs and InAs quantum dot (QD) structures and related semiconductor nanostructures having nanoscale heterointerfaces grown by the selective area metalorganic vapor-phase epitaxial (SA-MOVPE) method on partially masked GaAs substrates. GaAs QD arrays and wire–dot coupled structures having strong lateral confinement were fabricated on appropriately designed masked substrates. InAs QDs were also formed on various kinds of GaAs pyramidal and wire structures, where site-selective formation is demonstrated by the combination of self-assembling growth mode and selective area growth. The application of QDs to single-electron transistors using SA-MOVPE is also described.     

Electrically tunable g factors in quantum dot molecular spin states

We present a magnetophotoluminescence study of individual vertically stacked InAs/GaAs quantum dot pairs separated by thin tunnel barriers. As an applied electric field tunes the relative energies of the two dots, we observe a strong resonant increase or decrease in the g factors of different spin states that have molecular wave functions distributed over both quantum dots. We propose a phenomenological model for the change in g factor based on resonant changes in the amplitude of the wave function in the barrier due to the formation of bonding and antibonding orbitals.     

EFFECT OF DOPANT ON THE UNIFORMITY OF InAs SELF-ORGANIZED QUANTUM DOTS

Low-temperature photoluminescence studies have been performed on Si-doped and Bedoped self-organized InAs/GaAs quantum dot(QD) samples to investigate the effect of doping. When Si or Be is doped into the sample,a remarkable decrease in line-width is observed. We relate this phenomenon to a model that takes the Si or Be atoms as the nucleation centers for the formation of QDs. When Si or Be is doped, more smalll uniform quantum dots are formed. The result will be of significance for the application of self-organized InAs quantum dots in semiconductor devices.     

Intrinsic and Mn doped InAs quantum dots studied at the atomic scale by cross-sectional scanning tunneling microscopy

Cross-sectional Scanning Tunneling Microscopy (X-STM) is an ideal tool to study the structural properties of semiconductor nanostructures, such as InAs self-assembled quantum dots (QDs) and the properties of individual doping atoms at the atomic scale. The technique allows for a precise determination of the size, shape and composition of overgrown semiconductor nanostructures which can be part of a (complex) multilayer structure. In this paper we discuss our recent results on InAs QD structures that were capped by various methods in order to control their size and shape. We will show that the capping process does strongly affect the final QD structure and thus forms a very important step in the dot formation process. Recently people have started to investigate magnetically doped QDs. We have used our X-STM technique to study the incorporation of single Mn-impurities in InAs/GaAs QDs.     

Multi-color tunneling quantum dot infrared photodetectors operating at room temperature

Quantum dot structures designed for multi-color infrared detection and high temperature (or room temperature) operation are demonstrated. A novel approach, tunneling quantum dot (T-QD), was successfully demonstrated with a detector that can be operated at room temperature due to the reduction of the dark current by blocking barriers incorporated into the structure. Photoexcited carriers are selectively collected from InGaAs quantum dots by resonant tunneling, while the dark current is blocked by AlGaAs/InGaAs tunneling barriers placed in the structure. A two-color tunneling-quantum dot infrared photodetector (T-QDIP) with photoresponse peaks at 6 μm and 17 μm operating at room temperature will be discussed. Furthermore, the idea can be used to develop terahertz T-QD detectors operating at high temperatures. Successful results obtained for a T-QDIP designed for THz operations are presented. Another approach, bi-layer quantum dot, uses two layers of InAs quantum dots (QDs) with different sizes separated by a thin GaAs layer. The detector response was observed at three distinct wavelengths in short-, mid-, and far-infrared regions (5.6, 8.0, and 23.0 μm). Based on theoretical calculations, photoluminescence and infrared spectral measurements, the 5.6 and 23.0 μm peaks are connected to the states in smaller QDs in the structure. The narrow peaks emphasize the uniform size distribution of QDs grown by molecular beam epitaxy. These detectors can be employed in numerous applications such as environmental monitoring, spectroscopy, medical diagnosis, battlefield-imaging, space astronomy applications, mine detection, and remote-sensing.