Lateral ordering of quantum dots and wires in the (In,Ga)As/GaAs(100) multilayer structures

The surface morphology and optical properties of the (In,Ga)As/GaAs(100) multilayer structures with self-organized quantum dots and quantum wires, which were grown by molecular-beam epitaxy, are investigated. It is found that the ordered arrangement of quantum dots in the heterointerface plane starts to form during the growth of the first periods of the multilayer structure. As the number of periods increases, quantum dots line up in series and form wires along the \([0\bar 11]\) direction. An increase in the lateral ordering of the structures under consideration correlates with an increase in the optical emission anisotropy governed by relaxation anisotropy of elastic strains and by the shape of nano-objects. A possible mechanism of lateral ordering of quantum dots and wires in multilayer structures, which includes both anisotropy effects of the strain fields and adatom diffusion, as well as the elastic interaction of neighboring quantum dots, is discussed.     

Quantum light source devices of In(Ga)As semiconductor self-assembled quantum dots

A brief introduction of semiconductor self-assembled quantum dots (QDs) applied in single-photon sources is given. Single QDs in confined quantum optical microcavity systems are reviewed along with their optical properties and coupling characteristics. Subsequently, the recent progresses in In(Ga)As QDs systems are summarized including the preparation of quantum light sources, multiple methods for embedding single QDs into different microcavities and the scalability of single-photon emitting wavelength. Particularly, several In(Ga)As QD single-photon devices are surveyed including In(Ga)As QDs coupling with nanowires, InAs QDs coupling with distributed Bragg reflection microcavity and the In(Ga)As QDs coupling with micropillar microcavities. Furthermore, applications in the field of single QDs technology are illustrated, such as the entangled photon emission by spontaneous parametric down conversion, the single-photon quantum storage, the chip preparation of single-photon sources as well as the single-photon resonance-fluorescence measurements.     

Characterization of MOVPE-grown (Al, In, Ga) N heterostructures by quantitative analytical electron microscopy

The low pressure metalorganic vapor phase epitaxy growth of wurzite (Al, In, Ga)N heterostructures on sapphire substrates is investigated by quantitative analytical scanning transmission electron microscopy techniques like atomic number (Z-) contrast imaging and convergent beam electron diffraction (CBED). Especially (In, Ga)N quantum wells of different thicknesses as well as superlattices were analyzed with respect to defects, chemical composition variations, interface abruptness and strain (relaxation) effects. The interfaces in In_0.12Ga_0.88N/GaN quantum wells appear to be asymmetric. Additionally, we found composition variations of Δx_In≥0.03 within the InGaN quantum wells. The application of electron diffraction techniques (CBED) yields quantitative information on strain and relaxation effects. For the case of 17 nm thick InGaN quantum wells, we observed relaxation effects which are not present in the investigated thin quantum wells of 2 nm thickness. The experimentally obtained diffraction patterns were compared to simulations in order to get values for strain within the quantum wells. Additionally, the influence of dislocations on the digression of superlattices is investigated.     

Influence of composition and anneal conditions on the optical properties of (In, Ga)As quantum dots in an (Al, Ga)As matrix

The optical properties of structures containing InGaAs quantum dots in GaAs and AlGaAs matrices grown by molecular-beam epitaxy are investigated. It is shown that increasing the In content in the quantum dots has the effect of raising the energy of carrier localization and increasing the energy distance between the ground state and the excited states of carriers in the quantum dots. An investigation of the influence of postgrowth annealing on the optical properties of the structures shows that the formation of vertically coupled quantum dots and the use of a wide-gap AlGaAs matrix enhances the thermal stability of the structures. Moreover, high-temperature (830 °C) thermal annealing can improve the quality of the AlGaAs layers in structures with vertically coupled InGaAs quantum dots in an AlGaAs matrix. The results demonstrate the feasibility of using postgrowth annealing to improve the characteristics of quantum dot lasers. Fiz. Tekh. Poluprovodn. 33, 91–96 (January 1999)     

Elastic strains in GaAs/AlAs quantum dots studied by high resolution X-ray diffraction

We have studied a GaAs/AlAs quantum dot array using reciprocal space mapping around the (004) and (II3) reciprocal lattice points. Both the coherently and the diffusely scattered X-ray intensities were analysed by performing two-dimensional model calculations. From the distribution of the diffracted intensities we deduced the average strain status in the dots. From the numerical simulations it is evident that random elastic strain fields are present, which extend through almost the whole volume of the quantum dot. The simulations of the X-ray measurements revealed that the crystalline part of the dots is considerably smaller as scanning electron micrographs would indicate, namely 50 nm instead of 65 nm, respectively.     

Analysis of mechanisms of carrier emission in the p-i-n structures with In(Ga)As quantum dots

With the help of the photocurrent spectroscopy, the mechanism of emission of charge carriers from energy levels of the (In,Ga)As/(Al,Ga)As quantum dots of different design are studied. Thermal activation is shown to be the main mechanism of carrier emission from the quantum dots for the isolated layer of quantum dots separated by wide (Al,Ga)As spacer layers. At a small width of the (Al,Ga)As spacer layer, when electron binding of separate layers of the quantum dots in the vertical direction takes place, the role of the tunneling mechanism of carrier emission between the vertically coupled quantum dots increases.     

GaAs/InGaAs量子点应变场的TEM研究

运用透射电子显微术(TEM)对由分子束外延(MBE)制备的GaAs／InGaAs多层量子点样品进行观察和分析。利用对量子点周围应变场分布的模拟,定性解释了量子点形貌及其周围发现的凹陷区域。通过对量子点高分辨像显示的晶格错配和化学成分分析研究,解释了所研究样品中量子点尺寸逐层增大的现象。研究结果为量子点材料生长过程中应变场的控制提供了一些思路。     

Photoluminescence study of the In0.3Ga0.7As surface quantum dots coupling structure

Photoluminescence (PL) was investigated as functions of the excitation intensity and temperature for a coupling surface quantum dots (SQDs) structure which consists of one In0.3Ga0.7As SQDs layer being stacked on multi-layers of In0.3Ga0.7As buried quantum dots (BQDs). Accompanied by considering the localized excitons effect induced by interface fluctuation, carrier transition between BQDs and SQDs were analyzed carefully. The PL measurements confirm that there is a strong carrier transition from BQDs to SQDs and this transition leads to obvious different PL characteristics between BQDs and SQDs. These results are useful for future application of SQDs as surface sensitive sensors.     

Optoelectronic properties of heteronanostructures with combined layers of In(Ga)As/GaAs quantum wells and dots

The effect of the incorporation of an InGaAs quantum well into structures with InAs/GaAs quantum dots grown by gas-phase epitaxy on their optoelectronic properties is analyzed in the mode with increased growth-interruption time. It is established that the quantum-dot energy spectrum is weakly sensitive to variations in the thickness and composition of the double InGaAs/GaAs coating layer. The deposition of a quantum well onto a layer of quantum dots decreases the emission-barrier effective height in them. The conditions under which the quantum well can be used for protecting the quantum-dot active layer against penetration by defects generated during structure-surface anodic oxidation are determined.     

In0.5Ga0.5As/In0.5Al0.5As应变耦合量子点的形貌和光学性质

提出了利用分子束外延方法生长In0.5Ga0.5As/In0.5Al0.5As应变耦合量子点,并分析量子点的形貌和光学性质随GaAs隔离层厚度变化的特点.实验结果表明,随着耦合量子点中的GaAs隔离层厚度从2 nm增加到10 nm,In0.5Ga0.5As量子点的密度增大、均匀性提高,Al原子扩散和浸润层对量子点PL谱的影响被消除,而且InAlAs材料的宽禁带特征使其成为InGaAs量子点红外探测器中的暗电流阻挡层.由此可见,选择合适的GaAs隔离层厚度形成InGaAs/InAlAs应变耦合量子点将有益于InGaAs量子点红外探测器的研究.     

Effect of He+ ion irradiation on the photosensitivity spectra of In(Ga)As/GaAs quantum well and quantum dot heterostructures

The effect of He⁺ ion implantation on the photosensitivity spectra of InGaAs/GaAs quantum well and InAs/GaAs quantum dot heterostructures grown by metalorganic chemical vapor deposition (MOCVD) epitaxy is studied.     

Optical and morphological properties of In(Ga)As/GaAs quantum dots grown on novel index surfaces

Optical and morphological properties of self assembled In(Ga)As/GaAs quantum dot systems, grown on high index (N11) substrates, for a wide range of orientations, coverages and compositions, are presented and reviewed. The use of high Miller index substrate orientations permits to intervene on major quantum dots properties such as shape, size distribution, transition energy and emission polarisation, thus opening a wide range of device design opportunities.     

Arrays of strained InAs quantum dots in an (In Ga)As matrix, grown on InP substrates by molecular-beam epitaxy

Arrays of strained InAs islands in an (In, Ga)As matrix on an InP(100) substrate are synthesized by molecular-beam epitaxy, and their structural and optical properties are investigated. According to transmission electron microscope and high-energy electron diffraction data, the critical thickness corresponding to the onset of island growth is 3 monolayers. The resulting InAs islands are coherently strained, and their base diameter varies from 20 nm to 90 nm. The formation of islands produces in the photoluminescence spectra a dominant long-wavelength line, which shifts toward lower energies as the effective thickness of the InAs increases. The radiation emitted by the InAs islands spans a wavelength range of 1.65–2 μm. Fiz. Tekh. Poluprovodn. 31, 1256–1260 (October 1997)     

Self-ordered CdSe quantum dots in ZnSe and (Zn, Mn)Se Matrices Assessed by transmission electron microscopy and photoluminescence spectroscopy

Single and multilayer sheets of self-assembled CdSe [quantum dots (QDs)] were grown by means of molecular beam epitaxy in both ZnSe and (Zn_0.9Mn_0.1)Se matrices. Both types of structure were assessed by means of transmission electron microscopy in the scanning, high-resolution, and diffraction-contrast modes. Complementary results from wider sample areas were obtained by means of photoluminescence spectroscopy. In one of the samples analyzed, a fractional monolayer of MnSe was deposited immediately before the CdSe deposition. A second structure grown under identical conditions, but without the MnSe fractional monolayer, was also analyzed. This comparison provides direct evidence for an enhanced size and shape homogeneity of 3D QDs caused by the presence of a tiny amount of MnSe at the interface. In the multilayer structure, we observed the co-existence of highly strained quasi-2D QDs and CdSe rich aggregates with compositional modulations on certain crystallographic planes in close proximity.     

X-ray diffraction and high resolution transmission electron microscopy of 3C-SiC/AlN/6H-SiC(0001)

The structure and crystal quality of epitaxial films of SiC/AlN/6H-SiC(0001) prepared by chemical vapor deposition were evaluated by high resolution transmission electron microscopy (HRTEM) and x-ray diffraction techniques. Cross-sectional HRTEM revealed an abrupt AlN layer-6H-SiC substrate junction, but the transition between the AlN and SiC layers was much rougher, leading to the formation of a highly disordered SiC region adjacent to the interface. The AlN layer was relatively defect free, while the SiC layer contained many microtwins and stacking faults originating at the top SiC/AlN interface. The SiC layer was the 3C-polytype, as determined by double crystal x-ray rocking curves. The SiC layers were under in-plane compressive stress, with calculated defect density between 2–4×10⁷ defects/cm⁻².     

Electrical characterization of self-assembled In0.5Ga0.5As/GaAs quantum dots by deep level transient spectroscopy

We have investigated electron emission from self-assembled In_0.5Ga_0.5As/GaAs quantum dots (QDs) grown by molecular-beam epitaxy (MBE). Through detailed deep level transient spectroscopy comparisons between the QD sample and a reference sample, we determine that trap D, with an activation energy of 100 meV and an apparent capture cross section of 5.4×10⁻¹⁸ cm², is associated with an electron quantum level in the In_0.5Ga_0.5As/GaAs QDs. The other deep levels observed, M1, M3, M4, and M6, are common to GaAs grown by MBE.     

(In,Mn)As quantum dots: Molecular-beam epitaxy and optical properties

Self-assembled (In,Mn)As quantum dots are synthesized by molecular-beam epitaxy on GaAs (001) substrates. The experimental results obtained by transmission electron microscopy show that doping of the central part of the quantum dots with Mn does not bring about the formation of structural defects. The optical properties of the samples, including those in external magnetic fields, are studied.     

Lasing in structures with InAs quantum dots in an (Al, Ga)As matrix grown by submonolayer deposition

Structural and optical properties of structures with nanoscale InAs islands obtained by submonolayer deposition and embedded in an Al_xGa_1−xAs matrix is investigated. Deposition of several planes of InAs insertions results in formation of arrays of vertically correlated islands. The lateral size of the islands in a column is about 10 nm. Lasing via the ground states of the islands without external optical confinement is demonstrated.     

Effect of the electrochemical modification of a thin Ga(In)As cap layer on the energy spectrum of InAs/GaAs quantum dots

It was shown that the selective etching and anodic oxidation of a thin Ga(In)As cap layer makes it possible to decrease the ground-state transition energy in InAs/GaAs quantum dots from ～0.9 to ～0.7 eV due to the resulting partial stress relaxation. Similar processing of surface quantum dots leads to a decrease in the quantum-dot height that increases the transition energy.     

Subminiature emitters based on a single (111) In(Ga)As quantum dot and hybrid microcavity

The results of numerical modeling and investigation of a hybrid microcavity based on a semiconductor Bragg reflector and a microlens selectively positioned above a single (111) In(Ga)As quantum dot are presented. Emitters based on the hybrid microcavity demonstrate the effective pumping of a single quantum dot and high emission output efficiency. The microcavity design can be used to implement emitters of polarization- entangled photon pairs based on single semiconductor quantum dots.