A. A. Suvorova

Long-wavelength emission in structures with quantum dots formed in the stimulated decomposition of a solid solution at strained islands

When an array of strained InAs nanoislands formed on a GaAs surface is overgrown by a thin (1–10 nm) layer of an indium-containing solid solution, stimulated decomposition of the solid solution is observed. This process causes the formation of zones of elevated indium concentration in the vicinity of the nanoislands. The volume of newly formed InAs quantum dots increases as a result of this phenomenon, producing a substantial long-wavelength shift of the photoluminescence line. This effect is enhanced by lowering the substrate temperature, and it depends weakly on the average width of the band gap of the solid solution. The indicated approach has been used successfully in achieving room-temperature emission at a wavelength of 1.3 µm.

Local stresses induced by nanoscale As–Sb clusters in GaAs matrix

Microstructure of GaAs films grown by molecular-beam epitaxy at low temperature and delta doped with Sb was studied by transmission electron microscopy. The material contained 0.5 at.u200a% excess of arsenic that precipitated during post growth anneals. The Sb δ doping was found to strongly affect the microstructure of precipitates (clusters) and their ripening rate upon annealing. Segregation of Sb impurity in the clusters was revealed. In contrast to the well known pure As clusters, the As–Sb clusters induced strong local deformations in the surrounding GaAs matrix. Until a threshold diameter of 7–8 nm the clusters and surrounding matrix were coherently strained. Larger clusters were associated with dislocation loops of interstitial type. The cluster-loop orientation relationships were determined. Relaxation of local strains by formation of the dislocation loops was studied both experimentally and theoretically.

Enhanced As–Sb intermixing of GaSb monolayer superlattices in low-temperature grown GaAs

As–Sb compositional intermixing was studied by transmission electron microscopy (TEM) in GaAs films grown by molecular-beam epitaxy at low temperature (LT) and δ doped with antimony. The TEM technique was calibrated by imaging the as-grown films with δ layers containing various amounts of Sb. The calibration allowed us to deduce the effective As–Sb interdiffusion coefficient from apparent thickness of the Sb δ layers in the films subjected to isochronal anneals at 400–600u200a°C. The As–Sb intermixing in LT GaAs was found to be much enhanced when compared to conventional material. Its temperature dependence yields a diffusion coefficient of DAs–Sb=2×10−14u200aexp(−0.62±0.15u200aeV/kt)u200acm2u200as−1. Since the kick-out mechanism operating under equilibrium conditions is valid for As–Sb interdiffusion in GaAs, the enhanced intermixing was attributed to an oversaturation of arsenic self-interstitials in the LT GaAs films. The effective activation energy for As–Sb interdiffusion in LT GaAs seems to be reasonably close to the m...

In–Ga intermixing in low-temperature grown GaAs delta doped with In

Low-temperature grown GaAs films with indium delta layers are studied by transmission electron microscopy. The delta layers in the as-grown film are found to be as thick as four monolayers (ML) independently of a nominal In deposit of 0.5 or 1 ML, a thickness which reflects the film surface roughness during the low-temperature growth. A pronounced In–Ga intermixing is observed in the films subjected to 500–700u200a°C isochronal anneals. The In–Ga interdiffusion diffusivity is evaluated. The effective activation energy for In–Ga interdiffusion is found to be 1.1±0.3u200aeV which is significantly smaller than a value of 1.93 eV for a stoichiometric GaAs. The difference seems to result from a loss of the gallium vacancy supersaturation upon annealing, and is consistent with an annihilation enthalpy of 0.8 eV.

Capacitance-voltage profiling of Au/n-GaAs Schottky barrier structures containing a layer of self-organized InAs quantum dots

Capacitance-voltage measurements are used to obtain profiles of the free carrier distribution in Schottky barriers grown on uniformly doped n-GaAs hosts containing layers of self-organized InAs quantum dots. It is found that electrons accumulate at a depth of 0.54 µm, which corresponds to the depth of the quantum-dot layer. As the temperature drops below 90 K, a second peak appears in the concentration profile at 0.61 µm, which becomes dominant as the temperature continues to decrease. It is shown that the appearance of the second peak in the concentration profile is not due to electron density redistribution over the structure, but rather is observed when the rate of thermal emission of electrons from the quantum dots is slower than the angular frequency of the capacitance measurement signal.

Formation of InSb quantum dots in a GaSb matrix using molecular-beam epitaxy

Abstract InSb nanoislands in a GaSb matrix have been fabricated and their structural and luminescence properties have been studied. The deposition of ∼2 monolayers of InSb on the GaSb(100) surface has been found by atomic force microscopy to result in a 2D–3D growth mode transition and in a formation of 3D InSb islands with a size about 80 nm. Increase in the thickness of the InSb layer more than three monolayers causes a dramatic drop of the photoluminescence intensity due to plastical relaxation of the islands. Surprisingly transmission electron microscopy studies of InSb insertions with average InSb thickness below two monolayers demonstrate dense array of coherent 2D islands, or quantum dots, having a uniform lateral size of 30 nm. Bright luminescence due to these 2D islands is observed. We believe that these 2D multimonolayer islands can be used as precursor state for vertically coupled InSb-GaSb QD structures suitable for laser applications.

Stacked InAs/InGaAs quantum dot heterostructures for optical sources emitting in the 1.3 µm wavelength range

A method is proposed for growing stacked InAs/InGaAs self-organized quantum dots on GaAs substrates. The technique allows fabrication of structures exhibiting intense and narrow-line photoluminescence in the 1.3 µm wavelength region. The influence of growth conditions on structural and optical characteristics was studied. The proposed structures show promise in developing vertical-cavity surface-emitting devices.

Indium layers in low-temperature gallium arsenide: Structure and how it changes under annealing in the temperature range 500–700 °C

Transmission electron microscopy is used to study the microstructure of indium δ layers in GaAs(001) grown by molecular beam epitaxy at low temperature (200 °C). This material, referred to as LT-GaAs, contains a high concentration (≈1020 cm−3) of point defects. It is established that when the material is δ-doped with indium to levels equivalent to 0.5 or 1 monolayer (ML), the roughness of the growth surface leads to the formation of InAs islands with characteristic lateral dimensions <10 nm, which are distributed primarily within four adjacent atomic layers, i.e., the thickness of the indium-containing layer is 1.12 nm. Subsequent annealing, even at relatively low temperatures, leads to significant broadening of the indium-containing layers due to the interdiffusion of In and Ga, which is enhanced by the presence of a high concentration of point defects, particularly VGa, in LT-GaAs. By measuring the thickness of indium-containing layers annealed at various temperatures, the interdiffusion coefficient is determined to be DIn-Ga=5.1×10−12 exp(−1.08 eV/kT) cm2/s, which is more than an order of magnitude larger than DIn-Ga for stoichiometric GaAs at 700 °C.

Electron escape from self-assembled InAs/GaAs quantum dot stacks

Abstract Capacitance–voltage characteristics have been measured at various frequencies and temperatures for a Schottky barrier structure containing three sheets of self-assembled InAs quantum dots in an n-GaAs matrix. By changing the frequency of the measuring signal at fixed temperature, it is possible to control the ratio between the thermionic and the tunnel contributions to the electron escape from the quantum dots. An applied magnetic field reduces the thermionic emission rate and increases the importance of the tunnel part of escape of electrons from the dots due to the deepening of electron level in the dots by Landau quantization in the GaAs conduction band.

Optical properties of InAlAs quantum dots in an AlGaAs matrix

Abstract Structural and optical properties of In x Al 1− x As quantum dots (QD) embedded in an Al 0.30 Ga 0.70 As matrix are studied as a function of InAs mole fraction ( x ). Decreasing of x results in a sequential disappearance of bound electron and hole states in the QDs. There is a range of indium compositions were photoluminescence is determined by type-II optical transitions between electrons localized in a Al 0.3 Ga 0.7 As layer and holes localized in the QDs.