Yu. G. Musikhin

InAs/InGaAs quantum dot structures on GaAs substrates emitting at 1.3 μm

InAs self-organized quantum dots inserted in InGaAs quantum well have been grown on GaAs substrates by molecular beam epitaxy. The lateral size of the InAs islands has been found to be approximately 1.5 times larger as compared to the InAs/GaAs case, whereas the island heights and surface densities were close in both cases. The quantum dot emission wavelength can be controllably changed from 1.1 to 1.3 μm by varying the composition of the InGaAs quantum well matrix. Photoluminescence at 1.33 μm from vertical optical microcavities containing the InAs/InGaAs quantum dot array was demonstrated.

Continuous-wave operation of long-wavelength quantum-dot diode laser on a GaAs substrate

Continuous-wave operation near 1.3 /spl mu/m or a diode laser based on self-organized quantum dots (QDs) on a GaAs substrate is demonstrated. Multiple stacking of InAs QD planes covered by thin InGaAs layers allows us to prevent gain saturation and achieve long-wavelength lasing with low threshold current density (90-105 A/cm/sup 2/) and high output power (2.7 W) at 17/spl deg/C heatsink temperature. It is thus confirmed that QD lasers of this kind are potential candidates to substitute InP-based lasers in optical fiber systems.

Electronic structure of self-assembled InAs quantum dots in GaAs matrix

Capacitance–voltage characteristics have been measured at various frequencies and temperatures for structures containing a sheet of self-assembled InAs quantum dots in both n-GaAs and p-GaAs matrices. Analysis of the capacitance–voltage characteristics shows that the deposition of 1.7 ML of InAs forms quantum dots with electron levels 80 meV below the bottom of the GaAs conduction band and two heavy-hole levels at 100 and 170 meV above the top of the GaAs valence band. The carrier energy levels agree very well with the recombination energies obtained from photoluminescence spectra.

Volmer–Weber and Stranski–Krastanov InAs-(Al,Ga)As quantum dots emitting at 1.3 μm

Quantum dots (QDs) formed on GaAs(100) substrates by InAs deposition followed by (Al,Ga)As or (In,Ga,Al)As overgrowth demonstrate a photoluminescence (PL) peak that is redshifted (up to 1.3 μm) compared to PL emission of GaAs-covered QDs. The result is attributed to redistribution of InAs molecules in the system in favor of the QDs, stimulated by Al atoms in the cap layer. The deposition of a 1 nm thick AlAs cover layer on top of the InAs–GaAs QDs results in replacement of InAs molecules of the wetting layer by AlAs molecules, leading to a significant increase in the heights of the InAs QDs, as follows from transmission electron microscopy. This effect is directly confirmed by transmission electron microscopy indicating a transition to a Volmer–Weber-like QD arrangement. We demonstrate an injection laser based on this kind of QDs.

Control of the emission wavelength of self-organized InGaAs quantum dots: main achievements and present status

Recent achievements in controlling the electronic spectrum of InAs-based quantum dots (QDs) formed by self-organization phenomena during the initial stages of strained layer epitaxy are reviewed. Three different ways to exercise this control are discussed, based on variation of QD size with the amount of QD material deposited, tuning of the electronic levels in QDs by changing the matrix bandgap, and electronic coupling of neighbouring QDs vertically stacked in the growth direction. Possibilities to prevent thermal evaporation of carriers out of QD states and to tune the emission wavelength in the range 0.85-1.3 µm on GaAs substrates and up to 2 µm on InP substrates are demonstrated.

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.

Reversibility of the island shape, volume and density in Stranski-Krastanow growth

We report on reversible and irreversible phenomena in size-limited InAs island growth (SLIG) on GaAs(001) surface. We found that, with increasing the substrate temperature, the island density of the SLIG islands decreases, the lateral size of the islands increases and the islands strongly flatten. The average volume is either decreased or weakly affected. The total amount of InAs accumulated in quantum dots (QDs) strongly decreases in favour of the gas of In adatoms on the surface. Both unidirectional and reversible tuning of the substrate temperature after formation of the islands causes reversible changes in the island shape and density. We show the possibility of dramatically increasing the volume and the density of QDs approaching the strategically important 1.3 µm wavelength range via adatom condensation with cooling of the substrate after the formation of QDs. We also demonstrate that the substrate temperature cycling procedure may remarkably reduce the defect density in QD structures.

Optical and structural properties of self-organized InGaAsN/GaAs nanostructures

Structural and optical properties of thin InGaAsN insertions in GaAs, grown by molecular beam epitaxy using an RF nitrogen plasma source, have been investigated. Nitrogen incorporation into InGaAs results in a remarkable broadening of the luminescence spectrum as compared with that of InGaAs layer with the same indium content. Correspondingly, a pronounced corrugation of the upper interface and the formation of well defined nanodomains are revealed in cross-sectional and plan-view transmission electron microscope (TEM) images, respectively. Raising the indium concentration in InGaAsN (N<1 %) to 35 % results in the formation of well defined separated three-dimensional (3D) islands. The size of the nanodomains proves that the InGaAsN insertions in GaAs should be regarded as quantum dot structures even in the case of relatively small indium concentrations (25 %) and layer thicknesses (7 nm), which are below the values required for a 2D-3D transition to occur in InGaAs/GaAs growth. Dislocation loops have been found in TEM images of the structures emitting at 1.3 µm. They are expected to be responsible for the degradation of the luminescence intensity of such structures in agreement with the case of long-wavelength InGaAs-GaAs quantum dots.

Strain relaxation in stacked InAs/GaAs quantum dots studied by Raman scattering

We report a Raman scattering investigation of InAs vibrational modes in multiple layers of InAs self-assembled quantum dots in a GaAs matrix. The Raman peak associated with quantum-dot phonons shows a downward frequency shift as the interlayer spacing decreases. We attribute this frequency shift to the relaxation of the elastic strain in the stacked quantum-dot layers. From the phonon frequency shift, we estimate the magnitude of the strain in the quantum dot layers, which we relate to the energy of the photoluminescence emission of the dots.

Influence of metalorganic chemical vapor deposition growth conditions on In-rich nanoislands formation in InGaN/GaN structures

The influence of different growth conditions on the In distribution in ultrathin InGaN insertions in a GaN matrix is investigated by high-resolution transmission electron microscopy and an appropriate image evaluation technique. It is demonstrated that the indium distribution represents dense arrays of In-rich nanodomains inserted in a layer with a lower indium concentration. The sizes of the In-rich regions are about 4–5 nm at a growth temperature of 720 °C. Increasing the growth temperature leads to a strong decrease in the of nanoisland density and, also, a moderate decrease in their lateral size. Increasing the trimethylindium/trimethylgallium ratio strongly increases the density of the islands, but the lateral size remains weakly effected. The observations are in agreement with a thermodynamic model of island formation including entropy effects.