D. S. Sizov

The influence of heat treatment conditions on the evaporation of defect regions in structures with InGaAs quantum dots in the GaAs matrix

Structures with In(Ga)As quantum dots in the GaAs matrix obtained using molecular-beam epitaxy are investigated using photoluminescence (PL) measurements and transmission electron microscopy. The structures were subjected in situ to the procedure of the selective thermal elimination of defect regions. Based on the results of the analysis of luminescence properties, a method for evaluating the crystalline quality of structures using the measurements of PL intensity for the GaAs matrix at high temperatures (as high as 400 K) is suggested. Procedures for the elimination of defects are investigated, namely, the single-stage selective elimination of InAs defect islands at 600°C and a two-stage procedure. The latter procedure additionally includes selective overgrowth with a thin AlAs layer and high-temperature (650–700°C) heat treatment. The optimal conditions of the process, which permit the obtaining of structures with a relatively low defect density without a considerable decrease in the density of coherent quantum dots, are found.

High external differential efficiency and high optical gain of long-wavelength quantum dot diode laser

Abstract Long-wavelength (1.29 μm ) lasers grown on GaAs and based on several planes of self-organized quantum dots in an external quantum well demonstrate significant improvement of the external differential efficiency (88%) and the characteristic temperature (150 K ) . This is due to suppression of carrier pile-up in the waveguide region in combination with extended range of optical loss in which the ground-state lasing survives.

High efficiency (ηD>80%) long wavelength (λ>1.25 μm) quantum dot diode lasers on GaAs substrates

Diode lasers based on several layers of self-organized quantum dots (QD) on GaAs substrates were studied. The lasing wavelength lies in the range λ=1.25–1.29 μm, depending on the number of QD layers and optical losses. A record external differential efficiency of 88% and the characteristic temperature of threshold current, 145 K, were obtained. The internal losses, and also threshold and spectral characteristics, are correlated with the optical gain and radiative recombination efficiency, which are strongly dependent on the design of the active region and growth modes.

The emission from the structures with arrays of coupled quantum dots grown by the submonolayer epitaxy in the spectral range of 1.3–1.4 µm

Optical properties of structures with vertically coupled quantum dots grown by the combined submonolayer molecular-beam epitaxy were investigated. It is shown that the formation of the laterally coupled conglomerates of quantum dots are possible in upper rows for certain parameters of growth, with the corresponding photoluminescence emission being in the wavelength range of 1.3–1.4 µm at room temperature.

Analysis of the local indium composition in ultrathin InGaN layers

Experimental photoluminescence (PL) spectra and structural properties of the ultrathin InGaN insertions in an AlGaN matrix grown on a sapphire substrate were investigated. The PL emission mechanism was shown to be governed by the quantum dot (QD)-like indium-rich areas with size about 3 nm, which was determined from high resolution transmission electron microscopy image analysis. The local indium composition in QDs of the samples was estimated as about 26% using the suggested model of PL transition energy, which accounts for quantum confinement energy, spontaneous and piezoelectric fields.

Structural and optical properties of InAs quantum dots in AlGaAs matrix

Structural and optical properties of InAs quantum dots (QDs) grown in a wide-bandgap Al0.3Ga0.7As matrix is studied. It is shown that a high temperature stability of optical properties can be achieved owing to deep localization of carriers in a matrix whose band gap is wider than that in GaAs. Specific features of QD formation were studied for different amounts of deposited InAs. A steady red shift of the QD emission peak as far as ∼1.18 µm with the effective thickness of InAs in Al0.3Ga0.7As increasing was observed at room temperature. This made it possible to achieve a much higher energy of exciton localization than for QDs in a GaAs matrix. To obtain the maximum localization energy, the QD sheet was overgrown with an InGaAs layer. The possibility of reaching the emission wavelength of ~1.3 µm is demonstrated.

Effect of nonradiative recombination centers on photoluminescence efficiency in quantum dot structures

The influence of dislocations on photoluminescence (PL) intensity in structures with InAs-GaAs quantum dots (QD) has been studied. The structural characteristics of samples were studied by transmission electron microscopy in bright-field and weak-beam dark-field diffraction conditions. At temperatures below room temperature and for moderate excitation density, the PL intensity in a structure containing large clusters with dislocations was about the same as in a structure with a significantly lower density of clusters. In contrast, the measurement of PL intensity at elevated temperatures and high excitation densities allows an accurate estimation of the structural perfection of QD structures. The overgrowth of QDs with a thin (1–2 nm) GaAs layer with subsequent annealing reduces the density of clusters with dislocations and significantly improves the temperature stability of the PL intensity.

Influence of the carrier gas composition on metalorganic vapor phase epitaxy of gallium nitride

The influence of hydrogen and nitrogen as carrier gases on the rates of gallium nitride (GaN) growth and etching in the process of metalorganic vapor phase epitaxy (MOVPE) have been studied. Based on these data, the possible roles of hydrogen and nitrogen in the events on the surface of an epitaxial GaN layer are considered.

InGaAs nanodomains formed in situ on the surface of (Al,Ga)As

A new method for obtaining InGaAs nanodomains on the surface of GaAs or (Al,Ga)As is suggested. At the first stage, an InGaAs layer with a thickness above the critical value for dislocation formation is deposited onto the substrate surface by metalorganic CVD. Then the InGaAs film is coated with a thin AlAs layer and annealed at an elevated temperature. The “repulsion” of AlAs from plastically relaxed regions near dislocations and the high temperature stability of AlAs result in that evaporation is restricted to the regions containing defects. The self-organization effects favor the formation of an ordered array of coherent nanodomains that can be used for obtaining buried low-dimensional nanostructures and/or nanoheteroepitaxial inclusions.

Growth of AlGaN epitaxial layers and AlGaN/GaN superlattices by metal-organic chemical vapor deposition

Special features of metal-organic chemical vapor deposition of AlGaN epitaxial layers and AlGaN/GaN superlattices either in an Epiquip VP-50 RP research and development reactor (for a single wafer 2 in. in diameter) or in an AIX2000HT production-scale reactor (for up to six wafers 2 in. in diameter) are stud-ied. It is found that the dependence of the aluminum content in the solid phase on the trimethylaluminum (TMA) flux in a reactor levels off; this effect hinders the growth of the layers with a high aluminum content in both types of reactors and is more pronounced in the larger reactor (AIX2000HT). Presumably, this effect is a consequence of spurious reactions in the vapor phase and depends on the partial pressure of TMA in the reactor. The aluminum content in the layers can be increased not only by reducing the total pressure in the reactor but also by increasing the total gas flow through the reactor and reducing the trimethylgallium flux. The approaches described above were used to grow layers with a mole fraction of AlN as large as 20% in the AIX2000HT production-scale reactor at a pressure of 400 mbar (this fraction was as large as 40% at 200 mbar). AlGaN layers with the entire range of composition were grown in the Epiquip VP-50 RP reactor.