M. V. Maksimov

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.

Effect of p-doping of the active region on the temperature stability of InAs/GaAs QD lasers

A detailed study of the effect of p-doping of the active region on characteristics of long-wavelength InAs/GaAs QD lasers is performed. As the doping level increases, the characteristic temperature rises and the range of temperature stability for the threshold current density is broadened. In a laser doped with 2 × 1012 cm−2 acceptors per QD sheet, the characteristic temperature of 1200 K is obtained in the temperature range 15–75°C and the differential quantum efficiency is stable in the range 15–65°C. A maximum CW output power of 4.4 W is reached in an optimized structure.

Device characteristics of long-wavelength lasers based on self-organized quantum dots

The current state of the field of semiconductor lasers operating in the spectral range near 1.3 μm and with an active region represented by an array of self-organized quantum dots is reviewed. The threshold and temperature characteristics of such lasers are considered; the problems of overcoming the gain saturation and of an increase in both the differential efficiency and emitted power are discussed. Data on the response speed under conditions of direct modulation and on the characteristics of lasers operating with mode synchronization are generalized. Nonlinear gain saturation, the factor of spectral line broadening, and the formation of broad gain and lasing spectra are discussed.

Deep-level transient spectroscopy in InAs/GaAs laser structures with vertically coupled quantum dots

Indium arsenide/gallium arsenide structures with vertically coupled quantum dots imbedded in the active zone of a laser diode are investigated by deep-level transient spectroscopy (DLTS), and the capacitance-voltage characteristics are analyzed. The DLTS spectrum was found to undergo significant changes, depending on the temperature of preliminary isochronous annealing of the sample, TaTac, and on the cooling conditions, with a bias voltage Vb=0 or with an applied carrier pulse Vf>0. The changes are attributed to the onset of Coulomb interaction of carriers trapped in a quantum dot with point defects localized in the nearest neighborhoods of the quantum dots and also to the formation of a dipole when Ta0, or to the absence of a dipole when Ta>Tac and Vb=0. It is discovered that the tunneling of carriers from the deeper states of defects to the shallower states of quantum dots takes place in the dipole, and the carriers are subsequently emitted from the dots into bands.

High-power 1.5 µm InAs-InGaAs quantum dot lasers on GaAs substrates

Light-current, spectral, and temperature characteristics of long-wavelength (1.46–1.5 µm) lasers grown on GaAs substrates, with an active area based on InAs-InGaAs quantum dots, are studied. To reach the required lasing wavelength, quantum dots were grown on top of a metamorphic InGaAs buffer layer with an In content of about 20%. The maximum output power in pulsed mode was 7 W at room temperature. The differential efficiency of the laser, which had a 1.5-mm-long cavity, was 50%. The temperature dependence of the threshold current is described by a characteristic temperature of 61 K in the temperature range 10–73°C.

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.

Metamorphic lasers for 1.3-µm spectral range grown on GaAs substrates by MBE

A new method for the epitaxial formation of 1.3-µm injection lasers on GaAs substrates is reported. A metamorphic heterostructure with an In content of about 20% is deposited onto an intermediate buffer layer intended for mismatch strain relaxation. The laser active region is formed by quantum wells with a higher In content (about 40%). Lasers with 100-µm-wide stripes demonstrate room-temperature lasing at 1.29 µm with a minimum threshold current density of 3.3 kA cm2 (0.4 kA cm2 at T=85 K).

Injection heterolaser based on an array of vertically aligned InGaAs quantum dots in a AlGaAs matrix

Arrays of vertically aligned InGaAs quantum dots in a AlGaAs matrix have been investigated. It is shown that increasing the band gap of the matrix material makes it possible to increase the localization energy of quantum dots relative to the edge of the matrix band, as well as the states of the wetting layer. The use of an injection laser as the active region makes it possible to decrease the thermal filling of higher-lying states, and thereby decrease the threshold current density to 63 A/cm2 at room temperature. A model explaining the negative characteristic temperature section observed at low temperatures is proposed. The model is based on the assumption that a transition occurs from nonequilibrium to equilibrium filling of the states of the quantum dots.

Quantum dot injection heterolaser with ultrahigh thermal stability of the threshold current up to 50 °C

Gaseous phase epitaxy from metal organic compounds is used to obtain a low-temperature injection laser with an active region based on In0.5Ga0.5As/GaAs quantum dots. Optimizing the growth conditions and geometric parameters of the structure has made it possible to increase the range of ultrahigh thermal stability in the threshold current (the characteristic temperature is T0=385 K) up to 50 °C.

Photo-and electroluminescence in the 1.3-µm wavelength range from quantum-dot structures grown on GaAs substrates

A method is proposed to increase the emission wavelength from structures grown on GaAs substrates by inserting a strained InAs quantum dot array into an external InGaAs quantum well. The dependence of the luminescence peak position on the active region design was investigated for structures grown by this method. Room-temperature photo-and electroluminescence spectra in the 1.3-µm wavelength range are compared.