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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. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Composite InGaN/GaN/InAlN quantum dots (QDs) have been formed and studied. The structural properties of thin InAlN layers overgrown with GaN have been analyzed, and it is shown that 3D islands with lateral sizes of ∼(20-30) nm are formed in structures of this kind. It is demonstrated that deposition of a thin InGaN layer onto the surface of InAlN islands overgrown with a thin GaN layer leads to transformation of the continuous InGaN layer to an array of isolated QDs with lateral sizes of 20-30 nm and heights of 2-3 nm. The position of these QDs in the growth direction correlates with that of InAlN islands. [ABSTRACT FROM AUTHOR]

GaN-based structures with InGaN quantum dots in the active region emitting in the near-ultraviolet region are studied. In this study, two types of structures, namely, with InGaN quantum dots in a GaN or AlGaN matrix, are compared. Photoluminescence spectra are obtained for both types of structures in a temperature range of 80–300 K and at various pumping densities, and electroluminescence spectra are obtained for light-emitting (LED) structures with various types of active region. It is shown that the structures with quantum dots in the AlGaN matrix are more stable thermally due to the larger localization energy compared with quantum dots in the GaN matrix. Due to this, the LED structures with quantum dots in an AlGaN matrix are more effective. [ABSTRACT FROM AUTHOR]

InGaN quantum dot (QD) formation in a wide pressure range MOCVD reactor was studied. The existence of QDs and their lateral size (2–5 nm) were demonstrated using transmission electron microscopy and high spatial resolution (~ 100 nm) near-field magneto-photoluminescence spectroscopy. We found that an increase of the reactor pressure from 400 to 1000 mbar leads to an order of magnitude increase in light emission efficiency of the InGaN/GaN QDs accompanied by ~ 100 meV redshift of the emission wavelength. We explored stimulated phase separation (SPS) to control carrier localization and emission wavelength. The SPS was achieved by adding In in the matrix material. This leads to formation of extremely deep InGaN/InGaN QDs having energy localization up to ~ 0.8 eV, which was revealed from selectively excited far-field photoluminescence (PL) spectra. Without SPS the QD activation energy is found to be below 0.2 eV. A nonequilibrium carrier population strongly suppresses the temperature-induced shift of the PL emission in deep InGaN QDs. [ABSTRACT FROM AUTHOR]

Electronic and optical properties of ensembles of quantum dots with various energies of activation from the ground-state level to the continuous-spectrum region were studied theoretically and experimentally with the InGaN quantum dots as an example. It is shown that, depending on the activation energy, both the quasi-equilibrium statistic of charge carriers at the levels of quantum dots and nonequilibrium statistic at room temperature are possible. In the latter case, the position of the maximum in the emission spectrum is governed by the value of the demarcation transition: the quantum dots with the transition energy higher than this value feature the quasi-equilibrium population of charge carriers, while the quantum dots with the transition energy lower than the demarcation-transition energy feature the nonequilibrium population. A model based on kinetic equations was used in the theoretical analysis. The key parameters determining the statistic are the parameters of thermal ejection of charge carriers; these parameters depend exponentially on the activation energy. It is shown experimentally that the use of stimulated phase decomposition makes it possible to appreciably increase the activation energy. In this case, the thermal-activation time is found to be much longer than the recombination time for an electron-hole pair, which suppresses the redistribution of charge carriers between the quantum dots and gives rise to the nonequilibrium population. The effect of nonequilibrium population on the luminescent properties of the structures with quantum dots is studied in detail. [ABSTRACT FROM AUTHOR]

GaN-based structures with InGaN quantum dots in the active region, which emit in the blue and green spectral ranges, are studied. The structures grown by both the conventional method and with the use of special procedures of the growth of the active region were compared. The use of these special growth modes stimulates the activated phase decomposition, which leads to the formation of quantum dots with substantially larger localization depth of electrons. It is shown that the formation of such deep quantum dots, like the formation of larger inhomogeneities of the active region, substantially suppresses the lateral carrier transport. This effect improves the characteristics of light-emitting diode structures at a low injection level and increases the temperature stability of quantum efficiency. [ABSTRACT FROM AUTHOR]

Structures with InGaN/GaN quantum dots have been studied using photocurrent spectroscopy. The dynamic range of measurements is found to amount to four orders of magnitude under preservation of the signal-to-noise ratio at a level higher than ten. Within the experimental accuracy, the shape of the spectrum is independent of an applied reverse external bias, whereas the spectrum itself shifts to shorter wavelengths, which is attributed to the Franz–Keldysh effect. Variation of the temperature brings about a change in the spectrum shape. This effect is found to be different for structures grown under different conditions. This behavior can be attributed to homogeneous broadening of the electronic states, the statistics of charge carriers at the levels of quantum dots, and the effect of temperature on the position of these levels. © 2005 Pleiades Publishing, Inc. [ABSTRACT FROM AUTHOR]

The carrier statistics in LED structures with ultrathin multilayer InGaN insertions in a GaN matrix was studied. The optical data obtained indicate that an array of quantum dots (QDs) is formed in these structures. The QDs are scattered in size, which leads to an inhomogeneous broadening of the energy spectrum of carriers localized in the QDs. It is shown that, despite the suppressed transport of carriers between QDs, carriers are distributed among the levels of the QD array quasi-statistically at temperatures of about room temperature and higher. This makes it possible to describe the carrier injection and recombination in the device structures studied in terms of quasi-Fermi levels for electrons and holes. © 2005 Pleiades Publishing, Inc. [ABSTRACT FROM AUTHOR]

The dependence of properties of quantum dot (QD) arrays in an InAs/GaAs system on the InAs growth rate has been investigated theoretically and experimentally. The derived kinetic model of the formation of coherent nanoislands allows the calculation of the average size, surface density of islands, and wetting layer thickness as functions of the growth time and conditions. Optical properties of InAs/GaAs QDs have been studied for the case of two monolayers (ML) of the material deposited at different growth rates. Predictions of the theoretical model are compared with the experimental data. It is shown that with two ML of the deposited material the characteristic lateral size of QDs decreases and the thickness of the residual wetting layer increases with rising growth rate. [ABSTRACT FROM AUTHOR]

Structural and optical properties of InAs quantum dots (QDs) grown in a wide-bandgap (Al[SUB0.3]Ga[SUB0.7]As 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 ∼1.18μm with the effective thickness of InAs in Al[SUB0.3]Ga[SUB0.7]As 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 ∼1.3μm is demonstrated. [ABSTRACT FROM AUTHOR]

The structural and optical properties of ultrathin Ge insertions in an Si matrix were studied. Transmission electron microscopy revealed the spontaneous formation of arrays of disk-shaped quantum dots (QDs) with a small lateral size (3-10 nm) at a nominal Ge insertion thicknesses, from submonolayer to nearly critical, for the transition to 3D growth by the Stranski-Krastanow mechanism. Optical study revealed type-I band alignment in these structures, which results from the strong contribution of the electron-hole Coulomb interaction overpowering the repulsion potential for an electron existing in the Ge conduction band. The small lateral size of QDs lifts the selection rule prohibiting indirect recombination in the inverse k space. At the same time, the high surface density of QDs (10[sup 12]-10[sup 13] cm[sup -2]) and the possibility of their stacking with the use of ultrathin Si spacers allows the obtainment of an ultrahigh volume density of QDs (up to 10[sup 19] cm[sup -3]), which is necessary to achieve stimulated emission in Si. A sample with stacked QDs formed by 0.7-nm-thick Ge insertions exhibited a superlinear increase of the photoluminescence (PL) intensity, accompanied by narrowing of the PL line. The doping of Ge-Si structures with donors allows for a drastic increase in the PL intensity at high temperatures, which prevents depletion of the active region in weakly localized electrons. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Various structures with optical microcavities and active layers based on InGaAs/GaAs quantum dots MBE-grown on GaAs substrates were studied theoretically and experimentally. LEDs for the 1.3 µm spectral range with narrow spectral characteristics and low light beam divergence were fabricated. Vertical lasing at 1.3 µm was obtained in a structure with oxidized AlO/GaAs mirrors under injection pumping. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Mechanisms of InGaA1As solid solution decomposition stimulated by a purposely deposited layer of InAs quantum dots are studied. Decomposition of the solid solution results in an increase in the effective quantum dot size and the shift of the photoluminescence line to as far as 1.3 μm. When aluminum atoms are added to the solid solution, the effect of In atom "conservation" within the dots is observed, which also causes an increase in the effective dot size. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The feasibility of lasing at a wavelength close to 1.3 µm is demonstrated in InAs quantum-dot structures placed in an external InGaAs/GaAs quantum well. It is shown that the required wavelength can be attained with the proper choice of thickness of the InAs layer deposited to form an array of three-dimensional islands and with a proper choice of mole fraction of InAs in the InGaAs quantum well. Since the gain attained in the ground state is insufficient, lasing is implemented through excited states in the temperature interval from 85 K to 300 K in a structure based on a single layer of quantum dots. The maximum attainable gain in the laser structure can be raised by using three rows of quantum dots, and this configuration, in turn, leads to low-threshold (70 A/cm²) lasing through the ground state at a wavelength of 1.26 µm at room temperature. [ABSTRACT FROM AUTHOR]

The analytical form of the dependence of the gain on pump current density for lasers with an active region based on self-organized quantum dots is derived in a simple theoretical model. The proposed model is shown to faithfully describe experimental data obtained for laser diodes based on InGaAs quantum dots in an AlGaAs/GaAs matrix, as well as InAs quantum dots in an InGaAs/InP matrix. The previously observed gain saturation and switching of the lasing from the ground state to an excited state of the quantum dots are studied. The influence of the density of quantum-dot arrays on the threshold characteristics of lasers based on them is examined on the basis of this model. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 selforganized 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. [ABSTRACT FROM AUTHOR]

The modification produced in the structural and optical properties of vertically coupled In[sub 0.5]Ga[sub 0.5]As quantum dots in a GaAs matrix by increasing the number of deposited layers of quantum dots has been investigated. It was shown that the deposition of a sequence of In[sub 0.5]Ga[sub 0.5]As quantum-dot planes separated by narrow (of the order of the height of the quantum dots) GaAs layers gives rise to an interaction between neighboring vertically coupled quantum dots. This interaction shifts the photoluminescence line due to the recombination of nonequilibrium carriers via states of the quantum dots into the region of lower photon energies. [ABSTRACT FROM AUTHOR]

The experimental results of a study of the fine structure of the levels of excitons localized in InAlAs quantum dots in an AlGaAs matrix are reported. Transformations from optical orientation to alignment and from alignment to orientation, which occur due to the exchange splitting of a dipole-active excitonic doublet and are allowed by the low symmetry of a quantum dot, are observed in a longitudinal magnetic field (Faraday geometry). A comparison of theory with experiment for a self-organized ensemble of quantum dots enables a determination of the character of the distribution over the dipole orientations for resonance optical transitions. © 1997 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

It is shown that the self-organization effects appearing during the N-cycle deposition of In[sub 0.5]Ga[sub 0.5]As(12 Å)/GaAs(50 Å) lead to the formation of ordered arrays of vertically coupled quantum dots (VCQD's). The optical properties are investigated as a function of the number of deposition cycles (N = 1-10). In cases of injection heterostructure lasers with an active region based on VCQD's it is shown that increasing the number N of deposition cycles from 1 to 10 makes it possible to lower the threshold current density at 300 K from 950 to 97 A/cm² and to achieve a continuous lasing regime at room temperature with an output power of 160 mW per mirror. [ABSTRACT FROM AUTHOR]

The properties of GaAs-AlGaAs injection heterolasers with (In, Ga)As quantum dots (QDs) depend principally on the number N of planes with quantum dots imbedded in the active region. Lasing via the ground state of an exciton in a quantum dot at room temperature has been realized in structures with N > 3. For N = 1 at 300 K lasing occurs via an excited state of an exciton in a quantum dot or via a state of the wetting layer. As the number of planes with quantum dots increases, the threshold current density decreases and the differential quantum efficiency increases. The lowest threshold current density 97 A/cm² (λ = 1.05 µm, 300 K) was obtained in structures with maximum N (N = 10). [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Room-temperature photoluminescence (PL) at 1.55 µm from heterostructures with InAs/InGaAsN quantum dots (QDs) grown by MBE on GaAs substrates is demonstrated for the first time. The effect of nitrogen incorporated into InAs/InGaAsN QDs on the PL wavelength and intensity was studied. The integral intensity of PL from the new structure with InAs/(In)GaAsN QDs is comparable to that from a structure with InGaAsN quantum wells emitting at 1.3 µm. [ABSTRACT FROM AUTHOR]

Laser diode structures on GaAs substrates with an active region employing laterally associated InAs quantum dots obtained by low-temperature MBE exhibit electroluminescence at a wavelength of 1.55-1.6 µm in a temperature range from 20 to 260 K. [ABSTRACT FROM AUTHOR]

The optical properties of multilayer structures with quantum dots in the heteroepitaxial InAs/GaAs system have been studied. The structures were obtained by the method of submonolayer migration-stimulated epitaxy. It is shown that the optimized growth conditions provide room-temperature luminescence at a wavelength of 1.3 µm. [ABSTRACT FROM AUTHOR]

Spatially single-mode lasing in the wavelength range of 1.25-1.28 µm was accomplished in injection lasers on GaAs substrates. The peak output power is 110 mW at room temperature, and the differential quantum efficiency amounts to 37%. The active region of the laser is formed by an array of self-organizing InAs quantum dots. [ABSTRACT FROM AUTHOR]

It is shown that an array of two-dimensional islands is formed during the growth of ultrathin (∼1.5 monolayers) InSb layers on a GaSb (100) surface by molecular beam epitaxy. After the deposition of several InSb layers separated by narrow barriers, islands of subsequent rows are formed on top of islands of the first row (vertical correlation effect). The formation of islands is confirmed by analysis of the photoluminescence spectra. [ABSTRACT FROM AUTHOR]

The current dependence of the optical gain in lasers based on self-organized InGaAs quantum dots in a A1GaAs/GaAs matrix is investigated experimentally. A transition from lasing via the ground state of quantum dots to lasing via an excited state is observed. The saturated gain in the latter case is approximately four times greater than for the ground state. This result is attributable to the fourfold degeneracy of the excited level of quantum dots. The effect of the density of the quantum-dot array on the threshold characteristics is investigated. A lower-density array of dots is characterized by a lower threshold current density in the low-loss regime, because the transmission current is lower, while dense quantumdot arrays characterized by a high saturated gain are preferable at high threshold gains. [ABSTRACT FROM AUTHOR]

The photoluminescent properties of quantum dots formed in the deposition of an InSb thin film (1–3 monolayers) on GaAs(100) and GaSb(100) surface are investigated. The results indicate the importance of As–Sb substitution reactions in the formation of quantum dots on a GaAs surface. © 1997 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Accelerated degradation testing of long-wavelength (>1.25 μm) quantum-dot lasers made on GaAs substrates is carried out at a fixed current of 1.7 A, initial optical output of about 0.3 W, and a heat sink temperature of 60°C. No signs of degradation are revealed after testing for 450 h. The test bed is not sealed, inert gas purging is not performed, and the laser faces are not passivated. [ABSTRACT FROM AUTHOR]

The use of InAs quantum dots in an InGaAs matrix lattice-matched with an InP substrate can appreciably increase the emission wavelength of quantum-dot lasers. Lasing via quantumdot states at the 1.84 µm wavelength (77 K) was obtained for the first time at a threshold current density of 64 A/cm². [ABSTRACT FROM AUTHOR]

InAs self-organized quantum dots in In[sub 0.53]Ga[sub 0.47]As and In[sub 0.52]Al[sub 0.48]As matrices have been grown on InP substrates by molecular beam epitaxy. The dot size in InGaAs has been found to be 3–4 times larger, but the areal density about an order of magnitude smaller than that in InAlAs. Low-temperature photoluminescence (PL) of the InAs/InGaAs quantum dots is characterized by a narrow (35 meV) PL line as compared to that of InAs/InAlAs quantum dots (170 meV). Quantum dot formation increases the carrier localization energy as compared to quantum well structures with the same InAs thickness in a similar manner for both InAs/InGaAs and InAs/InAlAs structures. The effect of the barrier band gap on the optical transition energy is qualitatively the same for quantum well and quantum dot structures. The results demonstrate a possibility of controlling the quantum dot emission wavelength by varying the matrix composition. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

InAs/InGaAsN quantum dots have been grown by molecular beam epitaxy on GaAs substrates. Transmission electron microscopy shows the increase in the island size as compared to the InAs/InGaAs quantum dots. Room temperature photoluminescence at 1.55 μm has been demonstrated whose intensity was comparable to that of the InGaAsN/GaAs quantum wells emitting at 1.3 μm. The effect of nitrogen concentration on the PL peak position and intensity has been studied. [Copyright &y& Elsevier]