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The results of a study of nitrogen-containing active regions based on superlattices grown on GaAs substrates are presented. Active regions based on alternating InAs and GaAsN layers were fabricated by molecular-beam epitaxy using a nitrogen plasma source. Based on the XRD analysis, the thicknesses and average composition of superlattice layers are estimated. The study of dark-field images obtained by transmission electron microscopy showed the presence of interdiffusion of InAs into GaAsN. The results of a study of the photoluminescence and electroluminescence spectra at different pump levels are presented. Efficient electroluminescence is demonstrated near 1150 nm with a full width at half-maximum of about ~90 meV. [ABSTRACT FROM AUTHOR]

The spectral characteristics of a laser with an active region based on a ten-layer system of In(Ga)As/GaAs vertically correlated quantum dots with 4.5-nm GaAs spacer layers between InAs quantum dots are studied under the conditions of spontaneous and stimulated emission, depending on the current and the duration of pump pulses. Data obtained by transmission electron microscopy and electroluminescence and absorption polarization anisotropy measurements make it possible to demonstrate that the investigated system of tunnel-coupled InAs quantum dots separated by thin GaAs barriers represents a quantum-dot superlattice. With an increase in the laser pump current, the electroluminescence intensity increases linearly and the spectral position of the electroluminescence maximum shifts to higher energies, which is caused by the dependence of the miniband density-of-states distribution on the pump current. Upon exceeding the threshold current, multimode lasing via the miniband ground state is observed. One of the lasing modes can be attributed to the zero-phonon line, and the other is determined by the longitudinal-optical phonon replica of quantum-dot emission. The results obtained give evidence that, under conditions of the laser pumping of an In(Ga)As/GaAs quantum-dot superlattice, strong coupling between the discrete electron states in the miniband and optical phonons takes place. This leads to the formation of quantum-dot polarons, resulting from the resonant mixing of electronic states whose energy separation is comparable to the optical-phonon energy. [ABSTRACT FROM AUTHOR]

The polarization anisotropy of electroluminescence and absorption in a ten-layer system of vertically correlated InAs quantum dots separated by 8.6-nm-thick GaAs spacer layers is investigated experimentally. The quantum-dot system is built into a two-section laser structure with sections of equal length. It is found that the polarization anisotropy in this system is smaller than the anisotropy in similar systems with a single layer of quantum dots or quantum-dot molecules, but larger than that in a quantum-dot superlattice. The spectra of differential absorption in the structure under study for different strengths of the applied electric field are also investigated. The rate of variation in the Stark shift as a function of the electric field is determined, the results giving evidence of controlled quantum coupling between adjacent quantum dots in tenlayer vertically correlated InAs/GaAs quantum-dot systems with 8.6- and 30-nm-thick GaAs spacer layers. The measured polarization dependences are explained by the participation of heavy-hole ground states in optical transitions. This effect is defined by the two dimensional nature of the system under study. [ABSTRACT FROM AUTHOR]

The results of experimental studies concerning the optical polarization anisotropy of electroluminescence and absorption spectra of systems with a varied number of tunnel-coupled vertically correlated In(Ga)As/GaAs quantum dots (QDs), built into a double-section laser with equal-length sections, are presented. One such system is a QD superlattice exhibiting the Wannier-Stark effect. The involvement of heavyhole ground states in optical transitions for light polarized both in the plane perpendicular to the growth axis ( X-Y) and along the growth direction Z of the structure was observed. The degree of polarization anisotropy depends on the height of vertically correlated QDs and the QD superlattice: the total thickness of all In(Ga)As QD layers and GaAs spacers between the QDs, which is related to the Z component of the wave function of heavy-hole ground states for vertically correlated QDs and for the QD superlattice. [ABSTRACT FROM AUTHOR]

Electroluminescence and absorption spectra of a ten-layer InAs/GaAs quantum dot (QD) superlattice built in a two-section laser with sections of equal length is experimentally studied at room temperature. The thickness of the GaAs spacer layer between InAs QD layers, determined by transmission electron microscopy, is ∼6 nm. In contrast to tunnel-coupled QDs, QD superlattices amplify the optical polarization intensity and waveguide absorption of the TM mode in comparison with the TE mode. It is found that variations in the multimodal periodic spectrum of differential absorption of the QD superlattice structure are strongly linearly dependent on the applied electric field. Differential absorption spectra exhibit the Wannier-Stark effect in the InAs/GaAs QD superlattice, in which, in the presence of an external electric field, coupling of wave functions of miniband electron states is suppressed and a series of discrete levels called the Wannier-Stark ladder states are formed. [ABSTRACT FROM AUTHOR]

Electron and hole emission from states of a ten-layer system of tunneling-coupled vertically correlated InAs/GaAs quantum dots (QDs) is studied experimentally by capacitance—voltage measurements and deep-level transient spectroscopy. The thickness of GaAs interlayers separating sheets of InAs QDs was ≈3 nm, as determined from transmission electron microscope images. It is found that the periodic multimo-dal DLTS spectrum of this structure exhibits a pronounced linear shift as the reverse-bias voltage Ur applied to the structure is varied. The observed behavior is a manifestation of the Wannier—Stark effect in the InAs/GaAs superlattice, where the presence of an external electric field leads to the suppression of coupling between the wave functions of electron states forming the miniband and to the appearance of a series of discrete levels called Wannier—Stark ladder states. [ABSTRACT FROM AUTHOR]

The absorption of uncoupled and tunnel-coupled vertically correlated quantum dots (QDs), measured at room temperature, has been experimentally compared. It is revealed that matching of the laser wavelength and Stark shift for laser structures with tunnel-coupled QDs leads to resonant absorption with formation of bound and antibound exciton states with a splitting energy of ∼62 meV between them in QD molecules. For these states, an external field causes a large linear Stark shift (up to 68 meV). For uncoupled QDs, one resonant absorption peak with the formation of an exciton (for which the Stark shift does not exceed 13 meV) is observed. [ABSTRACT FROM AUTHOR]

Deep level transient spectroscopy (DLTS) measurements were performed to study electron emission from quantum states in a 20-layer Ge quantum-dot superlattice (QDSL) in a Ge/Si p-n heterostructure. It was established that the changes in the DLTS spectra depend heavily on the magnitude of the applied reverse bias U r . Three regions of the reverse bias U r were identified, corresponding to the manifestation of the three modes of the Wannier-Stark effect: Wannier-Stark ladder mode, Wannier-Stark localization, and nonresonant Zener tunneling mode. Furthermore, it was found that the appearance of DLTS peaks for all three modes is associated with electron emission from deep-level defects via Wannier-Stark localized states arising as a result of the splitting of the electron miniband of the Ge/Si QDSL. [ABSTRACT FROM AUTHOR]

Deep-level transient spectroscopy is used to study charge-carrier emission from the states of separate quantum dots in InAs/GaAs p–n heterostructures grown on (100)- and (311)A-oriented GaAs substrates in relation to the reverse-bias voltage U. It is established that the structures under consideration exhibit different bias-voltage dependences of the Stark shift for the energy levels of the quantum-dot states on the value of U. © 2005 Pleiades Publishing, Inc. [ABSTRACT FROM AUTHOR]

Deep-level transient spectroscopy is used to study the emission of holes from the states of a vertically coupled system of InAs quantum dots in p-n InAs/GaAs heterostructures. This emission was considered in relation to the thickness of a GaAs interlayer between two layers of InAs quantum dots and to the reverse-bias voltage Ur. It is established that hole localization at one of the quantum dots is observed for a quantum-dot molecule composed of two vertically coupled self-organized quantum dots in an InAS/GaAs heterostructure that has a 20-A-thick or 40-A-thick GaAs interlayer between two layers of InAs quantum dots. For a thickness of the GaAs interlayer equal to 100 A, it is found that the two layers of quantum dots are incompletely coupled, which results in a redistribution of the hole localization between the upper and lower quantum dots as the voltage Ur applied to the structure is varied. The studied structures with vertically coupled quantum dots were grown by molecular-beam epitaxy using self-organization effects. [ABSTRACT FROM AUTHOR]

The effect of In and Al on the properties of the As[sub Ga] defect complex (arsenic substituted for gallium at a crystal lattice site) in GaAs-based quantum dots (QDs) was investigated using the nonempirical quantum-chemical SCF–MO–LCAO technique. It is shown that an As[sub Ga] defect can exist in stable and metastable states. Raising the indium or aluminum content in QD enhances the probability of As[sub Ga] defect formation in the stable state; in case of In introduction, this effect is manifested more strongly. The activation energy of the transition between stable and metastable states varies between 0.886 and 2.049 eV, depending on the QD stoichiometry. The formation of an As[sub Ga] defect gives rise to two deep levels in the band gap. © 2004 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

The results of studies of hole energy states in vertically coupled quantum dots in InAs-GaAs p-n heterostructures by deep-level transient spectroscopy are reported. Spectra were recorded at different reverse-bias voltages. Levels related to bonding and antibonding s and p states of vertically coupled quantum dots were revealed. The energies of these states significantly depend on an external electric field applied to a heterostructure. This dependence was attributed to the quantum-dimensional Stark effect for the hole states of vertically coupled quantum dots. In addition to this, it was found that the energy of thermal activation of carriers from vertically coupled quantum dots depends on the conditions of isochronous annealing that was carried out both with the reverse bias switched-on and switched-off and both in the presence and absence of illumination. These changes, as in the case of isolated quantum dots, are typical of a bistable electrostatic dipole formed by carriers, localized in a coupled quantum dot, and ionized lattice point defects. The built-in electric field of this dipole reduces the energy barrier for the carriers in the coupled quantum dot. The investigated structures with vertically coupled quantum dots were grown using molecular-beam epitaxy taking account of self-assembling effects. [ABSTRACT FROM AUTHOR]

The electron emission from and capture by quantum dots in InGaAs/GaAs p-n heterostructures were studied using deep-level transient spectroscopy in relation to the conditions of isochronous annealing with a switched-on or -off bias voltage (U[sub ra] < 0 or U[sub ra] = 0). The results indicate that, as a result of annealing with U[sub ra] < 0, a dipole that consists of charge carriers localized in the quantum dots and the ionized lattice defects is formed. The electrostatic potential of this dipole reduces the barrier for the electron emission from and capture by a quantum dot. If the annealing is performed at U[sub ra] = 0, no dipole is formed and the band offset is controlled by the conditions of heteroboundary formation during the structure growth. The dependence of the barrier height on the filling-pulse duration was also observed; this dependence is related to the manifestation of the Coulomb blockade for the electron capture by the ground and excited states of a quantum dot. The structures with quantum dots studied were grown by gaseous-phase epitaxy from metal-organic compounds using self-organization effects. [ABSTRACT FROM AUTHOR]

A report is presented on the investigation of the influence of in situ annealing of the InGaAs layer in p-n InGaAs/GaAs structures grown by the metalloorganic chemical vapor deposition upon the formation of coherently strained three-dimensional islands. The structures were studied by the methods of capacitance-voltage measurements, deep-level transient spectroscopy, transmission electron microscopy, and photoluminescence. It is established that three-dimensional islands with misfit dislocations are formed in the unannealed structure A, while quantum dots are formed in the annealed structure B. The deep-level defects were investigated. In structure A, defects of various types (EL2, EL3 (I3), I2, HL3, HS2, and H5) are present in the electronaccumulation layer. Concentrations of these traps are comparable to the shallow donor concentration, and the number of hole traps is higher than that of the electron traps. On the in situ annealing, the EL2 and EL3 defects, which are related to the formation of dislocations, disappear, and concentrations of the other defects decrease by an order of magnitude or more. For structure A, it is established that the population of the quantum states in the islands is controlled by the deep-level defects. In structure B, the effect of the Coulomb interaction of the charge carriers localized in the quantum dot with the ionized defects is observed. [ABSTRACT FROM AUTHOR]

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, T[sub a] < T[sub ac] = 245 K or T[sub a] > T[sub ac], and on the cooling conditions, with a bias voltage V[sub b] = 0 or with an applied carrier pulse V[sub f] > 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 T[sub a] < T[sub ac] and cooling takes place with V[sub f] > 0, or to the absence of a dipole when T[sub a] > T[sub ac] and V[sub b] = 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. [ABSTRACT FROM AUTHOR]

Passive mode-locking is achieved in two sectional lasers with an active layer based on superlattice formed by ten layers of quantum dots. Tunnel coupling of ten layers changes the structural polarization properties: the ratio between the TE and TM polarization absorption coefficients is less by a factor of 1.8 in the entire electroluminescence spectrum range for the superlattice. [ABSTRACT FROM AUTHOR]

The Stark effect has been studied in multilayer InGaAs/GaAs laser structures with self-assembled quantum dots (QDs). A shift in the absorption edge depending on the reverse bias voltage has been measured in a two-section laser diode. The QD absorption edge shifts toward longer wavelengths with increasing electric field strength. It is established that the QD absorption depends on the polarization of light. The intensity at which TE-polarized luminescence in laser structures is studied is more than ten times higher than that of the TE-polarized emission component, which is explained by higher amplification of the TE mode. [ABSTRACT FROM AUTHOR]

Electron emission in a system of vertically coupled quantum dots (VCQDs) in InAs/GaAs p- n-heterostructures obtained by molecular beam epitaxy has been studied by means of deep-level transient spectroscopy (DLTS) as a function of the number of quantum dot (QD) rows and the reverse bias voltage. For a GaAs spacer thickness of d GaAs = 40 Å, the system occurs in a molecular state, irrespective of the number of QD rows. An increase in this number leads to a decrease in the Stark shift, which is probably related to a decrease in the lattice strain potential in the vicinity of VCQDs. [ABSTRACT FROM AUTHOR]