The Stark shift of the hole states in separate InAs/GaAs quantum dots grown on (100) and (311)A GaAs substrates

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.     

Metastable population of self-organized InAs/GaAs quantum dots

In the present work, we report on the investigation of a p-n heterostructure with InAs/GaAs quantum dots (QD) by capacitance-voltage and deep level transient spectroscopy. We have observed controllable and reversible metastable population of the energy states of quantum dots and interface in the structure containing one plane of InAs QDs as a function of temperature of isochronous annealing as well as under bias-on-bias-off cooling conditions and white light illumination. This effect was attributed to the change in the Fermi level position due to the hole capture on self-trapped defects similar to the DX center in GaAs after isochronous annealing and white light illumination.     

Electronic Raman scattering from holes in InAs/GaAs self-assembled quantum dots

A report is presented on the observation of hole excitations in unintentionally p-doped self-assembled InAs/GaAs quantum dots by resonant Raman spectroscopy. The small difference in the valence intraband energy values obtained by Raman and PL spectra is explained by the Coulomb interaction between electrons and holes. However, the reason why the maximum resonance occurs at a slightly higher energy than that of the hole excitation seen in Raman spectra is unknown.     

Photoelectric properties of GaAs/InAs heterostructures with quantum dots

The capacitive photovoltage and photoconductivity spectra of GaAs/InAs heterostructures with quantum dots is discussed. For these structures, which were fabricated by metallorganic gas-phase epitaxy, the photosensitivity spectrum has a sawtoothed shape in the wavelength range where absorption by the quantum dots takes place, which is characteristic of a δ-function-like density of states function. The spectra also exhibit photosensitivity bands associated with the formation of single-layer InAs quantum wells in the structure. An expression is obtained for the absorption coefficient of an ensemble of quantum dots with a prespecified size distribution. It is shown that the energy distribution of the joint density of states, the surface density of quantum dots, and the effective cross section for trapping a photon can all be determined by analyzing the photosensitivity spectrum based on this assumption. Fiz. Tekh. Poluprovodn. 31, 1100–1105 (September 1997)     

Pulsed laser annealing of self-organized InAs/GaAs quantum dots

The effect of pulsed laser annealing (PLA), using an excimer laser, on the luminescence efficiency of self-organized InAs/GaAs and In_0.4Ga_0.6As/GaAs quantum dots has been investigated. It is found that such annealing can enhance both the peak and integrated photoluminescence (PL) efficiency of the dots, up to a factor of 5–10 compared to as-grown samples, without any spectral shift of the luminescence spectrum. The improved luminescence is attributed to the annealing of nonradiative point and extended defects in and around the dots.     

Wannier-Stark states in a superlattice of InAs/GaAs quantum dots

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 U _r 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.     

Modified fermi-level pinning of the (100) GaAs surface through InAs quantum dots in different stages of overgrowth

The presence of InAs quantum dots on a {100} GaAs surface results in a pronounced increase of the Fermi level pinning energy. Using room-temperature photo-reflectance measurements combined with atomic force microscopy, we find that the presence of the quantum dots results in the Fermi level being pinned approximately ∼250 meV deeper in the bandgap, an effect which is reversed by either removing or overgrowing the dots. Both overgrowth and complete etching of quantum dots results in the disappearance of the polar InAs facets; we explain the change in Fermi level in terms of such facets. We also discuss the phase delay for the InAs related feature of the photoreflectance experiment as a detrapping of photo-generated electron-hole pairs in the dots.     

Temperature dependence of InAs/GaAs quantum dots solar photovoltaic devices

This paper presents the temperature dependence measurements characterisation of several InAs/GaAs quantum dots （QDs） solar cell devices. The devices with cylindrical geometry were fabricated and characterised on-wafer under 20 suns in a temperature range from 300°K to 430°K. The temperature dependence parameters such as open circuit voltage, short circuit density current, fill factor and efficiency are studied in detail. The increase of temperature produces an enhancement of the short circuit current. However, the open circuit voltage is degraded because the temperature increases the recombination phenomena involved, as well as reducing the effective band gap of the semiconductor.     

Temperature dependent optical properties of self-organized InAs/GaAs quantum dots

We report photoluminescence (PL), time-resolved PL, and PL excitation experiments on InAs/GaAs quantum dots (QDs) of different size as a function of temperature. The results indicate that both the inhomogeneous properties of the ensemble and the intrinsic properties of single QDs are important in understanding the temperature-dependence of the optical properties. With increasing temperature, excitons are shown to assume a local equilibrium distribution between the localized QD states, whereas the formation of a position-independent Fermi-level is prevented by carrier-loss to the barrier dominating thermally stimulated lateral carrier transfer. The carrier capture rate is found to decrease with increasing temperature and, at room temperature, long escape-limited ground state lifetimes of some 10 ps are estimated. PL spectra excited resonantly in the ground state transition show matching ground state absorption and emission, indicating the intrinsic nature of exciton recombination in the QDs. Finally, the PL excitation spectra are shown to reveal size-selectively the QD absorption, demonstrating the quantum-size effect of the excited state splitting.     

Self-organization of quantum dots in multilayer InAs/GaAs and InGaAs/GaAs structures in submonolayer epitaxy

The experimental results of RHEED and scanning tunneling microscopy investigations of multilayer structures of InGaAs/GaAs quantum dots, obtained by submonolayer epitaxy on singular and vicinal GaAS (100) substrates, are reported. The results presented show that spatial ordering of nano-objects exists in multilayer structures for InAs and heteroepitaxial InGaAs layers. Fiz. Tekh. Poluprovodn. 33, 733–737 (June 1999)     

Optical characterisation of MOVPE grown vertically correlated InAs/GaAs quantum dots

Structures with vertically correlated self-organised InAs quantum dots (QDs) in a GaAs matrix were grown by the low-pressure metal-organic vapour phase epitaxy (MOVPE) and characterised by different microscopic techniques. Photoluminescence in combination with photomodulated reflectance spectroscopy were applied for characterisation of QDs structures. We show that combination of both methods allows detecting optical transitions originating both from QDs and wetting (separation) layers, which can be than compared with those obtained from numerical simulations. On the basis of obtained results, we demonstrate that photoreflectance spectroscopy is an excellent tool for characterisation of QDs structures wetting layers and for identification of spacer thicknesses in vertically stacked QDs structures.     

Effects of external fields on the excitonic emission from single InAs/GaAs quantum dots

A low-temperature micro-photoluminescence (μ-PL) investigation of InAs/GaAs quantum dots (QDs) exposed to a lateral external electric field is reported. It is demonstrated that the QDs PL signal could be increased several times by altering the external and/or the internal electric field. The internal field in the vicinity of the dots could be altered by means of an additional infra-red laser. We propose a model, which is based on an essentially faster lateral transport of the charge carriers achieved in an external electric field. Consequently, also the capture probability into the dots and subsequently the dot luminescence is also enhanced. The results obtained suggest that the lateral electric fields play a major role for the dot luminescence intensity measured in our experiment.     

Built-in electric fields in InAs quantum dots grown on (N11) GaAs substrates

We report on the optical properties and carrier kinetics of a set of InAs self-assembled quantum dots on (N11)A/B GaAs substrates by means of cw and time-resolved PL. The cw-PL spectra show a blue shift of the PL band on different (N11) QD structures when increasing the carrier photoinjection. This is attributed to a photoinduced screening of the quantum confined Stark shift of the QD optical transition due to a large built-in electric field. The presence of an internal electric field also induces intrinsic optical non-linearity in time-resolved measurements. The analysis of the recombination kinetics shows that the carrier screening occurs inside the QDs, thus demonstrating the intrinsic nature of the built-in field. The dependence of the internal field on the substrate orientation and termination agrees with the presence of piezoelectric field and permanent dipole moment inside the QDs.     

Photocurrent in self-organized InAs quantum dots in 1.3 μm InAs/InGaAs/GaAs semiconductor laser heterostructures

The photocurrent spectra of InAs/InGaAs/GaAs laser heterostructures with self-organized InAs quantum dots (QDs) are studied. The study has been performed with a sample illuminated perpendicularly or in parallel to the QD plane. The optical density of QDs, maximum gain in the laser structure, radiative recombination time, and polarization characteristics of absorption have been determined from the experiment. The photocurrent spectra were correlated with specific features of lasing. The absorption spectrum is interpreted as a superposition of optical transitions between states of the discrete energy spectrum, those between the states of the continuous spectrum, and “mixed” transitions.     

Influence of GaAs spacer-layer thickness on quantum coupling and optical polarization in a ten-layer system of vertically correlated InAs/GaAs quantum dots

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.     

Luminescence properties of InAs/GaAs quantum dots prepared by submonolayer migration-stimulated epitaxy

The results of an investigation of the luminescence properties of an ensemble of InAs quantum dots, obtained by submonolayer migration-stimulated epitaxy on singular and vicinal GaAs(100) surfaces, are reported. The largest width at half-height of the photoluminescence line is observed in samples with a 3° disorientation, indicating that the size-variance of the quantum dots is largest in this case. Quasiequilibrium quantum dots are formed either with a long sample holding time in an arsenic flow or with a larger quantity of deposited indium. Fiz. Tekh. Poluprovodn. 31, 912–915 (August 1997)     

Resonances related to an array of InAs quantum dots and controlled by an external electric field

Photoluminescence of multilayer structures with InAs quantum dots grown in the p-n junction in GaAs by molecular-beam epitaxy is studied. Formation of vertical columns of quantum dots is verified by the data of transmission electron microscopy. It is shown that a natural increase in the size of quantum dots from layer to layer brings about their vertical coalescence at the upper part of a column. An unbalance of electronic levels caused by the enlargement of quantum dots was compensated by an external electric field, so that the resonance of ground electronic states in the column was attained. The onset of resonances was checked by the methods of steady-state and time-resolved photoluminescence. It is shown that, in the case of a resonance, the photoluminescence intensity and the radiative lifetime of excitons increase (up to 0.6–2 ns), while the time of tunneling of charge carriers becomes shorter (shorter than 150 ps). Outside the resonances, tunneling of electrons is appreciably enhanced owing to the involvement of longitudinal optical phonons. If only these phonons are involved, the time of nonresonance tunneling between quantum dots becomes shorter than the time of relaxation of charge carriers from the barrier (100 and 140 ps, respectively).     

Intraband photoconductivity induced by interband illumination in InAs/GaAs heterostructures with quantum dots

The effect of lateral intraband photoconductivity in undoped InAs/GaAs heterostructures with quantum dots (QDs) has been studied, with QD levels populated with carriers by means of interband optical excitation of varied power at different wavelengths. In the absence of interband illumination, no photoconductivity is observed in the mid-IR spectral range. At the same time, additional exposure of the structures to visible or near-IR light gives rise to a strong photoconductivity signal in the mid-IR spectral range (3?C5 ??m), associated with intraband transitions in QDs. The signal is observed up to a temperature of ??200 K. Use of interband optical pumping makes the intraband photoconductivity signal stronger, compared with similar structures in which doping serves to populate QD levels.     

Electric field effects on the carrier migration in self-assembled InAs/GaAs quantum dots

Electron and hole dynamics from self-assembled quantum dots (QDs) subject to vertical electric fields have been studied by observing the photoluminescence (PL) image on the sample surface. We have observed an asymmetric profile associated to migration of optically excited electron and holes in the quantum dot structure. The asymmetric profile is increased by rising the applied bias voltage. This behaviour was associated with charge accumulation at different regions of the QD layer plane due to an asymmetric electric field, upon which the QDs are immersed.     

Optical phenomena in InAs/GaAs heterostructures with doped quantum dots and artificial molecules

Spectra of intraband absorption of polarized mid-IR light were investigated in undoped, p-, and n-doped InAs/GaAs quantum dots (QDs) covered with an InGaAs layer. Optical matrix elements for intraband electron and hole transitions in QDs have been calculated for different polarizations of light, and a good agreement with the experimental data is obtained. It is shown that the intraband absorption of light by electrons strongly exceeds the absorption by holes. Photoluminescence spectra and TEM images of structures with artificial molecules formed by pairs of QDs were studied.