Nikolay A. Kalyuzhnyy

Transverse single-mode edge-emitting lasers based on coupled waveguides.

We report on the transverse single-mode emission from InGaAs/GaAs quantum well edge-emitting lasers with broadened waveguide. The lasers are based on coupled large optical cavity (CLOC) structures where high-order vertical modes of the broad active waveguide are suppressed due to their resonant tunneling into a coupled single-mode passive waveguide. The CLOC lasers have shown stable Gaussian-shaped vertical far-field profiles with a reduced divergence of ∼22° FWHM (full width at half-maximum) in CW (continuous-wave) operation.

High Intensity Low Temperature (HILT) performance of space concentrator GaInP/GaInAs/Ge MJ SCs

In the work, the results of an investigation of GaInP/GaInAs/Ge MJ SCs intended for converting concentrated solar radiation, when operating at low temperatures (down to −190 °C) are presented. A kink of the cell I-V characteristic has been observed in the region close to Voc starting from −20°C at operation under concentrated sunlight. The causes for its occurrence have been analyzed and the reasons for formation of a built-in potential barrier for majority charge carriers at the n-GaInP/n-Ge isotype hetero-interface are discussed. The effect of charge carrier transport in n-GaInP/n-pGe heterostructures on MJ SC output characteristics at low temperatures has been studied including EL technique.

Optical properties of hybrid quantum-confined structures with high absorbance

The methods of photoluminescence and photoconductivity spectroscopy and the spectroscopy of photocurrent of a p–i–n structure are used to study samples with hybrid quantum-confined medium “quantum well–dots” (QWD) structures grown on GaAs substrates. The significant contribution of QWD states, which extends the GaAs absorption range to 1075 nm, is found in the photoconductivity and photocurrent spectra. The absorption and luminescence of the quantum-confined structures possess characteristic features of quantum wells. Analysis of the photocurrent and photoconductivity spectra demonstrate that the excitation of carriers from localized QWD states has a combined nature: at temperatures lower than 100 K and an electric-field strength of below 40 kV/cm, excitation is possible via tunneling to the matrix, while at higher temperatures thermal activation coming into play. Also, lateral photoconductivity is observed in the layers of quantum-confined structures.

Counter-photo-electromotive force at heterointerfaces in MJ SC: Study by spectral method

The influence of p-n junction heating on dependence of the open circuit voltage on the photogenerated current density (Voc-Jg) has been observed and procedure for Voc correction has been suggested. Using this procedure the corrected Voc-Jg dependence has been obtained for triple-junction GaInP/GaAs/Ge solar cells. The shape of Voc-Jg dependence has been discussed, and it was suggested that it indicates the appearance of a counter EMF of about 10 mV at Jg = 100 A/cm2. It is assumed that this counter EMF is generated on some isotype heterointerface. The spectral method for determining the position of such heterointerface has been considered.

Light-emitting and photovoltaic devices based on quantum well-dots hybrid nanostructures

We report on optoelectronic devices based on novel type of active region - quantum well-dots (QWD) hybrid nanostructures. This hybrid type of the active region can be described as a quantum well, which has an ultradense array of narrow-gap In-rich regions with the size of 20-30 nm, which serve as the localization centers of charge carriers. Such QWD structures can be formed spontaneously during the MOVPE (metalorganic vapor phase epitaxy) deposition of InxGa1-xAs (0.3<; x<0.5) on GaAs substrate. Optimal average thickness and composition of InxGa1-xAs to achieve maximal PL intensity and photocurrent in QWD structures are determined. Characteristics of edge-emitting lasers based on 5 QWD layers are described. Advantages of using QWD medium in light-emitting and photovoltaic devices are discussed.

Current flow mechanism in GaAs solar cells with GaInAs quantum dots

Resistance-less dark IV-curves of GaAs solar cells with and without quantum-dimensional objects allied to both quantum dot and quantum well have been obtained and analyzed. Anomalous large increase of the pre-exponential factors was observed in tested (with quantum objects) p-n junctions. According to proposed interpretation embedding of quantum objects in p-n junction created new current flow mechanisms. As a result current flows throught recombination transitions inside quantum objects in space charge region of p-n junction.

Optimization of structural and growth parameters of metamorphic InGaAs/GaAs photoconverters grown by MOCVD

Metamorphic Ga0.76In0.24As heterostructures for PV converters of 1064 nm laser radiation have been grown by the MOCVD. Parameters of the GaInAs metamorphic buffer layer with a stepwise profile of In composition variation were calculated. Its epitaxial growth conditions have been optimized, which allowed improving collection of charge carriers from the n-GaInAs base region and obtaining the photo-response quantum yield of 83% at 1064 nm wavelength. It has been found that, due to discontinuity of valence bands at the In0.24Al0.76As- p/Ga0.76In0.24As-p heterointerface (window/emitter) a potential barrier for holes arises as a result of low carrier concentration in the wide-band-gap material. The use of InAlGaAs solid solution with Al concentration of < 40% has allowed raising the holes concentration in the wide-band-gap window, eliminating completely the potential barrier and reducing the device series resistance. Optimization of the PV converter metamorphic heterostructure has resulted in obtaining 1064 nm laser radiation conversion efficiency at the level of 38.5%.

Density Control of InP/GaInP Quantum Dots Grown by Metal-Organic Vapor-Phase Epitaxy

We investigated structural and emission properties of self-organized InP/GaInP quantum dots (QD) grown by metal organic chemical vapor deposition using an amount of deposited In from 7 to 2 monolayers (ML). In the uncapped samples, using atomic force microscopy (AFM), we observed lateral sizes of 100–200 nm, together with a bimodal height distribution having maxima at ∼5 and ∼15 nm, which we denoted as QDs of type A and B, respectively; and reduction of the density of the type-B dots from 4.4 to 1.6 μm–2. The reduction of the density of B-type dots were observed also using transmission electron microscopy of the capped samples. Using single dot low-temperature photoluminescence (PL) spectroscopy we demonstrated effects of Wigner localization for the electrons accumulated in these dots.

In0.8Ga0.2As Quantum Dots for GaAs Solar Cells: Metal-Organic Vapor-Phase Epitaxy Growth Peculiarities and Properties

The growth peculiarities of In0.8Ga0.2As quantum dots and their arrays on GaAs surface by metalorganic vapor-phase epitaxy are investigated. The bimodal size distribution of In0.8Ga0.2As quantum dots is established from the photoluminescence spectra recorded at different temperatures. The growth parameters were determined at which the stacking of 20 In0.8Ga0.2As quantum-dot layers in the active area of a GaAs solar cell makes it possible to enhance the photogenerated current by 0.97 and 0.77 mA/cm2 for space and terrestrial solar spectra, respectively, with the high quality of the p–n junction retained. The photogenerated current in a solar cell with quantum dots is higher than in the reference GaAs structure by ~1% with regard to nonradiative-recombination loss originating from stresses induced by the quantum-dot array.

Bimodality in Arrays of In 0.4 Ga 0.6 As Hybrid Quantum-Confined Heterostructures Grown on GaAs Substrates

Hybrid quantum-confined heterostructures grown by metal-organic vapor-phase epitaxy (MOVPE) via the deposition of In0.4Ga0.6As layers with various nominal thicknesses onto vicinal GaAs substrates are studied by photoluminescence spectroscopy and transmission electron microscopy. The photoluminescence spectra of these structures show the superposition of two spectral lines, which is indicative of the bimodal distribution of the size and/or shape of light-emitting objects in an array. The dominant spectral line is attributed to the luminescence of hybrid “quantum well–dot” nanostructures in the form of a dense array of relatively small quantum dots (QDs) with weak electron and hole localization. The second, lower intensity line is attributed to luminescence from a less dense array of comparatively larger QDs. Analysis of the behavior of the spectral line intensities at various temperatures showed that the density of larger QDs grows with increasing thickness of the InGaAs layer.