Sergey A. Mintairov

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.

Single-Mode Emission From 4–9-μm Microdisk Lasers With Dense Array of InGaAs Quantum Dots

A dense array of InGaAs quantum dots formed by MOCVD on a misoriented GaAs substrate has been used as an active medium of microdisk resonators of various types: a cylindrical disk, an undercut disk, and a suspended disk. Single-mode room temperature lasing in a 9-μm microdisk laser is demonstrated with a dominant line around 1.13 μm. Impact of the resonator design on the lasing threshold and its temperature behavior is discussed.

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.

High-power picosecond laser diodes based on different methods of fast gain control for high-precision radar applications

Current-pumped picosecond-range laser diodes with a peak power significantly exceeding that achievable from gainswitched lasers are of major interest for a large variety of commercial applications. A group of phenomena have been explored in which the peak transient gain is efficiently controlled by a fast reduction in the pumping current. Common to all these phenomena is the fact that the peak powers of the emitted picosecond optical pulses (15-100 ps) exceed that obtainable from gain-switched laser diodes by at least an order of magnitude, although the physical reasons for the high gain and the design principles of the semiconductor structures are different. The main problem in the realization of these picosecond modes in low-cost practical systems is the high sensitivity of the operation regime to structural and circuit parameters. A related problem is the questionable reproducibility of the fabrication processes used so far. Proper development of reliable high-power picosecond transmitters will require the use of more advanced fabrication methods and further study of the effect of structural parameters on the properties of the picosecond lasing mode. In this paper we report on a record value for the power density of the picosecond lasing (50W / 30ps) obtained from a laser diode chip of width 20 &mgr;m and give a qualitative interpretation of the operating mode. Use of the MOCVD process for diode fabrication should allow reproducible technology for picosecond laser diodes to be developed.

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.

Heterointerfaces in MJ SC: IV curves and their peculiarities

The heterointerface IV curves at different intensity illumination levels have been extracted from full IV curves of triple-junction GaInP/GaAs/Ge solar cell. For this purpose a special approach for multi-junction solar cells has been proposed. The solar cell equivalent circuit consisting of two parts – generation and connecting ones were considered. The generation part includes all photovoltaic p-n junctions, while connecting one incudes a resistance of the layers, spreading resistance, tunnel diodes and heterointerfaces. The equivalent circuit was used for analysis of «kink» free IV curves of Ge and GaAs solar cells. In this case the IV curves of connecting part are completely described by the spreading resistance. The analysis of GaInP/GaAs/Ge allows to obtain heterointerface IV curves and it has been shown that their shape depends on illumination level. The empirical formula was used for an approximation of heterointerface IV curves to show the nature of current flow mechanism which was established as tunneling one.

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%.