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Comparisons of buried InAs/GaAs and InAs/InP quantum dots (QDs) utilizing transmission electron microscopy display interesting parallels and differences between the two systems. The higher 7.2% misfit in the InAs/GaAs system produces small (~20 nm diameter) QDs with a majority displaying a predominately round shape. The lower 3.2% misfit in the InAs/InP system produces larger QDs (~35 nm diameter) with the majority also displaying a predominately round shape. In both systems, the size of the QDs can be varied by changes in growth procedures and the largest QDs in any population show evidence of faceting c 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Studies the single and multiple layers of self-assembled InAs quantum dots (QD) produced by the indium-flush technique by transmission electron microscopy. Development of a technique to reproducibly grow QD of uniform size and shape; Determination of the shapes and sizes of QD through the interpretation of the observed contrast in plan-view.

Focuses on a study which investigated the photoluminescence, electroluminescence, and optical gain on a single-quantum-well structure grown by metalorganic vapor-phase epitaxy. Information on electronic and optical devices based on quantum wells and superlattices; Methodology of the study; Results and discussion.

Discusses the engineering of quantum wells to control the energy shifts induced by semiconductor alloy intermixing. Insertion of semiconductor monolayers with a different band gap; Isotropic interdiffusion of the different atomic species of the semiconductor alloys.

Photon-recycling effects are studied experimentally in photovoltaic power converting III-V semiconductor devices designed with the vertical epitaxial heterostructure architecture (VEHSA). The responsivity of VEHSA structures with multiple thin GaAs n/p junctions is measured for various optical input powers and for different wavelength detuning values with respect to the peak of the spectral response. While the detuning of the optical excitation decreases the external quantum efficiency and the responsivity at low input powers, this study demonstrates that at high optical intensities, a large fraction of the performance can be recovered despite significant detuning values. The photon coupling effects therefore broaden the spectral range for which the VEHSA devices convert high-power optical inputs with high efficiencies into an electrical output having a preset voltage. The devices exhibit a near optimum responsivity of up to 0.645 A/W for tuned excitation conditions or at high optical intensities for spectral detuning values of up to ∼25 nm and corresponding to an external quantum efficiency of ∼94%. Efficiencies of 62.0% and 61.8% have been obtained for current-matched excitations and for a detuning of >10 nm, respectively. An output power of 5.87 W is reported and an open circuit voltage enhancement of 92 meV per n/p junction is measured compared to a device with a side by side planar architecture. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

We use apertureless scanning near-field optical microscopy to study the phase separation in chemical beam epitaxy grown Al0.1Ga0.9NxAs1−x alloys. Pits attributed to nitrogen-clustering observed on the Al0.1Ga0.9NxAs1−x surface grown at 420 °C become larger at higher growth temperatures, and 3D islands appear on the surface at 565 °C. Atomic force microscopy phase measurements reveal a composition difference between the islands and the pits, whereas the sample grown at 420 °C appears to be homogeneous. Confocal Raman spectra show that all the N atoms are bonded to Al instead of Ga. Using apertureless scanning near-field optical microscopy, the luminescence of a gold tip is mapped over the surface of the sample grown at 565 °C. We extract the shift of the tip's surface plasmon resonance and determine the variation in the refractive index between the islands and the pits to be close to 0.2. Numerical simulations of the tip luminescence while in contact with the sample predict a similar variation of ∼0.3 in the refractive indices between AlGaAs islands and AlN pits, a substantially smaller value than the difference in the bulk refractive indices of the two media (∼1.8), which we attribute to a convolution of material distribution in an uneven topography. The excellent agreement between simulation and experiments supports the hypothesis of nitrogen-clustering in the pits. [ABSTRACT FROM AUTHOR]

Photovoltaic power converting III—V semiconductor devices based on the Vertical Epitaxial HeteroStructure Architecture (VEHSA) design have been achieved with up to 20 thin p/n junctions (PT20). Open circuit photovoltages in excess of 23V are measured for a continuous wave monochromatic optical input power of ∼1W tuned in the 750 nm-875 nm wavelength range. Conversion efficiencies greater than 60% are demonstrated when the PT20 devices are measured near the peak of their spectral response. Noticeably, the PT20 structure is implemented with its narrowest ultrathin base having a thickness of only 24 nm. In the present study, the spectral response of the PT20 peaks at external quantum efficiency (EQE) of 89%/20 for an input wavelength of 841 nm. We also performed a detailed analysis of the EQE dependence with temperature and for VEHSA structures realised with a varied number of p/n junctions. The systematic study reveals the correlations between the measured conversion efficiencies, the EQE behavior, and the small deviations in the implementation of the optimal designs. Furthermore, we modeled the photovoltage performance of devices designed with thinner bases. For example, we derive that the narrowest subcell of a PT60 structure would have a base as thin as 8 nm, it is expected to still generate an individual subcell photovoltage of 1.14 V, and it will begin to feature 2-dimensional quantum well effects. [ABSTRACT FROM AUTHOR]

Optical to electrical power converting semiconductor devices were achieved with breakthrough performance by designing a Vertical Epitaxial Heterostructure Architecture. The devices are featuring modeled and measured conversion efficiencies greater than 65%. The ultrahigh conversion efficiencies were obtained by monolithically integrating several thin GaAs photovoltaic junctions tailored with submicron absorption thicknesses and grown in a single crystal by epitaxy. The heterostructures that were engineered with a number N of such ultrathin junctions yielded an optimal external quantum efficiencies approaching 100%/N. The heterostructures are capable of output voltages that are multiple times larger than the corresponding photovoltage of the input light. The individual nanoscale junctions are each generating up to ~1.2V of output voltage when illuminated in the infrared. We compare the optoelectronic properties of phototransducers prepared with designs having 5 to 12 junctions and that are exhibiting voltage outputs between >5V and >14 V. [ABSTRACT FROM AUTHOR]

Microfabrication cycles of III-V multijunction solar cells include several technological steps and end with a wafer dicing step to separate individual cells. This step introduces damage at lateral facets of the junctions that act as charge trapping centers, potentially causing performance and reliability issues, which become evenmore important with today's trend of cell size reduction. In this paper we propose a process of wet etching of microtrenches that allows electrical isolation of individual solar cells with no damage to the sidewalls. Etching with bromine-methanol, the solution that is typically used for nonselective etching of III-Vcompounds, results in the formation of unwanted holes on the semiconductor surfaces. We investigate the origin of holes formation and discuss methods to overcome this effect. We present an implementation of the isolation step into a solar cell fabrication process flow. This improved fabrication process opens the way for improved die strength, yield, and reliability. [ABSTRACT FROM AUTHOR]

Waveguides and passive integrated optics elements constructed from thin metal films of finite width embedded in a homogeneous background dielectric and propagating long-range surface plasmon polaritons (LRSPPs) have been characterized experimentally at free-space optical wavelengths near 1550 nm. Straight and curved waveguides, s-bends, four-port couplers, y-junctions, and Mach-Zehnder interferometers have been carefully characterized. Additionally, rigorous models based on modal decomposition have been proposed for all of these elements and validated via comparisons with the measurements. Excellent qualitative and quantitative agreement between theory and experiment is observed for all structures over all measurement wavelengths. It is hoped that this work will help further establish this new integrated optics technology, taking it beyond demonstrators. [ABSTRACT FROM PUBLISHER]

Reports on hidden symmetries in the energy levels of excitonic quantum dots, or artificial atoms. Possibilities for quantum dots that may form the basis of new generations of lasers; Importance of understanding the internal electronic structure of the dots; Observations of electrons and holes in a single quantum dot; Hidden symmetries in the energy function that describes the multi-particle system.

The optical performance of external-cavity lasers based on InAs/InGaAsP quantum dot laser diodes is investigated. The broad electroluminescence reveals a gain spectrum with full width at half maximum of at least 175 nm. By fabricating as-cleaved ridge lasers of different length and width, tuning ranges as high as 110 nm have been achieved at wavelengths encompassing 1.55 μm. The tuning ranges and efficiencies obtained are limited by internal losses and competition between the external-cavity lasing modes and the laser diode natural lasing modes. The laser diode length is found to affect both the wavelength tuning range and the threshold current density, which is consistent with a quantum-dot type density of states. [ABSTRACT FROM AUTHOR]

Five stacked layers of InAs quantum dots (QDs) embedded in quaternary InGaAsP are grown on (100) InP substrate to form a laser diode. The QD ensemble has a density of 1.5×10[sup 10] cm[sup -2] and emits light at ∼1.6 μm at 77 K. Lasing wavelength and threshold current density can be shifted by changing the cavity length of the laser diode and the latter reaches a value as low as 49 A/cm2 at 77 K for a gate size of 2000 μm×150 μm. Temperature dependence of the threshold current is observed implying the presence of thermionic emission increasing with temperature. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We study how the optical properties of InAs self-assembled quantum dots (QDs) grown on GaAs substrate are affected when using AlGaAs barriers to increase the carrier confinement. The inhomogeneous broadening of the QD ensemble is found to increase when ternary aluminum alloys are used next to or within the QDs. By growing thin GaAs spacers to separate the QDs from the AlGaAs barriers, we obtain QD ensembles which exhibit little photoluminescence quenching and well-defined excited states up to room temperature. Postgrowth rapid thermal annealing is then used to intermix these InAs/GaAs/AlGaAs QDs and diffuse the Al towards the QDs. In contrast with QDs having thick binary GaAs barriers, the inhomogeneous broadening of QDs with nearby AlGaAs barriers is not decreased with intermixing, leading to unresolved excited state peaks when the interdiffusion length becomes comparable to the GaAs spacer thickness. © 2001 American Institute of Physics. We study how the optical properties of InAs self-assembled quantum dots (QDs) grown on GaAs substrate are affected when using AlGaAs barriers to increase the carrier confinement. The inhomogeneous broadening of the QD ensemble is found to increase when ternary aluminum alloys are used next to or within the QDs. By growing thin GaAs spacers to separate the QDs from the AlGaAs barriers, we obtain QD ensembles which exhibit little photoluminescence quenching and well-defined excited states up to room temperature. Postgrowth rapid thermal annealing is then used to intermix these InAs/GaAs/AlGaAs QDs and diffuse the Al towards the QDs. In contrast with QDs having thick binary GaAs barriers, the inhomogeneous broadening of QDs with nearby AlGaAs barriers is not decreased with intermixing, leading to unresolved excited state peaks when the interdiffusion length becomes comparable to the GaAs spacer thickness. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Self-assembled strained semiconductor nanostructures have been grown on GaAs substrates to fabricate quantum dot infrared photodetectors. State-filling photoluminescence experiments have been used to probe the zero-dimensional states and revealed four atomic-like shells (s,p,d,f) with an excitonic intersublevel energy spacing which was adjusted to ∼60 meV. The lower electronic shells were populated with carriers by n doping the heterostructure, and transitions from the occupied quantum dot states to the wetting layer or to the continuum states resulted in infrared photodetection. We demonstrate broadband normal-incidence detection with a responsivity of a few hundred mA/W at a detection wavelength of ∼5 μm. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Discusses the effects of alloy intermixing in quantum-dot (QD) ensembles with sharp adjustable shells using the state-filling spectroscopy. Rapid thermal annealing on samples of self-assembled QD grown with different intersublevel energy spacings; Indication of the intense and sharp shell structures observed in photoluminescence.

Discusses the evolution of the energy levels in quantum dot ensembles with different densities. Lateral interactions for quantum dot spacings of hundreds of nanometers; Decrease in the photoluminescence intensity.

Describes the fabrication of quantum-dot laser diodes with up to five well-defined electronic shells using self-assembled quantum dots (QD) grown by molecular-beam epitaxy. Observation of lasing in the upper QD shells for small-gain media; Progression of lasing towards the QD ground states for longer cavity lengths.

Investigates the reduction of indium gallium arsenide/gallium arsenide semiconductor laser facet temperatures. Role of reflectance modulation in facet temperature determination; Effect of ion-implantation enhanced quantum well intermixing on temperature; Significance of reduced photoabsorption in optical power levels.

Examines the indium arsenide self-assembled quantum dots (QD) on indium phosphide substrate by molecular beam epitaxy. Range of the spectral region; Characterization of QD using photoluminescence and transmission electron microscopy; Influence of buffer layer on the island formation of residual strain.

Examines visible luminescence from self-assembled semiconductor quantum dots grown by molecular beam epitaxy. Calculation of planar densities, dot heights and wetting layer thickness; Analysis of the reflection high energy electron diffraction patterns; Variation in the ground state energy levels of quantum dots.

Investigates the energy levels of nanometer size indium gallium arsenide/ GaAs quantum dots (QD) and selective excitation of the photoluminescence. Synthesis of QD structures with diameters comparable to electrons' de Broglie wavelength; Use of coherent islanding growth effect in highly strained semiconductors; Observation of positions of QD peaks.

Examines the time-resolved optical characterization of indium gallium arsenide/gallium arsenide quantum dots. Details of the photoluminescence efficiency of the quantum dots; Increase in radiative decay at low temperature due to excitonic recombination; Analysis of the emission and absorption energies of the quantum dots.