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III–V quantum dot systems are widely studied to implement the promising intermediate band solar cell concept. However, current performances are quite lower than theoretical expectations, and severa...

In the present work, we have systematically investigated the design of Asymmetric SPAcer-layer Tunnel (ASPAT) diodes using numerical modeling employing SILVACO Atlas software. After reproducing and validating physical models using experimental data obtained from fabricated ASPAT diodes, a developed SILVACO model was used to evaluate the effect of different structure designs on two key parameters, namely the junction resistance and curvature coefficient at zero bias. The impact of the variation in the spacer layers thicknesses, emitter, collector, their doping profile, and the barrier width was thoroughly explored. It was found that the spacers and the barrier thickness are the most critical design parameters affecting the junction resistance and curvature coefficient. The insertion of an InxGa1-xAs QW layer adjacent to the AlAs barrier, combined with the appropriate choice of spacers and AlAs barrier thicknesses, contributed to an increase in the curvature coefficient by a factor of three while maintaining a low junction resistance. The cut-off frequency of a 4 × 4 μm2 ASPAT device is predicted to be > 500 GHz at zero bias. A comprehensive study is presented to guide researchers working on ASPAT devices, providing valuable prediction for appropriate design parameters toward maximizing the microwave/mm-wave device performance.

In this work, we investigate the optical properties of InAs quantum dots (QDs) capped with composite In 0.15 Al 0.85 As/GaAs 0.85 Sb 0.15 strain-reducing layers (SRLs) by means of high-resolution X-ray diffraction (HRXRD) and photoluminescence (PL) spectroscopy at 77 K. Thin In 0.15 Al 0.85 As layers with thickness t = 20 Å, 40 Å, and 60 Å were inserted between the QDs and a 60-Å-thick GaAs 0.85 Sb 0.15 layer. The type II emissions observed for GaAs 0.85 Sb 0.15 -capped InAs QDs were suppressed by the insertion of the In 0.15 Al 0.85 As interlayer. Moreover, the emission wavelength was blueshifted for t = 20 Å and redshifted for t ≥ 40 Å resulting from the increased confinement potential and increased strain, respectively. The ground state and excited state energy separation is increased reaching 106 meV for t = 60 Å compared to 64 meV for the QDs capped with only GaAsSb SRL. In addition, the use of the In 0.15 Al 0.85 As layers narrows significantly the QD spectral linewidth from 52 to 35 meV for the samples with 40- and 60-Å-thick In 0.15 Al 0.85 As interlayers.

The optical quality enhancement of a GaAs template grown on Silicon substrate using InGaAs/GaAs Dislocation Filters (DFs) combined with an annealing step have been assessed optically using Photoluminescence (PL) of embedded InAs/InGaAs QDs within a GaAs matrix. An annealing temperature after the growth of the DFs of 690 °C was shown to be optimum, giving an enhanced PL emission from the embedded InAs QDs in terms of intensity and Full Width at Half Maximum (FWHM). InAs quantum dots capped with GaAsSb grown at different temperatures were grown on the optimized GaAs template on Si. The prepared samples were characterized by PL, Atomic Force Microscopy (AFM), excitation power and dependent PL at 77 K and 300 K. At 77 K, the InAs/GaAsSb grown at 484 °C showed a type II band alignment with an emission wavelength of 1297 nm, which is shorter than the emission obtained from the reference sample, where the GaAsSb-capped dots were grown on GaAs (1357 nm). By reducing the GaAsSb capping layer growth temperatures to 465 °C and 450 °C, the wavelength at 77 K was extended to 1375 nm and 1480 nm respectively, resulting from an increased Sb content in the capping layer. At 300 K, a long wavelength emission of 1623 nm have been achieved.

The effect of antimony incorporation in In0.11Ga0.89As overgrown layers on the optical properties of InAs/GaAs quantum dots grown by molecular beam epitaxy has been studied. The results of photoluminescence, high resolution X-ray diffraction, atomic force microscopy, power and temperature-dependent photoluminescence from 77 K to 300 K have been correlated to characterize the grown samples. The wavelength emission increases with Sb content, and it reaches ∼1367 nm and does not increase further because of Sb incorporation's saturation in the In0.11Ga0.89As layer. To redshift further the wavelength to 1432 nm, a reduction of the growth temperature of the In0.11Ga0.89As1-ySby layer from 500 °C to 440 °C was necessary. This achievement is accompanied by the transformation of the QDs distribution to 2 QDs families. The extracted activation energies suggest that the PL quenching is mainly attributed to the thermal transfer of carriers in the QDs to the higher energy levels in the defects located within the In0.11Ga0.89As1-ySby SRLs. The variation of PL FWHM with temperature was interpreted by carrier thermalization and capture among different QDs within the same QD family or between small and large QD families.

In this work we present a solar cell structure where the concept of intermediate band is exploited by a high energy barrier AlGaAs material with embedded InAs-based quantum dots via a multistep growth approach. In this way the intrinsic issues related to different surface kinetics of involved species (Ga, In and Al adatoms) and affecting crystal quality are successfully overcome. With respect to energy band engineering of the cell, this growth approach introduces a two-dimensional quaternary layer and consequently an additional energy band, between the host junction and the dot energy levels. This band results strongly related to the quantum dot states by thermal transferring and inter-level filling processes. Moreover, low temperature (up to 100 K) photocurrent generation via additional infrared absorption is promoted by the employed band engineering, thus representing an effective method to extend intermediate band solar cell design flexibility.

Self-organized InAs QDs have been grown by Molecular Beam Epitaxy with different strain reducing layers (SRL) having nearly similar lattice mismatches to GaAs. The used SRLs are GaAsSb, InGaAs and InGaAsSb. Atomic Force Microscopy (AFM), transmission electron microscopy (TEM), high resolution X-ray diffraction (HRXRD), power dependent photoluminescence (PL) from low to room temperature have been employed for the characterization of the grown samples. Our results revealed that the emission wavelength reaches 1.32 μm when the InAs QD are either covered by InGaAs or GaAsSb with a PL spectrum broadening when using GaAsSb. However, the incorporation of only 1% of Sb in the InGaAs SRL extends further the wavelength to 1.37 μm without altering the PL spectrum. For more quantitative analysis of the observed results, the QDs size dependence on the SRL type has been estimated by tuning the theoretical transition energies, obtained by solving the single band effective mass Schrodinger equation for an ellipsoidal QD through changing the dot size to fit the experimental transition energies. Indeed, in addition to the QDs strain reduction, the type of alloy capping layer is found to seriously alter the QD size and aspect ratio and consequently the energy separation between the first and second excited state. However, from the temperature dependence of the PL, the thermionic emission activation energies were found to be close to separation between the ground and first excited state independently of the SRL type.

In the present work we report on the synthesis of tetragonal phase of stannite Cu2FeSnS4 powder from Sn(II) and Sn(IV) using a solvent free melt method using a mixture of Cu, Fe, Sn(II) / Sn(IV) O-ethylxanthates and annealed at different temperatures. The as-synthesized powders were characterized by powder X-ray diffraction (p-XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), UV-Vis absorption spectroscopy, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy, which confirm the succesful synthesis of stannite CFTS. Optical measurements shows that Cu2FeSnS4 powders have visible light absorption onsets in the far red with direct band gap energies in the range 1.32 eV - 1.39 eV which are suitable for acting as efficient absorber layers in solar cells. Electronic characterisation of these materials deposited as thin films by spin coating show that they are p type semiconductors with respectable carrier mobilities of ca. 60 cm2 V-1 s-1 with carrier densities in the order of 1014 cm-1.

In this work physical models of GaAs/AlAs and InGaAs/AlAs Asymmetric Spacer Layer Tunnel Diodes (ASPATs) with thin AlAs barriers were created using Silvaco ATLAS software. DC and RF characteristics of these ASPAT designs were obtained using the ATLAS semiconductor-insulator-semiconductor model. The simulated results were validated against measured results of fabricated ASPATs, grown using molecular beam epitaxy (MBE) with the same epitaxial designs. The models showed good scaling with device mesa area and good matching to both the DC and RF characteristics of the fabricated devices. Once experimentally validated models of the ASPAT diodes were produced, simulations were performed to investigate new metamorphic ASPAT devices in which InGaAs/AlAs devices are grown on GaAs substrates. These devices are the first demonstration of In 0.53 Ga 0.47 As/AlAs ASPATs grown on a GaAs platform. This has the potential to reduce cost and is a step forward for the manufacturability of low cost mmwave/THz detector devices. The simulations were compared with fabricated metamorphic ASPATs to better understand these new devices.

In the present work we report on the synthesis of tetragonal stannite Cu

We present two novel barrier-well heterostructure diodes, for use as zero bias detectors in microwave and mm-wave applications. One based on the GaAs platform and one based on In0.53Ga0.47As lattice matched to InP. This is achieved by adding quantum wells to As ymmetric Sp acer Layer T unnel (ASPAT) diodes next to the barrier. The DC characteristics of these new diodes were simulated in SILVACO Atlas TCAD software using experimentally validated physical models based upon conventional ASPAT diodes. The highest extracted curvature coefficients of these diodes at zero bias were 33 V−1 and 35 V−1 for GaAs and In0.53Ga0.47As based structures respectively. A C-V analysis was performed, and it was found that the addition of quantum wells reduced the zero-bias capacitance of these devices when compared with the standard ASPAT diode. The new diodes exhibited estimated cut-off frequencies of 532 GHz and 800 GHz for the GaAs and In0.53Ga0.47As based structures respectively. The lower cut off frequencies for the In0.18Ga0.82As/AlAs/GaAs devices is largely due to their inherently higher series resistance

We have investigated the effect of antimony incorporation in GaAsSb as a capping layer on the optical properties of InAs quantum dots grown by Molecular Beam Epitaxy. Atomic Force Microscopy (AFM), High Resolution X-Ray Diffraction (HRXRD), photoluminescence (PL) and power dependent PL at 77 K and 300 K have been used for the characterization of the grown samples. Our analysis showed that the emission wavelength increases with Sb content and reaches ∼1.5 μm for Sb concentration of 22%. To achieve this wavelength, a reduction of the growth temperature of the GaAsSb layer from 500 °C to 440 °C was necessary. The wavelength increase is accompanied by a transition from a type I to type II band alignment and a broadening of the PL spectrum to a value of ∼237 nm for an excitation power of 100 mW. This broadening is attributed to the QD size inhomogeneity increase and Sb atoms redistribution during the in-situ annealing during the growth of the barriers at elevated temperature. Our results show the potential of the InAs/GaAsSb system in the development of broadband light sources and super-luminescent light emitting diodes in 1.2–1.5 μm wavelength range.

In this study, the optical properties of InAs quantum dots (QDs) were characterized using photoluminescence (PL) measurements. The QDs, capped with GaAs and GaAs1−xSbx (x = 6%) strain-reducing layer (SRL), were grown by Molecular Beam Epitaxy. Temperature-dependent photoluminescence (TDPL) of both ground state (GS) and first excited state (ES) was carried out through the analysis of the PL peak position as well as the integrated PL intensity. The temperature dependence of the integrated PL intensity shows the carrier trapping in the potential barrier at the interface between the capping layer and QDs in both samples at low temperatures for the GS and ES. The Excitation density dependent photoluminescence (EDPL) showed a redshift of the GS and ES PL peak energies with increasing excitation density. We attribute this variation to the bandgap renormalization (BGR) effect. The potential barrier reduction for the GaAsSb-capped QDs increases carrier injection efficiency inside the QDs, giving rise to a larger BGR effect compared to the QDs capped with GaAs. With increasing temperature, BGR redshift varies considerably for the GaAs- capped QDs but less for the Q with GaAsSb SRL. This effect was explained using the population rate of carriers inside the QDs while taking into account the nonradiative recombination process for the two samples. Furthermore, the variation of the integrated intensity with the excitation power density grows superlinearly for the two samples for a temperature range from 10 K to 220 K. This behavior was explained by the random capture of carriers in the dots. The sample with GaAsSb SRL having a smaller potential barrier has reduced superlinear dependence at low temperatures. Losses mechanism has a significant impact on increasing the superlinear dependence at high temperatures. The ES showed a stronger superlinearity compared to the GS. This study helps to understand the optical mechanisms in some devices, such as QD lasers.

We present work on a novel In0.53Ga0.47As/AlAs metamorphic asymmetric spacer layer tunnel (mASPAT) diode structure, which was grown on GaAs by solid source molecular beam epitaxy. mASPAT diodes with different mesa sizes were fabricated and tested following growth under optimal conditions. The measured I–V characteristics of these tunneling devices showed rectifying behavior resulting from the asymmetric design of the epitaxial spacer layers. The extracted curvature coefficient, junction resistance, and leakage currents at −1 V resulted in an estimated theoretical cut-off frequency f t at zero bias exceeding 180 GHz for 4 × 4 μm2 mesa devices. The obtained results demonstrate the potential use of mASPAT devices on GaAs as a low-cost alternative to devices fabricated on InP substrates for high-volume zero bias microwave and millimeter-wave detectors.

Double-quantum-well GaInNAsSb laser structures grown on GaSb substrate emitting between 2.3 and 3.3 μm are proposed, and modeled self-consistently using a commercial software that combines gain calculation with 2-D simulations of carrier transport and waveguiding. The model is calibrated using experimental results obtained from conventional GaInAsSb lasers emitting at 2.3 μm. The simulated results show that the incorporation of a small concentration of nitrogen in the conventional InGaAsSb can extend the wavelength to 3.3 μm with a reasonable threshold current density increase. This is obtained as a result of preserving good confinement for holes as compared to conventional InGaAsSb 2.3-μm laser. The threshold current density increase with wavelength is attributed mainly to nonuniform distribution of carriers due to the high-conduction band offset. This drawback of the structure can be overcome by incorporating AlGaInAsSb material as a barrier and waveguide instead of AlGaAsSb. The simulation shows that the bandgap of the AlGaInAsSb quinternary material should be reduced linearly when increasing the wavelength to maintain a good laser performance in terms of emitted optical power.

The electrical and optical properties of InGaN–GaN light-emitting diodes (LEDs) emitting in the 400–500-nm range and having a v-shaped quantum well (VSQW) and a u-shaped quantum well (USQW) are numerically investigated using APSYS simulation program. The simulation results showed that the devices containing VSQW have superior performance in terms of optical power and internal quantum efficiency droop compared to those with USQW. The optical power of the LEDs containing USQW increases gradually and reaches a maximum at 460 nm; however, the optical power of the LEDs with VSQW improves gradually, and the maximum is obtained in a window from 420 to 436 nm as a result of radiative recombination enhancement. The simulation results suggest that the higher performance of the VSQW is due to piezoelectric field reduction and an enhancement of electron and hole wave functions overlap.

In this work, we demonstrate the use of a digitally alloyed GaAsSb as a strain reducing layer (SRL) to tune the optical properties of InAs quantum dots grown on GaAs substrates. The GaAsSb digital alloy was made using a short period GaAs/GaSb superlattice. Increasing the Sb-shutter duty cycle (Sb-SDC) from 0 to 0.35 redshifts the emission wavelength from 1220 nm to 1492 nm at 300 K. The wavelength red shift is attributed mainly to the increased Sb content in the GaAsSb SRL as confirmed by high resolution X-ray diffraction. Power dependent photoluminescence showed a type II band alignment for the samples grown with a Sb-SDC of 0.25 and 0.35. Reversing the growth order of GaAs and GaSb layers and maintaining the same Sb-SDC (0.25) drastically alters the optical properties of the QDs. A large wavelength blue shift of 143 nm and a transformation from a type II to type I emission were observed as a result of GaSb intermixing with InAs QDs.

Multiple-quantum well InGaAs laser structures emitting at 2 $$\upmu $$ m with different barriers are modeled using commercial software that combines gain calculation with 2-D simulations of carrier transport and waveguiding. The model is calibrated using experimental results. The simulated results show a non-uniform distribution of carriers in different quantum wells with InGaAlAs barriers which affects their contribution to the gain. The carrier uniformity and a reduction in threshold current density are observed when we use an InGaAs barrier material. The quantum well number was varied from 2 to 4 in both structures and a comparison of the threshold current and its variation with temperature were investigated.

Presented are the results of a comparative analysis to identify abundant, non-toxic binary materials with potential applicability for photovoltaics. Materials other than the conventional Si, CdTe, and Cu(In,Ga)Se 2 (CIGS) are examined. The screening is based on the materials’ bulk properties and a set of environmental, physical, and chemical criteria. The screening process is detailed and the properties and applicability of the screened materials are discussed.

We self-consistently analyze the effect of quantum well (QW) number on the performance of semiconductor lasers based on GaSb material emitting at 2.3 mum. The analysis is performed with commercial software that combines gain calculation with 2-D simulations of carrier transport and wave guiding. The laser model implemented in the software was calibrated using experimental results. Excellent agreement between simulation and experimental results was achieved. The calibrated model was then used to simulate the performance of these lasers as a function of the number of QWs. The analysis includes the cavity length dependence of the threshold current density, the characteristic temperature, and the emitted optical power reduction at a constant current with increasing temperature. Our simulation shows that the transparency current density and the gain parameter increase with a rate of ~16 A/cm2 and 6.82 cm-1 per QW, respectively. The internal loss is mainly due to free carrier in the p-type layers. The simulation shows that the optimum number of QWs for the characteristic temperature, threshold current, and optical power reduction depends strongly on the cavity length.

Due to their delta-like density of states, quantum dots (QDs) were expected to improve laser device performances with respect to quantum wells (QWs). Nevertheless, some important drawbacks limit this technology. For instance, QD laser still suffers from a low value of the modal gain, due to the low areal density of QDs, and inhomogeneous broadening, especially when multistacked layers are used. In this paper, we demonstrate that a linear increase of the QD modal gain with the QD layers number, as typically achieved in multi-QW lasers, is possible by a careful control of the Stranski-Krastanov QDs growth and QDs stacking optimization. A low-transparency current density of 10 A/cm(2) per QD layer and a modal gain of 6 cm(-1) per QD layer Were achieved from laser structures containing up to seven QD layers. We demonstrate 10-Gb/s direct modulation (until a temperature of 50 degrees C and high T-0 (110 K) from a single-mode device containing six QD layers.

This paper relies on the design and fabrication of CdSe/ZnS core/shell colloidal nanocrystals (NCs) cylindrical microcavities for microphotonics applications. The fabrication technology relies on the release of the strain in strained heterostructures, combined with technological processes to improve the hydrophilic properties of the inner surface of tubes thus allowing their efficient filling with a nanocrystals blend. The fabricated microtubes can be also suitable as microneedles for biological applications, as well as cylindrical pipelines for fluid transportation on the micro and nanoscale. (c) 2007 Elsevier B.V. All rights reserved.

In this work the analysis, fabrication and optical characterization of a circular two-dimensional photonic crystal (2D-CPC) nano-resonator based on a air/GaAs/air slab waveguide are presented. Four InAs/InGaAs quantum dots (QDs) stacked layers emitting around 1300 nm were embedded in a GaAs waveguide layer grown on an Al0.7Ga0.3As sacrificial layer. The nano-resonator was realized by drilling 2D-PC air holes arranged in a circular lattice configuration through electron beam lithography (EBL), inductively coupled plasma (ICP) etching and wet selective etching of the Al0.7Ga0.3As sacrificial layer in order to release the membrane. The spectral response of the active circular nano-cavity has been simulated by using a three dimensional finite-difference time-domain method (3D-FDTD) as a function of both the inner and outer holes radius/period ratios of the photonic crystal structure. Good agreement between the calculated resonance and the experimental results, recorded from the nano-cavity by a mu PL setup, has been achieved. (c) 2007 Elsevier B.V. All rights reserved.

International audience; Hydrostatic pressure and spontaneous emission techniques have been used to examine the important recombination mechanisms in type-I GaInAsSb/GaSb quantum well lasers. High pressure results indicate that Auger recombination dominates the threshold current of 2.11 μm and 2.37 μm devices and is the origin of their temperature sensitivity around room temperature. While the characteristics of the 2.37 μm devices are much improved by the suppression of the CHSH Auger process, since its spin-orbit splitting energy is greater than its band gap, other important Auger processes such as CHHL and CHCC persist. In the larger band gap 2.11 μm devices, an increase in threshold current with pressure is observed suggesting that CHSH Auger is present in these devices at atmospheric pressure and contributes to performance degradation at these shorter wavelengths.

The performances of laser diodes operating in continuous wave regime, in terms of lifetime, output power and emission wavelength, are strongly limited by internal Joule heating. Using a CCD-based thermoreflectance system, we have obtained thermal images of laser diodes operating at various biases. Separate calibration procedures have been implemented to obtain quantitative temperatures on the gold-covered ridge waveguides and on the emission facets, thus providing a complete picture of the surface temperature of the device. The continuity of the obtained temperatures is a strong indication of the consistency of the measurements. Even in quasi-static regime, temperature inhomogeneities have been detected, which might lower the permanent damage threshold of the devices.

Temperature-dependent optical gain, loss, and internal quantum efficiency have been measured for continuous-wave operated 2- μm InP laser from 10 °C to 70 °C using the Hakki-Paoli method. Modal gain and loss were extracted from the measured amplified spontaneous emission. The optical loss is constant with temperature, while the material gain decreases from 1128 cm-1 at 10 °C to 427 cm-1 at 70 °C. The internal efficiency decreases from 74% to 37% at 10 °C and 50 °C , respectively. The temperature sensitivity of threshold current is due to the decrease of the differential gain and a degradation of internal quantum efficiency.

We report on the growth and characterization of low threshold 1.32-μm quantum dots (QDs) laser diodes. The quantum dot active region was optimised to get the highest photoluminescence emission and the lowest Full Width at Half Maximum (FWHM). From samples containing multilayer QDs and using the Limited-Area Photoluminescence (LAPL) technique we have shown that the gain of an N-layer structure is higher than N times that of a single layer. This enhancement is attributed to the increase of the quantum dot density in the upper layers and also to the use of the high growth temperature spacer layer. Broad area laser diodes were processed from the grown samples containing three layers of InAs QDs grown directly on GaAs and capped with 4-nm-thick In x Ga 1-x As layer. Than measurements were performed at room temperature under pulsed excitation. The laser diodes operate at room temperature and emit between 1.29 and 1.32-μm which is beyond the strategic telecommunication wavelength. The characteristic temperature is around 80 K and very stable in the hole range of the operating temperature (from 0 to 90 °C). The internal quantum efficiency is 53% and the modal gain per QD layer was estimated to be ~ 6 cm -1 . For an infinite cavity length a threshold current density of 8 A/cm 2 per QD layer was obtained. From the calculation of the optical confinement of QDs, we have estimated a material gain of 1979 cm -1 .

High performance 1300 nm lasers based on self-organized InAs/InGaAs quantum dots (QDs) are reported. By optimizing the QD growth parameters and decreasing the waveguide thickness, a high modal gain and a low transparency current density of 32 cm(-1) and 35 A cm(-2), respectively, were obtained from a device containing five stacked QD layers. The internal quantum efficiency is as high as 90%.

In this letter, we report on experimental results of directly modulated single-transverse mode 1.3-mu m InAs-InGaAs quantum-dot (QD) lasers in a wide temperature range. A 3.125-Gb/s data modulation over temperature with an extinction ratio up to 10 dB is reported. Moreover, 10-Gb/s eye patterns at 15 degrees C and 50 degrees C and 5-Gb/s modulation in the whole explored temperature range (15 degrees C-85 degrees C) are demonstrated. These results were obtained by exploiting heterostructures containing six layers of high modal gain InAs QDs grown without incorporation of p-doping in the active region or tunnelling injection structure implementation. QD lasers exhibited a saturation modal gain as high as 36.3 cm(-1), ground state lasing from short cavities down to 400-mu m length and a characteristic temperature of about 110 K in a large temperature range between 15 degrees C and 85 degrees C.

In this work two quantum dot (QD) solar cell structures have been proposed and compared as potential solutions for the realization of the Intermediate Band Solar Cell concept: the well known dot/barrier material system InAs / GaAs and an engineered InAlGaAs/AlGaAs combination. The Al-based structures have been obtained by a suitably developed growth procedure with the aim of increasing island density and engineering the absorption spectrum and the energy band profile in the near infrared region. Along with tunability of the confined electron energy levels, the proposed Al-based structures exhibit transport features, such as reduced edge recombination losses and lower reverse saturation current density with respect to the InAs/GaAs QD system, which can be useful for enhancing device performances. © 2014 AEIT.

Diode lasers emitting at 2.26 /spl mu/m, based on the InGaAsSb-AlGaAsSb materials system, are reported. These devices exhibit high internal quantum efficiency of 78% and low threshold current density of 184.5 A/cm/sup 2/ for a 2-mm-long cavity. Output power up to 700 mW (/spl ap/550 mW) has been obtained at 280 K (300 K) in continuous-wave operation with 100 /spl mu/m/spl times/1 mm lasers. These devices have been coated with an antireflection on the output facet and are mounted epilayer down on a copper block. The working temperature was maintained by a thermoelectric Peltier cooling element.

In this paper, c-plane stepped- and graded- InGaN/GaN multiple quantum wells (MQWs) are grown using plasma assisted molecular beam epitaxy (PAMBE) by in situ surface stoichiometry monitoring (i-SSM). Such a technique considerably reduces the strain build-up due to indium clustering within and across graded-MQWs; especially for QW closer to the top which results in mitigation of the quantum-confined Stark effect (QCSE). This is validated by a reduced power dependent photoluminescence blueshift of 10 meV in graded-MQWs as compared to a blueshift of 17 meV for stepped-MQWs. We further analyze microstrain within the MQWs, using Raman spectroscopy and geometrical phase analysis (GPA) on high-angle annular dark-field (HAADF)-scanning transmission electron microscope (STEM) images of stepped- and graded-MQWs, highlighting the reduction of ~1% strain in graded-MQWs over stepped-MQWs. Our analysis provides direct evidence of the advantage of graded-MQWs for the commercially viable c-plane light-emitting and laser diodes.

We report the dry etch of GaSb(001) by inductively coupled plasma reactive ion etcher. Silicon Oxide, deposited by PECVD, was used as a mask. The oxide layer proved to be almost unaffected compared to the GaSb, when using chlorine compound gases as etchants (Cl2, BCl3, and SiCl4) as well as argon. This provides high selectivity for GaSb to the mask layer. The sample holder has no silicon that may contribute to the etching process. Etching using Cl2 + Ar showed increase in the etching rate as the chlorine ratio increases; however, the process led to grassy surface and chemical like reaction. The use of SiCl4+Cl2+Ar mixture with low chlorine ratio resulted in anisotropic etch with smooth sides. It has been found for this case that the increase of the chlorine ratio led to an increased etching rate as well. The repeat of previously reported result by Swaminathan et al. [Thin Solid Films 516 (2008) 8712.] yet with a sample holder not having silicon, proved the effect of Si-contribution in producing vertical profile etch with smooth surfaces.

GaSb-based quantum well lasers have been optimized for high coupling efficiency into an optical system. The use of a V-shaped Weaker Waveguide in the n-cladding layer is shown to dramatically reduce vertical beam divergence without any performance degradation compared to a conventional broad-waveguide laser structure. Starting from a broad waveguide laser structure design which gives low threshold current and a large vertical far-field (VFF), the structure was modified to decrease the VFF while maintaining a low threshold-current density. In a first step the combination of a narrow optical waveguide and reduced refractive index step between the waveguide and the cladding layers reduces the VFF from 67o to 42o. The threshold current density was kept low to a value of ~190 A/cm2 for 1000×100 μm2 devices by careful adjustment of the doping profile in the p-type cladding layer. The insertion of a V-Shaped Weaker Waveguide in the n-cladding layer is shown to allow for further reduction of the VFF to a value as low as 35o for better light-coupling efficiency into an optical system without any degradation of the device performance.

International audience; Thermal characterization of semiconductor lasers is an important issue for optoelectronics. This paper presents our thermoreflectance measurements on two different types of laser diodes : classical ridge laser diodes and vertical cavity surface emitting lasers(VCSELs). We first studied the external temperature increase in ridge diodes in order to determine inhomogeneity. Then,we tried to determine the inner temperature of the VCSELs.

We report the results of an extensive optical investigation by continuous-wave and time resolved photoluminescence experiments on double GaInAsSb∕AlGaAsSb quantum wells emitting at 2.3μm at room temperature. We have found that, at low temperature (

Inthiswork,theanalysis,fabricationandopticalcharacterizationofatwo-dimensionalcircularphotoniccrystal(2D-CPC)nano-resonatorbasedonanair/GaAs/airslabwaveguidearepresented.FourInAs/InGaAsquantumdots(QDs)stackedlayersemittingaround1300nmatroomtemperaturewereembeddedinaGaAswaveguide layer grown on an Al 0.7 Ga 0.3 As layer and GaAs substrate. The patterning of the structure andthe membrane release were achieved by using electron beam lithography, ICP plasma etching and selectivewetetchingoftheAlGaAssacrificiallayer.Themicro-luminescencespectrumrecordedfromthefabricatednano-cavityshowsanarrowopticaltransitionattheresonancewavelengthofabout1282nmwithaFWHMand Q-factor of 6.2 A and more than 2000, respectively.˚c 2007 Elsevier Ltd. All rights reserved. Keywords: Nano-cavity; Quantum dot; Photonic crystal 1. IntroductionElectromagnetic resonant cavities, which trap light within a finite volume, are essentialcomponents for the fabrication of high efficiency photonic devices such as ultra small opticalfilters, single-photon sources [1,2] and low-threshold lasers [3]. In a micro-cavity and in weakcoupling regimes, according to the Purcell factor relationship [4], an increase in the Q-factorand a reduction of the modal volume V

High saturation modal gain InAs/InGaAs quantum dot (QD) lasers operating at 1.3 micron have been successfully demonstrated, thus enabling high frequency operation, high temperature stability and ultra-small cavity lasers.

We have investigated the ultrafast carrier dynamics in Molecular Beam Epitaxy (MBE)-grown InAs/InGaAs/GaAs quantum dots emitting at 1.3 mu m by means of time resolved photoluminescence upconversion measurements with a time resolution of about 200 fs. The detection energies scan the spectral region from the energy of the quantum dot excitonic transition up to the barrier layer absorption edge. We found, under high excitation intensity, that the intrinsic electronic states are populated mainly by carriers directly captured from the barrier. (C) 2007 Elsevier Ltd. All rights reserved.

The authors report a detailed investigation of the structural and optical properties of vertically stacked InAs quantum dots embedded in an (In,Ga)As quantum well by means of transmission electron microscopy and time resolved photoluminescence based on the upconversion technique. By comparing the optical features of quantum dot samples of different barrier thicknesses (nominal values between 5 and 65 nm), they have found evidence for electronic coupling among the quantum dots, featured by an increase of radiative lifetime and a relatively blueshifted emission peak for the thinnest spacer layer sample.

In this paper, we study an unconventional kind of quasi-three-dimensional (3-D) photonic crystal (PhC) with circular lattice pattern: it consists of air holes in a GaAs material (n = 3.408) along circular concentric lines. This particular PhC geometry has peculiar behavior if compared with the traditional square and triangular lattices, but it is difficult to model by using conventional numerical approaches such as wave expansion method. The resonance and the radiation aspects are analyzed by the 3-D finite-element method (FEM). The model, based on a scattering matrix approach, considers the cavity resonance frequency and evaluates the input-output relationship by enclosing the photonic crystal slab (PhCS) in a black box in order to define the responses at different input-output ports. The scattering matrix method gives important information about the frequency responses of the passive 3-D crystal in the 3-D spatial domain. A high sensitivity of the scattering parameters to the variation of the geometrical imperfection is also observed. The model is completed by the quality factor (Q-factor) estimation. We fabricated the designed circular photonic crystal over a slab membrane waveguide embedding InAs/GaAs quantum dots emitting around 1.28 mu m. Good agreement between numerical and experimental results was found, thus validating the 3-D FEM full-wave investigation.

In this contribution we show the fabrication of functional hybrid organic-semiconductor microtubes obtained by combining the self-rolling of In0.2Ga0.8As/GaAs layers with the deposition of polymer multilayers obtained by Layer-by-Layer (LbL) self-assembly. It was demonstrated that the polymer multilayers are compatible with the selective etching exploited for the channel formation and that reactivity to an external stimulus (e.g. pH) is maintained after the hybrid structure formation. (C) 2008 Elsevier B.V. All rights reserved.

In this work, we report on the competition between two-step two photon absorption, carrier recombination, and escape in the photocurrent generation mechanisms of high quality InAs/GaAs quantum dot intermediate band solar cells. In particular, the different role of holes and electrons is highlighted. Experiments of external quantum efficiency dependent on temperature and electrical or optical bias (two-step two photon absorption) highlight a relative increase as high as 38% at 10K under infrared excitation. We interpret these results on the base of charge separation by phonon assisted tunneling of holes from quantum dots. We propose the charge separation as an effective mechanism which, reducing the recombination rate and competing with the other escape processes, enhances the infrared absorption contribution. Meanwhile, this model explains why thermal escape is found to predominate over two-step two photon absorption starting from 200 K, whereas it was expected to prevail at lower temperatures (70 K), solely on the basis of the relatively low electron barrier height in such a system. VC 2016 AIP Publishing LLC.

In this work, we show that a modal gain as high as 42cm-1 could be achieved from a device containing 7 InAs/InGaAs stacked QD layers. The enhancement of the modal gain is accompanied by a decrease of the threshold and transparency current densities per QD layer down to 16 and 7 A cm-2, respectively. Moreover, a linear increase of the modal gain with the number of QD layers is observed. The obtained high modal gain allows us to achieve good temperature stability and high-speed operation in a wide temperature range. Modulation eye diagrams under 10 Gb/s are achieved in 15degC-50degC range in a single- transverse mode 1.3mum QD lasers. High characteristic temperature of about 110 K between 15degC and 85degC are reported.

We report on the fabrication and characterization of single-transverse mode 1.3 μm InAs/InGaAs QD lasers in a wide temperature range. Open eye patterns up to 10 Gb/s are reported, whereas a characteristic temperature of about 110 K has been measured in the whole temperature range (15°C - 85°C). These results were obtained by exploiting heterostructures containing six-layers of high modal gain InAs quantum dots grown without incorporation of p-doping. QD lasers exhibited a saturation modal gain as high as 6 cm-1 per QD layer, which linearly increases with the numbers of the quantum dot layers.

We have investigated the ultra-fast carrier dynamics in Molecular Beam Epitaxy (MBE)-grown InAs/InGaAs/GaAs quantum dots (QDs) emitting at 1.3 μm by time resolved photoluminescence (TRPL) upconversion measurements with a time resolution of about 200 fs. Changing the detection energies in the spectral region from the energy of the quantum dots excitonic transition up to the barrier layer absorption edge, we have found that, under high excitation intensity, the intrinsic electronic states are populated mainly by carriers directly captured from the barrier.