Showing papers by "Jan Misiewicz published in 2013"

Selective excitation of Eu3+ in the core of small β-NaGdF4 nanocrystals

Abstract: The influence of the nanocrystal matrix on the optical properties of lanthanide dopants is investigated with europium ions used as local crystal field probes. The analysis is performed on small NaYF4 and NaGdF4 nanocrystals obtained by the thermolysis of the corresponding metal trifluoroacetates. An important role in the synthesis is played by trioctylphosphine oxide which induces the crystallization of nanocrystals with small diameters (∼5 to 6 nm). In such small particles, the energy transfer from gadolinium to europium ions is studied with photoluminescence, photoluminescence excitation and time-resolved experiments. We demonstrate that excited gadolinium ions efficiently transfer their energy to europium, and their photoluminescence spectra depend on the nanocrystal size. This is contrary to the direct excitation of Eu3+ ions, which produces size-dependent emission corresponding to the surface to volume ratio of europium sites. Finally, we propose that Gd3+ ions transfer their energy mainly to the Eu3+ in the core of the nanocrystals. These observations provide a base for the optically controlled emission from only the core of the nanocrystals.

Photoreflectance, photoluminescence, and microphotoluminescence study of optical transitions between delocalized and localized states in GaN 0.02 As 0.98 , Ga 0.95 In 0.05 N 0.02 As 0.98 , and GaN 0.02 As 0.90 Sb 0.08 layers

Abstract: Optical transitions in as-grown and annealed GaN${}_{0.02}$As${}_{0.98}$, Ga${}_{0.95}$In${}_{0.05}$N${}_{0.02}$As${}_{0.98}$, and GaN${}_{0.02}$As${}_{0.90}$Sb${}_{0.08}$ bulklike layers have been studied with photoreflectance (PR), photoluminescence (PL), and microphotoluminescence (\ensuremath{\mu}PL) in a broad range of temperatures. The exciton binding energy and the temperature dependence of the energy gap have been determined from PR measurements. In addition, the alloy-related broadening of optical transitions has been analyzed and compared for the three alloys. The largest alloy inhomogeneities have been observed for GaN${}_{0.02}$As${}_{0.90}$Sb${}_{0.08}$. In \ensuremath{\mu}PL spectra of GaN${}_{0.02}$As${}_{0.98}$ and Ga${}_{0.95}$In${}_{0.05}$N${}_{0.02}$As${}_{0.98}$ layers measured at low temperatures and low excitation conditions, sharp PL lines of 100--300-\ensuremath{\mu}eV widths have been clearly observed \ensuremath{\sim}10--20 meV below the energy gap of these alloys. Analyzing the temperature quenching of the sharp PL lines, they have been attributed to the recombination of localized excitons trapped at deep donor (acceptor)-like states. Such lines were not well resolved for GaN${}_{0.02}$As${}_{0.90}$Sb${}_{0.08}$ layers. The analysis of thermal quenching of localized exciton emission suggests that for this alloy excitons are localized mostly on alloy content fluctuations. With the increase in the excitation power an additional PL band has been observed in \ensuremath{\mu}PL spectra at the higher-energy side. This band corresponds to the optical transitions between delocalized states which are observed in photoreflectance spectra. The emission of localized excitons without resolved sharp lines has been also observed with macro-PL measurements at low temperatures. In addition, the donor trap--valence band (DT-VB) recombination has been identified for the three alloys in macro-PL spectra. It has been clearly observed that the intensity of DT-VB recombination is the weakest for GaN${}_{0.02}$As${}_{0.90}$Sb${}_{0.08}$ alloys whereas for GaN${}_{0.02}$As${}_{0.98}$ and Ga${}_{0.95}$In${}_{0.05}$N${}_{0.02}$As${}_{0.98}$ alloys it is very similar. The observed differences between the three alloys and the role of postgrowth annealing have been analyzed and discussed in the context of the influence of III-V host on alloy inhomogeneities, formation of native point defects in III-V-N alloys, and their influence on the optical quality of III-V-N alloys.

Temperature dependence of the band gap of GaSb1-xBix alloys with 0 < x <= 0.042 determined by photoreflectance

Abstract: GaSb1−xBix layers with 0 < x ≤ 0.042 have been studied by photoreflectance in 15–290 K temperature range. We found that due to the incorporation of Bi atoms into the GaSb host, the E0 band gap-related transition redshifts (∼30 meV per 1% Bi) and significantly broadens. The shift of the E0 transition in the temperature range 10–270 K has been found to be ∼70 meV, very similar to the energy shift in GaSb over the same temperature range. We analyzed the energy and broadening of the E0 transition using the Varshni and Bose-Einstein formulas and found that the Varshni and Bose-Einstein parameters of GaSb1−xBix are similar to those of GaSb. Moreover we concluded that the inhomogeneities in GaSb1−xBix alloys is less important than in dilute bismide arsenides since Bi atoms are more similar to Sb atoms (in electronegativities and ionic sizes).

On the nature of carrier relaxation and ion–ion interactions in ultrasmall β-NaYF4:Eu3+ nanocrystals – effect of the surface

Abstract: The purely hexagonal phase of ultrasmall (∼10 nm) NaYF4 nanocrystals (NCs), containing different Eu concentrations, has been obtained by a modified co-thermolysis method. Detailed investigations of the excitation and relaxation mechanisms of the Eu ions in such NCs are reported. Based on the photoluminescence excitation, absorbance, photoluminescence and emission decay times measured as a function of the excitation wavelengths, it has been shown that two Eu sites with different excitation and relaxation characteristics are present in the case of ultrasmall NaYF4 NCs. It has been shown that, when the Eu concentration increases, strong ion–ion interactions influence the relaxation phenomena in Eu ions, changing their optical properties. Moreover, these ion–ion interactions enable connections between the surface ions and the internal ones via energy transfer from the surface to the NCs core. Furthermore, it has been proposed that the different kinetic properties of the surface Eu ions are mainly caused by the formation of a charge transfer state between the ions and ligand groups attached to the NCs surface.

Band structure and the optical gain of GaInNAs/GaAs quantum wells modeled within 10-band and 8-band kp model

Abstract: The band structure and optical gain have been calculated for GaInNAs/GaAs quantum wells (QWs) with various nitrogen concentrations within the 10-band and 8-band kp models. Two approaches to calculate optical properties of GaInNAs/GaAs QWs have been compared and discussed in the context of available material parameters for dilute nitrides and the conduction band nonparabolicity due to the band anti-crossing (BAC) interaction between the N-related resonant level and the conduction band of a host material. It has been clearly shown that this nonparabolicity can be neglected in optical gain calculations since the dispersion of conduction band up to the Femi level is very close to parabolic for carrier concentrations typical for laser operation, i.e., 5 × 1018 cm−3. This means that the 8-band kp model when used to calculate the optical gain is very realistic and much easier to apply in QWs containing new dilute nitrides for which the BAC parameters are unknown. In such an approach, the energy gap and electron effective mass for N-containing materials are needed, instead of BAC parameters. These parameters are available experimentally much easier than BAC parameters.

NaYF4 nanocrystals with TOPO ligands: synthesis-dependent structural and luminescent properties

Abstract: A comprehensive characterization of NaYF4 nanocrystals synthesized in trioctylphosphine oxide has been reported in order to present an effective method of monodisperse, small, hexagonal nanocrystal synthesis in a high boiling organic solvent via a co-thermolysis pathway. We observed the influence of temperature, Na/Y precursors ratio and time of the synthesis on the nanocrystals size, shape and crystal structure. For that purpose, we characterized the structure of as-synthesized nanocrystals by X-ray diffraction and transmission electron microscopy. Moreover, all nanocrystals were doped with Eu3+ ions, which were used as an optical crystal field probe. We applied photoluminescence, PL excitation and absorbance spectra to determine the influence of crystal symmetry, surface to volume ratio and ligands on the optical properties of doped Eu3+ ions. It was found that trioctylphosphine oxide reduces the free-energy barrier and stimulates the NaYF4 crystallization in the hexagonal phase, even at relatively low temperature. A similar effect was observed when the excess of sodium trifluoroacetate precursors was used. Moreover, the presented nanocrystal evolution within synthesis time confirmed that at suitable conditions NaYF4 crystallized in the hexagonal phase within less than 5 min. Optical spectroscopy investigations confirmed the high quality of small β-NaYF4:Eu3+ nanocrystals, which are promising candidates for e.g. optical markers in the visible wavelength range.

Exciton and biexciton dynamics in single self-assembled InAs/InGaAlAs/InP quantum dash emitting near 1.55 μm

Abstract: Exciton and biexciton dynamics in a single self-assembled InAs/In0.53Ga0.23Al0.24As/InP(001) quantum dash emitting near 1.55 μm has been investigated by micro-photoluminescence and time-resolved micro-photoluminescence at T = 4.2 K. The exciton and biexciton fine structure splitting of ∼60 μeV, the biexciton binding energy of ∼3.5 meV, and the characteristic exciton and biexciton decay times of 2.0 ± 0.1 ns and 1.1 ± 0.1 ns, respectively, have been determined. The measurement of the biexciton and exciton cross-correlation statistics of the photon emission confirmed the cascaded relaxation process. The exciton-to-biexciton decay time ratio and a small fine structure splitting suggest carrier localization within the investigated quantum dash.

Electronic structure, morphology and emission polarization of enhanced symmetry InAs quantum-dot-like structures grown on InP substrates by molecular beam epitaxy

Abstract: The optical and structural properties of a new kind of InAs/InGaAlAs/InP quantum dot (QD)-like objects grown by molecular beam epitaxy have been investigated. These nanostructures were found to have significantly more symmetrical shapes compared to the commonly obtained dash-like geometries typical of this material system. The enhanced symmetry has been achieved due to the use of an As2 source and the consequent shorter migration length of the indium atoms. Structural studies based on a combination of scanning transmission electron microscopy (STEM) and atom probe tomography (APT) provided detailed information on both the structure and composition distribution within an individual nanostructure. However, it was not possible to determine the lateral aspect ratio from STEM or APT. To verify the in-plane geometry, electronic structure calculations, including the energy levels and transition oscillator strength for the QDs have been performed using an eight-band k·p model and realistic system parameters. The results of calculations were compared to measured polarization-resolved photoluminescence data. On the basis of measured degree of linear polarization of the surface emission, the in-plane shape of the QDs has been assessed proving a substantial increase in lateral symmetry. This results in quantum-dot rather than quantum-dash like properties, consistent with expectations based on the growth conditions and the structural data.

Carrier relaxation dynamics in InAs/GaInAsP/InP(001) quantum dashes emitting near 1.55 μm

Abstract: Carrier relaxation dynamics in self-assembled InAs/Ga0.2In0.8As0.4P0.6/InP(001) quantum dashes (QDashes) has been studied by degenerated time-resolved pump-probe and non-resonant photoluminescence spectroscopy at low temperature. These experiments yielded information about the exciton ground-state recombination lifetime reaching ∼1.75 ns. Additionally, the excited-to-ground state relaxation time has been measured to increase significantly from ∼40 ps up to ∼250 ps with the QDash size. Slow down of the intra-band relaxation time is interpreted in terms of the internal dash-to-dash energy-band-structure variation. The carrier dynamics in this case is most likely governed by the acoustic-phonon-mediated scattering processes.

Correlation between matrix structural order and compressive stress exerted on silicon nanocrystals embedded in silicon-rich silicon oxide

Abstract: Silicon nanocrystals embedded in a silicon oxide matrix were deposited by radio frequency reactive magnetron sputtering. By means of Raman spectroscopy, we have found that a compressive stress is exerted on the silicon nanocrystal cores. The stress varies as a function of silicon concentration in the silicon-rich silicon oxide layers varies, which can be attributed to changes of nanocrystal environment. By conducting the Fourier transform infrared absorption experiments, we have correlated the stresses exerted on the nanocrystal core to the degree of matrix structural order.

On the origin of emission and thermal quenching of SRSO:Er3+ films grown by ECR-PECVD

Abstract: Silicon nanocrystals embedded in a silicon-rich silicon oxide matrix doped with Er3+ ions have been fabricated by electron cyclotron resonance plasma-enhanced chemical vapor deposition. Indirect excitation of erbium photoluminescence via silicon nanocrystals has been investigated. Temperature quenching of the photoluminescence originating from the silicon nanocrystals and the erbium ions has been observed. Activation energies of the thermally activated quenching process were estimated for different excitation wavelengths. The temperature quenching mechanism of the emission is discussed. Also, the origin of visible emission and kinetic properties of Er-related emission have been discussed in details.

Impact of wetting-layer density of states on the carrier relaxation process in low indium content self-assembled (In,Ga)As/GaAs quantum dots

Abstract: Carrier relaxation processes have been studied in low indium content self-assembled (In,Ga)As/GaAs quantum dots (QDs). Temperature-dependent photoluminescence of the wetting layer (WL) and QDs, and QD photoluminescence rise time elongation from $\ensuremath{\sim}$100 to $\ensuremath{\sim}$200 ps in the range of 10--45 K, indicated a complex carrier relaxation scheme. It involves localization of carriers/excitons in the WL, their temperature-mediated release, and subsequent transfer between the states of the WL and QD ensemble. These observations are explained by a thermal hopping model, in which electron-hole pairs are redistributed within two separate sets of zero-dimensional states of considerably different densities connected by a two-dimensional mobility channel.

Temperature dependence of photoluminescence from InNAsSb layers: The role of localized and free carrier emission in determination of temperature dependence of energy gap

Abstract: The temperature dependence of energy gap-related emission from InNAsSb layers was studied by Fourier transform infrared photoluminescence (PL) spectroscopy. The shape of PL peak was analyzed using a theoretical expression, which takes into account both the localized and free carrier emission. Proper accounting for those two effects is very important for an accurate determination of the Varshni and Bose-Einstein parameters from PL data. It is shown that nitrogen incorporation has a very week effect on the temperature induced bandgap reduction in InNAsSb alloys and that the Varshni and Bose-Einstein parameters are very close to those observed in InAs and InSb.

Contactless electroreflectance studies of surface potential barrier for N- and Ga-face epilayers grown by molecular beam epitaxy

Abstract: Two series of N- and Ga-face GaN Van Hoof structures were grown by plasma-assisted molecular beam epitaxy to study the surface potential barrier by contactless electroreflectance (CER) A clear CER resonance followed by strong Franz-Keldysh oscillation of period varying with the thickness of undoped GaN layer was observed for these structures This period was much shorter for N-polar structures that means smaller surface potential barrier in these structures than in Ga-polar structures From the analysis of built-in electric field it was determined that the Fermi-level is located 027 ± 005 and 060 ± 005 eV below the conduction band for N- and Ga-face GaN surface, respectively

Effect of arsenic on the optical properties of GaSb-based type II quantum wells with quaternary GaInAsSb layers

Abstract: Optical properties of molecular beam epitaxially grown type II “W” shaped GaSb/AlSb/InAs/GaIn(As)Sb/InAs/AlSb/GaSb quantum wells (QWs) designed for the active region of interband cascade lasers have been investigated. Temperature dependence of Fourier-transformed photoluminescence and photoreflectance was employed to probe the effects of addition of arsenic into the original ternary valence band well of GaInSb. It is revealed that adding arsenic provides an additional degree of freedom in terms of band alignment and strain tailoring and allows enhancing the oscillator strength of the active type II transition. On the other hand, however, arsenic incorporation apparently also affects the structural and optical material quality via generating carrier trapping states at the interfaces, which can deteriorate the radiative efficiency. These have been evidenced in several spectroscopic features and are also confirmed by cross-sectional transmission electron microscopy images. While arsenic incorporation into type II QWs is a powerful heterostructure engineering tool for optoelectronic devices, a compromise has to be found between ideal band structure properties and high quality morphological properties.

Contactless electroreflectance studies of free exciton binding energy in Zn1-xMgxO epilayers

Abstract: Contactless electroreflectance (CER) has been applied to study optical transitions in Zn1-xMgxO layers with magnesium concentration ≤44%. CER resonances related to free exciton and band-to-band transitions were clearly observed at room temperature. For ZnO the two transitions are separated by the energy of ∼65 meV, which is attributed to the free exciton binding energy in ZnO. Due to magnesium incorporation, the CER resonances broaden and shift to blue. The energy separation between excitonic and band-to-band transitions increases up to ∼100 meV when the magnesium concentration reaches 22%. For larger magnesium concentrations, CER resonances are significantly broadened and the excitonic transition is no longer resolved in the CER spectrum.

Verification of band offsets and electron effective masses in GaAsN/GaAs quantum wells: Spectroscopic experiment versus 10-band k·p modeling

Abstract: Optical transitions in GaAs1−xNx/GaAs quantum wells (QWs) have been probed by two complementary techniques, modulation spectroscopy in a form of photoreflectance and surface photovoltage spectroscopy. Transition energies in QWs of various widths and N contents have been compared with the results of band structure calculations based on the 10-band k·p Hamiltonian. Due to the observation of higher order transitions in the measured spectra, the band gap discontinuities at the GaAsN/GaAs interface and the electron effective masses could be determined, both treated as semi-free parameters to get the best matching between the theoretical and experimental energies. We have obtained the chemical conduction band offset values of 86% for x = 1.2% and 83% for x = 2.2%, respectively. For these determined band offsets, the electron effective masses equal to about 0.09 mo in QWs with 1.2% N and 0.15 mo for the case of larger N content of 2.2%.

Surface properties of c-plane GaN grown by plasma-assisted molecular beam epitaxy

Abstract: Two series of GaN van Hoof structures with different thicknesses of an undoped GaN cap layer were grown under metal-rich conditions by plasma-assisted molecular beam epitaxy. These were then investigated by contactless electroreflectance (CER) to study the Fermi-level position of the (0001) GaN surface after growth as well as after chemical treatment using Piranha solution. The first and second series of samples were grown on GaN/sapphire templates and high-pressure bulk GaN crystals, respectively. A clear CER resonance followed by Franz–Keldysh oscillations (FKOs) of various periods was clearly observed for both sample series before and after chemical treatment. The Fermi-level position of the GaN surface was determined from the analysis of FKOs related to the built-in electric field in the undoped GaN layer. For the as-grown GaN surface, the Fermi level was found to be located 0.42 and 0.57 eV below the conduction band in samples grown on GaN/sapphire templates and high-pressure bulk GaN crystals, respectively. For the Piranha-etched GaN surfaces, the Fermi level was pinned at almost the same energy (0.49 and 0.48 eV) in each of the two sets of samples. This means that this cleaning procedure, which is commonly used before device processing, is able to saturate the surface states at a certain level of Fermi-level pinning.

Influence of AlN layer on electric field distribution in GaN/AlGaN/GaN transistor heterostructures

Abstract: Distribution of built-in electric field in GaN/AlGaN/GaN transistor heterostructures without and with AlN layer was studied theoretically (solving the Schrodinger and Poisson equation) and experimentally (measuring contactless electroreflectance (CER) spectra and analyzing the AlGaN-related Franz-Keldysh oscillation). It is shown that the AlN layer changes very strongly the distribution of electric field in such heterostructures. This change can be very well predicted if the surface boundary conditions for self-consistent Schrodinger-Poisson calculations in GaN/AlGaN/GaN heterostructures are known. These conditions can be determined/verified by CER measurements of AlGaN-related Franz-Keldysh oscillation, which depends on the built-in electric field in AlGaN layer.

Formation and optical properties of semiconducting thick Ca silicide films and Si/CaxSi/Si heterostructures on Si(111) substrate

Abstract: Si/Ca2Si/Si(111) and Si/Ca3Si4/Si(111) heterostructures have been formed by procedure of layer-by-layer deposition of Ca at 130 °C or 500 °C on previously formed layer of amorphous Si or on a polycrystalline Si layers. Features of electronic structure, optical and electrical properties were established by in situ and ex situ methods. Strong direct interband transitions at 0.89 and 0.912 eV were firstly found for Ca3Si4 film grown at 500 °C. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Ion?ion interaction in two-dimensional nanoporous alumina filled with cubic YAlO3?:?Tb3+ matrix

Abstract: Terbium doped YAlO3 composites, with terbium concentrations up to 2.11 at%, were fabricated by co-precipitation in porous anodic alumina films grown on silicon. The presence of the cubic YAlO3 phase was confirmed by x-ray diffraction and FT-IR analysis. The fabricated samples demonstrate photoluminescence (PL) within the range 350?640?nm, which is associated with f?f transitions from the 5D3, 5G6 and 5D4 levels of Tb3+ ion. Additionally, a broad and fast- decaying PL band in the blue range has been observed, which is associated with defect states. Based on PL excitation spectra, the main excitation channel of Tb3+ ions is due to 4f?5d transitions. Excitation bands observed at 235, 270 and 320?nm have been related to the permitted low-spin 5d2 [LS], 5d1 [LS] and forbidden high-spin 5d1 [HS] states, respectively. The PL decay spectra have been measured, with the results analysed using the maximum entropy method; a strong indication of ion?ion interaction has been observed. The distribution of Tb3+ ions in the PAA?:?YAlO3 structure has been proposed and its influence on optical properties of Tb3+ ions has been discussed.

Enhancement of activation energies of sharp photoluminescence lines for GaInNAs quantum wells due to quantum confinement

Abstract: It is shown that localized emission from GaInNAs quantum wells (QW) is composed of sharp photoluminescence (PL) lines. Spectral position of these PL lines varies in a very broad range (∼150 meV) but the activation energy of each line is the same within the experimental uncertainty and equals ∼11 meV. This value is higher than in bulk GaInNAs (∼6 meV) and corresponds very well to electron–hole attraction in GaInNAs QW. It means that the source of these sharp PL lines are excitons localized on deep centres, which can recombine radiatively while the thermal energy is smaller than the Coulomb attraction between electrons and holes. Because of this the localized emission composing of sharp PL lines is observed at low temperatures and quenched much faster for GaInNAs layers than GaInNAs/GaAs QWs.

Theoretical Studies of the Influence of Temperature on Photoluminescence Dynamics in GaInNAs/GaAs Quantum Wells

Abstract: Dynamics of excitons in GaInNAs/GaAs quantum wells (QWs) is studied theoretically within a model of hopping excitons. In this model the temporal evolution of photoluminescence (PL) is described by the system of rate equations which takes into account hopping of excitons between randomly generated localizing states. In this work we study the influence of temperature on such characteristic features of PL as the decay and rise time of PL signal as well as their spectral dependences. It is clearly shown that our model reproduces experimental data very well.

Temperature dependent surface photovoltage spectra of type I GaAs1−xSbx/GaAs multiple quantum well structures

Abstract: We present temperature dependent surface photovoltage spectra of GaAs1−xSbx/GaAs multiple quantum well structures. Our previous studies [Sitarek et al., J. Appl. Phys. 105, 123523 (2009)] have identified all features present in the surface photovoltage spectra and showed weak type-I band alignment in the investigated GaAsSb/GaAs system. By analyzing the changes in the relative intensity of features near the energy of fundamental transitions caused by the temperature variations, we are able to determine the energy difference between electronic states localized in GaAsSb quantum well and conduction band edge in the GaAs barrier. In addition, the Bose-Einstein parameters that describe the temperature dependences of 1hh-1e transitions are evaluated and discussed.

Enhancement of photoluminescence from GaInNAsSb quantum wells upon annealing: improvement of material quality and carrier collection by the quantum well.

Abstract: In this study we apply time resolved photoluminescence and contactless electroreflectance to study the carrier collection efficiency of a GaInNAsSb/GaAs quantum well (QW). We show that the enhancement of photoluminescence from GaInNAsSb quantum wells annealed at different temperatures originates not only from (i) the improvement of the optical quality of the GaInNAsSb material (i.e., removal of point defects, which are the source of nonradiative recombination) but it is also affected by (ii) the improvement of carrier collection by the QW region. The total PL efficiency is the product of these two factors, for which the optimal annealing temperatures are found to be ~700 °C and ~760 °C, respectively, whereas the optimal annealing temperature for the integrated PL intensity is found to be between the two temperatures and equals ~720 °C. We connect the variation of the carrier collection efficiency with the modification of the band bending conditions in the investigated structure due to the Fermi level shift in the GaInNAsSb layer after annealing.

GaAs-Based Quantum Well Exciton-Polaritons beyond 1 μm

Abstract: GaAs-Based Quantum Well Exciton-Polaritons beyond 1 μm M. Pieczarka, P. Podemski, A. Musiaa, K. Ryczko, G. S¦k, J. Misiewicz, F. Langer, S. Hofling, M. Kamp and A. Forchel Institute of Physics, Wrocaaw University of Technology, Wybrze»e Wyspia«skiego 27, 50-370 Wrocaaw, Poland Technische Physik, University of Wurzburg and Wilhelm-Conrad-Rontgen-Research Center for Complex Material Systems (RCCM), Am Hubland, D-97074 Wurzburg, Germany

Lateral Coupling within the Ensemble of InAs/InGaAlAs/InP Quantum Dashes

Abstract: In this work we investigate the electronic structure of coupled quantum dashes. The respective con ned state energy levels are calculated for various cross-sectional shapes and sizes of the dashes and in function of the lateral distance between them. The results are confronted with the existing experimental data on the optical transitions in such structures. It has been concluded that for realistic system parameters (geometry and spatial in-plane separation) the obtained direct coupling is weak and in most of the applications the dashes can be considered individually, as long as the ensemble is strongly inhomegeneous.

Recent advances in GaSb-based structures for mid-infrared emitting lasers: spectroscopic study

Abstract: There are reviewed the optical properties of two kind of active regions of mid infrared laser devices both grown on GaSb substrates: GaInAsSb/AlGaInAsSb type I QWs for laser diodes and InAs/GaInAsSb type II QWs for interband cascade lasers. There are presented their crucial optical properties and the related current challenges with respect to the device performances. This covers such issues as spectral tenability of the emission via the structure parameters, the band gap discontinuities, carrier loss mechanisms and oscillator strengths. For that, spectroscopic techniques have been used (photoluminescence and its temperature dependence, and photoreflectance) and combined with the energy level calculations based on effective mass approximation and kp theory. Eventually, the potential for further material optimization and prospects for the improved device performances are also discussed. Keywords: GaSb-based structures, quantum well, type II quantum well, mid infrared, semiconductor laser, gas sensing, interband cascade laser