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We report two GaInNAs/GaAs semiconductor disk lasers emitting at the wavelengths of 1180 nm and 1220 nm. The lasers generated 5 W and 7 W output powers, respectively, at a mount temperature of 15 deg. C. Both the gain mirrors were grown by molecular beam epitaxy and consisted of a GaAs/AlAs distributed Bragg reflector and an active region with 10 GaInNAs/GaNAs/GaAs QWs.
We have examined the responses of GaAs, InP, InAs, and AlAs to 30 keV focused ion beam (FIB) irradiation and applied a unified model that consistently explains the observed effects. Nanodots were observed to form on GaAs, InP, and InAs under irradiation at normal incidence, while nanodots are not observed on AlAs. The FIB response and nanodot formation behavior of each material is discussed with regard to a few basic material properties and a model for nanodot creation and growth by the action of preferential sputtering and Ostwald ripening. The model predicts the development of a stable average nanodot size with increasing ion dose, with the average nanodot size depending on the excess group III adatom yield, adatom surface diffusion rate, and surface tension. These predictions qualitatively agree with the experimentally observed trends for GaAs and InP. They also agree for the initial nanodot formation on InAs, but this material system exhibits a sudden transition in the nanodot size distribution. The model predicts that nanodots will have difficulty forming and growing on AlAs, which is also in agreement with our experimental results.
We report on Raman scattering experiments on InAs/Al{sub x}Ga{sub 1-x}As quantum dot heterostructures with 0{<=}x{<=}0.6. The samples were prepared by using molecular beam epitaxy (MBE) and atomic layer MBE for the growth of different layers. For x>0, we detected several lines originating from the Al{sub x}Ga{sub 1-x}As alloy. These can be related to scattering from GaAs-like and AlAs-like phonons with q congruent with 0, and weaker scattering from disorder-activated phonons with q{ne}0. In particular, we identified a line at {approx}250 cm{sup -1} as due to disorder-activated longitudinal optical phonons in the alloy. This conclusion is different than the attribution of this line to scattering from dots and, consequently, we do not recognize the possibility of deriving any information about the actual composition of the dots from an analysis of this line as proposed by other authors.
We report weak antilocalization (WAL) measurements in a nearly symmetric In{sub 0.53}Ga{sub 0.47}As/In{sub 0.52}Al{sub 0.48}As quantum well. In this system, an enhancement of the spin-orbit scattering time, equivalently the weakening of the WAL effect, is expected at carrier densities N{sub s} where equal magnitudes between the Rashba and Dresselhaus parameters ({alpha} = {+-}{beta}) are realized. Our magnetotransport measurements indeed exhibited a weakening of the WAL effect as a function of N{sub s}, though the weakening took place at only one carrier density. Our interpretation is that the value of {beta} is so small that the two carrier densities corresponding to {alpha} = {beta} and {alpha} = -{beta} are very close to each other. It turned out that the deduced values for the bulk Dresselhaus parameter {gamma} in In{sub 0.53}Ga{sub 0.47}As/In{sub 0.52}Al{sub 0.48}As quantum wells is considerably smaller than the k{center_dot}p value of 27 eVA{sup 3}.
We investigated double-doped InGaAs/InAlAs heterostructures with high indium compositions. The heterostructures were grown by molecular beam epitaxy on GaAs(001) with metamorphic step-graded buffer layers. The magneto-transport characterization was performed by using Hall-bar devices. We observed non-monotonic magneto-resistance oscillations, which indicate a single two-dimensional electron gas (2DEG) with multiple-subband occupation or a 2DEG bilayer. We also observed weak-antilocalization in all samples, which is an evidence of spin-orbit coupling.
A study of lasers with two different heterostructures has revealed that the polarisation state of laser radiation depends strongly on the specific sample at pump currents corresponding to the regular behaviour of the light-current characteristic. Thus, the deviation of the radiation polarisation from linear can be used as a sensitive characteristic of the laser quality. It is found that anomalies in the power-current characteristic (the so-called kinks) are accompanied by a significant decrease in the degree of polarisation. The measured degrees of polarisation of spontaneous emission from samples (at pump currents much below the threshold) are in good agreement with the theoretical estimate obtained within the three-band model of optical transitions. (lasers)
The influence of the amplified luminescence (AL) and spreading of nonequilibrium charge carriers on the threshold, dynamic, and power characteristics of high-power InGaAs/AlGaAs laser diode arrays (LDAs) is studied. It is found that, depending on the near-field fill factor, the contribution of AL-induced recombination to the lasing threshold of LDAs may reach 11%. It is shown that the losses of the LDA pump energy, associated with the AL, increase with the injection current growth above its threshold value because of the increase in the intensity of radiation, propagating normally to the LDA cavity axis. (lasers)
Light absorption by GaAs/AlAs heterostructures with a layer of self-assembled InAs quantum dots (QDs) at resonant tunneling through an energy-selected QD has been investigated. A high sensitivity of the current through this selected tunneling channel to the absorption of single photons with a wavelength {lambda} {<=} 860 nm up to a temperature of 50 K is demonstrated; this sensitivity is caused by the Coulomb effect of the photoexcited holes captured by surrounding QDs on the resonance conditions. It is shown that single-photon absorption can discretely change the current through the system under study by a factor of more than 50. The captured-hole lifetimes have been measured, and a model has been developed to qualitatively describe the experimental data. It is also demonstrated that the InAs monolayer can effectively absorb photons. The properties of the heterostructure studied can be used not only to detect photons but also to design logical valves and optical memory devices.
The results of studies of the characteristics of contacts with Schottky barriers on a pseudomorphic AlGaAs/InGaAs heterostructure are presented. The Ti-based Schottky diodes were created by a standard pHEMT technology. It is shown that the parameters of the fabricated contacts with Schottky barriers are close to the characteristics of ideal contacts based on aluminum or molybdenum created in the process of molecular beam epitaxy. It is also shown that as compared to the diodes based on AlGaAs homostructures, the measured Schottky barrier height is smaller and the ideality factor is higher in diodes created on pseudomorphic heterostructures. Along with the well-known methods for determining the Schottky-barrier height, we tested a technique based on the measurement of the threshold voltage in diodes with a two-dimensional electron gas.
Structural and magnetic properties of GaAs thin films with embedded nanoclusters were investigated as a function of the annealing temperature and Mn content. Surprisingly, the presence of two kinds of nanoclusters with different structure was detected in most of the samples independently of the thermal processing or Mn content. This proved that the presence of a given type of clusters cannot be assumed a priori as is reported in many papers. The fraction of Mn atoms in each kind of cluster was estimated from the extended X-ray absorption fine structure analysis. This analysis ruled out the possibility of the existence of nanoclusters containing a hypothetic MnAs cubic compound-only (Ga,Mn)As cubic and hexagonal MnAs clusters were detected. Moreover the bimodal distribution of Mn magnetic moments was found, which scales with the estimated fraction of Mn atoms in the cubic and hexagonal clusters. - Graphical abstract: After thermal processing of GaMnAs layers the clear evidence of formation of clusters with two structures is provided, which scales with the Mn magnetic moment. The structure of clusters cannot be a priori assumed as it is commonly done. Highlights: > Granular GaMnAs layers in the function of Mn content and thermal processing were studied. > Five independent methods provided clear evidence of two types of cluster formation. > The magnetic moment of clusters scale with fraction of Mn in given type of clusters. > Formed in all samples cubic clusters consist of GaMnAs not cubic MnAs. > Commonly used processing does not provide clusters with one crystalline structure.
In this work, we present for the first time the spectroscopic properties of perovskite nanocrystals CaTiO{sub 3}: Nd{sup 3+}, measured at room and liquid nitrogen temperature. Samples were prepared by the sol-gel method and annealed at 700 and 1000 deg. C. The concentration of Nd{sup 3+} ranged from 0.5% to 5%. Average nanocrystallite size primarily depends on the annealed temperature and is about 25 and 50 nm for 700 and 1000 deg. C, respectively. The absorption, emission and excitation spectra (monitored at 1078 nm) as well as the decay time profile of the emission from the {sup 4}F{sub 3/2} energy level of Nd{sup 3+} are obtained. The increasing amount of Nd{sup 3+} ions decreases the lifetime of the {sup 4}F{sub 3/2} level and that changes from 146 {mu}s for 0.5% to 50 {mu}s for 5% of Nd{sup 3+} concentration. The strongest emission was observed at two regions: 1050-1120 nm and 865-930 nm and was assigned to the {sup 4}F{sub 3/2}{yields}{sup 4}I{sub 11/2} and {sup 4}F{sub 3/2}{yields}{sup 4}I{sub 9/2} transitions, respectively. The spectroscopic quality parameter of neodymium in the CaTiO{sub 3} lattice has been calculated from the emission spectra. The influence of luminescent properties depending on the annealing temperature and concentration of neodymium ions is discussed. - Graphical abstract: The 300 K emission spectrum of CaTiO{sub 3}: 1% Nd{sup 3+} annealed at 700 deg. C and measured at 300 K on InGaAs detector. Highlights: > Nano-CaTiO{sub 3}: Nd{sup 3+} phosphor has been prepared by using the sol-gel method. > The spectroscopic quality parameter in CaTiO{sub 3} depends on Nd{sup 3+} concentration. > Neodymium strongly influence the structural and spectroscopic properties of CaTiO{sub 3}.
There are many difficulties limiting the further development of monolithic multi-junction solar cells, such as the growth of lattice-mismatched material and the current matching constraint. As an alternative approach, the light-splitting photovoltaic system is investigated intensively in different aspects, including the energy loss mechanism and the choice of energy bandgaps of solar cells. Based on the investigation, a two-dual junction system has been implemented employing lattice-matched GaInP/GaAs and InGaAsP/InGaAs cells grown epitaxially on GaAs and InP substrates, respectively. (author)
This work proposes a new optoelectronic measurement of quantum well (QW) thickness and applies it to doped and undoped In{sub 0.53}Ga{sub 0.47}As/In{sub 0.52}Al{sub 0.48}As multiple-QW structures. Near-infrared spectroscopic identification of the interband optical transition at 100-300 K gave the eigenenergies of the conduction band in the QW. Evaluation of the QW thickness involved analysis of the effective mass at the corresponding eigenenergy. QW thicknesses in the range of 5.45-20.8 nm were determined in six different wafers. These thicknesses agreed well with the QW thicknesses estimated by double-crystal x-ray diffraction within almost two monolayers. This measurement was used to determine the distance of potential boundaries confining the electron wave functions.
Real-time three-dimensional reciprocal space mapping (3D-RSM) measurement during In{sub 0.12}Ga{sub 0.88}As/GaAs(001) molecular beam epitaxial growth has been performed to investigate anisotropy in relaxation processes along [110] and [110] directions caused by alpha and beta misfit dislocations (MDs). Anisotropies, strain relaxation, and crystal quality in both directions were simultaneously evaluated via the position and broadness of 022 diffraction in 3D-RSM. In the small-thickness region, strain relaxation caused by alpha-MDs is higher than that caused by beta-MDs, and therefore crystal quality along [110] is worse than that along [110]. Rapid relaxation along both [110] and [110] directions occurs at almost the same thickness. After rapid relaxation, anisotropy in strain relaxation gradually decreases, whereas crystal quality along [110] direction, presumably due to beta-MDs, becomes better that along [110] direction and the ratio does not decay with thickness.
Composition modulations are observed by transmission electron microscopy in In{sub 0.53}Ga{sub 0.37}Al{sub 0.10}As barrier layers that overgrow both single- and multilayer InAs quantum wire structures grown on an InP substrate. Indium-rich (gallium-deficient) regions were observed in the region of the barrier layer lying directly above individual quantum wires, while indium-deficient (gallium-rich) regions were detected in the barrier above the gaps between adjacent underlying quantum wires. The magnitude of such modulation was typically 7% (atomic percent) for both indium and gallium as estimated from the energy dispersive x-ray analysis. The origin of such composition modulations was determined by modeling the chemical potential distribution for indium and gallium on the growth front of the barrier layer at the initial capping stage of the quantum wires with finite element simulations. It is found that the number and positions of the indium-rich regions are determined by the combined effects of strain and surface energy distributions on the barrier material capping the quantum wires. Moreover the estimated magnitudes of the composition modulation for both indium and gallium from the finite element models are in good agreement with the experimental observations. This method provides a simple way to understand the origin of, and to estimate the magnitude of the quantum wire-induced composition modulation in the barrier layer.
A giant magnetocaloric effect was found in series of Mn{sub 1-x}Co{sub x}As films epitaxied on GaAs (001). The maximum magnetic entropy change caused by a magnetic field of 4 T is as large as 25 J/kg K around room temperature, which is about twice the value of pure MnAs film. The observed small thermal hysteresis is more suitable for practical application. Growing of layered Mn{sub 1-x}Co{sub x}As films with Co concentration changing gradually may draw layered active magnetic regenerator refrigerators closer to practical application. Our experimental result may provide the possibility for the combination of magnetocaloric effect and microelectronic circuitry.
The authors report the coupling of single InGaAs quantum dots (QDs) to the surface plasmon resonance of a metal nanocrystal. Clear enhancement of the photoluminescence (PL) in the spectral region of the surface plasmon resonance is observed which splits up into distinct emission lines from single QDs in micro-PL. The hybrid metal-semiconductor structure is grown by molecular beam epitaxy on GaAs (100) utilizing the concept of self-organized anisotropic strain engineering for realizing ordered arrays with nanometer-scale precise positioning of the metal nanocrystals with respect to the QDs.
In this work, we present a study of InAs quantum dots deposited on InGaAs metamorphic buffers by molecular beam epitaxy. By comparing morphological, structural, and optical properties of such nanostructures with those of InAs/GaAs quantum dot ones, we were able to evidence characteristics that are typical of metamorphic InAs/InGaAs structures. The more relevant are: the cross-hatched InGaAs surface overgrown by dots, the change in critical coverages for island nucleation and ripening, the nucleation of new defects in the capping layers, and the redshift in the emission energy. The discussion on experimental results allowed us to conclude that metamorphic InAs/InGaAs quantum dots are rather different nanostructures, where attention must be put to some issues not present in InAs/GaAs structures, namely, buffer-related defects, surface morphology, different dislocation mobility, and stacking fault energies. On the other hand, we show that metamorphic quantum dot nanostructures can provide new possibilities of tailoring various properties, such as dot positioning and emission energy, that could be very useful for innovative dot-based devices.