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The long-wave infrared (LWIR) interband cascade detector with type-II superlattices (T2SLs) and a gallium-free ("Ga-free") InAs/InAsSb (x = 0.39) absorber was characterized by photoluminescence (PL) and spectral response (SR) methods. Heterostructures were grown by molecular beam epitaxy (MBE) on a GaAs substrate (001) orientation. The crystallographic quality was confirmed by high-resolution X-Ray diffraction (HRXRD). Two independent methods, combined with theoretical calculations, were able to determine the transitions between the superlattice minibands. Moreover, transitions from the trap states were determined. Studies of the PL intensity as a function of the excitation laser power allowed the identification of optical transitions. The determined effective energy gap (Eg) of the tested absorber layer was 116 meV at 300 K. The transition from the first light hole miniband to the first electron miniband was red-shifted by 76 meV. The detected defects' energy states were constant versus temperature. Their values were 85 meV and 112 meV, respectively. Moreover, two additional transitions from acceptor levels in cryogenic temperature were determined by being shifted from blue to Eg by 6 meV and 16 meV, respectively. [ABSTRACT FROM AUTHOR]

InSb self‐assembled quantum dots (QDs) have the possibility to increase the luminescence intensity of light‐emitting diodes operating in the midinfrared region. As a first step toward the midinfrared luminescence, the single‐layer InSb QDs are grown on GaSb (100) substrates by molecular beam epitaxy and the effects of V/III ratio and InSb coverage on size and density of InSb QDs and photoluminescence (PL) characteristics are investigated. As the V/III ratio increases, the distribution of QD height broadens and goes from monomodal to multimodal. The increase in the number of large QDs and their multimodal distribution are thought to be due to the ripening process. The PL intensity is maximum at a V/III ratio of 2 with PL wavelength of 1.76 μm. As the InSb coverage increases at a V/III ratio of 2, QD size and density increase while maintaining a monomodal distribution in QD size. However, the PL peak intensity decreases with increasing InSb coverage due to the increase of nonradiative QDs. The highest PL peak intensity at an InSb coverage of 2.0 monolayer and a V/III ratio of 2 with PL wavelength of 1.69 μm are obtained. The present single‐layer InSb QD shows the emission in the near‐infrared region. [ABSTRACT FROM AUTHOR]

The results of photoluminescence (PL) study of As-doped Cd0.3Hg0.7Te solid solutions layers grown by molecular beam epitaxy on a Si substrate are presented. Analysis of the PL spectra obtained at different temperatures and excitation laser powers allows one to judge the nature of the observed peaks. The activation of arsenic in annealed samples was established, as a result of which small acceptor levels are formed. The effectiveness of arsenic as an acceptor impurity for cadmium-mercury tellurides has been confirmed. [ABSTRACT FROM AUTHOR]

In this paper, the authors report strain-balanced M-structures InAs/GaSb/AlSb/GaSb superlattice growth on GaSb substrates using two kinds of interfaces (IFs): GaAs-like IFs and InSb-like IFs. The in-plane compressive strain of 60-period and 100-period InAs/GaSb/AlSb/GaSb with different InAs (1) and AlSb (1) monolayers are investigated. The M-structures InAs/GaSb/AlSb/GaSb represent type II superlattices (T2SL) and at present are under intensive investigation. Many authors showtheoretical and experimental results that such structures can be used as a barrier material for a T2SL InAs/GaSb absorber tuned for long-wave infrared detectors (8 µm-14 µm). Beside that, M-structure can also be used as an active material for short-wave infrared detectors to replace InAs/GaSb which, for this region of infrared, are a big challenge from the point of view of balancing compression stress. The study of InAs/GaSb/AlSb/GaSb superlattice with the minimal strain for GaSb substrate can be obtained by a special procedure of molecular beam epitaxy growth through special shutters sequence to form both IFs. The authors were able to achieve smaller minimal mismatches of the lattice constants compared to literature. The high-resolution X-ray diffraction measurements prove that two types of IFs are proper for balancing the strain in such structures. Additionally, the results of Raman spectroscopy, surface analyses of atomic force microscopy, and differential interference contrast microscopy are also presented. The numerical calculations presented in this paper prove that the presence of IFs significantly changes the energy gap in the case of the investigated M-structures. The theoretical results obtained for one of the investigated structures, for a specially designed structure reveal an extra energy level inside the energy gap. Moreover, photoluminescence results obtained for this structure prove the good quality of the synthesized M-structures, as well as are in a good agreement with theoretical calculations. [ABSTRACT FROM AUTHOR]

In this paper, the features of the formation of bulk InGaN layers with an indium content of ~ 60% in the immiscibility gap of InGaN ternary solid solutions by the method of molecular-beam epitaxy with plasma nitrogen activation are studied. The structures under study were grown on sapphire substrates, while the epitaxy temperature and the ratio of metal (In + Ga) and activated (atomic) nitrogen fluxes were varied. It has been demonstrated that the rates of thermal decomposition and phase separation for In0.6Ga0.4N ternary solutions depend nonmonotonically on the growth temperature in the range Tgr = 430–470°C. It is shown that InGaN thermal decomposition processes occur on the growth surface and lead to the appearance of surface phases of metallic In and binary InN, while phase separation leads to the appearance of InGaN phases of various compositions throughout the volume of the deposited InGaN layer. It is shown that, in the temperature range under study, phase separation is determined by surface diffusion, which can be suppressed by growth under highly nitrogen-enriched conditions, which made it possible to obtain homogeneous InGaN layers with an In content of In ~ 60% during high-temperature (Tgr = 470°C) growth. It is shown that the suppression of InGaN thermal decomposition processes is decisive in achieving effective interband luminescence of the obtained structures, while the presence of phase separation affects the radiative properties of InGaN layers to a lesser extent, at least in the region of low (T = 77 K) temperatures. [ABSTRACT FROM AUTHOR]

Experimental results of studying the InAs islands formation of silicon surface by molecular beam epitaxy are presented. It has been found that, InAs islands with both bimodal and uniform size distributions can be formed depending on the Si surface relief and the presence of nanopits. The possibility of fabricating heterostructures with InAs quantum dots demonstrating photoluminescence in the region of 1.65 μm, was showed. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) from Be-doped bulk GaN crystals grown by the High Nitrogen Pressure Solution method was studied and compared with PL from GaN:Be layers on sapphire grown by molecular beam epitaxy and metalorganic chemical vapor deposition techniques. The yellow luminescence band in the latter is caused by the isolated BeGa acceptor (the YLBe band), while the broad yellow band in bulk GaN:Be crystals is a superposition of the YLBe band and another band, most likely the CN-related YL1 band. The attribution of the yellow band in bulk GaN:Be crystals to the BeGaON complex (a deep donor) is questioned. [ABSTRACT FROM AUTHOR]

InGaN/GaN single quantum wells were grown by molecular beam epitaxy on the silicon substrate onto thin AlN and GaN buffer layers. The InGaN/GaN structure is porosified using a combination of SixNy nanomasking and sublimation and compared with a non-porous reference. The photoluminescence efficiency at room temperature of the porosified sample is improved by a factor reaching 40 compared with the reference sample. Plan-view and cross-sectional transmission electron microscopy images reveal that the remaining material is free of dislocation cores. The regions around dislocations are, thus, preferentially sublimated. This explains the strong photoluminescence improvement of nanoporous InGaN/GaN samples. [ABSTRACT FROM AUTHOR]

The results of a study of photoluminescence (PL) of epitaxial films of Hg0.3Cd0.7Te and Hg0.7Cd0.3Te solid solutions grown by molecular beam epitaxy are presented. A comparison of PL data with the results of optical transmission measurements and structural and microscopic studies showed that in terms of the degree of disorder of the solid solution, the studied Hg0.7Cd0.3Te films are not inferior in quality to the material synthesized by other methods. For Hg0.3Cd0.7Te films, PL data revealed significant composition fluctuations and the presence of acceptor states, which indicates the need to optimize the technology. [ABSTRACT FROM AUTHOR]

The internal quantum efficiency of GaAs/AlGaAs- and InGaAs/AlGaAs-heterostructures for infrared light emitter diodes has been determined. The influence of the growth conditions of heterostructures grown by the molecular beam epitaxy and post-growth annealing on the quantum efficiency of heterostructures has been investigated. It has been shown that it is possible to increase the quantum luminescence efficiency of the studied heterostructures up to 75–80% at the average power by the combined optimization of these processes. [ABSTRACT FROM AUTHOR]

Erbium (Er)‐doped III‐V semiconductors are promising for optoelectronic devices due to the unique intra‐4f electronic transitions of Er3+; however, the Er‐related luminescence at room temperature has always been challenging to obtain. In this work, High crystalline quality ErAs:GaAs films are grown with self‐assembled ErAs nanoparticles embedded within the GaAs matrix by molecular beam epitaxy. Rich photoluminescence spectra in a broad wavelength ranging from near‐ultraviolet to near‐infrared are observed at room temperature and confirmed to be Er‐related. A carrier‐mediated energy transfer up‐conversion mechanism is proposed, and the pump power‐dependent photoluminescence results reveal that the excited‐state absorption is governed by a single photonic process. The efficient interaction and energy transfer between the semiconductor matrix and Er3+ ions offered by this unique nanocomposite material provide a promising route to achieve novel optoelectronic devices with the potential to cover a broad wavelength range. [ABSTRACT FROM AUTHOR]

In our past research, Group-III nitride nanopillar crystals were grown vertically on the multi-crystalline silicon substrate and applied to surface-emitting LEDs. To discuss the further possibilities of this kind of LEDs, such as application in long-wavelength LEDs, Mg-doped GaN and InGaN as important parts of the LEDs were grown under various conditions. Different testing experiments were carried out on these GaN-related samples to clarify their surface morphologies and luminescence characteristics. It was confirmed that Mg was successfully doped into GaN grown on the multi-crystalline silicon substrate. InGaN with a wide range of In-contents was obtained by using the same substrate. It is believed that these materials can be applied to various optoelectronic devices, especially large-area light-emitting devices with emissions of more colors, by analyzing the various properties. [ABSTRACT FROM AUTHOR]

The temperature-dependent intensities of the exciton (X) and biexciton (XX) peaks from single GaAs cone–shell quantum dots (QDs) are studied with micro photoluminescence (PL) at varied excitation power and QD size. The QDs are fabricated by filling self-assembled nanoholes, which are drilled in an AlGaAs barrier by local droplet etching (LDE) during molecular beam epitaxy (MBE). This method allows the fabrication of strain-free QDs with sizes precisely controlled by the amount of material deposited for hole filling. Starting from the base temperature T = 3.2 K of the cryostat, single-dot PL measurements demonstrate a strong enhancement of the exciton emission up to a factor of five with increasing T. Both the maximum exciton intensity and the temperature T x , m a x of the maximum intensity depend on excitation power and dot size. At an elevated excitation power, T x , m a x becomes larger than 30 K. This allows an operation using an inexpensive and compact Stirling cryocooler. Above T x , m a x , the exciton intensity decreases strongly until it disappears. The experimental data are quantitatively reproduced by a model which considers the competing processes of exciton generation, annihilation, and recombination. Exciton generation in the QDs is achieved by the sum of direct excitation in the dot, plus additional bulk excitons diffusing from the barrier layers into the dot. The thermally driven bulk-exciton diffusion from the barriers causes the temperature enhancement of the exciton emission. Above T x , m a x , the intensity decreases due to exciton annihilation processes. In comparison to the exciton, the biexciton intensity shows only very weak enhancement, which is attributed to more efficient annihilation processes. [ABSTRACT FROM AUTHOR]

The effect of cooling conditions in the plasma-assisted molecular-beam epitaxy growth on the structural and optical properties of InGaN nanostructures is studied. It is shown that cooling of the samples without nitrogen plasma contributes to the suppression of phase separation in InGaN nanostructures. The integrated intensity of photoluminescence from these nanostructures increased by a factor of 2. [ABSTRACT FROM AUTHOR]

Studies and analysis of samples with heteroepitaxial structure based on a InGaAs solid solution grown by molecular beam epitaxy on a GaAs substrate are presented. The composition and thickness of the layers of the structure were determined by photoluminescent spectroscopy at room temperature and scanning electron microscopy, respectively. Transmission spectra were measured on an IR Fourier spectrometer. An analytical model of the spectral characteristics of the studied structures has been developed. By solving the inverse problem, the structural parameters of the structure and the composition of the InGaAs active layer were determined by the fitting method. A comparative analysis of experimental and theoretical data showed a small spread of values for the thickness (less than 65 nm) and the composition of the absorbing layer (less than 0.04). The correctness and speed of the developed method of characterization of semiconductor structures without plate destruction is shown. [ABSTRACT FROM AUTHOR]

Epitaxial layers of SiC 80 nm thick were grown on a silicon substrate by molecular beam epitaxy at 950°C. The Raman spectra contain a peak at 793 cm–1, which corresponds to the transverse optical phonon mode of the cubic SiC polytype. It was shown that the increase of photocurrent in the ranges of 1.25–1.4 and 1.5–2.0 eV is due to defects in the heterostructure of SiC/Si. It was found that the photoluminescence spectra of SiC/Si and Pt2Si/SiC/Si heterostructures contain two main emission bands in the blue (2.8 eV) and red (1.9 eV) regions. [ABSTRACT FROM AUTHOR]

The results of a comparative study on how the design of multiple quantum structures containing a parabolic barrier profile affects optical properties are presented. All quantum well (QW) structures were grown by molecular beam epitaxy (MBE) on semi-insulating GaAs substrates. The investigated samples consisted of (i) double parabolic quantum wells (type A) or (ii) multiple (two or three) rectangular quantum wells surrounded by parabolic barriers (type B). The optical quality of samples was characterized performing room-temperature (RT-PL) and temperaturedependent photoluminescence (TD-PL) measurements. The investigation aimed at the optimization of a multiple quantum well (MQW) structure design for application in the gain region of near infrared (NIR) laser diodes (LDs) revealed benefits of both double parabolic quantum wells and a mixed design (rectangular MQW with parabolic barriers). The PL band position for all samples was registered in the vicinity around 1.19 eV, which corresponds to the Bi content in QW of ~4.4%. It was shown that all structures of type A exhibit an intense emission, while the intensity of photoluminescence measured for the samples of type B depends on the number of QWs. The weaker intensity of the PL signal from two QWs inserted between parabolic barriers was explained by a larger point defect density at low temperature grown inner GaAs barriers. The room-temperature PL intensity of the structure with three GaAsBi QWs embedded in one parabolic AlGaAs barrier was the highest one. The shift of PL peak position to lower energies (1.16 eV) was attributed to the slightly higher bismuth concentration, 4.9%. [ABSTRACT FROM AUTHOR]

This paper presents results of the characterisation of type I GaSb/AlSb superlattices (SLs) with a thin GaSb layer and varying thicknesses of an AlSb layer. Nextnano software was utilized to obtain spectral dependence of absorption and energy band structure. A superlattice (SL) with an energy bandgap of ~ 1.0 eV and reduced mismatch value was selected for experimental investigation. SLs with single (sample A) and double (sample B) AlSb barriers and a single AlSb layer (sample C) were fabricated using molecular beam epitaxy (MBE). Optical microscopy, high-resolution X-ray diffractometry, and photoluminescence were utilized for structural and optical characterisation. The presence of satellite and interference peaks in diffraction curves confirms the high crystal quality of superlattices. Photoluminescence signal associated with the superlattice was observed only for sample B and contained three low-intensity peaks: 1.03, 1.18, and 1.25 eV. The first peak was identified as the value of the energy bandgap of the SL. Other two peaks are related to optical transitions between defect states located at the interface between the SL and the top AlSb barrier. The time-dependent changes observed in the spectral characteristics are due to a modification of the SL/AlSb interface caused by the oxidation and hydroxylation of the AlSb layer. [ABSTRACT FROM AUTHOR]

We investigate the beneficial effects of rapid thermal annealing on structure and photoluminescence of PbSe thin films on GaAs (001) grown below 150 °C, with a goal of low temperature integration for infrared optoelectronics. Thin films of PbSe deposited on GaAs by molecular beam epitaxy are epitaxial at these reduced growth temperatures, yet the films are highly defective with a mosaic grain structure with low angle and dendritic boundaries following coalescence. Remarkably, we find that rapid thermal annealing for as short as 180 s at temperatures between 300 and 425 °C in nitrogen ambient leads to extensive re-crystallization and transformation of these grain boundaries. The annealing at the same time dramatically improves the band edge luminescence at 3.7 μm from previously undetectable levels to nearly half as intense as our best conventionally grown PbSe films at 300 °C. We show using an analysis of laser pump-power dependent photoluminescence measurements that this dramatic improvement in the photoluminescence intensity is due to a reduction in the trap-assisted recombination. However, we find it much less correlated with improved structural parameters determined by x-ray diffraction rocking curves, thereby pointing to the importance of eliminating point defects over extended defects. Overall, the success of rapid thermal annealing in improving the luminescent properties of low growth temperature PbSe is a step toward the integration of PbSe infrared optoelectronics in low thermal budget, back end of line compatible fabrication processes. [ABSTRACT FROM AUTHOR]

Aluminium Gallium Nitride (AlyGa1-yN) quantum dots (QDs) with thin sub-µm AlxGa1-xN layers (with x > y) were grown by molecular beam epitaxy on 3 nm and 6 nm thick hexagonal boron nitride (h-BN) initially deposited on c-sapphire substrates. An AlN layer was grown on h-BN and the surface roughness was investigated by atomic force microscopy for different deposited thicknesses. It was shown that for thicker AlN layers (i.e., 200 nm), the surface roughness can be reduced and hence a better surface morphology is obtained. Next, AlyGa1-yN QDs embedded in Al0.7Ga0.3N cladding layers were grown on the AlN and investigated by atomic force microscopy. Furthermore, X-ray diffraction measurements were conducted to assess the crystalline quality of the AlGaN/AlN layers and examine the impact of h-BN on the subsequent layers. Next, the QDs emission properties were studied by photoluminescence and an emission in the deep ultra-violet, i.e., in the 275–280 nm range was obtained at room temperature. Finally, temperature-dependent photoluminescence was performed. A limited decrease in the emission intensity of the QDs with increasing temperatures was observed as a result of the three-dimensional confinement of carriers in the QDs. [ABSTRACT FROM AUTHOR]

Experimental and theoretical assessments of phonon and optical characteristics are methodically accomplished for comprehending the vibrational, structural, and electronic behavior of InP1−xSbx/n-InAs samples grown by Gas-Source Molecular Beam Epitaxy. While the polarization-dependent Raman scattering measurements revealed InP-like doublet covering optical modes ( ω L O I n P ~ 350 cm−1, ω T O I n P ~ 304 cm−1) and phonons activated by disorders and impurities, a single unresolved InSb-like broadband is detected near ~195 cm−1. In InP1−xSbx, although no local vibrational (InSb:P; x → 1) and gap modes (InP:Sb; x → 0) are observed, the Raman line shapes exhibited large separation between the optical phonons of its binary counterparts, showing features similar to the phonon density of states, confirming "two-mode-behavior". Despite the earlier suggestions of large miscibility gaps in InP1−xSbx epilayers for x between 0.02 and 0.97, our photoluminescence (PL) results of energy gaps insinuated achieving high-quality single-phase epilayers with x ~ 0.3 in the miscibility gap. Complete sets of model dielectric functions (MDFs) are obtained for simulating the optical constants of binary InP, InSb, and ternary InP1−xSbx alloys in the photon energy (0 ≤ E ≤ 6 eV) region. Detailed MDF analyses of refractive indices, extinction coefficients, absorption and reflectance spectra have exhibited results in good agreement with the spectroscopic ellipsometry data. For InP0.67Sb0.33 alloy, our calculated lowest energy bandgap E0 ~ 0.46 eV has validated the existing first-principles calculation and PL data. We feel that our results on Raman scattering, PL measurements, and simulations of optical constants provide valuable information for the vibrational and optical traits of InP1−xSbx/n-InAs epilayers and can be extended to many other technologically important materials. [ABSTRACT FROM AUTHOR]

In this paper, for the first time, we report a strong effect of the arsenic pressure used for the high-rate GaAs capping of self-assembled InAs quantum dots on their optical properties. A 140 nm red shift of the photoluminescence peak position is observed when the overgrowth arsenic pressure increases threefold. We explain this behavior in terms of different intensities of quantum dot decomposition, which occurs during the overgrowth under different conditions. When the arsenic pressure is sufficiently high, a GaAs capping layer is formed by deposited species with a low impact on initial quantum dots. At a low arsenic pressure, arsenic deficiency leads to the intensive intermixing caused both by the enhanced Ga/In atom exchange and by the consumption of arsenic atoms belonging to quantum dots for the GaAs capping layer formation. As a result of the overgrowth, quantum dots are divided into families with a large (high pressure) and a small (low pressure) average size, yielding long-wave (1.23 µm) and short-wave (1.09 µm) photoluminescence peaks, respectively. Thus, a significant influence of the overgrowth arsenic pressure on the characteristics of InAs quantum dots is evidenced in this study. [ABSTRACT FROM AUTHOR]

The GaAs1–xBix semiconductor alloy allows one to achieve large bandgap reduction and enhanced spin–orbit splitting energy at dilute Bi quantities. The bismide is currently being developed for near- to mid-infrared lasers, multi-junction solar cells, and photodetectors. In this structure–property relationship study of GaAsBi alloys, we report polarization dependent photoluminescence that reaches a polarization ratio up to 2.4 at room temperature. Polarization dependence is also presented using transmittance spectra, birefringence, and linear dichroism. The optical anisotropy observations agree with the predictions of point symmetry reduction in the CuPtB-type ordered GaAsBi phase. The structural ordering is investigated experimentally from the atomic scale in molecular-beam epitaxy (MBE) grown samples on exact and miscut (001) GaAs substrates, as well as on (001) Ge. The latter sample is composed of anti-phase domains in which the ordering axes are rotated by 90° angles. Since the conditions stabilizing the CuPtB ordered phase fall within the typical MBE growth regime of dilute bismides, the optical anisotropy in GaAsBi alloys is expected to be ubiquitous. These findings are important for the future development of GaAsBi-based optoelectronics and also provide new means to analyze structurally complex bismide alloys. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) spectroscopy of nonpolar m-plane InGaN thin films with indium composition up to 21% and nearly strain-balanced In0.09Ga0.91N/Al0.19Ga0.81N superlattices grown by plasma-assisted molecular beam epitaxy was performed as a function of temperature. The experimental transition energies are consistently lower than the calculation based on structural parameters extracted from x-ray diffraction measurements. This indicates the presence of indium composition fluctuations in InGaN and hence local bandgap reduction that produces charge localization centers. The spectral width of the low-temperature PL of our m-plane InGaN/AlGaN superlattices is narrower than previously reported for m-plane InGaN/GaN quantum wells grown by MOCVD. The PL integrated intensity drops rapidly, though, as the temperature is increased to 300 K, indicating strong non-radiative recombination at room temperature. Time-resolved PL at low temperatures was performed to characterize the relaxation time scales in an undoped and a doped superlattice. [ABSTRACT FROM AUTHOR]

GaN/AlN heterostructures with thicknesses of one monolayer (ML) are currently considered to be the most promising material for creating UVC light-emitting devices. A unique functional property of these atomically thin quantum wells (QWs) is their ability to maintain stable excitons, resulting in a particularly high radiation yield at room temperature. However, the intrinsic properties of these excitons are substantially masked by the inhomogeneous broadening caused, in particular, by fluctuations in the QWs' thicknesses. In this work, to reduce this effect, we fabricated cylindrical nanocolumns of 50 to 5000 nm in diameter using GaN/AlN single QW heterostructures grown via molecular beam epitaxy while using photolithography with a combination of wet and reactive ion etching. Photoluminescence measurements in an ultrasmall QW region enclosed in a nanocolumn revealed that narrow lines of individual excitons were localized on potential fluctuations attributed to 2-3-monolayer-high GaN clusters, which appear in QWs with an average thickness of 1 ML. The kinetics of luminescence with increasing temperature is determined via the change in the population of localized exciton states. At low temperatures, spin-forbidden dark excitons with lifetimes of ~40 ns predominate, while at temperatures elevated above 120 K, the overlying bright exciton states with much faster recombination dynamics determine the emission. [ABSTRACT FROM AUTHOR]

Highly ordered lateral composition modulation (LCM) is obtained in InAs/AlAs short-period superlattices (SPS) grown by molecular beam epitaxy and its effect on photoluminescence (PL) is studied. The formation of LCM and modulation length can be resolved by x-ray diffraction. Furthermore, atomic-resolution scanning transmission electron microscopy results reveal both the composition and strain distribution in the modulated and unmodulated samples and demonstrate a clear transition of strain redistribution due to LCM formation, showing that LCM is a preferential route over dislocation formation for strain relief in these strain-compensated SPSs. The LCM leads to a large redshift in PL emissions up to 0.3 eV, which we attribute to the In-rich regions in the modulated InAs/AlAs SPS. This finding facilitates a deeper understanding of these modulated SPSs. [ABSTRACT FROM AUTHOR]

The droplet epitaxy of indium gallium nitride quantum dots (InGaN QDs), the formation of In–Ga alloy droplets in ultra-high vacuum and then surface nitridation by plasma treatment, is firstly investigated by using plasma-assisted molecular beam epitaxy. During the droplet epitaxy process, in-situ reflection high energy electron diffraction patterns performs the amorphous In–Ga alloy droplets transform to polycrystalline InGaN QDs, which are also confirmed by the characterizations of transmission electron microscopy and X-ray photoelectron spectroscopy. The substrate temperature, In–Ga droplet deposition time, and duration of nitridation are set as parameters to study the growth mechanism of InGaN QDs on Si. Self-assembled InGaN QDs with a density of 1.33 × 1011 cm−2 and an average size of 13.3 ± 3 nm can be obtained at the growth temperature of 350 °C. The photoluminescence emissions of uncapped InGaN QDs in wavelength of the visible red (715 nm) and infrared region (795 and 857 nm) are observed. The formation of high-indium composition of InGaN QDs via droplet epitaxy technique could be applied in long wavelength optoelectronic devices. [ABSTRACT FROM AUTHOR]

Hybrid nanostructures based on InGaN nanowires with decorated plasmonic silver nanoparticles are investigated in the present study. It is shown that plasmonic nanoparticles induce the redistribution of room temperature photoluminescence between short-wavelength and long-wavelength peaks of InGaN nanowires. It is defined that short-wavelength maxima decreased by 20%, whereas the long-wavelength maxima increased by 19%. We attribute this phenomenon to the energy transfer and enhancement between the coalesced part of the NWs with 10–13% In content and the tips above with an In content of about 20–23%. A proposed Fröhlich resonance model for silver NPs surrounded by a medium with refractive index of 2.45 and spread 0.1 explains the enhancement effect, whereas the decreasing of the short-wavelength peak is associated with the diffusion of charge carriers between the coalesced part of the NWs and the tips above. [ABSTRACT FROM AUTHOR]

Strain-free GaAs cone–shell quantum structures (CSQS) with widely tunable wave functions (WF) are fabricated using local droplet etching (LDE) during molecular beam epitaxy (MBE). During MBE, Al droplets are deposited on an AlGaAs surface, which then drill low-density (about 1 × 10 7 cm−2) nanoholes with adjustable shape and size. Subsequently, the holes are filled with GaAs to form CSQS, where the size can be adjusted by the amount of GaAs deposited for hole filling. An electric field is applied in growth direction to tune the WF in a CSQS. The resulting highly asymmetric exciton Stark shift is measured using micro-photoluminescence. Here, the unique shape of the CSQS allows a large charge–carrier separation and, thus, a strong Stark shift of up to more than 16 meV at a moderate field of 65 kV/cm. This corresponds to a very large polarizability of 8.6 × 10 − 6 eVkV − 2 cm2. In combination with simulations of the exciton energy, the Stark shift data allow the determination of the CSQS size and shape. Simulations of the exciton–recombination lifetime predict an elongation up to factor of 69 for the present CSQSs, tunable by the electric field. In addition, the simulations indicate the field-induced transformation of the hole WF from a disk into a quantum ring with a tunable radius from about 10 nm up to 22.5 nm. [ABSTRACT FROM AUTHOR]

To determine the band alignment at the GaSb1-xBix/GaSb interface, a set of GaSb1-xBix/GaSb quantum wells (QWs) of various widths (7, 11, and 15 nm) and contents (Bi ≤ 12%) were grown by molecular beam epitaxy and investigated by photoreflectance (PR) spectroscopy. In PR spectra, the optical transitions related to both the ground and the excited states in the QW were clearly observed. It is a direct experimental evidence that the GaSb1-xBix/GaSb QW is a type-I QW with a deep quantum confinement in both the conduction and valence bands. From the comparison of PR data with calculations of energies of QW transitions performed for the varying valence band offset (VBO), the best agreement between experimental data and theoretical calculations has been found for the VBO ∼50 ± 5%. A very similar VBO was obtained from ab initio calculations. These calculations show that the incorporation of Bi atoms into a GaSb host modifies both the conduction and valence band: the conduction-band position changes linearly at a rate of ∼15–16 meV per % Bi and the valence band position changes at a rate of ∼15–16 meV per % Bi. The calculated shifts of valence and conduction bands give the variation of VBO between GaSb1-xBix and GaSb in the range of ∼48%–52%, which is in good agreement with conclusions derived from PR measurements. In addition, it has been found that the electron effective mass reduces linearly with the increase in Bi concentration (x): m e f f G a S b B i = m e f f G a S b − 0.2 x , where m e f f G a S b is the electron effective mass of GaSb. Moreover, a strong photoluminescence (PL) was observed and a negligible Stokes shift (less than a few meV) between the PL peak and the fundamental transition in the PR spectrum was detected for all QWs at low temperatures. It means that the investigated QWs are very homogeneous, and the carrier localization for this alloy is very weak in contrast to other dilute bismides. [ABSTRACT FROM AUTHOR]

We carry out the surface potential (SP) measurements of Si-doped InN nanorods (NRs) grown by plasma-assisted molecular beam epitaxy. Photoluminescence and photoemission spectroscopic studies reveal that the Si-doped InN nanorods possess surface electron accumulation. To estimate the SP value of the InN nanorods, a contact potential difference is measured using Kelvin probe force microscopy (KPFM). In order to avoid the influence of the surface adsorbed species, KPFM measurements were carried out at a high vacuum condition of ∼7.5 × 10−7 mbar. The SP value of the Si-doped InN nanorods is found to depend on the size of nanorods. The size-dependent SP value of the Si-doped InN nanorods is attributed to the variation in the downward surface band bending caused by the change in the sheet carrier density of surface electron accumulation. The change in surface band bending is the result of the variation in the free-electron distribution with a size of the NRs. [ABSTRACT FROM AUTHOR]

The bandgaps of short period ZnO/(Zn)MgO superlattices deposited on c-, m-, and a-ZnO substrates were examined both theoretically and experimentally. Ab initio calculations showed that the bandgaps of c-oriented polar superlattices are smaller than those of nonpolar ones; however, this is mainly due to different geometric configurations, because the influence of internal electric fields existing in polar superlattices is not very significant. The calculations revealed that for 5–6 MgO monolayers in the barriers, the bandgap values become independent of the barrier thickness, which suggests that such superlattices can be treated as sets of isolated ZnO wells. In the experimental part of this work, it is demonstrated that short period ZnO/MgO and ZnO/ZnMgO superlattices can be grown successfully on differently oriented crystalline bulk ZnO substrates using molecular beam epitaxy. The bandgaps of the superlattices were determined from low temperature photoluminescence measurements. It is shown that they agree well with the theoretical results. [ABSTRACT FROM AUTHOR]

We present a study of germanium as an alternative to silicon for n-type doping of cubic GaN. We find that Ge is a well-suited donor impurity. Our layers were grown by plasma-assisted molecular beam epitaxy on 3C-SiC/Si (001) substrates. Germanium-doped layers were fabricated with donor concentrations ranging over several orders of magnitude up to 3.7 × 1020 cm−3. For comparison, silicon-doped layers with donor concentrations of up to 3.8 × 1019 cm−3 were also grown. Incorporation of germanium into the cubic GaN layers was verified by time-of-flight secondary ion mass spectrometry. The crystalline quality of our layers was analyzed using high-resolution x-ray diffraction. Germanium- as well as silicon-doped layers with donor concentrations above 1019 cm−3 exhibited an increase of the dislocation density with increasing dopant concentration. The surface topography of our layers was investigated by atomic force microscopy. Comparable values for the surface roughness were measured for germanium- as well as silicon-doped layers. Optical properties were investigated by photoluminescence spectroscopy at 13 K. Doping with silicon resulted in a spectrally slightly narrower luminescence than doping with germanium. Donor concentrations and carrier mobilities were determined by Hall effect measurements at room temperature and we observe 20% higher electron mobilities for Ge-doping compared to Si-doping in the case of high dopant concentrations. [ABSTRACT FROM AUTHOR]

Thin GaTe films were grown by molecular beam epitaxy (MBE) on GaAs(001) substrates. X-ray powder diffraction confirmed the coexistence of the h- and m-GaTe phases in all grown layers. A quantitative correlation between the MBE growth conditions and the phase composition of the grown films is established, and the upper limit of the MBE growth temperature for thin GaTe/GaAs(001) films is experimentally determined. New data are presented confirming the defect-related origin of the broad emission line with an energy maximum of ~1.45–1.46 eV, which dominates the low-temperature photoluminescence spectra of the grown GaTe/GaAs(001) layers. [ABSTRACT FROM AUTHOR]

We studied the influence of the arsenic pressure during low-temperature GaAs overgrowth of InAs quantum dots on their optical properties. In the photoluminescence spectrum of quantum dots overgrown at a high arsenic pressure, we observed a single broad line corresponding to unimodal size distribution of quantum dots. Meanwhile, two distinct peaks (~1080 and ~1150 nm) at larger wavelengths are found in the spectra of samples with quantum dots overgrown at a low arsenic pressure. We attributed this phenomenon to the high-pressure suppression of atom diffusion between InAs islands at the overgrowth stage, which makes it possible to preserve the initial unimodal size distribution of quantum dots. The same overgrowth of quantum dots at the low arsenic pressure induces intensive mass transfer, which leads to the formation of arrays of quantum dots with larger sizes. Integrated photoluminescence intensity at 300 K is found to be lower for quantum dots overgrown at the higher arsenic pressure. However, a difference in the photoluminescence intensity for the high- and low-pressure overgrowths is not so significant for a temperature of 77 K. This indicates that excess arsenic incorporates into the capping layer at high arsenic pressures and creates numerous nonradiative recombination centers, diminishing the photoluminescence intensity. [ABSTRACT FROM AUTHOR]

Interband photoluminescence was obtained for structures with multiple quantum wells (MQWs) with different content of germanium and tin. Peak position in photoluminescence spectra obtained from the MQW of Ge Si Sn /Si shifts to the long wavelength region with an increase in the Ge content in the solid solution and is observed in the energy range 0.85-0.68 eV for the germanium content from 30 to 78 . Thus, the shift of the peak along the wavelength was observed from 1.46 to 1.82 m, and the total spectral range of MQW luminescence covered by these structures was 1.3–2.1 m. An even more significant shift of the MQW photoluminescence peak to the long-wavelength region was achieved by increasing the tin content. Increasing the fraction of Sn from 7 to 14 while keeping the 30 Ge fraction constant led to a shift of the peak from 0.85 to 0.75 eV. A simultaneous increase in the content of both tin and germanium in the solid solution (up to 14 and 79%, respectively) made it possible to obtain a photoluminescence peak with an energy of 0.58 eV, which corresponds to the radiation wavelength of 2.14 m. A sharp "red" shift in the position of the photoluminescence peak with increasing temperature was discovered and its value reached 50 meV when the sample heating temperature was changed from 11 to 60–80 K. Such a significant shift in the position of the MQW photoluminescence peak is explained within the framework of a model that assumes that at low temperatures, charge carriers are randomly localized on spatial inhomogeneities of the MQW, and as the temperature increases, they are redistributed and transition to a thermodynamically equilibrium state with the lowest energy. [ABSTRACT FROM AUTHOR]

The refinement of interface structure in InGaAs/AlGaAs multiple quantum wells (MQWs) through molecular beam epitaxy (MBE) is crucial for enhancing their luminescence performance. The influencing mechanisms of the GaAs insertion layer (ISL) thicknesses on the interface structure and luminescence properties of InGaAs/AlGaAs MQWs were investigated by varying the GaAs ISL thickness variations and optimizing growth parameters. In our designed InGaAs/AlGaAs MQWs structures, GaAs ISL with thickness variables of 0 nm, 0.5 nm, 1 nm, 2 nm, and 4 nm were inserted between the interfaces of the InGaAs well layer and the AlGaAs barrier layer. Through experimental characterizations. The introduction of the GaAs ISL resulted in a gradual transition of the MQWs surface morphology from a three-dimensional (3D) island structure to a typical step-flow surface morphology, leading to the reduction in surface roughness and enhancement of hetero-interfaces quality. Moreover, the GaAs ISL with thickness exceeding 2 nm effectively inhibited the segregation of indium atoms, promoting the ordering of interface alloys and decreasing the polarization degrees from 3.76 % to 2.09 %. Temperature-dependent PL measurements indicated that the GaAs ISL could reduce interface defects and the density of non-radiative recombination centers, thereby enhancing the radiative recombination efficiency of MQWs and ultimately increasing the peak intensity of photoluminescence by 3.4 times. This work is significance for understanding the luminescent properties of InGaAs/AlGaAs MQWs, which provides reference highly efficient optoelectronic devices development. • Optimized MBE growth improves InGaAs/AlGaAs MQWs quality. • GaAs insertion layer suppresses intermixing, enhances interface quality. • Optimized GaAs insertion layer thickness increased MQWs PL intensity 3.4x. [ABSTRACT FROM AUTHOR]

In this paper, for the first time, we report a strong effect of the arsenic pressure used for the high-rate GaAs capping of self-assembled InAs quantum dots on their optical properties. A 140 nm red shift of the photoluminescence peak position is observed when the overgrowth arsenic pressure increases threefold. We explain this behavior in terms of different intensities of quantum dot decomposition, which occurs during the overgrowth under different conditions. When the arsenic pressure is sufficiently high, a GaAs capping layer is formed by deposited species with a low impact on initial quantum dots. At a low arsenic pressure, arsenic deficiency leads to the intensive intermixing caused both by the enhanced Ga/In atom exchange and by the consumption of arsenic atoms belonging to quantum dots for the GaAs capping layer formation. As a result of the overgrowth, quantum dots are divided into families with a large (high pressure) and a small (low pressure) average size, yielding long-wave (1.23 µm) and short-wave (1.09 µm) photoluminescence peaks, respectively. Thus, a significant influence of the overgrowth arsenic pressure on the characteristics of InAs quantum dots is evidenced in this study.

We use two-wavelength-excited photoluminescence (PL) to investigate nonradiative recombination (NRR) centers in GaAs:N δ-doped superlattice (SL) structures grown by molecular beam epitaxy. The change in photoluminescence (PL) intensity due to the superposition of below-gap excitation at energies of 0.75, 0.80, 0.92, and 0.95 eV and above-gap excitation at energies of 1.69 or 1.45 eV into the GaAs conduction band and the E− band implies the presence of NRR centers inside the GaAs:N δ-doped SL and/or GaAs layers. The change in PL intensity as a function of the photon number density of below-gap excitation is examined for both bands, which enables us to determine the distribution of NRR centers inside the GaAs:N δ-doped SL and GaAs layers. We propose recombination models to explain the experimental results. Defect-related parameters that give a qualitative insight into the samples are investigated systematically by fitting the rate equations to the experimental data. [ABSTRACT FROM AUTHOR]

We report a systematic study of high quality GaAs growths on on-axis (001) GaP/Si substrates using molecular beam epitaxy. Various types of dislocation filter layers and growth temperatures of initial GaAs layer were investigated to reduce the threading dislocation densities in GaAs on GaP/Si. Electron channeling contrast imaging techniques revealed that an optimized GaAs buffer layer with thermal cycle annealing and InGaAs/GaAs dislocation filter layers has a threading dislocation density of 7.2×106 cm-2, which is a factor of 40 lower than an unoptimized GaAs buffer. The root-mean-square surface roughness was greatly decreased from 7.8 nm to 2.9 nm after the optimization process. A strong enhancement in photoluminescence intensity indicates that the optimized GaAs template grown on on-axis (001) GaP/Si substrates is a promising virtual substrate for Si-based optoelectronic devices. [ABSTRACT FROM AUTHOR]

We study the transport of photogenerated carriers in molecular beam epitaxy (MBE)-grown gallium arsenide/aluminum gallium arsenide (GaAs/AlGaAs) coupled (CDQW) and uncoupled (UDQW) double quantum wells. Photoluminescence (PL) spectroscopy was used to investigate the optical properties and establish differences in the tunneling properties between the CDQW and UDQW. Terahertz time-domain spectroscopy (THz-TDS) measurements have shown that the emissions from the CDQW and UDQW were 57 % and 31 % of the THz emission of p-InAs, 800 nm excitation wavelength. The higher THz emission from the CDQW is attributed to the tunneling of electrons from the NW to the WW leading to a larger dipole moment. Furthermore, excitation wavelength-dependent THz-TDS measurements have shown that when the NW is not photoexcited, high-frequency components appear in the frequency spectra. These results provide insights on the possible development of DQWs as THz optoelectronic devices. It also demonstrates the application of THz-TDS in investigating tunneling dynamics in DQWs in conjunction with established optical characterization techniques, such as PL spectroscopy. [ABSTRACT FROM AUTHOR]

The impact of growth rate on the optical and crystal quality of InGaAs/AlGaAs multi-quantum wells (MQWs) and InGaAs single layer grown by molecular beam epitaxy (MBE) is studied. Photoluminescence (PL) and high-resolution X-ray diffraction (HRXRD) are applied for evaluation of the optical, crystal quality and interfaces smoothness. The room-temperature PL integral intensity increases by 220 % when the growth rate decreases from 2 to 1 Å/s. HRXRD analysis reveals the growth rate of 1 Å/s improves crystal and interface quality of MQWs, as well as significantly enhances PL intensity. But further reduce growth rate to 0.5 Å/s, the PL integral intensity decreases, instead. At lower growth rate, more impurities can incorporate into InGaN layer and act as nonradiative recombination centers. Growth rate can effectively control crystal quality, interfaces smoothness and impurity content and thus determine the optical quality of InGaAs/AlGaAs MQWs. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) and photoreflectance (PR) spectroscopy were used for the optical study of arsenic doping of HgCdTe grown by molecular beam epitaxy (MBE). Un-doped and arsenic-doped material with cadmium telluride molar fraction x = 0.29 grown under similar conditions and subjected to similar types of annealing was studied. The PL spectra of the un-doped material featured signatures of intrinsic defects associated with mercury vacancy, excessive tellurium, and related complexes. In the arsenic-doped material, optical signatures of these defects appeared to be suppressed. After arsenic activation, shallow (7–8 meV) acceptor levels were found in the material. These were attributed to the dopant activation, which was confirmed with electrical studies showing p -type conductivity with hole concentration ∼1016 cm−3. The studies showed that the actual pattern of arsenic doping in HgCdTe can be indeed screened by intrinsic defects, which are inherent to MBE-grown HgCdTe and tend to interact with the dopant. • Un-doped and arsenic-doped HgCdTe films were grown with molecular-beam epitaxy on Si substrates. • Photoluminescence (PL) and photoreflectance (PR) techniques were used for the optical study of the material. • The PL spectra of un-doped material featured signatures of mercury vacancy, excessive tellurium, and related complexes. • In arsenic-activated films, dopant-related shallow (7–8 meV) acceptor levels were found. [ABSTRACT FROM AUTHOR]

One drawback of self-organized quantum dots is their low optical gain. The development of new shaping methods that would allow higher gain, for example, due to a higher surface density of the QD array, remains an important task to date. In the present work, arrays of quantum dots have been formed by substituting phosphorous atoms with arsenic ones in a thin InGaP epilayer. Based on transmission electron microscopy data, the surface density was estimated to be about 1.3 × 1012 cm−2, which is one of the highest values for quantum dots on GaAs. The influence of an additional InGaAs epilayer (quantum well) on the luminescence properties of quantum dots was investigated in a wide temperature range and different optical pumping levels. It was found that placing the quantum well under the layer of quantum dots has little effect on the central emission wavelength, while quantum dots covered with the InGaAs demonstrate a strong red shift of the luminescence spectrum. A transition from a nonequilibrium to an equilibrium distribution of charge carriers over quantum-dot states, related to the lateral transport of charge carriers over quantum dot array, similar to that previously observed in In(Ga)As Stranski-Krastanow quantum dots, was revealed. This is manifested in a change in temperature dependencies of the linewidth and peak position of the photoluminescence band. For the structures under study, this transition occurs at a temperature of about 125 K. Rapid thermal annealing of the structures allows to increase the integrated photoluminescence intensity of quantum dots up to 4 times, while the maximum of the spectrum remains almost unchanged, provided that the annealing temperature does not exceed 620 °C. [ABSTRACT FROM AUTHOR]

In this work, we have decoupled the radiative and nonradiative contributions to recombination by developing analytical models to fit observed non-monoexponential photoluminescence decay and bulk-lifetime temperature-dependence, which has enabled absolute measurement of the effective radiative efficiency and characterization of the dominant defect in lightly doped GaAs samples grown using molecular-beam-epitaxy. The measured effective radiative efficiency values under low level injection conditions range from (88.9 ± 0.1)% and (81.2 ± 0.1)% at 77K to (0.028 ± 0.001)% and (0.034 ± 0.001)% at 700 K, with 300K values of (17.3 ± 0.1)% and (10.5 ± 0.1)% in the p-type and n-type samples, respectively. A common defect-signature is observed in the temperaturedependence of the nonradiative lifetime in samples of both doping types, and our measured parameters are consistent with characteristics previously measured for the EL2 defect using deep-level transient spectroscopy. [ABSTRACT FROM AUTHOR]

Series of 100 nm-thick pseudomorphic GaAsBi layers with the Bi content varying from 0.97 to 11.2% have been grown by molecular beam epitaxy (MBE) on the semi-insulating GaAs(100) substrates buffered with an ultra-thin up to 20 nm thick GaAs layer. The main attention in this work was focused on the investigation of relaxation in the Bi induced compressively-strained GaAsBi layers containing a various content of Bi. The lattice parameters of GaAs- Bi compound and the Bi concentration have been evaluated from high resolution X-ray diffraction measurements. The relaxation values of GaAsBi layers ranging from 0.4 to 3.5% were obtained analyzing the symmetric and asymmetric reciprocal space maps of (004) and (115) planes, respectively. Also, the complex study was performed to clarify the relaxation effect on structural, morphological and optical properties of bismide layers. Optical measurements revealed a significant reduction of the energy band gap from 1.34 to 0.92 eV for the layers containing 0.97-8.6% of Bi in the GaAs lattice. [ABSTRACT FROM AUTHOR]