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In this study, we investigate the tradespace between the improvement of mid-infrared light-emitting diode efficiency through microcavity enhancement vs reduction of Auger recombination for different W-superlattice thicknesses. Several sample designs are modeled and then grown and fabricated to test the tradespace at different W-superlattice thicknesses down to the quantum well limit. In a half-cavity, with a single reflector from the top metal contact, intermediate thickness W-superlattices gave the highest efficiencies, outperforming those in the W-quantum well limit across the entire measured current range. Experimentally, we report wallplug efficiencies of 0.4% for a room temperature 3.2 μm device. W-superlattices of intermediate thickness were also found to be optimal for a full-cavity device with a bottom distributed Bragg reflector added. The resonant full cavity did strongly improve the peak spectral radiance, with a measured increase of four to five times for a 3.6 μm device, and a value that is >250 times larger than previously reported. [ABSTRACT FROM AUTHOR]

Ternary semiconductors such as InGaN thin films, quantum wells, and superlattices commonly exhibit alloy fluctuations that become increasingly pronounced with higher In-content. The thickness fluctuations of quantum wells and their thin cap-layers further introduce nanoscale inhomogeneities that alter the potential landscape. In this work, we present a combined theoretical and experimental study of InGaN single quantum wells with thin GaN cap-layers to unravel the influence of cap-layer thickness, compositional inhomogeneity, and thickness fluctuations on their electronic and optical properties. A pronounced spectral shift of quantum well emission for thin cap-layers between 1 and 10 nm is observed by micro-photoluminescence spectroscopy. The origin of this shift is explained by calculations of electronic band profiles and probability density overlap of carriers in the quantum well. The impact of alloy fluctuations and homogeneity for different cap-layer thicknesses is studied on both the microscale and nanoscale using UV micro-Raman scattering and tip-enhanced Raman spectroscopy (TERS). On the microscale, the alloy composition as determined by micro-Raman mapping appears very homogeneous except for the thinnest 1 nm cap-layer where small fluctuations are visible. On the nanoscale, TERS reveals local fluctuations on a 20–30 nm length scale. The influence of the cap-layer thickness on the TERS spectra is discussed regarding both the nanoscale homogeneity and the depth resolution of the near-field Raman scattering technique. Our results demonstrate the capabilities of TERS to resolve nanoscale thickness fluctuations and compositional inhomogeneities in ultra-thin semiconductor layers, even when they are buried by thin cap-layers with thicknesses below 10 nm. [ABSTRACT FROM AUTHOR]

While it is known that a resonant amplification of T c in two-gap superconductors can be driven by using the Fano–Feshbach resonance tuning the chemical potential near a Lifshitz transition, little is known on tuning the T c resonance by cooperative interplay of the Rashba spin–orbit coupling (RSOC) joint with phonon mediated (e-ph) pairing at selected k-space spots. Here, we present first-principles quantum calculation of superconductivity in an artificial heterostructure of metallic quantum wells with 3 nm period where quantum size effects give two-gap superconductivity with RSOC controlled by the internal electric field at the interface between the nanoscale metallic layers intercalated by insulating spacer layers. The key results of this work show that fundamental quantum mechanics effects including RSCO at the nanoscale [Mazziotti et al., Phys. Rev. B, 103, 024523 (2021)] provide key tools in applied physics for quantitative material design of unconventional high temperature superconductors at ambient pressure. We discuss the superconducting domes where T c is a function of either the Lifshitz parameter (η) measuring the distance from the topological Lifshitz transition for the appearing of a new small Fermi surface due to quantum size effects with finite spin–orbit coupling and the variable e-ph coupling g in the appearing second Fermi surface linked with the energy softening of the cut off ω 0. [ABSTRACT FROM AUTHOR]

The growth of InGaAs quantum wells (QWs) epitaxially on InP substrates is of great interest due to their wide application in optoelectronic devices. However, conventional molecular beam epitaxy requires substrate temperatures between 400 and 500 °C, which can lead to disorder scattering, dopant diffusion, and interface roughening, adversely affecting device performance. Lower growth temperatures enable the fabrication of high-speed optoelectronic devices by increasing arsenic antisite defects and reducing carrier lifetimes. This work investigates the low-temperature epitaxial growth of InAs/GaAs short-period superlattices as an ordered replacement for InGaAs quantum wells, using migration-enhanced epitaxy (MEE) with low growth temperatures down to 200–250 °C. The InAs/GaAs multi-quantum wells with InAlAs barriers using MEE grown at 230 °C show good single crystals with sharp interfaces, without mismatch dislocations found. The Raman results reveal that the MEE mode enables the growth of (InAs)4(GaAs)3/InAlAs QWs with excellent periodicity, effectively reducing alloy scattering. The room temperature (RT) photoluminescence (PL) measurement shows the strong PL responses with narrow peaks, revealing the good quality of the MEE-grown QWs. The RT electron mobility of the sample grown in low-temperature MEE mode is as high as 2100 cm2/V∗s. In addition, the photoexcited band-edge carrier lifetime was about 3.3 ps at RT. The high-quality superlattices obtained confirm MEE's effectiveness for enabling advanced III-V device structures at reduced temperatures. This promises improved performance for applications in areas such as high-speed transistors, terahertz imaging, and optical communications. [ABSTRACT FROM AUTHOR]

We present a theoretical analysis of mid-infrared radiative recombination in InAs/GaSb superlattices (SLs). We employ a semi-analytical plane wave expansion method in conjunction with an 8-band k ⋅ p Hamiltonian to compute the SL electronic structure, paying careful attention to the identification and mitigation of spurious solutions. The calculated SL eigenstates are used directly to compute spontaneous emission spectra and the radiative recombination coefficient B . We elucidate the origin of the relatively large B coefficients in InAs/GaSb SLs which, despite the presence of spatially indirect (type-II-like) carrier confinement, are close to that of bulk InAs and compare favourably to those calculated for mid-infrared type-I pseudomorphic and metamorphic quantum well structures having comparable emission wavelengths. Our analysis explicitly quantifies the roles played by carrier localisation (specifically, partial delocalisation of bound electron states) and miniband formation (specifically, miniband occupation and optical selection rules) in determining the magnitude of B and its temperature dependence. We perform a high-throughput optimisation of the room temperature B coefficient in InAs/GaSb SLs across the 3.5–7 μ m wavelength range, quantifying the dependence of B on the relative thickness of the electron-confining InAs and hole-confining GaSb layers. This analysis provides guidance for the growth of optimised SLs for mid-infrared light emitters. Our results, combined with the expected low non-radiative Auger recombination rates in structures having spatially indirect electron and hole confinement, corroborate recently observed high output power in prototype InAs/GaSb SL inter-band cascade light-emitting diodes. [ABSTRACT FROM AUTHOR]

Asymmetric molecular bonds possess a microscopic second-order nonlinear optical polarizability p(2). Crystals built from them possess a macroscopic second-order nonlinear optical susceptibility, χ(2), if their structure lacks centrosymmetry. χ(2) can be enhanced by introducing additional asymmetry at the meta-structural level. Here, we use a dipole matrix formalism to calculate χ(2) of asymmetric GaAs/AlGaAs coupled quantum well structures at telecommunication frequencies, for which interband (rather than previously considered intersubband) optical transitions govern optical nonlinearities. Using unit cell and envelope wavefunctions and considering all possible transitions between two bound electron and two bound hole states, we predict tenfold enhancement in χ(2) in previously underexplored ranges of quantum well asymmetry and coupling barrier thickness. This work paves the way toward enhanced, tailorable second-order optical nonlinearities for semiconductor digital alloy and superlattice structures. [ABSTRACT FROM AUTHOR]

In order to shift the light emission of nitride quantum structures towards the red color, the technological problem of low In incorporation in InGaN−based heterostructures has to be solved. To overcome this problem, we consider superlattices grown on InGaN buffers with different In content. Based on the comparison of the calculated ab initio superlattice band gaps with the photoluminescence emission energies obtained from the measurements on the specially designed samples grown by metal-organic vapor phase epitaxy, it is shown that by changing the superlattice parameters and the composition of the buffer structures, the light emission can be shifted to lower energies by about 167 nm (0.72 eV) in comparison to the case of a similar type of superlattices grown on GaN substrate. The importance of using superlattices to achieve red emission and the critical role of the InGaN buffer are demonstrated. [ABSTRACT FROM AUTHOR]

A strong polarization electric field (PEF) in III‐nitride semiconductors has adverse effects on the performance of III‐N light‐emitting diodes (LEDs). However, to reduce the PEF of III‐N LEDs based on the polar plane (c‐plane) remains a big challenge. Here, a weak PEF blue LED on c‐plane GaN with an InGaN/AlGaN superlattices (SLs) quantum barrier and InGaN quantum well grown by metal‐organic chemical vapor deposition is proposed. The InGaN/SLs multiple quantum wells (MQWs) show a significant reduction of PEF compared with traditional InGaN/GaN MQWs. A low electric field of 0.5 MV cm−1 is obtained, which is confirmed by power‐dependent photoluminescence (PL) and time‐resolved PL measurements. The proposed LEDs with InGaN/AlGaN SLs barriers present a low efficiency droop, a great wavelength stability, and a high −3 dB E‐O modulation bandwidth up to 1.01 GHz. This work demonstrates the role of InGaN/AlGaN SLs barriers in reducing the PEF of III‐N LEDs and provides a feasible technical route to achieve the weak PEF LEDs on c‐plane, which can be expanded and applied to the preparation of high‐performance, long wavelength, and deep UV III‐N LEDs. [ABSTRACT FROM AUTHOR]

The rocksalt structure of ZnO has a very promising bandgap for optoelectronic applications. Unfortunately, this high-pressure phase is unstable under ambient conditions. This paper presents experimental results for rocksalt-type ZnO/MgO superlattices and theoretical considerations of the critical thickness of MgxZn1−xO layers. The correlations between the layer/spacer thickness ratio, elastic strain, chemical composition, and critical thickness are analyzed. The Matthews and Blakeslee model is revisited to find analytic conditions for the critical layer thickness resulting in phase transition. Our analysis shows that due to the decrease in misfit stresses below some critical limit, the growth of multiple quantum wells composed of rocksalt ZnO layers and MgO spacers is possible only for very large layer/spacer thickness ratios. [ABSTRACT FROM AUTHOR]

The production of superlattices with pseudomorphically strained quantum wells (QWs) {InxGa1–xAs/GaAs} grown by molecular beam epitaxy on GaAs substrates with (100), (110), and (111)A crystallographic surface orientations is reported. The quality of the crystal structure of epitaxial samples is assessed using the atomic force microscopy of their surface. The manifestation of a piezoelectric field in the photoluminescence spectra is reported. [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]

Bandgap engineering and quantum confinement in semiconductor heterostructures provide the means to fine‐tune material response to electromagnetic fields and light in a wide range of the spectrum. Nonetheless, forming semiconductor heterostructures on lattice‐mismatched substrates is a challenge for several decades, leading to restrictions for device integration and the lack of efficient devices in important wavelength bands. Here, it is shown that the van der Waals epitaxy of 2D GaSe and InSe heterostructures occur on substrates with substantially different lattice parameters, namely silicon and sapphire. The GaSe/InSe heteroepitaxy is applied in the growth of quantum wells and superlattices presenting photoluminescence and absorption related to interband transitions. [ABSTRACT FROM AUTHOR]

Weakly coupled semiconductor superlattices under DC voltage bias are nonlinear systems with many degrees of freedom whose nonlinearity is due to sequential tunneling of electrons. They may exhibit spontaneous chaos at room temperature and act as fast physical random number generator devices. Here we present a general sequential transport model with different voltage drops at quantum wells and barriers that includes noise and fluctuations due to the superlattice epitaxial growth. Excitability and oscillations of the current in superlattices with identical periods are due to nucleation and motion of charge dipole waves that form at the emitter contact when the current drops below a critical value. Insertion of wider wells increases superlattice excitability by allowing wave nucleation at the modified wells and more complex dynamics. Then hyperchaos and different types of intermittent chaos are possible on extended DC voltage ranges. Intrinsic shot and thermal noises and external noises produce minor effects on chaotic attractors. However, random disorder due to growth fluctuations may suppress any regular or chaotic current oscillations. Numerical simulations show that more than 70% of samples remain chaotic when the standard deviation of their fluctuations due to epitaxial growth is below 0.024 nm (10% of a single monolayer) whereas for 0.015 nm disorder suppresses chaos. [ABSTRACT FROM AUTHOR]

Quaternary dilute nitride compound semiconductors like Ga As y P 1 − x − y N x and Ga 1 − z In z P 1 − x N x are lattice matched with silicon when y = 4.7 * x − 0.1 and z = 2.2 * x − 0.044 and also have direct bandgaps (with N > 0.6%), thus allowing for monolithic integration of III-V optoelectronics with silicon technology as well as III-V/Si tandem junction solar cells. By applying an eight-band k.p strained (tensile or compressive) Hamiltonian and a Band Anti-crossing model (to account for small amounts of nitrogen impurities) to the conduction band, the electronic band structure and the dispersion relation of these alloys can be determined near the center of Brillouin Zone. In this work, by minimizing the total mechanical energy of the stack of alternating layers of Ga P 1 − x N x and Ga As 1 − x N x , we have evaluated the ratio of thickness of the respective layers for a strain-balanced superlattice Ga As 1 − x N x / Ga P 1 − x N x structure on silicon. We calculated the confinement energies and the corresponding states of the respective carriers inside a quantum well (with and without resonantly coupled) or in the miniband of a superlattice structure as a function of the nitrogen composition using a transfer matrix approach under the envelope function approximation. Incorporating only a small amount of nitrogen (<5%), the bandgap of these lattice matched structures fulfils the optimum bandgap requirement of (1.65–1.8) eV for III-V/Si tandem solar cells and optoelectronic devices. The optical-absorption coefficient, in both symmetric and asymmetric quantum wells, is then evaluated with respect to nitrogen composition and temperature by using the Fermi-golden rule for both TE and TM polarization of incident light, including the effect of excitons and thermal broadening. [ABSTRACT FROM AUTHOR]

We investigate the In content in single monolayer (ML)-thick InxGa1-xN quantum wells (QWs) as a function of the growth temperature ranging from 650 °C to 480 °C, stacked in a superlattice (SL). The SLs were grown by plasma-assisted molecular beam epitaxy using high N-flux. For the evaluation of the indium concentrations, scanning transmission electron microscopy high angle annular dark field (STEM-HAADF) studies were combined with local lattice parameter measurements obtained from high-resolution transmission electron microscopy (HRTEM) images. The mean In content in the QWs increases from 11% to 23% when the growth temperature decreases from 650 °C to 610 °C. Further decrease in the growth temperature results in a saturation of the mean In content. Our experiments show that a substantial reduction of the growth temperature is not a practical way to obtain pseudomorphically grown InN MLs on GaN(0001). The InGaN QW thickness is limited to 1 ML and is not affected by a change of growth temperature. For two SL structures grown at constant temperatures of 640 °C and 600 °C, increase in the In content in the QWs causes a shift in the peak emission from 382 to 395 nm, as was measured by cathodoluminescence at 7 K. The application of X-ray diffraction studies to analyze the composition of InGaN ML-thick QWs in SLs is discussed. [ABSTRACT FROM AUTHOR]

A deep-ultraviolet (DUV) laser diode (LD) consisting of specifically designed cladding layers involving superlattice nitride alloy has been proposed. Simulation studies of different cladding layers were carried out using Crosslight software. It was found that the proposed structure effectively suppresses the leakage of the optical field from the active region and the optical confinement coefficient is 1.45 times higher than that of the conventional structure. The proposed structure has a significant increase in laser power with a low threshold current. Moreover, the introduction of novel cladding layer suppresses the electron and hole leakage from the multiple quantum well (MQW) region, which provides an attractive solution for increasing the stimulated recombination rate in the MQW region leading to the improvement in the performance of the DUV LD. [ABSTRACT FROM AUTHOR]

Modelling of quantum cascade laser (QCL) structures, despite a regular progress in the field, still remains a complex task in both analytical and numerical aspects. Computer simulations of such nanodevices require large operating memories and effective algorithms to be applied. Promisingly, by applying semi-analytical polynomial approximation method to computing potential, wave functions and electron charge distribution, accurate results and quick convergence of the self-consistent solution for the Schrödinger and Poisson equations are reachable. Additionally, such an approach makes the respective numerical models competitively effective. For contemporary QCL structures, with quantum wells quite typically forming complex systems, a special approach to determining self energies and coefficients of approximating polynomials is required. Under this paper we have analysed whether the polynomial approximation method can be successfully applied to solving the Schrödinger equation in QCL. A new algorithm for determining self energies has been proposed and a new method has been optimised for the researched structures. The developed solutions have been implemented as a new module for the finite model of the superlattice (FMSL) and tested on the QCL emitting light in the mid-infrared range. [ABSTRACT FROM AUTHOR]

Generating large topologically protected surface currents using conventional III–V infrared materials such as InAsSb/InGaSbAs quantum wells (QWs) and superlattices (SLs) has been important. In such materials, topological states can be formed at the edge by hybridizing ordinary electronic band structures. However, achieving large surface currents out of these materials is still difficult due to low emission currents and high carrier defects. In this work, we present two hybridized topological structures: one for the 6.22 Å metamorphic QWs and the other for the 6.10 Å pseudomorphic SLs. Both structures are tailored for the same hybridization gap (Δ) of ∼60 meV and optimized for the minimum crystal defects. While the QW grown on metamorphic buffers generates a significant amount of mismatch-related crystal defects, the SL grown on lattice-matched buffers produces an excellent crystalline-quality. Quasiparticle interference mapping and calculations on a SL sample show good agreement of the band structure. [ABSTRACT FROM AUTHOR]

This paper advocates for an innovative approach designed for estimating optoelectronic properties of quantum structures utilizing Tight-Binding (TB) theory. Predicated on the comparative analysis between estimated and actual properties, the study strives to validate the efficacy of this proposed technique; focusing notably on the computation of bandgap energy. It is observed that preceding methodologies offered a restricted accuracy when predicting complex structures like super-lattices and quantum wells. To address this gap, we propose a methodology involving three distinct phases using orbital interaction parameters (OIPs) and the TB theory. The research employed Aluminium Arsenide (AlAs) and Gallium Arsenide (GaAs) as the primary bulk materials. Our novel approach introduces a computation framework that first focuses on bulk computation, subsequently expanding to super-lattice structures. The findings of this research demonstrate promising results regarding the accuracy of predicated optoelectronic properties, particularly the cut-off wavelength. This study paves the way for future research, potentially enhancing the precision of the proposed methodology and its application scope within the field of quantum optoelectronics. • Novel approach that optimizes the Orbital Interaction Parameters to enhance the estimation of optoelectronic properties within the ETBM. [ABSTRACT FROM AUTHOR]

Self‐assembly of colloidal nanocrystals into ordered superlattices is a powerful approach to enable novel collective properties which are not available in individual colloids. However, to date, it remains a major challenge to develop a practical route to modulate such collective properties for potential photonic applications. Herein, it is shown that the collective emission properties in colloidal quantum well (CQW) superlattices, including emission color and anisotropy, can be effectively modulated in a binary host–guest architecture. The experimental and theoretical results reveal that excitons of the host (i.e., the undoped CQWs) generated by photoexcitation can be controllably harvested by the guest (i.e., the Cu‐doped CQWs) for light emission, owing to an exciton hopping assisted exciton trapping process. Such a nano‐building block with tunable collective optical properties may enlighten novel colloidal material‐based photonic applications, including optical anti‐counterfeiting, next‐generation liquid crystal displays, and multifunctional biological markers. [ABSTRACT FROM AUTHOR]

We investigate designed InN/GaN superlattices (SLs) grown by plasma-assisted molecular beam epitaxy on c-plane GaN templates in situ by line-of-sight quadrupole mass spectroscopy and laser reflectivity, and ex situ by scanning transmission electron microscopy, X-ray diffraction, and photoluminescence (PL). The structural methods reveal concordantly the different interface abruptness of SLs resulting from growth processes with different parameters. Particularly crucial for the formation of abrupt interfaces is the Ga to N ratio that has to be bigger than 1 during the growth of the GaN barriers, as Ga-excess GaN growth aims at preventing the unintentional incorporation of In accumulated on the growth surface after the supply of InN, that extends the (In,Ga)N quantum well (QW) thickness. Essentially, even with GaN barriers grown under Ga-excess yielding to 1 monolayer (ML) thick QWs, there is a real discrepancy between the designed binary InN and the actual ternary (In,Ga)N ML thick QWs revealed by the above methods. The PL emission line of the sample with atomically abrupt interfaces peaks at 366 nm, which is consistent with the In content measured to be less than 10%. [ABSTRACT FROM AUTHOR]

InGaN/GaN quantum wells (QWs) with sub-nanometer thickness can be employed in short-period superlattices for bandgap engineering of efficient optoelectronic devices, as well as for exploiting topological insulator behavior in III-nitride semiconductors. However, it had been argued that the highest indium content in such ultra-thin QWs is kinetically limited to a maximum of 33%, narrowing down the potential range of applications. Here, it is demonstrated that quasi two-dimensional (quasi-2D) QWs with thickness of one atomic monolayer can be deposited with indium contents far exceeding this limit, under certain growth conditions. Multi-QW heterostructures were grown by plasma-assisted molecular beam epitaxy, and their composition and strain were determined with monolayer-scale spatial resolution using quantitative scanning transmission electron microscopy in combination with atomistic calculations. Key findings such as the self-limited QW thickness and the non-monotonic dependence of the QW composition on the growth temperature under metal-rich growth conditions suggest the existence of a substitutional synthesis mechanism, involving the exchange between indium and gallium atoms at surface sites. The highest indium content in this work approached 50%, in agreement with photoluminescence measurements, surpassing by far the previously regarded compositional limit. The proposed synthesis mechanism can guide growth efforts towards binary InN/GaN quasi-2D QWs. [ABSTRACT FROM AUTHOR]

We report a GaSb-based type-I quantum well cascade diode laser emitting at nearly 2-μm wavelength. The recycling of carriers is realized by the gradient AlGaAsSb barrier and chirped GaSb/AlSb/InAs electron injector. The growth of quaternary digital alloy with a gradually changed composition by short-period superlattices is introduced in detail in this paper. And the quantum well cascade laser with 100-μm-wide, 2-mm-long ridge generates an about continuous-wave output of 0.8 W at room temperature. The characteristic temperature T0 is estimated at above 60 K. [ABSTRACT FROM AUTHOR]

The paper presents a theoretical study of the modulation instability of a continuous or quasicontinuous optical probe in a three level quantum well system under electromagnetically induced transparency. The modulation instability is affected by the control field detuning, as well as even-order dispersion and by the strength of Kerr (third-order) and quintic (fifth-order) nonlinearities. The fourth-order dispersion reduces the bandwidth over which modulation instability occurs, whereas the quintic nonlinearity saturates the growth of the modulation instability. Detuning the control field from resonance can significantly reduce the growth of the modulation instability at both low and high power levels. At low powers, the system becomes stable against modulation instability for small detuning of the control field and at high powers modulation instability disappears for larger detuning. [ABSTRACT FROM AUTHOR]

Damped oscillations of the current were observed in the transient current pulse characteristics of a 30-period weakly coupled GaAs/AlGaAs superlattice (SL). The switching time of the current is exponentially decreased as the voltage is verged towards the current discontinuity region indicating that the space charge necessary for the domain boundary formation is gradually accumulated in a certain SL period in a timescale of several hundreds ns. The spectral features in the electroluminescence spectra of two connected in parallel SL mesas correspond to the energy of the intersubband transitions and the resonance detuning of subbands caused by charge trapping in the quantum wells (QWs) residing in a region of the expanded domain boundary. The obtained results support our understanding of the origin of self-oscillations as a cyclic dynamics of the subband structure in the QWs forming the expanded domain boundary. [ABSTRACT FROM AUTHOR]

The internal quantum efficiency (IQE) of InGaN/GaN multiple quantum wells (MQWs) with blue light emission was improved by inserting an InGaN/GaN superlattice (SL) beneath the MQWs. While the SL technique is useful for improving the light-emitting diode (LED) performance, its effectiveness from a multilateral point of view requires investigation. V-shaped pits (V-pits), which generate a potential barrier and screen the effect of the threading dislocation, are one of the candidates for increasing the light emission efficiency of LEDs exceptionally. In this research, we investigated the relationship between the V-pit and SL and revealed that the V-pit diameter is strongly correlated with the IQE by changing the number of SL periods. Using scanning near-field optical microscopy and photoluminescence measurements, we demonstrated the distinct presence of the potential barrier formed by the V-pits around the dislocations. The relationship between the V-pit and the number of SL periods resulted in changing the potential barrier height, which is related to the V-pit diameter determined by the number of SL periods. In addition, we made an attempt to insert pit expansion layers (PELs) composed of combination of SL and middle temperature grown GaN layer instead of only SL structure. As a result of the evaluation of LEDs using SL or PEL, the EL intensity was strongly related to pit diameter regardless of the structures to form the V-pits. In addition, it was clear that larger V-pits reduce the efficiency droop, which is considered to be suppression of the carrier loss at high injection current. [ABSTRACT FROM AUTHOR]

In this work, a mid-wavelength infrared separate absorption and multiplication avalanche photodiode (SAM-APD) with 100% cut-off wavelength of ~ 5.0 µm at 200 K grown by molecular beam epitaxy was demonstrated. The InAsSb-based SAM-APD device was designed to have electron dominated avalanche mechanism via the band structure engineered multi-quantum well structure based on AlAsSb/GaSb H-structure superlattice and InAsSb material in the multiplication region. The device exhibits a maximum multiplication gain of 29 at 200 K under -14.7 bias voltage. The maximum multiplication gain value for the MWIR SAM-APD increases from 29 at 200 K to 121 at 150 K. The electron and hole impact ionization coefficients were derived and the large difference between their value was observed. The carrier ionization ratio for the MWIR SAM-APD device was calculated to be ~ 0.097 at 200 K. [ABSTRACT FROM AUTHOR]

The strength of the interlayer exchange coupling in [Fe/MgO] N (001) superlattices with 2 ≤ N ≤ 10 depends on the number of bilayer repeats (N). The exchange coupling is antiferromagnetic for all the investigated thicknesses while being nine times larger in a sample with N = 4 as compared to N = 2. The sequence of the magnetic switching in two of the samples (N = 4, N = 8) is determined using polarized neutron reflectometry. The outermost layers are shown to respond at the lowest fields, consistent with having the weakest interlayer exchange coupling. The results are consistent with the existence of quantum well states defined by the thickness of the Fe and the MgO layers as well as the number of repeats (N) in [Fe/MgO] N (001)superlattices. [ABSTRACT FROM AUTHOR]

The article explores the artificial visual systems enabled by quasi–two-dimensional electron gases in oxide super lattice nanowires. It mentions the rapid development of artificial intelligence techniques ignites the emerging demand on accurate perception and understanding of optical signals from external environments via brain-like visual systems.

Heterostructures with multiple strongly coupled quantum wells, such as super-multiperiod (SMP) superlattices (SLs), are promising semiconductor devices, which may contain hundreds or even thousands of layers with 100 or more periods synthesized by molecular beam epitaxy (MBE) to high structural perfection. The proposed characterization method employs matched application of high-resolution x-ray diffractometry (XRD), reflectometry (XRR), and, for the first time, the deep XRR (DXRR) allows the study of SMP structures, as well as high-accuracy determination of the thicknesses of layers, roughness/diffuseness of boundaries using the rigorous scattering theory, and composition of solid solutions. Combining these methods with scanning transmission electron microscopy (STEM) enables characterization of SMP SLs and independent determination of these same parameters. The differences between the expected and obtained layer thicknesses by XRD and XRR were 1%–3% for AlGaAs/GaAs structures. The samples were characterized by sharp interfaces with the root-mean-square width of the transition layers of the order of a few Ĺ, which is consistent with the XRR/DXRR and STEM analysis. Based on the data obtained for the thicknesses of layers, the composition of Al0.3Ga0.7As has been accurately determined by the x-ray methods. These results may be considered as the first step in the analysis of MBE-grown SMP structures with a number of periods up to 1000. [ABSTRACT FROM AUTHOR]

Elastically strained supermultiperiod (100–1000 periods) AlGaAs/GaAs superlattices with different doping levels and slightly differing period thicknesses have been investigated. The proposed technique of characterization consisting of combined application of deep X-ray reflectometry based on a rigorous calculation method, as well as the well-known method of high-resolution X-ray reflectometry, has made it possible to investigate 100-period structures with 2-nm-wide Al0.3Ga0.7As barriers and 10-nm-wide GaAs wells and determine with a high accuracy the layer thicknesses and spread of interfaces. This achievement can be considered as a first step in further analysis of thick structures using bright synchrotron radiation source. The difference between the expected and obtained by the proposed method layer thicknesses is several percent, including that for samples with a high doping level (up to 1018 cm–3). All supermultiperiod structures are characterized by sharp interfaces with a standard deviation of about 0.1 nm. Based on the obtained data on the thicknesses, one can accurately determine the layer compositions using high-resolution X-ray diffraction. [ABSTRACT FROM AUTHOR]

To investigate the effect of the miniband formation on the optical absorption spectrum, we adopted two non-destructive methodologies of piezoelectric photothermal (PPT) and photoreflectance (PR) spectroscopies for strain-balanced InGaAs/GaAsP multiple quantum-well (MQW) and superlattice (SL) structures inserted GaAs p-i-n solar cells. Because the barrier widths of the SL sample were very thin, miniband formations caused by coupling the wave functions between adjacent wells were expected. From PR measurements, a critical energy corresponding to the inter-subband transition between first-order electron and hole subbands was estimated for MQW sample, whereas two critical energies corresponding to the mini-Brillouin-zone center (Γ) and edge (π) were obtained for SL sample. The miniband width was calculated to be 19 meV on the basis of the energy difference between Γ and π. This coincided with the value of 16 meV calculated using the simple Kronig-Penney potential models. The obtained PPT spectrum for the SL sample was decomposed into the excitonic absorption and inter-miniband transition components. The latter component was expressed using the arcsine-like signal rise corresponding to the Γ point in the mini-Brillouin zone that was enhanced by the Sommerfeld factor. The usefulness of the PPT methodology for investigating the inserted MQW and/or SL structure inserted solar cells is clearly demonstrated. [ABSTRACT FROM AUTHOR]

It clearly demonstrates that computer modeling of the dispersion and optical response of semiconductor quantum wells and nanostructures provides a wide field for such kind of activity. Semiconductor quantum wells and nanostructures have been the main quantum and classical physical objects in condensed matter physics for over half a century, since the discovery of the two-dimensional electron gas in silicon MOSFETs and size quantization in thin bismuth films. This Special Issue of I Nanomaterials i is devoted to the latest news in the physics of Semiconductor Quantum Wells and Nanostructures connected with different aspects of contemporary condense matter physics. [Extracted from the article]

The electrical properties, electroluminescence (EL) power output and deep trap spectra were studied before and after 5 MeV electron irradiation of near-UV single-quantum-well (SQW) light-emitting diodes (LED) structures differing by the presence or absence of InAlN superlattice underlayers (InAlN SL UL). The presence of the underlayer is found to remarkably increase the EL output power and the radiation tolerance of LEDs, which correlates with a much lower and more slowly changing density of deep traps in the QW region with radiation dose, and the higher lifetime of charge carriers, manifested by higher short-circuit current and open-circuit voltage in current–voltage characteristics under illumination. The observed phenomena are explained by the capture of native defects segregated at the growing surface by In atoms in the underlayer which traps them in the underlayer and prevents their penetration into the QW region. [ABSTRACT FROM AUTHOR]

Direct integration of III–V semiconductor light sources on silicon is an essential step toward the development of portable, on-chip infrared sensor systems. Driven by the presence of characteristic molecular fingerprints in the mid-infrared (MIR) spectral region, such systems may have a wide range of applications in infrared imaging, gas sensing, and medical diagnostics. This paper reports on the integration of an InAs virtual substrate and high crystalline quality InAs/InAsSb multi-quantum wells on Si using a three-stage InAs/GaSb/Si buffer layer. It is shown that the InAs/GaSb interface demonstrates a strong dislocation filtering effect. A series of strained AlSb/InAs dislocation filter superlattices was also used, resulting in a low surface dislocation density of approximately 4 × 107 cm−2. The InAs/InAsSb wells exhibited a strong photoluminescence signal at elevated temperatures. Analysis of these results indicates that radiative recombination is the dominant recombination mechanism, making this structure promising for fabricating MIR Si-based sensor systems. [ABSTRACT FROM AUTHOR]

The thermoelectric and physical properties of superlattices consisting of modulation doped Ge quantum wells inside Si1-yGey barriers are presented, which demonstrate enhancements in the thermoelectric figure of merit, ZT, and power factor at room temperature over bulk Ge, Si1-yGey, and Si/Ge superlattice materials. Mobility spectrum analysis along with low temperature measurements indicate that the high power factors are dominated by the high electrical conductivity from the modulation doping. Comparison of the results with modelling using the Boltzmann transport equation with scattering parameters obtained from Monte Carlo techniques indicates that a high threading dislocation density is also limiting the performance. The analysis suggests routes to higher thermoelectric performance at room temperature from Si-based materials that can be fabricated using micro- and nano-fabrication techniques. [ABSTRACT FROM AUTHOR]

We report on the observation of intersubband absorption in GaN/AlN quantum well superlattices grown on [formula]-oriented GaN. The absorption is tuned in the 1.5-4.5 μm wavelength range by adjusting the well thickness. The semipolar samples are compared with polar samples with identical well thickness grown during the same run. The intersubband absorption of semipolar samples shows a significant red shift with respect to the polar ones due to the reduction of the internal electric field in the quantum wells. The experimental results are compared with simulations and confirm the reduction of the polarization discontinuity along the growth axis in the semipolar case. The absorption spectral shape depends on the sample growth direction: for polar quantum wells the intersubband spectrum is a sum of Lorentzian resonances, whereas a Gaussian shape is observed in the semipolar case. This dissimilarity is explained by different carrier localization in these two cases. [ABSTRACT FROM AUTHOR]

We report strong mid-infrared absorption of in-plane polarized light due to heavy-to-light hole intersubband transitions in the valence band of C-doped GaAs quantum wells with AlAs barriers. The transition energies are well reproduced by theoretical calculations including layer inter-diffusion. The inter-diffusion length was estimated to be 8 ± 2 Å, a value that is consistent with electron microscopy measurements. These results highlight the importance of modeling the nanoscale structure of the semiconductors for accurately reproducing intra-band transition energies of heavy carriers such as the holes. [ABSTRACT FROM AUTHOR]

A new type of photovoltaic device where GaAs/GaInNAs multiple quantum wells (MQW) or superlattice (SL) are inserted in the i-region of a GaAs p-i-n solar cell (SC) is presented. The results suggest the device can reach record efficiencies for single-junction solar cells. A theoretical model is developed to study the performance of this device. The conversion efficiency as a function of wells width and depth is modeled for MQW solar cells. It is shown that the MQW solar cells reach high conversion efficiency values. A study of the SL solar cell viability is also presented. The conditions for resonant tunneling are established by the matrix transfer method for a superlattice with variable quantum wells width. The effective density of states and the absorption coefficient for SL structure are calculated in order to determinate the J-V characteristic. The influence of superlattice length on the conversion efficiency is researched, showing a better performance when width and cluster numbers are increased. The SL solar cell conversion efficiency is compared with the maximum conversion efficiency obtained for the MQW solar cell and shows an efficiency enhancement. [ABSTRACT FROM AUTHOR]

It is well-known that it is crucial to insert either a single InGaN underlayer or an InGaN superlattice (SLS) structure (both with low InN content) as a pre-layer prior to the growth of InGaN/GaN multiple quantum wells (MQWs) served as an active region for a light-emitting diode (LED). So far, this growth scheme has achieved a great success in the growth of III-nitride LEDs on c-plane substrates, but has not yet been applied in the growth of any other orientated III-nitride LEDs. In this paper, we have applied this growth scheme in the growth of semi-polar (11–22) green LEDs, and have investigated the impact of the SLS pre-layer on the optical performance of semi-polar (11–22) green LEDs grown on patterned (113) silicon substrates. Our results demonstrate that the semi-polar LEDs with the SLS pre-layer exhibit an improvement in both internal quantum efficiency and light output, which is similar to their c-plane counterparts. However, the performance improvement is not so significant as in the c-plane case. This is because the SLS pre-layer also introduces extra misfit dislocations for the semi-polar, but not the c-plane case, which act as non-radiative recombination centres. [ABSTRACT FROM AUTHOR]

Hot electrons generated by the absorption of high-energy photons typically thermalize by dissipating energy through phonon mediated relaxation pathways. Existence of non-equilibrium optical phonon populations (hot phonons) can suppress thermalization of electrons by facilitating a stable hot carrier population. In this work, we derive optical phonon scattering rates in InAs/AlSb superlattices using first-principles calculations and compare them with bulk InAs. Computations are performed for the two shortest period superlattices with equal widths of InAs and AlSb. While in pure InAs, the highest optical phonon scattering rates reach 5 cm−1; in the superlattice, the peak scattering rates of optical phonons with frequencies above the energy gap, are much smaller, reaching ∼1 cm−1. These lower scattering rates in the superlattice have important implications for design of high efficiency hot carrier solar cells and explain some of the robust hot carriers observed recently in InAs/AlAsSb quantum wells. [ABSTRACT FROM AUTHOR]

In the presented work, the influence of the quantum well and barrier thicknesses on optical characteristics of InGaAs/AlInAs superlattices was reported. Six different structures of In0.53Ga0.47As/Al0.48In0.52As superlattices lattice-matched to InP were grown by low pressure metal organic vapour phase epitaxy (LP-MOVPE). Optical properties of the obtained structures were examined by means of photoluminescence spectroscopy. This technique allows quick, simple and non-destructive measurements of radiative optical transitions in different semiconductor heterostructures. The analysis of recorded photoluminescence spectra revealed the influence of the quantum well and barrier thicknesses on the emission line energy. [ABSTRACT FROM AUTHOR]

Promising semiconductor devices, such as quantum-cascade lasers and similar to them heterostructures with multiple strongly-coupled quantum wells, may contain hundreds and even thousands of layers. Independent methods of photoluminescence (PL) and transmission electron microscopy (TEM), along with X-ray diffractometry and reflectometry, make it possible to characterize super-multiperiod (SMP—superlattices with 100 and more periods) structures and to determine, with high accuracy, the thicknesses of layers, the roughness/diffusion of interfaces and the composition of solid solutions. The difference between the expected and measured using PL and TEM thicknesses of the layers of GaAs/AlGaAs samples synthesized by molecular beam epitaxy was 5–10%. The SMP structures are characterized by stable thicknesses of the layers in depth and sharp boundaries with a width of transition interfaces of the order of several Å that is consistent with the X-ray analysis. Based on the data obtained on thicknesses of layers, by comparing the theoretical and experimental values of photoluminescence intensities it is possible to accurately determine the composition of Al0.3Ga0.7As which corresponds to the X-ray diffraction data. [ABSTRACT FROM AUTHOR]

Heterostructures with strongly-coupled multiple quantum wells, such as super-multiperiod superlattices with high perfection, may contain hundreds of layers, whose thicknesses can vary by orders of magnitude. The proposed method of characterization, consisting of the matched application of high-resolution X-ray diffractometry and reflectometry, makes it possible to study super-multiperiod structures of various types, including those with long periods and thin layers, and with high accuracy to determine the thicknesses of layers and roughness/diffuseness of boundaries. The difference between the expected and resulting thicknesses of the layers was 2–7% and 1–3% for the type I (InAs/GaAs) and type II (GaAs/Al0.3Ga0.7As) samples, respectively. Both types of structures are characterized by sharp interfaces with the RMS width of the transition layers of the order of several Å. Based on the best solution of inverse scattering problems, it is possible to determine with high accuracy both the morphology of the layers and their composition. That can be considered as the first step in the analysis of structures with a very large number of periods. [ABSTRACT FROM AUTHOR]

Optical properties of intentionally disordered multiple quantum well (QW) system embedded in a wide AlGaAs parabolic well were investigated by photoluminescence (PL) measurements as functions of the laser excitation power and the temperature. The characterization of the carriers localized in the individual wells was allowed due to the artificial disorder that caused spectral separation of the photoluminescence lines emitted by different wells. We observed that the photoluminescence peak intensity from each quantum well shifted to high energy as the excitation power was increased. This blue-shift is associated with the filling of localized states in the valence band tail. We also found that the dependence of the peak intensity on the temperature is very sensitive to the excitation power. The temperature dependence of the photoluminescence peak energy from each QW was well fitted using a model that takes into account the thermal redistribution of the localized carriers. Our results demonstrate that the band tails in the studied structures are caused by alloy potential fluctuations and the band tail states dominate the emission from the peripheral wells. [ABSTRACT FROM AUTHOR]

We study energy-band engineering with nitrogen delta (δ)-doping in GaAs-related quantum structures. A tight-binding calculation indicates that the band structure can be engineered by introducing the one-dimensional doping profile of nitrogen into GaAs. Using molecular beam epitaxy, we prepare δ-doped samples of AlGaAs/GaAs quantum wells and GaAs/δ-doped nitrogen superlattice structures at the growth temperature 560 °C. Photoluminescence obtained from the samples shows a clear redshift of the spectral peak positions dependent on the nitrogen coverage. The transition energies of the superlattice structures agree well with those obtained from photoreflectance, indicating the feasibility of band modification with a single or a multiple nitrogen δ-doped layer. [ABSTRACT FROM AUTHOR]

The internal well polarization field in InGaN quantum wells (QWs), surrounded by strain-compensated (InxGa1-xN)/(Al0.065Ga0.935N) ultra-thin superlattice (SC-SL) barriers with different indium composition, is investigated. The indium composition of InGaN constituent of superlattice barriers has been varied in the range from 0.04 to 0.18. It is observed that the increase of indium composition of InGaN into the barrier results in a strong blue-shift of the peak wavelength of the room-temperature photoluminescence (RT-PL) and the significant increase in the intensity of the luminescence emission until too much indium is added into InGaN layers of superlattice barriers. From the bias-dependent photoluminescence measurements, it is determined that the blue-shift and intensity increase of the emission are caused by the decrease of well polarization field as the indium composition in InGaN of SC-SL barrier increases. In case of In0.16Ga0.84N containing SC-SL barriers, the well internal polarization field is greatly reduced to -0.33 MV/cm from -1.5 MV/cm with respect to typical GaN barriers, indicating that the internal field reduction similar to that obtained in semi-polar InGaN/GaN quantum wells can be obtained by applying the strain-compensating barrier to polar substrates. [ABSTRACT FROM AUTHOR]

An epitaxied AlGaAs/GaAs/AlGaAs heterostructure is studied by using the low frequency noise technique over a wide range of temperature from 300 K down to 4 K as a function of bias. Emphasis is placed on the generation—recombination noise because it is present on a wide range of temperature and frequency. Thermal activation energies and capture cross sections of traps responsible for this kind of noise are extracted from an arrhenius plot. Six kinds of traps have been identified and their electrical nature has been suggested. [ABSTRACT FROM AUTHOR]

Interband-cascade infrared photodetectors (ICIPs), composed of discrete superlattice absorbers, are demonstrated at temperatures up to 350 K with a cutoff wavelength near 5 μm at 80 K to beyond 7 μm above room temperature. The peak responsivity exceeds 200 mA/W, higher than the values reported from early interband cascade laser structures, suggesting a significantly enhanced quantum efficiency of the superlattice absorbers. A theoretical model, originally developed for quantum well infrared photodetectors (QWIPs), is applied to ICIPs to analyze their device performance. The Johnson-limited and background-limited detectivities are extracted and indicate that background-limited performance temperatures for two ICIP structures are 126 and 105 K at 5 μm. It is expected that optimized ICIPs will provide improved performance by combining the advantages of conventional photodiodes and the discrete nature of QWIPs and IC lasers. [ABSTRACT FROM AUTHOR]