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Excitons are expected to be a high‐efficiency emission source in UV light‐emitting devices. However, the damping of the excitonic laser oscillation has been reported under conditions where the excitonic states are expected to be populated in the conventional theory. In order to understand the exciton dynamics under the thermal nonequilibrium state, a theoretical model including various energy species in semiconductors such as electrons, phonons, and photons is required. Herein, a 2D phononic–excitonic–radiative model is constructed to analyze the exciton dynamics in a 2D system. 2D excitons with four principal quantum number states and the continuum in the lowest energy level of the AlN/GaN/AlN quantum wells are considered. It is found that the 2D phonon significantly augments the excitation transition rate. When the high recombination rate corresponding to stimulated emission is considered, the exciton binding energy of 108 meV is not enough to reduce the population in the high‐order discreet states and the continuum states, while the binding energy of 215 meV corresponding to the one monolayer GaN has an advantage of reducing these populations. The analysis of population flux has an advantage in discussing the increase in the kinetic energy transfer to the 1S exciton. [ABSTRACT FROM AUTHOR]

To investigate nonradiative recombination processes in indium gallium nitride (InGaN)‐based red light‐emitting diodes (LEDs), an InGaN‐based red LED with a hybrid quantum well (QW) structure consisting of red and blue single quantum wells (SQWs) is characterized by micro‐photoluminescence (μ‐PL) spectroscopy. The μ‐PL mapping of the red emission reveals numerous dark spots with various sizes and contrasts. Not only the red and blue (from a blue SQW) but green emission bands are observed at some red dark spots, suggesting that indium (In) segregation is one of the causes of nonradiative recombination in the red emission. Comparing the blue and green emission images to the red emission image reveals that the dark spots in the intensity map of the red emission can be classified into four types. Through this correlative analysis, the red dark spots associated with the dark areas in the intensity map of the blue emission are attributed to the major nonradiative recombination centers in the red emission. [ABSTRACT FROM AUTHOR]

Radiative and nonradiative recombination processes are investigated in the temperature range from 10 to 500 K for AlGaN quantum wells on epitaxially laterally overgrown AlN/sapphire templates. Time‐integrated photoluminescence (PL) spectroscopy under selective excitation conditions demonstrates that the decrease in the radiative recombination efficiency with increasing temperature is one of the causes of the thermal droop in AlGaN‐based deep‐ultraviolet (DUV) light‐emitting diodes. Time‐resolved PL spectroscopy indicates that not only the decreasing nonradiative recombination lifetime but increasing radiative recombination lifetime with increasing temperature contributes to the thermal droop. The temperature dependence of the radiative recombination lifetime is discussed, revealing that luminescence linewidth is a valuable criterion for designing efficient AlGaN‐based DUV emitters. [ABSTRACT FROM AUTHOR]

The internal quantum efficiency (IQE) and cathodoluminescence intensity line profile of AlGaN multiple quantum well (MQW) structures on low‐dislocation face‐to‐face annealed sputtered AlN (FFA Sp‐AlN) and on conventional metalorganic vapor‐phase epitaxy‐grown AlN (MOVPE‐AlN) templates are evaluated and the effect of the number of quantum wells (QWs) on the IQE is discussed. The higher IQE in the FFA samples is probably due to the lower threading dislocation (TD) density; however, the IQE also increases with the number of QWs although the TD density remains constant. Effective diffusion length increases with the number of QWs, indicating that the AlGaN MQW layer helps to suppress point defect diffusion, resulting in IQE increase. Furthermore, the segregation of point defects into the TDs and point defect diffusion via the TDs may explain the difference in the IQE improvement rate between the MQWs on the FFA Sp‐AlN and MOVPE‐AlN templates. [ABSTRACT FROM AUTHOR]

The polarized photoluminescence emitted on the edge of a series of aluminum‐rich (Al,Ga)N‐AlN quantum wells (QWs) grown by molecular beam epitaxy on AlN templates deposited by metal organic chemical vapor deposition on c‐plane sapphire is measured. The contrast and the principal axis of the emission diagrams for 2 nm‐thick (Al,Ga)N QWs grown for aluminum compositions between 40% and 90% are studied. The light is emitted on the edge of the QWs at wavelengths going from 280 nm down to 209 nm. The emission diagram, a change from oblate to prolate with respect to the in‐plane orientation, for an aluminum composition is found to occur around 72%, that is, at an emission wavelength of about 235 nm. The orientations and shapes of the edge‐emission diagrams indicate that the fluctuations of the composition of the (Al,Ga)N confining layer are deep enough for producing intravalence band mixings. This property, that acts in concert with the built‐in strain and quantum‐confined Stark effect, contributes to the anisotropy of the light emission when the aluminum composition reaches 60–70%, that is, for an emission wavelength of 260–235 nm. [ABSTRACT FROM AUTHOR]

We analyze the asymmetric doping‐dependent electron mobility μ of GaAs/InGaAs/GaAs quantum well field‐effect transistor (QWFET) structure. We consider doping concentrations, nd1 and nd2, in the substrate and surface barriers, respectively, and study μ as a function of nd2, taking (nd1 + nd2) unchanged. An increase in nd2 decreases nd1, yielding interesting changes in the occupation of subbands. For well width W < 164 Å, μ is due to single subband occupancy (SSO). Around W = 164 Å, there occurs first SSO, then double subband occupancy (DSO), and again SSO with an increase in nd2. Near the transition of subbands, abrupt discontinuities in μ arise due to inter‐subband effects. Thus, high to low and then high values of μ are obtained, displaying almost flat‐like variations, symmetric about |nd2 − nd1| = 0. As W becomes wider, complete DSO occurs throughout the range of nd2 having reduced μ. Alternatively, keeping nd1 unchanged and by increasing nd2, μ raises due to enhanced N s , with a drop near the transition from SSO to DSO. Under SSO, μ is controlled by the ionized impurity and alloy disorder scatterings, while under DSO, the impurity scattering determines μ. Our analysis on μ can help to examine the inter‐subband effects on device characteristics of the QWFET system. [ABSTRACT FROM AUTHOR]

The effect of tensile and compressive strain on low‐temperature electron transport (τt) and quantum (τq) lifetimes are analyzed as function of well width (w) in InxGa1−xAs/In0.52Al0.48As modulation‐doped double quantum well (MD‐DQW)‐based high electron mobility transistors structures. The DQW system can be made either lattice‐matched or strained, tensile and compressive, by considering x = 0.53, 0.41, and 0.75, respectively. Results show that the nonlinearity in τt and τq relates to the uneven dependence on the ionized impurity (ii), interface roughness (ir), and alloy disorder (al) scatterings. As w increases, τt enhances. The improvement in τt is substantial for the strained system compared to the unstrained. Further, for w < 94 Å, the ir‐scattering is more dominating under the compressive than the tensile strained structure, leading to τt (x = 0.41) > τt (x = 0.75). Conversely, τq is dominated by ii‐scattering exhibiting τq (x = 0.75) > τq (x = 0.53) > τq (x = 0.41). A hybrid structure is also proposed with one well tensile (x = 0.41) and the other as compressively (x = 0.75) strained, resulting in a step‐like MD‐DQW structure. As a result, only a single sub‐band is occupied leading to a twofold improvement in τt for w = 150 Å compared to the symmetric structures. [ABSTRACT FROM AUTHOR]

The shape complexity of a particle confined in the circular quantum well with an external homogeneous perpendicular magnetic field is investigated. First, the Shannon entropy and the averaging electron probability density, as well as the shape complexities in the position and momentum spaces for several quantum states, are computed and discussed. The quantum confinement effect on the shape complexity of this system is extensively explored. Then, the influence of the magnetic field on the shape complexities of the particle confined in the circular quantum well is discussed. It is demonstrated that the shape complexity exhibits scaled invariance under the influence of the magnetic field. However, when the radius of the quantum well is fixed, the shape complexity of the given quantum state changes with the magnetic field strength. Results suggest that the shape complexity of a particle confined in the quantum well can be changed by varying both the strength of the magnetic field and the size of the well. This study has some practical applications in quantum information measurement of the confined particle and can guide future researches for the disorder characteristics of confined particle in semiconductor quantum dots. [ABSTRACT FROM AUTHOR]

The antiparallel current‐induced spin polarization (CISP) in an inversion‐symmetric double‐quantum‐well structure with the antiparallel Rashba effective magnetic field is calculated by solving the Boltzmann equation which is derived for the distribution operator in layer‐pseudospin space in the relaxation‐time approximation. The antiparallel CISP in an analytical form, obtained for constant momentum relaxation time in the limit of large Fermi energy, exhibits factor‐two increase with increasing the interlayer‐tunneling strength. This increase originates from the correction (to the semiclassical CISP) created by the subband mixing due to the momentum dependence of the Rashba effective magnetic field. The present interlayer‐coupling dependence of the antiparallel CISP is successfully explained as the layer‐pseudospin Hanle effect in which the spin in the original Hanle effect is replaced by the layer pseudospin. The present finding suggests that the correction by band mixing can be significant in the transport properties of various systems with the momentum‐dependent effective magnetic field. [ABSTRACT FROM AUTHOR]

The ongoing miniaturization of semiconductor devices renders charge‐carrier transport along interfaces increasingly important. The characteristic length scales in state‐of‐the‐art semiconductor technology span only a few nanometers. Consequently, charge‐carrier transport inevitably occurs directly at interfaces between adjacent layers rather than being confined to a single material. Herein, charge‐carrier diffusion is systematically studied in prototypical active layer systems, namely, in type‐I direct‐gap quantum wells and in type‐II heterostructures. The impact of internal interfaces is revealed in detail as charge‐carrier diffusion takes place much closer to or even across the internal interfaces in type‐II heterostructures. Type‐I quantum wells and type‐II heterostructures exhibit comparable diffusion rates given similar inhomogeneous exciton linewidths. Consequently, the changes in the structural quality of the interfaces are responsible for changes in diffusion and charge‐carrier transport along interfaces rather than the existence of the interfaces themselves. [ABSTRACT FROM AUTHOR]

The growth and characterization of (101¯1) semipolar GaN buffer, InGaN multiple quantum wells (MQWs), and light‐emitting diode (LED) structure on patterned silicon‐on‐insulator (SOI) substrates, implementing the aspect ratio technique (ART), are reported. The early growth stages of GaN result in continuous and uniform stripes with small height variations that cause the formation of chevrons. Three coalescence strategies are tested to improve surface morphology and optical quality. Scanning electron microscopy identifies no crack formation but undulations of the surface. A roughness of ≈10 nm is measured by atomic force microscopy on large areas. The impact of MQW growth temperature shows similar surface morphology in terms of undulations and roughness. Room temperature photoluminescence spectra show wavelength emission redshifting when decreasing the MQW growth temperature. Room‐temperature cathodoluminescence (CL) highlights first the presence of threading dislocations (TDs) in between the coalescence boundary despite the use of the ART technique. Second, CL shows a spatially homogeneous emission wavelength of around 485 nm only perturbed by lower‐wavelength emission (455 nm) arising from the chevrons. Blue LED structures exhibit uniform emission wavelength at 450 nm, having a crack‐free surface, and roughness of ≈5 nm. These results pave the way for the fabrication of semipolar micro‐LEDs on SOI substrates. [ABSTRACT FROM AUTHOR]

Lateral ambipolar diffusion in an InGaN/GaN single quantum well (SQW) structure grown on bulk GaN is studied by microphotoluminescence (μPL) investigations. The analysis is done via pinhole scans, that is, by decoupling the excitation area from the detection area and scanning the latter one. Knowing the size of the excited region, conclusions about the carrier transport in the in‐plane layer of the QW can be drawn. In this study, a diffusion length Ld up to 12 μm is observed at room temperature. Furthermore, the energy of the spectral peak is analyzed as a function of distance, showing a pronounced blueshift at the excitation center. An increase in distance to the injection region is accompanied by a redshift. The indication of peak energy is used to relate the drop of PL intensity with increasing radius to actual diffusion of charge carriers. In addition, temperature‐dependent studies of the diffusion behavior are done, covering the range between 10 K and room temperature. No significant change of diffusion length can be seen with temperature, indicating that temperature dependencies of carrier lifetime and of mobility compensate each other. [ABSTRACT FROM AUTHOR]

Herein, the canonical Fowler–Nordheim theory is extended by computing the zero‐temperature transmission probability for the more general case of a barrier described by a fractional power law. An exact analytical formula is derived, written in terms of Gauss hypergeometric functions, that fully capture the transmission probability for this generalized problem, including screened interaction with the image potential. First, the quality of approximation against the so far most advanced formulation of Fowler–Nordheim, where the transmission is given in terms of elliptic integrals, is benchmarked. In the following, as the barrier is given by a power law, in detail, the dependence of the transmission probability on the exponent of the power law is analyzed. The formalism is compared with results of numerical calculations and its possible experimental relevance is discussed. Finally, it is discussed how the presented solution can be linked in some specific cases with an exact quantum‐mechanical solution of the quantum well problem. [ABSTRACT FROM AUTHOR]

Scanning tunneling electroluminescence (STL) microscopy is performed on a 3 nm‐thick InGaN/GaN quantum well (QW) with [In] = 0.23 such that the main light emission occurs in the green. The technique is used to map the radiative recombination properties at a scale of a few nanometers and correlate the local electroluminescence map with the surface topography simultaneously imaged by scanning tunneling microscopy. While the expected green emission is observed all over the sample, measurements performed on a 500 nm × 500 nm area around a 150 nm‐large and 2.5 nm‐deep hexagonal defect reveal intense emission peaks at higher energies close to the defect edges, features which are not visible in the macrophotoluminescence spectrum of the sample. Via a fitting of the local tunneling electroluminescence spectra, quantitative information on the fluctuations of the intensity, peak energy, width, and phonon replica intensity of the different spectral contributions is obtained, which provides information on carrier localization in the QW. This procedure also indicates that the carrier diffusion length on the probed area of the QW is shorter than 50 nm. [ABSTRACT FROM AUTHOR]

In their last article [Phys. Lett. A 2021, 389, 127091], the authors have extended the k⋅p approach by considering crossings with the spin–orbit interaction (SOI) up to the second order and studied the spin texture and Berry phase of the heavy‐mass holes (HHs) confined in the SiGe 2D quantum well system. HHs cause quasi‐degeneracy via the intersub‐band interaction (ISI) working as Dirac‐like singularity, leading to the energy‐dependent Berry phase "quantized" by π. This π quantization is understandable by counting the number of quasi‐degenerate points in the Brillouin zone alongside recognizing the sign of the Berry curvature there. Herein, the remaining holes of the light‐mass holes (LHs) and separate holes (SHs) are studied. The spin textures and the Berry curvatures and phases are explored comprehensively, focusing on the competition between the Rashba and Dresselhaus SOIs via ISI. Comparisons with HHs elucidate that LHs and SHs cause similar quasi‐degeneracy through ISI, functioning as the Dirac‐like singularity. As such, the Berry phase is quantized by π, confirming the validity of the counting model irrespective of hole type. In some peculiar cases, HHs and LHs cause a possible sign inversion in the Berry phase, leading to zero Berry phase against temperature. [ABSTRACT FROM AUTHOR]

The effect of the external electric field on the ground state binding energy and self‐polarization of a hydrogenic donor impurity in quantum wells (QWs) made of different materials is calculated within the effective mass approximation using a variational scheme. The variations of binding energy and self‐polarization depending on well width, electric field, and impurity position have been studied in detail. For each QW made of different materials, it has been observed that the binding energy decreases with the increase of the electric field, whereas the self‐polarization increases. Also, it has been observed that InP/In1−xGaxP has higher binding energy values among the structures discussed. It is seen that material selection has a noticeable effect on self‐polarization and binding energy in QW‐based structures. [ABSTRACT FROM AUTHOR]

Temperature‐dependent continuous‐excitation and time‐resolved photoluminescence are studied to probe carrier localization and recombination in nearly strain‐balanced m‐plane In0.09Ga0.91N/Al0.19Ga0.81N multi‐quantum wells grown by plasma‐assisted molecular‐beam epitaxy. An average localization depth of 21 meV is estimated for the undoped sample. This depth is much smaller than the reported values in polar structures and m‐plane InGaN quantum wells. As part of this study, temperature and magnetic field dependence of time‐resolved photoluminescence is performed. At 2 K, an initial fast decay time of ≈0.3 ns is measured for both undoped and doped structures. The undoped sample also exhibits a slow decay component with a time scale of 2.2 ns. The existence of two relaxation paths in the undoped structure can be attributed to different localization centers. The fast relaxation decays are relatively insensitive to external magnetic fields, while the slower relaxation time constant decreases significantly with increasing magnetic fields. The fast decay time scale in the undoped sample is likely due to indium fluctuations in the quantum well. The slow decay time may be related to carrier localization in the barriers. The addition of doping leads to a single fast decay time likely due to stronger exciton localization in the InGaN quantum wells. [ABSTRACT FROM AUTHOR]

The role of intra‐band hole relaxation and tunneling of electrons in the carrier relaxation in Al0.7Ga0.3As/GaAs quantum well (QW) is studied by performing pump‐probe transient reflectivity measurements with a femtosecond laser at room temperature. The ultrafast optical response of the QW sample is measured with an aim of understanding various ultrafast processes occurring on the time scale of ∼100 fs to 10 ps. Carriers are injected into the QW layer via a resonant photoexcitation of electron–hole pairs enabling a sharp rise of the signal. Carrier relaxation inside QW is dominated by non‐radiative processes like phonon‐mediated intra‐band relaxation of holes and tunneling of electrons out of the QW plane and their subsequent capture by the midgap states in the AlGaAs barrier. Subsequent to the initial thermalization within 100 fs, carrier relaxation is mainly driven by the tunneling of electrons and trapping by the midgap states over a time scale of 3–8 ps. In between, photoexcited holes are seen to relax from light to heavy hole states of QW within 1–2 ps, leading to a delayed rise of transient reflectivity signal. Systematic efforts are made to isolate various contributions which showed phonon re‐absorption leading to saturation effect in intraband relaxation. [ABSTRACT FROM AUTHOR]

Circularly polarized photoluminescence (PL) spectroscopy measured at 19 K on GaAs/AlGaAs symmetric and asymmetric double quantum wells (DQW) is reported. The PL is obtained by exciting the sample with a circularly polarized (left or right) laser in order to create an initial unbalanced distribution of electron spins in the conduction band and, in this way, obtain the electron spin lifetime τs. The effects of the excitation laser wavelength are estimated by exciting with laser wavelengths of 701.0, 787.0, 801.5, and 806.5 nm. The increase of τs with the excitation wavelength is attributed to the lower initial quasimomentum k of the excited carriers, which also reduces spin–orbit relaxation processes. τs is found to be higher in asymmetric DQWs: this is attributed to the wider QWs in these samples, which reduces spin relaxation due to the Dresselhaus mechanism. In addition, a smaller contribution from the Rashba mechanism is also detected by comparing samples with built‐in electric fields of different orientations defined by doped barrier layers. [ABSTRACT FROM AUTHOR]

The calculation of material gain and performance characteristics of compound III–V semiconductor quantum well (QW) lasers has been developed, incorporating various approximations for the band structure. As the accuracy and sophistication of the band structure are increased, the computational time and mathematical complexity rise accordingly. The modeling of QW lasers is examined to determine the criteria for the accurate use of a parabolic approximation for the full band structure in the calculation of the material gain and performance characteristics of a conventional III–V QW. A comparison of the cosine approximation of band structure in highly mismatched alloys is also presented. [ABSTRACT FROM AUTHOR]

The results of the growth and characterization of an 8 monolayers (MLs) thick Zn1−xCdxSe/ZnSe quantum well (QW) with quite high Cd content (x = 0.70) are presented. At room temperature (RT), the QW presents yellow excitonic emission at 2.179 eV (569 nm, same color as the yellow line of a Kr ion laser). Despite the large Cd content, the RT photoluminescence spectrum shows a well‐defined, symmetric excitonic peak with relatively narrow full width at half maximum, indicating a good structural quality of the QW that is attributed to the sequential layer‐by‐layer growth mode achieved by the use of the submonolayer pulsed beam epitaxy (SPBE) technique. The ZnSe barriers are grown by molecular beam epitaxy (MBE); the QW heterostructure is deposited on top of a GaAs(001) substrate at 275 °C. Scanning transmission electron microscopy indicates a homogeneous smooth QW layer. The evolution of the excitonic emission energy with increasing temperature in the 19–300 K range shows the well‐known S‐shaped behavior that is interpreted in terms of exciton migration. [ABSTRACT FROM AUTHOR]

The luminescence characteristics of the hexagonal GaN (h‐GaN) and cubic GaN (c‐GaN) on micropatterned Si(100) substrates are explored. Microstripes of InGaN/GaN multiple quantum wells in the cubic and hexagonal phases are grown on V‐grooved Si(100) substrate. The crystal phases are identified by X‐ray diffraction and selective area electron diffraction, which shows the top surface is c‐GaN(001) phase at the center and h‐GaN(11¯01) at the side regions. Then, the energy dispersive X‐ray spectra of the indium content demonstrate the indium content in cubic InGaN/GaN multiple quantum wells (c‐MQWs) is higher than that in hexagonal InGaN/GaN MQWs (h‐MQWs). In addition, photoluminescence and cathodoluminescence measurements reveal that a cubic InGaN/GaN quantum well has produced longer wavelengths. The c‐MQWs could incorporate higher indium content and realize longer wavelength emissions, which has great potential to realize red light emitting diode. [ABSTRACT FROM AUTHOR]

Microdisk resonators offer a small mode volume and are therefore commonly investigated for their suitability as platforms for low‐threshold lasing. In addition, the inherent strong confinement of light not only renders such structures promising candidates for efficient single‐photon sources, but even motivates a key role for integrated photonic circuitry. Herein, ZnSe‐based microdisks with ZnCdSe quantum well (QW) structures in ZnMgSe barriers are analyzed. In microphotoluminescence (μPL), stimulated emission into whispering gallery modes (WGMs) is demonstrated. Scanning electron microscopy (SEM) is used to determine the edge roughness of a multitude of resonators using a fitting routine, revealing a correlation between the edge roughness and overall optical quality of the resonators. These results are confirmed by calculations based on the boundary element method using the measured roughness as input parameter. To avoid the common problem of degradation under excitation in ZnSe‐based structures, an alternative fabrication technique is introduced, which is new to this material system. It yields supported disks in direct contact with an Al2O3 layer on the substrate, which enhances the mechanical and thermal stability of the resonator. The occurrence of WGMs in supported ZnSe:Cl resonators is demonstrated in μPL experiments and confirmed by theoretical calculations. [ABSTRACT FROM AUTHOR]

While growth processes of InxGa1−xN/GaN quantum well (QW) structures on the Ga face of GaN buffer layers are already optimized to obtain high quantum efficiency, the growth on N‐face has gained momentum only in the past years. Compared with Ga face InxGa1−xN layers are more stable on N face, and the surface can easily be structured by wet chemical etching, which usually leads to the formation of pyramids on the surface. This allows a new way to realize nanooptical light emitters, which offers the possibility to produce structures with similar emission properties. First, InxGa1−xN/GaN (single or multi‐) QW structures on N‐face GaN are grown. In a second step, pyramids are formed by KOH etching. Pyramids with smooth side facets of the type (11¯01¯) are demonstrated and sharp tips in the nanometer range can be achieved without any sign of damage. Transmission electron microscopy (TEM) reveals that InxGa1−xN quantum dot‐like structures are present in the pyramids and in photoluminescence narrow emission lines are observed. The etching process depends on electrolyte composition and temperature, defects at the surface, and surface morphology. A better control of this process is required to achieve reproducible nanostructures. [ABSTRACT FROM AUTHOR]

Integrated Si photonics has the potential to revolutionize the processing of information between different integrated chips, as well as within a single chip itself. By performing at least a part of the task with photons rather than electrons, new opportunities for broad‐band low‐power communication and computing are created. Herein, the theoretical description of the linear electro‐optic (EO), or Pockels, effect and a newly elucidated design rule for materials evaluation is summarized. Possible applications of Si‐integrated optical elements based on perovskite oxides and their heterostructures are also discussed. In particular, the Pockels effect in BaTiO3 films grown on Si and intersubband transitions in Si‐integrated perovskite quantum wells (QWs) is described. [ABSTRACT FROM AUTHOR]

The electronic transport properties of a 2D normal metal superconductor are studied by a nonequilibrium Green's function method. The normal metal is the non‐inverted HgTe/CdTe quantum well doped by electrons. It is found that Andreev reflection (AR) is enhanced in the weak disorder regime while weakened in the strong disorder regime. However, the AR coefficient can be perfect (TA=1) with vanished fluctuations in the moderate disorder regime, whereas other scattering processes are forbidden. The mechanism goes that Anderson disorder leads to a topological Anderson insulator with bulk states localized while edge states still extensive. Multiple ARs and spin‐momentum locking cooperate to vanish the normal reflection and leave AR only. A detailed numerical study shows that fixing the Fermi surface in conduction band, a large enough junction width, and a strong coupling between the normal and superconducting leads are necessary for perfect AR. [ABSTRACT FROM AUTHOR]

The single‐photon blockade effect arising in a cavity containing a quantum well and interacting with squeezed light, resulting from a down‐conversion process through a nonlinear medium, is investigated. Optical parametric oscillator (OPO) materials with positive and negative second‐order susceptibilities are considered. By solving the master equation analytically in the weak‐driving limit and calculating the second‐order equal‐time correlation function, it is found that strong photon antibunching can be achieved in this system with weak excitonic nonlinearity or squeezed light via a destructive quantum interference mechanism. The optimal conditions for the photon blockade are derived analytically and discussed in details. Unlike the excitonic nonlinearity, a strong photon blockade occurs with the squeezed light at total or quasitotal resonance, even in the weak coupling regime, using an OPO material with negative second‐order nonlinear susceptibility. By acting on the driving coherent pump frequency, the control of the cavity output is achieved and the scheme can serve as a tunable single‐photon emission source. [ABSTRACT FROM AUTHOR]

The ground‐state properties of the excitonic condensation state in the effects of the mass imbalance are discussed in the framework of the spinless extended Falicov–Kimball model, in which the hopping of the f electrons is involved. Using the unrestricted Hartree–Fock approximation, a set of self‐consistent equations are obtained so the excitonic condensate order parameter is determined. In this sense, the real part of the optical conductivity is analytically calculated based on the Kubo formula. Phase diagram releases a complex phase structure of the excitonic condensation state with a strong depression of its Bardeen–Cooper–Schrieffer (BCS)–Bose–Einstein condensation (BEC) crossover in the influence of the mass imbalance. The impression of the mass imbalance affecting the excitonic condensate is also addressed in the signature of the optical conductivity. The finding delivers the natural scenario of the excitonic condensation state applied for a wide range of materials, from transition‐metal dichalcogenides to double‐layer systems such as a semiconductor double quantum well, double‐bilayer graphene, or double‐layer graphene. [ABSTRACT FROM AUTHOR]

The influence of the growth substrate on the internal quantum efficiency (IQE) of deep ultraviolet light‐emitting diodes is studied. Two nominally identical Al‐rich AlGaN/AlN multi‐quantum‐well (MQW) structures grown by metal–organic vapor phase epitaxy (MOVPE) on different substrates are investigated. The first MQW structure is grown on a native AlN substrate, whereas the second one is deposited on an AlN template on sapphire. By the combination of atomic force microscopy (AFM), photoluminescence (PL), and cathodoluminescence (CL) spectroscopy, it is demonstrated that the dislocation‐mediated spiral growth of MQWs on sapphire results in the more efficient localization of carriers. This effect helps to prevent nonradiative carrier recombination at point defects, improving the IQE of the structure. [ABSTRACT FROM AUTHOR]

The intensity of photoluminescence (PL) changes when an additional below‐gap excitation (BGE) light modifies the electronic occupation of one of the crystalline defects acting as nonradiative recombination (NRR) levels and shifts the balance between radiative and NRR rates. The photon energy of above‐gap excitation (AGE) light selects the layer and determines the depth of inspection, whereas that of BGE corresponds to the energy distribution of the NRR levels. The essential advantage of two‐wavelength excited PL (TWEPL) lies on a systematic combination of BGE spectroscopy and AGE spectroscopy for a variety of materials. It provides the noncontacting and nondestructive detection of NRR levels, distributing in a whole wafer as well as in a microscopic volume. Density and the electron and hole capture rates of NRR level 1, Nt1, Cn1, and Cp1, and those of level 2, Nt2 and Cp2, are determined consistently by the TWEPL measurement with the aid of time‐resolved PL. A brief review is given on the principle of the TWEPL, basic model of analysis, and experimental results of detecting NRR levels in GaAs/AlGaAs, InGaAs/GaAs, InAs/GaAs, GaN, InGaN, AlGaN, and GaPN. The method gives us a guiding compass toward the efficiency improvement of electronic devices from which PL is observable. [ABSTRACT FROM AUTHOR]

The impact of radiative and non‐radiative processes on the photovoltaic response of GaAs/AlGaAs multiple‐quantum wells is investigated by temperature, excitation power, and chopping‐frequency‐dependent surface photovoltage (SPV) measurements. It is shown that a systematic study of SPV amplitude along with phase spectra can provide important information on thermal escape, carrier localization, and spatial redistribution of charge carriers. Characteristic features in SPV‐phase spectra related to quantum well (QW) transitions and their variation under an external perturbation are explained by the electron–electron interaction. It is found that a many‐body interaction inside the QW is responsible for the capture of charges at the heterointerfaces, which builds up an interface electric field and hinders the drift/diffusion of charges. Such an effect becomes dominant under higher excitation powers, causing a sub‐linear enhancement of photovoltage amplitude and an increase in phase delay as a function of photon flux. From the chopping‐frequency‐dependent SPV measurements, it is concluded that carriers during the drift in barrier layers are repeatedly captured by point defects and other QWs, which enhances the effective time response of the photovoltage signal. Results obtained in this study are beneficial in the engineering of quantum structures for high‐efficiency photovoltaics and non‐invasive characterization of electro‐optical processes. [ABSTRACT FROM AUTHOR]

The carrier dynamics of Al‐rich AlGaN/AlN quantum well (QW) structures in the presence of strong carrier localization is reported. Excitation density‐dependent photoluminescence (PL) measurements at low temperatures reveal a clear correlation between the onset of efficiency droop and the broadening of the time‐integrated PL spectra. While the droop onset is heavily impacted by the localization strength, the PL emission broadening is observed almost exclusively on the high energy side of the emission spectrum. Spectrally resolved PL decay transient measurements reveal a strong dependency of the carrier lifetimes on the emission photon energy across the spectrum, consistent with a distribution of localized states, as well as on the temperature, depending on the localization strength of the investigated structure. The characteristic "S"‐shaped temperature dependence of the PL emission energy is shown to be directly correlated to the thermal redistribution of carriers between localized states. Based on these findings, the role of carrier localization in the recombination processes in AlGaN QW structures is underlined and its implications for efficiency droop are discussed. [ABSTRACT FROM AUTHOR]

For indirect‐bandgap germanium (Ge), strain‐engineering provides a controllable means to transform Ge into direct‐bandgap material with strongly enhanced light emission efficiency. Flexible electronics offer a powerful platform for introducing external strain into semiconductor nanomembranes (NMs) through deformation with flexible substrates. So far, the Ge/GexSi1−x alloy quantum well (QW) NMs have not been sufficiently investigated to comprehend how the external deformation tunes the wavelength and light‐emitting properties. Herein, a theoretical model based on the elasticity theory and k·p method is established to analyze the strain‐optoelectronic properties of the externally deformed Ge/GexSi1−x QW NMs via neutral plane (NP) modulation in both embedded and sticked form. Calculated results reveal that the elastic strain is linearly distributed, inducing tilt of the band edge similar to the electric field. Considering the fracture limit of the material, the NP layout determines the critical bending curvature that the NM can achieve. Large tunable range of wavelength Δλ of 1615 nm and enhanced transverse electric (TE)/transverse magnetic (TM) peak gain of 949/3778 cm−1 are predicted for r = 1000 nm (the distance between NP and QW) under the bending limit. Further calculations conclude that larger r allow for smaller bending curvature to achieve wider range of wavelength and more enhanced optical gain. [ABSTRACT FROM AUTHOR]

Temperature‐dependent transport of photoexcited charge carriers through a nominally undoped, c‐plane GaN layer toward buried InGaN quantum wells is investigated by continuous‐wave and time‐resolved photoluminescence spectroscopy. The excitation of the buried InGaN quantum wells is dominated by charge carrier diffusion through the GaN layer; photon recycling contributes only slightly. With temperature decreasing from 310 to 10 K, the diffusion length in [0001¯] direction increases from 250 to 600 nm in the GaN layer. The diffusion length at 300 K also increases from 100 to 300 nm when increasing the excitation power density from 20 to 500 W cm−2. The diffusion constant decreases from the low‐temperature value of ∼7 to 1.5 cm2 s−1 at 310 K. The temperature dependence of the diffusion constant indicates that the diffusivity at room temperature is limited by optical phonon scattering. Consequently, higher diffusion constants in GaN‐based devices require a reduced operation temperature. To increase diffusion lengths at a fixed temperature, the effective recombination time has to be prolonged by reducing the number of nonradiative recombination centers. [ABSTRACT FROM AUTHOR]

Temperature‐dependent polarized photoluminescence from anisotropically strained c‐plane InGaN/GaN multiple quantum wells on stripe‐shaped cavity‐engineered sapphire substrate is theoretically and experimentally investigated. Polarization ratios decrease from 0.98 to 0.74, and emission peak shifts increase from 0 to 50.9 meV with increasing temperature from 10 to 300 K, respectively. Theoretical calculations based on k·p perturbation theory reveal that the temperature dependence of polarized optical behaviors is attributed to the modified valence band structures and hole distributions in each subband. Theoretical results are in good agreement with the experimental results over temperature range from 10 to 300 K, providing in‐depth understanding for the strain‐induced valence band modification of III‐nitride semiconductors. [ABSTRACT FROM AUTHOR]

The performance of a transistor laser (TL) structure consisting of SiGeSn emitter, SiGeSn base incorporating a GeSn quantum well (QW) and a GeSn collector is studied in the present work. The Sn concentration is chosen to yield direct band gap for all the layers. An earlier model developed for InGaAs QW in GaAs base, that could satisfactorily explain most of the experimental data, is employed to calculate terminal currents, threshold current, and light power output. The model solves continuity equation, considers virtual states above QW, Fermi Golden rule to calculate gain, Fermi statistics for occupancy of subbands and broadening of states. A low value of threshold base current is predicted. [ABSTRACT FROM AUTHOR]

Dense arrays of carrier localizing indium‐rich regions (referred to as quantum well‐dots, QWDs) formed inside an indium‐depleted residual quantum well by metalorganic vapor phase epitaxial deposition of 4–16 monolayers (ML) of InxGa1−xAs (0.3 < x < 0.5) on 6° misoriented GaAs (100) substrates are studied. It is shown that in addition to QWDs the deposited layers may contain other objects with size and shape similar to conventional self‐organized quantum dots (QDs). Transmission electron microscopy and photoluminescence studies reveal that the density of QDs grows with the increase in indium composition and average amount of deposited InGaAs. The QWDs show efficient absorption in the optical region of 900–1100 nm, whereas QDs do not contribute to photocurrent spectra and result in a waste of carriers. Optimal indium composition x to form QWDs with high structural and optical quality is about 0.4. For x = 0.3 the QWDs are not yet well developed, while for x = 0.5 the density of QDs becomes too high. [ABSTRACT FROM AUTHOR]

We propose a coaxial cylindrical quantum well wire which is composed of layers of a dilute magnetic semiconductor (DMS) and a nonmagnetic semiconductor (NMS). Using effective mass approximation, we present a numerical calculation of the eigenenergies and eigenstates of electrons in the heterostructure. The variation of the spin‐dependent energy levels is influenced by the giant Zeeman splitting and potential energies due to a vector potential for sufficiently low and high magnetic fields, respectively. It is found that crossing and anticrossing behaviors of energy levels can be controlled by the thickness of a NMS layer. The ballistic transport of the structure is also investigated. The ranges of the Fermi energy for obtaining full spin polarization are suggested in this work. [ABSTRACT FROM AUTHOR]

The low‐energy electron beam irradiation (LEEBI) effect at 80 K on the optical properties of InGaN/GaN multiple quantum well structures has been studied. It is shown that the new emission band formed under LEEBI is related to quantum wells. At 80 K the total intensity of new and initial emission bands does not change under LEEBI. The intensity of the 3.3 eV line is shown to increase due to LEEBI. The results obtained are explained assuming a modification of properties of small local regions, which strongly localize the excess carriers. [ABSTRACT FROM AUTHOR]

We systematically investigate and compare the structural and optical properties of InGaN nanocolumns (NCs) and InGaN/GaN single quantum structures (SQSs) fabricated on top of GaN NCs as functions of the GaN column diameter. Each InGaN/GaN SQS comprises an In‐rich InGaN single quantum dot at the center of a semipolar Ga‐rich InGaN single quantum well. The InGaN NCs and InGaN/GaN SQSs qualitatively exhibit similar optical properties. However, the specific diameters at which the photoluminescence spectra change from single‐peak emission patterns to double‐peak patterns are different. Based on scanning transmission electron microscopy observations of the InGaN layers formed on the GaN NCs with different growth times, a model for the growth of InGaN on GaN NCs is proposed and the origin of the difference in the specific diameters is elucidated. Finally, when QSs are introduced to the InGaN‐based NCs, the radiative recombination probability increases while the nonradiative and surface recombination probabilities decrease, thus leading to high emission efficiency. [ABSTRACT FROM AUTHOR]

In this work we compare luminescence results obtained on InGaN/GaN multiple quantum well (QW) structures with 10 and 30 QWs. The aim is to increase the intensity of faster blue QW emission and decrease the luminescence of the QW defect band, showing a slower luminescence decay time, which is undesired for fast scintillator applications. We demonstrate that increasing the number of InGaN QWs is an efficient method to reach this goal. The luminescence improvement of the sample with higher number of QWs is explained by the influence of the increased size of V‐pits with increased QW number. Thinner QWs on the side wall of V‐pits serve as barriers which separate carriers from dislocations penetrating through the V‐pit centre, supressing thus the non‐radiative and radiative recombination on defects. Based on cathodoluminescence (CL) results, the scintillator structure design is discussed. Scintillator structures with higher number of QWs can take advantage of both, improved luminescence efficiency and thicker active region. [ABSTRACT FROM AUTHOR]

We report on the observation of biexciton luminescence from single quantum‐confined structures in N‐polar InGaN/GaN multiple quantum wells (MQWs) grown on c‐plane sapphire by metalorganic vapor phase epitaxy. Sharp emission lines are observed at different positions of the sample by micro‐photoluminescence (μ‐PL) mapping. The density of sharp emission lines is ≈0.3 μm−2, which is comparable with the inversion domain (ID) density found from transmission electron microscope observation, suggesting that the sharp emission lines originate from single quantum‐confined structures formed by the combination of quantum well and IDs in N‐polar InGaN/GaN MQWs. It is found from the excitation power dependence of the PL intensity of two adjacent sharp lines that the intensity of biexciton luminescence at the lower energy side shows a quadratic dependence on the excitation power while that of exciton luminescence at the higher energy side increased linearly with increasing excitation power. The biexciton binding energy is found to be 0.8 meV. [ABSTRACT FROM AUTHOR]

We carried out spot cathodoluminescence (CL) with scanning electron microscopy (SEM‐CL) of InGaN multiple quantum well (MQW) structures with an In molar fraction of 18% grown on c‐plane sapphire at room temperature, in which formation of a potential barrier around threading dislocations was confirmed in previous spatially resolved photoluminescence measurements with scanning near‐field optical microscopy (SNOM‐PL). We confirmed the suitability of CL measurements for evaluating the local potential barriers near the dislocations. Local emissions around the V‐pits were observed in monochromatic CL mapping images acquired at the higher‐energy side of InGaN emissions at RT and 78 K. The density of the higher‐energy emission regions was smaller than the V‐pit density, indicating the effect of nonradiative recombination. Spot CL spectra around the V‐pits showed shoulders on the higher‐energy side of the InGaN peaks. These observations agree with the SNOM‐PL results acquired from the same samples, indicating that SEM‐CL can be used to evaluate potential barriers. The emission energy difference between the InGaN and higher‐energy emissions estimated by spot CL spectra was compared with that estimated by SNOM‐PL spectra. The energy difference as a function of pit diameter showed a similar trend for both measurement methods, but was slightly smaller for the SEM‐CL results, suggesting that the potential barrier height evaluation is not necessarily quantitative. [ABSTRACT FROM AUTHOR]

The carrier transfer via non‐resonant tunneling is of great significance for many devices like the quantum cascade laser. In this study time‐resolved photoluminescence is used for an investigation of this effect in asymmetric double quantum wells for low temperatures. The Al content in these asymmetric double quantum wells was determined by HRXRD to x = 0.64 ± 0.03. The growth of the asymmetric cubic GaN/AlxGa1−xN double quantum wells was performed by a radio‐frequency plasma‐assisted molecular beam epitaxy. As a substrate, 3C‐SiC (001) on top of Si (001) was used. Three samples with different barrier thickness d were analysed (1 nm, 3 nm, 15 nm). The two quantum wells are designed with the thicknesses 2.5 nm and 1.35 nm. Thus, three expected emission bands measured in luminescence can be resolved. The maximum intensities are 3.49 eV (wide well), 3.73 eV (narrow well) and 4.12 eV (AlGaN). A correlation between the carrier lifetimes of the quantum wells (QWs) and the barrier width is found. Exploiting rate equations, the intensity ratio of both QW emissions is calculated. The coupling of the two QWs starts below 3 nm barrier thickness, above this value there is no coupling. [ABSTRACT FROM AUTHOR]

We investigated nanoscopic spectroscopy of blue and green InGaN multiple quantum wells (MQWs) by scanning near‐field optical microscopy (SNOM). The photoluminescence (PL) spectra showed emission peaks related to GaN and a main InGaN peak both in the blue and green MQWs. In addition, local emission peaks appeared on the higher‐energy side of the main InGaN peak (high‐E emission) in only the green InGaN MQW. The PL intensity map acquired from the high‐E emission in the green MQW was compared with the GaN emission intensity map, and there was no clear correlation between the high‐E emission regions and the dark spots in the GaN emission intensity map at room temperature (RT). The high‐E emissions are not due to potential barriers, but to other defects in the quantum wells. This may be because there is no high‐E emission related to the potential barrier at RT owing to the small V‐pit size, which reduces the suppression of carrier diffusion into dislocations especially at RT. Thus, the SNOM‐PL is measured for the green MQW at low temperature. In contrast to RT, there was a clear correlation between the high‐E emission regions and the dark spots in the GaN emission intensity map at 40 K, indicating that potential barriers formed around threading dislocations. The potential barrier height estimated from the SNOM‐PL spectra of the green MQW was sufficient to suppress carrier diffusion into threading dislocations. [ABSTRACT FROM AUTHOR]

We study the asymmetric doping dependence of electron mobility µ in an Al xGa1− xAs based square-parabolic double quantum well (SPDQW) high electron mobility transistor (HEMT) structure. We consider the square (parabolic) well and doping concentration Nd1 ( Nd2) in the barrier toward the substrate (surface) of the structure. Keeping Nd1 fixed, variation of Nd2 leads to the resonance of subband states of the individual wells at a Nd2 > Nd1. The mobility is calculated as a function of Nd2 by considering ionized impuritiy ( ii-), interface roughness ( ir-) and alloy disorder ( al-) scatterings. We show that near resonance of the subband states, the dip in µ arises due to the substantial change in the ir- and al-intersubband scattering rate matrix elements. The dip in μ is significant in SPDQW because of the sharp variations of subband wave functions between the wells due to the asymmetric potential profiles. Enhancement of the well width and central barrier width increases μ and sharpness of the dip in μ. Keeping Nd2 constant and Nd1 varying, the resonance occurs at Nd1 < Nd2 with similar nonlinearities in μ. Our results of µ can be utilized for the analysis of the performance of the HEMT devices. [ABSTRACT FROM AUTHOR]

We report on the growth and characterization of optical quality multiple quantum well structures of Zn xCd1− xSe/Zn xCd yMg1− x− ySe on an ultra-thin Bi2Se3/CdTe virtual substrate on c-plane Al2O3 (sapphire). Excellent quality highly oriented films grown along the (111) direction were achieved as evidenced by reflection high energy electron diffraction and X-ray diffraction studies. We also observed room temperature and 77 K photoluminescence emission with peak energies at 77 K of 2.407 eV and linewidths of 56 meV comparable to those achieved on structures grown on InP. Exfoliation of the structures is also possible due to the van der Waals bonding of Bi2Se3. Exfoliated (substrate free) films exhibit photoluminescence emission nearly identical to that of the supported film. Additionally, contactless electroreflectance measurements show good agreement with simulations of the multiple quantum well structure as well as evidence of excited state levels. These results open new avenues of research for substrate independent epitaxy and the possibility of ultra-thin electronics. [ABSTRACT FROM AUTHOR]

Waveguide modes in semipolar InGaN quantum-well (QW) laser diodes (LDs) with cleaved-facet cavity mirrors have been theoretically investigated using a 4 × 4 transfer matrix technique. It is shown that the waveguide mode can be controlled by changing the refractive-index contrast in the waveguide structures, although previous studies suggest that the waveguide modes are either ordinary or extraordinary due to the birefringence of the material itself, and that optical gain cannot be fully utilized for lasing because the polarization direction which determines optical gain in the devices does not coincide with the polarization direction in the waveguide mode. This behavior can be explained based on the competition between the layer-structural effect and the material birefringence. From the results of our calculations, it is concluded that the utilization of a high refractive-index-contrast waveguide structure is effective in enhancing the optical gain in semipolar LDs with cleaved-facet cavity mirrors. [ABSTRACT FROM AUTHOR]

A method to characterize the interface of InGaN/GaN quantum wells by X-ray reflectivity is presented. The interface roughness can be obtained from the ratio of diffuse to specular scatterings obtained on a transverse [ABSTRACT FROM AUTHOR]

Here, we study the mechanisms underlying a method used to limit the formation of trench defects in InGaN/GaN quantum well structures by using H2 in the carrier gas for the growth of GaN barriers. The method leads to a complete removal of the trench defects by preventing the formation of basal-plane stacking faults from which trench defects originate, as well as preventing the formation of stacking mismatch boundaries. The penalty paid for the absence of trench defects is the formation of InGaN wells with gross well-width fluctuations where the H2 gas has etched away the indium locally. Where a fully formed trench defect (stacking mismatch boundary opened as V-shaped ditch) already exists in the structure, the GaN barrier growth method using H2 results in a strongly disturbed structure of the quantum well stack in the enclosed region, with the quantum wells and barriers being in places significantly thinner than their counterparts in the surrounding material. [ABSTRACT FROM AUTHOR]