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Wurtzite ScxAl1−xN/GaN (x = 0.13–0.18) multi-quantum wells grown by molecular beam epitaxy on c-plane GaN are found to exhibit remarkably strong and narrow near-infrared intersubband absorption in the technologically important 1.8–2.4 μm range. Band structure simulations reveal that, for GaN wells wider than 3 nm, the quantized energies are set by the steep triangular profile of the conduction band caused by intrinsic polarization fields. As a result, the intersubband transition energies provide unique and direct access to essential ScAlN polarization parameters. Measured infrared absorption indicates that the spontaneous polarization difference of the presumed lattice-matched Sc0.18Al0.82N/GaN heterostructure is smaller than the theoretically calculated value. The intersubband transition energies are relatively insensitive to the barrier alloy composition indicating negligible variation of the net polarization field in the probed 0.13–0.18 Sc composition range. [ABSTRACT FROM AUTHOR]

GaN/AlxGa1−xN quantum wells were grown by molecular beam epitaxy on high quality bulk (0001) GaN substrates. The quantum well thickness was set in the 6–8 nm range to favor the photoluminescence emission of indirect excitons. Indeed, such excitons are known to be spatially indirect due to the presence of the internal electric field which spatially separates the electron and hole wave functions. The growth conditions were optimized in view of minimizing the photoluminescence peak broadening. In particular, the impact of growth temperature (up to 900 °C) on the surface morphology, structural, and photoluminescence properties was studied. The diffusion of indirect excitons on the scale of tens of micrometers was measured with a micro-photoluminescence setup equipped with a spatially resolved detection. A dedicated model and its analysis allow us to extract from these measurements the exciton diffusion constant and to conclude on the optimum growth conditions for the GaN/AlxGa1−xN quantum well structures suited for studies of quantum collective effects in indirect exciton liquids. [ABSTRACT FROM AUTHOR]

The effect of the GaAsP strain-compensating layer on type-II GaAs1−xSbx/InyGa1−yAs was investigated. GaAsSb/InGaAs multiple quantum wells (MQWs) without and with GaAsP strain-compensating layers were grown by molecular beam epitaxy. Increasing Sb or In compositions can extend photoluminescence (PL) emission at longer wavelength along with the highly induced compressive strain in the QWs. The power-dependent PL measured at low temperature reveals the type-II band characteristics of the GaAs1−xSbx/InyGa1−yAs system. A detailed analysis of the experimental data reveals that the GaAsP layers compensate the compressive strain of GaAsSb/InGaAs. The type-II QWs with GaAsP layers, (8 nm) GaAs0.84Sb0.16/(2.5 nm) In0.3Ga0.7As/(10 nm) GaAs0.85P0.15, emits PL at ∼1.1 μm, up to 210 K, while the PL of those strained sample without GaAsP vanishes at lower temperature. In view of the described sample, x-ray diffraction (XRD) analysis along with the simulation shows the validity of the procedure, resulting in nearly matched parameters of QW thicknesses and material compositions—(8.9 nm) GaAs0.835Sb0.165/(2.3 nm) In0.3Ga0.7As/(10.3 nm) GaAs0.85P0.15, with those of the designed QW. The thicknesses of QW from the TEM image, (8.6 nm) GaAsSb/(3.1 nm) InGaAs/(10.1 nm) GaAsP, agree well with the XRD results. [ABSTRACT FROM AUTHOR]

We discuss the electrical properties of molecular beam epitaxy (MBE) grown, modulation doped, Ge1− x Sn x quantum well devices. A consequence of the epitaxial growth process is that electronic disorder is introduced even in modulation doped quantum well structures and electrical transport properties that are characteristic of a high level of disorder are apparent. MBE growth of this material also results in the surface segregation of elemental β -Sn in the way that has been observed utilizing other epitaxial growth methods. A thermally activated, p-type mobility is a clear feature of the electrical properties with generally temperature independent hole densities ∼1012 cm−2 from the measured Hall effect and coming from the modulation doping. We present a discussion of Hall effect measurements in this disordered regime. The percolation carrier density in MBE modulation doped GeSn is in the region of ∼1 × 1012 cm−2 although Hall measurements in this regime are difficult to quantify when the resistivity >(h / e 2). In this notation h is Planck's constant and e is the unit of charge. Conductivities (σ) as low as ∼0.028 × (e 2/ h) × square can be measured in the four-contact ac configuration and the temperature dependence indicates a mobility edge in these p-type devices below ∼2 × 1012 cm−2. At lower temperatures (<∼1 K) the presence of a Coulomb gap can be determined using dc transport, constant voltage measurements where small ac current excitation is not available experimentally. This two-contact configuration can determine σ down to ∼10−6 × (e 2/ h), deep into the localization regime, revealing a hopping conductivity dominated system. We discuss the relevance of these electrical properties for MBE grown GeSn devices. [ABSTRACT FROM AUTHOR]

HgTe has been considered to be one of the most versatile topological materials. Depending on the in‐plane strain, Weyl and Dirac semi‐metal, as well as topological insulator phases, are feasible. Here, for the first time it is reported, that an InAs:S substrate promotes an initial 2D growth of a ZnTe thin layer and subsequent high crystalline quality ZnTe/CdTe superlattices serve as a smooth and continuous virtual substrate for the growth of (Cd,Hg)Te/HgTe/(Cd,Hg)Te heterostructures with unstrained quantum well HgTe (topological insulator) and compressively strained bulk HgTe (Weyl semimetal) by molecular beam epitaxy. Compared with the superlattices previously grown on GaAs substrates, the quantum well and bulk heterostructures exhibit homogeneous surfaces with root mean square roughnesses of 0.88–0.93 nm, which is three times lower than those observed for 3D islands (2.7–3.4 nm) on GaAs substrates. Additionally, magnetotransport measurements confirm high electronic quality and demonstrate that the S‐doped InAs substrate can be used as an effective back gate. These results manifest a (big) step forward toward the improvement of micro‐ and nanometer‐sized top‐ and back‐gated device fabrication on topological materials. [ABSTRACT FROM AUTHOR]

The misfit dislocation formation related to plastic strain relaxation in Si or Ge quantum well layers in SiGe heterostructures for spin qubits tends to negatively affect the qubit behaviors. Therefore, it is essential to understand and then suppress the misfit dislocation formation in the quantum well layers in order to achieve high-performance qubits. In this work, we studied the misfit dislocation propagation kinetics and interactions by annealing the strained Si or Ge layers grown by molecular beam epitaxy. The annealing temperatures are from 500 to 600 °C for Si layers and from 300 to 400 °C for Ge layers. The misfit dislocations were investigated by electron channeling contrast imaging. Our results show that the misfit dislocation propagation is a thermally activated process. Alongside, the blocking and unblocking interactions during misfit dislocations were also observed. The blocking interactions will reduce the strain relaxation according to theoretical calculation. These observations imply that it is possible to suppress the misfit dislocation formation kinetically by reducing the temperatures during the SiGe heterostructure epitaxy and post-epitaxy processes for developing well-functional SiGe-based spin qubits. [ABSTRACT FROM AUTHOR]

III-V lasers based on self-assembled quantum dots (QDs) have attracted widespread interest due to their unique characteristics, including low threshold current density (Jth), low sensitivity to backreflections, and resistance to threading dislocations. While most work to date has focused on 1.3 μm InAs/GaAs QDs, InP QDs have also aroused interest in lasers emitting at visible wavelengths. Molecular beam epitaxy (MBE) enables the growth of high-density InP/AlGaInP QDs on exact (001)-oriented GaAs substrates but requires a relatively low substrate temperature of <500 °C. The low substrate temperature used for phosphide growth in MBE leads to degraded optical properties and makes post-growth annealing a crucial step to improve the optical quality. Here, we report the exceptional thermal stability of InP/AlGaInP QDs grown using MBE, with up to 50× improvement in room temperature photoluminescence intensity with the optimization of annealing temperature and time. We also demonstrate the room temperature pulsed operation of InP multiple quantum dot (MQD) lasers on GaAs (001) emitting close to 735 nm with Jth values of 499 A/cm2 after annealing, a factor of 6 lower than their as-grown counterparts and comparable to such devices grown by MOCVD. In0.6Ga0.4P single quantum well (SQW) lasers on GaAs (001) also exhibit a substantial reduction in Jth from 340 A/cm2 as-grown to 200 A/cm2 after annealing, emitting at 680 nm under pulsed operation conditions. This work shows that post-growth annealing is essential for realizing record-performance phosphide lasers on GaAs grown by MBE for applications in visible photonics. [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]

This investigation explores the structural and electronic properties of low-temperature-grown (InAs)4(GaAs)3/Be-doped InAlAs and InGaAs/Be-doped InAlAs multiple quantum wells (MQWs), utilizing migration-enhanced epitaxy (MEE) and conventional molecular beam epitaxy (MBE) growth mode. Through comprehensive characterization methods including transmission electron microscopy (TEM), Raman spectroscopy, atomic force microscopy (AFM), pump–probe transient reflectivity, and Hall effect measurements, the study reveals significant distinctions between the two types of MQWs. The (InAs)4(GaAs)3/Be-doped InAlAs MQWs grown via the MEE mode exhibit enhanced periodicity and interface quality over the InGaAs/Be-InAlAs MQWs grown through the conventional molecule beam epitaxy (MBE) mode, as evidenced by TEM. The AFM results indicate lower surface roughness for the (InAs)4(GaAs)3/Be-doped InAlAs MQWs by using the MEE mode. Raman spectroscopy reveals weaker disorder-activated modes in the (InAs)4(GaAs)3/Be-doped InAlAs MQWs by using the MEE mode. This originates from utilizing the (InAs)4(GaAs)3 short period superlattices rather than InGaAs, which suppresses the arbitrary distribution of Ga and In atoms during the InGaAs growth. Furthermore, pump–probe transient reflectivity measurements show shorter carrier lifetimes in the (InAs)4(GaAs)3/Be-doped InAlAs MQWs, attributed to a higher density of antisite defects. It is noteworthy that room temperature Hall measurements imply that the mobility of (InAs)4(GaAs)3/Be-doped InAlAs MQWs grown at a low temperature of 250 °C via the MEE mode is superior to that of InGaAs/Be-doped InAlAs MQWs grown in the conventional MBE growth mode, reaching 2230 cm2/V.s. The reason for the higher mobility of (InAs)4(GaAs)3/Be-doped InAlAs MQWs is that this short-period superlattice structure can effectively suppress alloy scattering caused by the arbitrary distribution of In and Ga atoms during the growth process of the InGaAs ternary alloy. These results exhibit the promise of the MEE growth approach for growing high-performance MQWs for advanced optoelectronic applications, notably for high-speed optoelectronic devices like THz photoconductive antennas. [ABSTRACT FROM AUTHOR]

InGaAs/GaAsSb "W" quantum wells with GaAsP barriers are grown on GaAs (001) substrates by molecular beam epitaxy. We investigate the effect of the Sb composition in GaAsSb on the photoluminescence (PL) wavelength. X-ray rocking curve (XRC) measurements and simulations are performed to investigate the material composition and layer thickness. Low-temperature PL spectra are consistent with the XRC results. At the lowest Sb composition of 6%, the PL intensity is the strongest, and room-temperature PL is realized at ∼1100 nm. By increasing the Sb composition in the GaAsSb layer, low-temperature (20 K) PL emits at longer wavelength up to ∼1400 nm at 21% Sb while the PL intensity is the weakest. The XRC is also degraded. The strained bandgap simulation reveals the type-I to type-II band alignment transition as the Sb composition is ≥9%. [ABSTRACT FROM AUTHOR]

InAs quantum wells (QWs) are promising material systems due to their small effective mass, narrow bandgap, strong spin–orbit coupling, large g-factor, and transparent interface to superconductors. Therefore, they are promising candidates for the implementation of topological superconducting states. Despite this potential, the growth of InAs QWs with high crystal quality and well-controlled morphology remains challenging. Adding an overshoot layer at the end of the metamorphic buffer layer, i.e., a layer with a slightly larger lattice constant than the active region of the device, helps to overcome the residual strain and provides optimally relaxed lattice parameters for the QW. In this work, we systematically investigated the influence of overshoot layer thickness on the morphological, structural, strain, and transport properties of undoped InAs QWs on GaAs(100) substrates. Transmission electron microscopy reveals that the metamorphic buffer layer, which includes the overshoot layer, provides a misfit dislocation-free InAs QW active region. Moreover, the residual strain in the active region is compressive in the sample with a 200 nm-thick overshoot layer but tensile in samples with an overshoot layer thicker than 200 nm, and it saturates to a constant value for overshoot layer thicknesses above 350 nm. We found that electron mobility does not depend on the crystallographic directions. A maximum electron mobility of 6.07 × 105 cm2/Vs at 2.6 K with a carrier concentration of 2.31 × 1011 cm−2 in the sample with a 400 nm-thick overshoot layer has been obtained. [ABSTRACT FROM AUTHOR]

Spectroscopy of shallow donors is a tool to test theoretical models and to reveal properties of semiconductors. In this work we consider intra-shallow impurity transitions by studying a CdTe/(Cd, Mg)Te structure grown by a molecular beam epitaxy in which both a CdTe quantum well and (Cd, Mg)Te barries are uniformly doped with iodine donors. Measurements of a photocurrent (PC) at the far-infrared were carried out at 4.2 K and magnetic fields B up to 7 T with the energy of photons originated from a molecular laser in the range 2.2 meV–12.8 meV. Spectra (a PC signal vs. B, at a constant energy of photons) show lines which position does not depend on the photon energy but shifts with the in-plane electric field. These dependencies, which do not follow a well-established picture of shallow donor magnetospectroscopy in quantum wells, are explained within a model which unifies the role of fluctuations of the electrostatic potential and a magnetic-field induced electron localization. [ABSTRACT FROM AUTHOR]

With the rapid advancement of Artificial Intelligence-driven object recognition, the development of cognitive tunable imaging sensors has become a critically important field. In this paper, we demonstrate an infrared (IR) sensor with spectral tunability controlled by the applied bias between the long-wave and mid-wave IR spectral regions. The sensor is a Quantum Well Infrared Photodetector (QWIP) containing asymmetrically doped double QWs where the external electric field alters the electron population in the wells and hence spectral responsivity. The design rules are obtained by calculating the electronic transition energies for symmetric and antisymmetric double-QW states using a Schrödinger–Poisson solver. The sensor is grown and characterized aiming detection in mid-wave (~5 µm) to long-wave IR (~8 µm) spectral ranges. The structure is grown using molecular beam epitaxy (MBE) and contains 25 periods of coupled double GaAs QWs and Al0.38Ga0.62As barriers. One of the QWs in the pair is modulation-doped to provide asymmetry in potential. The QWIPs are tested with blackbody radiation and FTIR down to 77 K. As a result, the ratio of the responsivities of the two bands at about 5.5 and 8 µm is controlled over an order of magnitude demonstrating tunability between MWIR and LWIR spectral regions. Separate experiments using parameterized image transformations of wideband LWIR imagery are performed to lay the framework for utilizing tunable QWIP sensors in object recognition applications. [ABSTRACT FROM AUTHOR]

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

The 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]

InAs quantum wells are grown epitaxially as being embedded in (Ga,In)As–(Al,In)As double heterojunctions on InP(001). Despite an extensive gradual composition variation introduced in the buffer layer to overcome the mismatch of the lattices between InAs and InP, the layers relax during the epitaxy, as manifested by a cross-hatch pattern. The mobility of the two-dimensional (2D) electron gases in the quantum wells is nonetheless large, fairly unaffected by the presence of misfit dislocations generated in the lattice relaxation. The strain states in the epitaxial layer is analyzed using the X-ray diffraction. Transmission electron microscopy reveals that the relaxation is localized in the buffer layer, leaving the epitaxial growth in the quantum-well region coherent. The second subband of the 2D state is shown to be occupied when the sheet electron density is relatively low as the higher subbands are confined by the (In,Al)As layers. [ABSTRACT FROM AUTHOR]

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

Various designs of dielectric waveguides made of heterostructures with CdHgTe quantum wells grown by molecular beam epitaxy have been studied to generate stimulated emission in the 15–30 μm wavelength range. The reduction of radiation losses in optimized structures has made it possible to reduce the threshold intensity of the generation of stimulated emission to ~100 W/cm2. Modernized growth technology has ensured the reduction of the residual cadmium content in HgCdTe quantum wells to 2.5%, which has allowed us to increase the threshold energy of Auger recombination, as well as the maximum temperature for the observation of stimulated emission at interband transitions above 100 K. The results obtained are prerequisites for the implementation of coherent radiation sources exceeding in characteristics of lead–tin chalcogenide lasers used in the 15–30 μm spectral range. [ABSTRACT FROM AUTHOR]

Here, we present a systematical investigation of AlN/GaN double-barrier resonant tunneling diodes (RTDs) grown by plasma-assisted molecular beam epitaxy on metal-organic chemical vapor deposition GaN-on-sapphire templates. The processed devices featured an active region composed of 2.5 nm GaN quantum well sandwiched by two 1.5 nm AlN barriers and RTD mesa diameter ranging from 1 to 20 μm. Room temperature current–voltage characteristics exhibited a repeatable negative differential resistance (NDR) free of degradation and hysteresis after 1000 times subsequently up-to-down voltage sweeps across different sizes. High peak-to-valley current ratios of 1.93 and 1.58 were obtained at room temperature for 1 and 12 μm diameter devices, respectively, along with peak current densities of 48 and 36 kA/cm2 corresponding to peak voltages of 4.65 and 5.9 V. The peak current density decreased quickly initially and then was less susceptible to this averaging effect with increasing the device diameter. Temperature-dependent measurements revealed that the valley current density displayed a positive relationship to the temperature, and an abruptly increasement was observed for the devices with a diameter of 20 μm when the temperature rose over 230 K. We attributed this abnormal phenomenon to the increased contribution from acoustic and longitudinal optical (LO) phonon scattering, especially for the LO phonon scattering. The area dependence of electrical performance suggested that the leakage pathway through dislocations played a vital role for charge transport and there existed a threshold of dislocation density for NDR characteristics. These results promote further study for future implementation of III-nitride-based RTD oscillators into high-frequency and high-power terahertz radiation. [ABSTRACT FROM AUTHOR]

By taking advantage of the GaAs (631) corrugation self-assembled on top of multi-quantum well heterostructure interfaces, the modulation of the confined state wave functions (eigenstates) has been achieved, attaining quasi-one-dimensional or fractional dimension eigenstates. Two different theoretical approaches were used to compute the energy shift of subband optical transitions as a function of the interface corrugation geometrical configuration. For large nominal quantum well widths and small corrugation amplitude, the perturbation theory was employed, while a modified Lanczos algorithm assisted us to calculate the shifts when the corrugation amplitude was comparable to the nominal quantum well width. Experimentally, the heterostructures were grown by molecular beam epitaxy on (001) and (631) oriented substrates, where the quasi-one-dimensional ordering was reached by changing the As to Ga molecular beam fluxes ratio. It was found that the corrugated interfaces (i) break the wave function's in-plane symmetry, allowing transitions that, in principle, must be forbidden and (ii) induce blue shifts or red shifts in the order of 10 meV to the energy spectrum of the quantum wires depending on the lateral and vertical periodicities, exhibiting the presence of a lateral confinement system. The main result is the effective modulation of eigenstates through the interface corrugation control. Additionally, it was found that the interface modulation effect is greater for harmonic (n > 1) heavy (and light) hole subbands than for the ground states. [ABSTRACT FROM AUTHOR]

InAs quantum wells (QWs) are promising material systems due to their small effective mass, narrow bandgap, strong spin–orbit coupling, large g-factor, and transparent interface to superconductors. Therefore, they are promising candidates for the implementation of topological superconducting states. Despite this potential, the growth of InAs QWs with high crystal quality and well-controlled morphology remains challenging. Adding an overshoot layer at the end of the metamorphic buffer layer, i.e., a layer with a slightly larger lattice constant than the active region of the device, helps to overcome the residual strain and provides optimally relaxed lattice parameters for the QW. In this work, we systematically investigated the influence of overshoot layer thickness on the morphological, structural, strain, and transport properties of undoped InAs QWs on GaAs(100) substrates. Transmission electron microscopy reveals that the metamorphic buffer layer, which includes the overshoot layer, provides a misfit dislocation-free InAs QW active region. Moreover, the residual strain in the active region is compressive in the sample with a 200 nm-thick overshoot layer but tensile in samples with an overshoot layer thicker than 200 nm, and it saturates to a constant value for overshoot layer thicknesses above 350 nm. We found that electron mobility does not depend on the crystallographic directions. A maximum electron mobility of 6.07 × 105 cm2/Vs at 2.6 K with a carrier concentration of 2.31 × 1011 cm−2 in the sample with a 400 nm-thick overshoot layer has been obtained.

The use of an indium surfactant considerably alters the composition and morphology of low-temperature non-polar m-plane (10 1 ¯ 0) AlxGa1−xN (x ∼ 0.2) and of silicon-doped AlGaN/InGaN multiple quantum wells grown by plasma-assisted molecular beam epitaxy. This paper compares heterostructures grown with indium surfactant with those grown under conventional stoichiometric and gallium-rich conditions at the relatively low temperature necessary for growth of In0.16Ga0.84N quantum wells (565 °C). Stoichiometric growth results in rough, inhomogeneous AlGaN layers that are unsuitable for optical devices. Gallium-rich growth produces a smoother AlGaN layer, reduced inhomogeneities, and sharper interfaces as compared to stoichiometric growth. However, due to the low temperature, gallium-rich growth leads to the formation of an unintentional GaN layer on top of each AlGaN barrier, reducing the energies of confined electronic states in the quantum wells. An indium surfactant enables two-dimensional AlGaN growth at low temperature, producing atomically flat surface morphology and sharp heterostructure interfaces. Indium surfactant assisted epitaxy also eliminates the high aluminum alloy inhomogeneities observed with conventional stoichiometric and gallium-rich growth. Even though partial indium incorporation into the AlGaN layer is found at the studied temperatures, the high-quality, uniform non-polar In0.055Al0.19Ga0.755N/In0.16Ga0.84N quantum wells grown with indium surfactant display bright and narrow photoluminescence that is essential for device applications. [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]

We report on structural and optical studies of metamorphic InAs(Sb)/In(Ga,Al)As quantum well (QW) heterostructures with different designs of the active region, grown by molecular beam epitaxy on GaAs substrates and emitting in the mid-IR spectral range (3.0–3.5 μm) at room temperature. The influence of the thickness of the InGaAs/InAlAs superlattice waveguide and design of the InSb/InAs/InGaAs QW on stress balance in such metamorphic structures, their luminescent properties, and density of extended defects in the active region is discussed. The peculiarities of electron and hole energy spectra of the active region vs stress and design are studied theoretically in the framework of the 8-band Kane model and verified experimentally by Fourier-transform infrared photoreflectance spectroscopy. Despite that optimized metamorphic heterostructures are characterized by the extended defect density in the active region of just about 107 cm−2, carrier confinement in the QW has a stronger impact on their mid-IR photoluminescence intensity at room temperature. [ABSTRACT FROM AUTHOR]

The effect of Furnace Annealing (FA) and Rapid Thermal annealing (RTA) on the structural and optical properties of GaAs1 − xBix/GaAs single quantum wells grown on (001) and (311)B substrates by molecular beam epitaxy was investigated. The structural properties were investigated by high-resolution x-ray diffraction (HR-XRD) and Transmission Electron Microscopy. The Bi concentration profiles were determined by simulating the HR-XRD 2θ−ω scans using dynamical scattering theory to estimate the Bi content, lattice coherence, and quality of the interfaces. The Bi composition was found to be similar for both samples grown on (001) and (311)B GaAs substrates. However, the simulations indicate that the Bi composition is not only limited in the GaAsBi quantum well (QW) layer but also extends out of the GaAsBi QW toward the GaAs barrier. Photoluminescence (PL) measurements were performed as a function of temperature and laser power for samples with a nominal Bi composition of 3%. PL spectra showed that (001) and (311)B samples have different peak energies at 1.23 eV and 1.26 eV, respectively, at 10 K. After RTA at 300 °C for 2 min, the PL intensity of (311)B and (001) samples was enhanced by factors of ∼2.5 and 1.75, respectively. However, for the (001) and (311)B FA samples, an enhancement of the PL intensity by a factor of only 1.5 times could be achieved. The enhancement of PL intensity in annealed samples was interpreted in terms of PL activation energies, with a reduction in the alloy disorder and an increase in the Bi cluster. [ABSTRACT FROM AUTHOR]

Powerful emitters of ultraviolet C (UVC) light in the wavelength range of 230–280 nm are necessary for the development of effective and safe optical disinfection technologies, highly sensitive optical spectroscopy and non-line-of-sight optical communication. This review considers UVC emitters with electron-beam pumping of heterostructures with quantum wells in an (Al,Ga)N material system. The important advantages of these emitters are the absence of the critical problem of p-type doping and the possibility of achieving record (up to several tens of watts for peak values) output optical power values in the UVC range. The review consistently considers about a decade of world experience in the implementation of various UV emitters with various types of thermionic, field-emission, and plasma-cathode electron guns (sources) used to excite various designs of active (light-emitting) regions in heterostructures with quantum wells of AlxGa1−xN/AlyGa1−yN (x = 0–0.5, y = 0.6–1), fabricated either by metal-organic chemical vapor deposition or by plasma-activated molecular beam epitaxy. Special attention is paid to the production of heterostructures with multiple quantum wells/two-dimensional (2D) quantum disks of GaN/AlN with a monolayer's (1 ML~0.25 nm) thickness, which ensures a high internal quantum efficiency of radiative recombination in the UVC range, low elastic stresses in heterostructures, and high-output UVC-optical powers. [ABSTRACT FROM AUTHOR]

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

Molecular beam epitaxy growth and optical properties of GaN quantum disks in AlN nanowires were investigated, with the purpose of controlling the emission wavelength of AlN nanowire-based light emitting diodes. Besides GaN quantum disks with a thickness ranging from 1 to 4 monolayers, a special attention was paid to incomplete GaN disks exhibiting lateral confinement. Their emission consists of sharp lines which extend down to 215 nm, in the vicinity of AlN band edge. The room temperature cathodoluminescence intensity of an ensemble of GaN quantum disks embedded in AlN nanowires is about 20% of the low temperature value, emphasizing the potential of ultrathin/incomplete GaN quantum disks for deep UV emission. [ABSTRACT FROM AUTHOR]

Red, green, and blue light InxGa1−xN multiple quantum wells have been grown on GaN/γ-LiAlO2 microdisk substrates by plasma-assisted molecular beam epitaxy. We established a mechanism to optimize the self-assembly growth with ball-stick model for InxGa1-xN multiple quantum well microdisks by bottom-up nanotechnology. We showed that three different red, green, and blue lighting micro-LEDs can be made of one single material (InxGa1-xN) solely by tuning the indium content. We also demonstrated that one can fabricate a beautiful InxGa1-xN-QW microdisk by choosing an appropriate buffer layer for optoelectronic applications. [ABSTRACT FROM AUTHOR]

The strain distribution and the electronic band structure of InAs quantum dots (QDs) embedded in asymmetrical (Al)GaAs barriers were studied by numerical analysis based on the finite element method. The outlines of the structures were designed considering experimental outcomes such as QDs morphology, wetting layer thickness, and the composition of the materials observed for the molecular beam epitaxial growth and capping of InAs/(Al)GaAs samples. The Al content in the AlGaAs alloy encapsulating material prompted variations on the island's shape, so regular and truncated pyramidal QDs were simulated. According to the simulations, higher values of positive biaxial strain tensor εxx were obtained above the apex zone in pyramidal QDs as compared to truncated ones. The heavy hole and light hole bands intercalated relative positions along the internal QDs profile, a consequence of the compressive and tensile strain distribution inside the pyramidal QDs. The biaxial strain and the elastic energy analyzed above the apex zone and below the islands are important for the vertical correlation probability, and we found dependence on the shape of the nanostructure and the distance from the top of the islands to the surface spacer. Finally, those nanoislands for which the capping procedure did not change the geometry, showed a higher number of confined eigenstates, which is required for many optoelectronic applications. [ABSTRACT FROM AUTHOR]

The nanoscale intrinsic electrical properties of in-plane InAs nanowires grown by selective area epitaxy are investigated using a process-free method involving a multi-probe scanning tunneling microscope. The resistance of oxide-free InAs nanowires grown on an InP(111)B substrate and the resistance of InAs/GaSb core–shell nanowires grown on an InP(001) substrate are measured using a collinear four-point probe arrangement in ultrahigh vacuum. They are compared with the resistance of two-dimensional electron gas reference samples measured using the same method and with the Van der Pauw geometry for validation. A significant improvement of the conductance is achieved when the InAs nanowires are fully embedded in GaSb, exhibiting an intrinsic sheet conductance close to the one of the quantum well counterpart. [ABSTRACT FROM AUTHOR]

Solid‐state‐based quantum technologies, such as electronic spin‐qubits, constitute a leading approach to the realization of quantum computation. Electronic spin‐qubits hosted in semiconductor heterostructures demand the highest crystalline quality, specifically with respect to the structure and formation of in‐plane misfit dislocations (MDs). Here, the formation mechanisms of MD networks in such Si/SiGe heterostructures are investigated. For this purpose, strained Si layers on relaxed Si0.7Ge0.3 with thicknesses above the critical thickness are grown by molecular beam epitaxy to allow for the formation of MDs. MDs are mapped out by electron channeling contrast imaging and the experimental results are compared to MD networks calculated by Monte Carlo simulations. The simulations show that the ratio between the threading dislocation density (TDD) and the average length of MDs is a constant for a given set of parameters, here 2.9 for sufficiently large dislocation statistics and simultaneous extension of MDs. Further, the fractal dimension of the MD network determined by the box‐counting method as an alternative quantity to characterize MD networks is introduced. This allows to infer the formation mechanism of MDs in real devices and hence the quantification of resulting average relaxation within the Si‐quantum well. [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]

The studies of the interband electron transition energy in multiple Hg1-xCdxTe/Hg1-yCdyTe quantum wells (MQWs) at room temperature were carried out. The MQWs were grown on the (013) GaAs substrate by molecular beam epitaxy, with the layer composition and thickness being measured by the in-situ ellipsometric parameters measurement at the nanometer level. The Hg1-xCdxTe barrier composition and width were x = 0.69 and 30 nm, respectively. The Hg1-yCdyTe well composition was y = 0.06–0.10, and the width varied in the range of 2.7–13 nm. The experimental data of the interband electron transition energy were determined by the absorption spectral analysis. The calculation of the interband electron transition energy was carried out on the basis of the four-band Kane model. A good agreement between the experimental and calculated data was obtained. It was shown that MQWs may be used as a photosensitive material for creating infrared optoelectronic devices operating in different modes in the range of 3–10 μm at room temperature. [ABSTRACT FROM AUTHOR]

InAs/GaSb double quantum wells separated by a 100 Å AlSb middle barrier are grown by molecular beam epitaxy. We report a nanofabrication technique that utilizes the surface Fermi level pinning position in InAs [ E f s (InAs) ] for realizing independent electric contacts to each well. In particular, separate Ohmic contacts to the upper InAs quantum well are achieved by selectively etching down to the InAs, while contacts to the lower GaSb quantum well are obtained by the depletion method. For the latter, the upper InAs quantum well is locally pinched off by top etched trenches capped with a remaining 2–3 nm InAs layer. As a result of a relatively low E f s (InAs) , applying a negative bias gate potential will create a conducting hole channel in GaSb and, hence, a separate Ohmic contact to the lower quantum well. This method is demonstrated with experiment and the support of a self-consistent band bending calculation. A number of experiments on separately probing Coulomb and tunnel-coupled InAs/GaSb systems now become accessible. [ABSTRACT FROM AUTHOR]

This article describes GaN/AlN heterostructures for ultraviolet-C (UVC) emitters with multiple (up to 400 periods) two-dimensional (2D)-quantum disk/quantum well structures with the same GaN nominal thicknesses of 1.5 and 16 ML-thick AlN barrier layers, which were grown by plasma-assisted molecular-beam epitaxy in a wide range of gallium and activated nitrogen flux ratios (Ga/N2*) on c-sapphire substrates. An increase in the Ga/N2* ratio from 1.1 to 2.2 made it possible to change the 2D-topography of the structures due to a transition from the mixed spiral and 2D-nucleation growth to a purely spiral growth. As a result, the emission energy (wavelength) could be varied from 5.21 eV (238 nm) to 4.68 eV (265 nm) owing to the correspondingly increased carrier localization energy. Using electron-beam pumping with a maximum pulse current of 2 A at an electron energy of 12.5 keV, a maximum output optical power of 50 W was achieved for the 265 nm structure, while the structure emitting at 238 nm demonstrated a power of 10 W. [ABSTRACT FROM AUTHOR]

The molecular beam epitaxy growth and optical properties of the III-V semiconductor alloy InAsSbBi are investigated over a range of growth temperatures and V/III flux ratios. Bulk and quantum well structures grown on the (100) on-axis and offcut GaSb substrates are examined. Bismuth readily incorporates at growth temperatures around 300 °C but results in materials with limited optical quality. Conversely, higher growth temperatures around 400 °C yield improved optical performance but with limited Bi incorporation. Photoluminescence spectroscopy is used to examine the optical properties and bandgap energies of InAsSbBi layers grown at temperatures from 400 to 430 °C using 0.91 and 0.94 As/In flux ratios, 0.10 and 0.12 Sb/In flux ratios, and 0.05 and 0.10 Bi/In flux ratios. Emission is observed from low to room temperature with peaks ranging from 3.7 to 4.6 μm. The relationships between Bi incorporation, surface morphology, growth temperature, and group-V flux are examined. Large concentrations of Bi-rich surface features are observed on samples where the incident Bi flux neither fully incorporates nor desorbs but instead accumulates on the surface and coalesces into droplets. [ABSTRACT FROM AUTHOR]

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

Fabry–Perot laser diodes based on (Al, Ga)As and Ga(As, Bi) with single or multiple parabolic or rectangular-shaped quantum wells (QWs) emitting at the 780–1100 nm spectral range were fabricated and investigated for optimization of the laser QW design and composition of QWs. The laser structures were grown using the molecular beam epitaxy (MBE) technique on the n-type GaAs(100) substrate. The photolithography process was performed to fabricate edge-emitting laser bars of 5 μm by 500 μm in size. The temperature-dependent power-current measurements showed that the characteristic threshold current of the fabricated LDs was in the 60–120 mA range. Light and current characteristics were almost linear up to (1.2–2.0) Ith. Low-frequency 10 Hz–20 kHz electrical and optical noise characteristics were measured in the temperature range from 70 K to 290 K and showed that the low-frequency optical and electrical noise spectra are comprised of 1/f and Lorentzian-type components. The positive cross-correlation between optical and electrical fluctuations was observed. [ABSTRACT FROM AUTHOR]

The optical properties of nanowire-based InGaN/GaN multiple quantum wells (MQWs) heterostructures grown by plasma-assisted molecular beam epitaxy are investigated. The beneficial effect of an InGaN underlayer grown below the active region is demonstrated and assigned to the trapping of point defects transferred from the pseudo-template to the active region. The influence of surface recombination is also investigated. For low InN molar fraction value, we demonstrate that AlO x deposition efficiently passivate the surface. By contrast, for large InN molar fraction, the increase of volume non-radiative recombination, which we assign to the formation of additional point defects during the growth of the heterostructure dominates surface recombination. The inhomogeneous luminescence of single nanowires at the nanoscale, namely a luminescent ring surrounding a less luminescent centre part points towards an inhomogeneous spatial distribution of the non-radiative recombination center tentatively identified as intrinsic point defects created during the MQWs growth. These results can contribute to improve the performances of microLEDs in the visible range. [ABSTRACT FROM AUTHOR]

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LEDs based on hexagonal InGaN/GaN quantum wells are dominant technology for many lighting applications. However, their luminous efficacy for green and amber emission and at high drive currents remains limited. Growing quantum wells instead in the cubic phase is a promising alternative because, compared to hexagonal GaN, it benefits from a reduced bandgap and is free of the strong polarization fields that can reduce the radiative recombination rate. Initial attempts to grow cubic GaN in the 1990s employed molecular beam epitaxy, but now, metal-organic chemical vapor deposition can also be used. Nonetheless, high phase purity requires careful attention to growth conditions and the quantification of any unwanted hexagonal phase. In contrast to hexagonal GaN, in which threading dislocations are key, at its current state of maturity, the most important extended structural defects in cubic GaN are stacking faults. These modify the optical properties of cubic GaN films and propagate into active layers. In quantum wells and electron blocking layers, segregation of alloying elements at stacking faults has been observed, leading to the formation of quantum wires and polarized emission. This observation forms part of a developing understanding of the optical properties of cubic InGaN quantum wells, which also offer shorter recombination lifetimes than their polar hexagonal counterparts. There is also growing expertise in p-doping, including dopant activation by annealing. Overall, cubic GaN has rapidly transitioned from an academic curiosity to a real prospect for application in devices, with the potential to offer specific performance advantages compared to polar hexagonal material. [ABSTRACT FROM AUTHOR]

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

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]

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

An AlGaAs/GaAs multiple-quantum-well based resonant Bragg structure was designed to match the optical Bragg resonance with the exciton-polariton resonance at the second quantum state in the GaAs quantum wells. The sample structure with 60 periods of AlGaAs/GaAs quantum wells was grown on a semi-insulating GaAs substrate by molecular beam epitaxy. Angle- and temperature-dependent photoluminescence, optical reflectance, and electro-reflectance spectroscopies were employed to study the resonant optical properties of the Bragg structure. Broad and enhanced optical and electro-reflectance features were observed when the Bragg resonance was tuned to the second quantum state of the GaAs quantum well excitons, manifesting a strong lightmatter interaction. From the electro-optical experiments, we found the electro-reflectance features related to the transitions of x(e2-hh2) and x(e2-hh1) excitons. The excitonic transition x(e2-hh1), which is prohibited at zero electric field, was allowed by a DC bias due to the brake of symmetry and increased overlap of the electron and hole wave functions caused by the electric field. By tuning the Bragg resonance frequency, we have observed the electro-reflectance feature related to the second quantum state up to room temperature, which evidences a robust light-matter interaction in the resonant Bragg structure. [ABSTRACT FROM AUTHOR]

Magnetoresistance measurements were performed on a 30 nm-thick SnTe quantum well (QW) grown by molecular beam epitaxy on the BaF2 substrate in the temperature range of 1.9–50 K. The weak antilocalization (WAL) effect was observed at low temperatures and low magnetic fields as a result of the strong spin–orbit coupling present in the QW. Using the Hikami–Larkin–Nagaoka equation, we analyzed the experimental data and found that the WAL effect is not purely 2D but composed of 2D and 3D channels that exist within the QW structure. The spin–orbit and phase coherence mechanisms are also extracted, and a general view of the transport properties of the QW is also provided. [ABSTRACT FROM AUTHOR]

We report on the epitaxial growth of antimony-based GaSb/Ga1−xInxSb/GaSb quantum wells (QWs) on the (111) plane. Different from the more common (001) heterostructures, (111) structures offer certain advantages in electronic properties, such as the existence of the strain-induced piezoelectric effect, as well as a large effective mass of hole carriers along the growth axis. In the present work, we found that high-quality QWs can be successfully grown using a GaSb buffer layer on a vicinal GaAs (111)B substrate. Due to the presence of the piezoelectric field and the surface states, hole carriers are introduced in the QW without intentional doping. Electrical transport measurements on an x = 0.1 sample at 300 mK show that the QW has a two-dimensional hole gas (2DHG) density of 6.7 × 1011 cm−2 and a mobility of 28 000 cm2/Vs. The 2DHG exhibits clear Shubnikov–de Haas oscillations and the integer quantum Hall effect under a perpendicular magnetic field. [ABSTRACT FROM AUTHOR]