Intersubband Transition Energies Research Articles

Conduction-band engineering of polar nitride semiconductors with wurtzite ScAlN for near-infrared photonic devices

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.

Optical rectification and second harmonic generation in CdSe/MgSe asymmetric double quantum wells

In this study, we report theoretically the effect of well, barrier widths and polarization on optical properties of intersubband transitions (ISBT) in CdSe/MgSe asymmetric quantum wells (ADQWs). Eigenenergies and their corresponding wave functions have been calculated by solving numerically the Schrödinger equation. The second harmonic generation and the optical rectification including intersubband transition energies have been discussed. Obtained results revealed that intersubband transition depends strongly on the quantum wells and the barrier widths as well as the stark effect. With appropriate intensity, optical rectification can reach great magnitude. We hope that the numerical results of our research are valuable theoretically and experimentally to our scientific community in nonlinear optics.

A particular behavior of levels energy evolution and theoretical study of the quantum probability distribution in GaAs/AlGaAs asymmetric stepped semiconductor quantum wells for terahertz optoelectronics

The study of the evolution of the energy levels as a function of the step layer’s width Lw2 in an asymmetric stepped quantum well AlxGa1-xAs/GaAs/AlyGa1-yAs/AlxGa1-xAsx=24%,y=14%, revealed a particular behavior, marked by the presence of plateaus within certain deep well’s width, Lw2, ranges. We initially computed the eigenenergies and eigenfunctions by numerically solving the Schrödinger equation. This was done within the effective mass and envelope function approximation, taking into account the influence of non-parabolicity. To explain the origin of the plateaus and their impact on intersubband transition energies, we determined the distribution of the probability density of the presence of the electron in each region of the stepped quantum well. The results of these calculations demonstrated that the electron's probability density is higher in the step well than in the deep one, particularly for electrons with energy levels above 113.47 meV. This elevation in probability density exhibits maximum values that remain nearly constant within specific Lw2 domains, providing an explanation for the observed energy plateaus. Such plateaus allow more energy tunability and thus open up additional possibilities for applications in the THz domain either in the linear or nonlinear optics.

Intersubband Transitions in Lead Halide Perovskite-Based Quantum Wells for Mid-Infrared Detectors.

Due to their excellent optical and electrical properties as well as versatile growth and fabrication processes, lead halide perovskites have been widely considered as promising candidates for green energy and applications related to optoelectronics. Here, we investigate their potential applications at infrared wavelengths by modeling the intersubband transitions in perovskite-based quantum well systems. Both single-well and double-well structures are studied, and their energy levels as well as the corresponding wave functions and intersubband transition energies are calculated by solving the one-dimensional Schrödinger equations. Via adjustment of the quantum well and barrier thicknesses, the intersubband transition energies can be tuned to cover a broad infrared wavelength range. We also find that the lead halide perovskite-based quantum wells possess high absorption coefficients. The widely tunable transition energies and high absorption coefficients of the perovskite-based quantum well systems, combined with their unique material and electrical properties, may enable an alternative material system for infrared photodetector applications.

Effects of pressure, temperature, and electric field on linear and nonlinear optical properties of InxGa1-xAs/GaAs strained quantum dots under indium segregation and In/Ga intermixing phenomena

Linear and nonlinear optical properties of InxGa1-xAs/GaAs lens-shaped quantum dots were investigated, taking into account simultaneous effects of indium segregation, In/Ga intermixing, temperature, pressure, external electric field, strains, and structure dimensions. Muraki's model was used for the indium segregation and a Gaussian distribution along the growth axis and radial direction for the atoms intermixing phenomenon. All simulations were carried out using the Finite Difference Method and the effective mass approximation. Nonlinear optical properties associated with refractive index changes and light absorption were evaluated and discussed on the basis of the calculated intersubband transition energies. The results showed that the mentioned parameters have a significant effect in controlling and tuning the nonlinear optical properties. It is shown that the indium segregation inside the wetting layer brought about a blue shift, whereas inhomogeneous indium distribution inside the QDs caused a red shift. Moreover, the nonlinear optical properties underwent a prominent stark-effect, and the resonant energy's red or blue shift is related to the electric field orientation. Furthermore, the nonlinear optical properties achieved a red (blue) shift with the increase in temperature (pressure). Thus, under the combined effects of the mentioned parameters, our theoretical results were correlated with the experimental data from photoluminescence spectra.

Intersubband Transitions in Nonpolar ZnO/BeMgZnO Quantum Wells: Effects of Physical Dimension, Concentration and Donor Level

Polarizaton properties of ZnO well layers on BeMgZnO barrier layers grown in polar and semipolar orientations have been investigated. Cases of relaxed and strained barrier layers are considered. It is found that the polarizaton difference at the interfaces leads to a built-in electric field inside the well layer as much as 8 MV cm^(-1) in magnitude. Nonpolar ZnO/BeMgZnO quantum wells have been studied in terms of intersubband transitions. The calculations have covered Be and Mg concentrations up 0.18 and 0.5, respectively. It has been found that intersubband transition (ISBT) energies ranging from 50 to 700 meV are possible. The effect of barrier thickness on the ISBT energies has been studied. The results indicate insignificant changes in ISBT energies compared to the energies.

Carrier screening effects on intersubband nonlinear optical rectification in wurtzite InGaN/GaN coupling quantum wells

Carrier screening effects on an optical rectification (OR) susceptibility of InGaN/GaN coupling quantum well (CQW) structures are theoretically investigated by using effective mass theory. The intersubband (ISB) transition energy E21 is shown to be strongly affected by the carrier screening. It has the value of about 0.1–0.13 eV, which is comparable to that (∼0.12 eV) observed in GaAs/AlGaAs system. The peak energy of the OR coefficient is redshifted greatly by the carrier screening effect with increasing carrier density. Also, the peak intensity of the OR coefficient rapidly increases with increasing carrier density because the OR coefficient is proportional to carrier density.

Piezo-phototronic intersubband terahertz devices based on layer-dependent van der Waals quantum well

Piezo-phototronic effect on intersubband transition of valence band is studied by the hybrid kp theory tight-binding model approach. Piezoelectric properties of multilayer MoS2 show the layer-dependent behaviors, which offer new way to design quantum well by strain induced polarization. In this work, we design a kind of van der Waals quantum well based on multilayer MoS2. The intersubband transition energy can be regulated within terahertz by strain. The transition strength is studied by calculating oscillator strength. The novel structures not only offer a new degree of freedom for piezotronic and piezo-phototronic materials and devices, but also pave a new way to design optoelectronic nanoscale devices.

Optical Properties of Gaas/Alxga1-xas Superlattice Under E-Field for Quantum Cascade Laser Application

Optical properties of GaAs/AlxGa1-xAs superlattice are studied dependent on quantum well thickness of gain region and doping density of injector layers underperformed electric field. Conduction band alignment of the superlattice is obtained by using effective mass approximation. 1d-Schrodinger formula is solved by using FDM. Intersubband transition energies, linear (nonlinear and total) absorption coefficients and linear (nonlinear and total) refractive index changes are plotted under applied electric field intensity. Intersubband transition energy of electron from second excited state to first excited state shows 147 meV. It is found that -45 kV/cm electric field intensity and 5 nm layer thickness of last quantum well of the gain region are the best values for studied structure. After that, linear absorption coefficient is investigated dependent on carrier number in the injector region under electric field. It is found that carrier number over 5 𝑥 1016 𝑐𝑚−2 can causes huge internal absorption of the radiative emission obtained in gain region due to increase in linear absorption coefficient by factor 10. As a conclusion, total absorption coefficient and total refractive index change are calculated for optimized parameters.

Defect Tolerance of Intersubband Transitions in Nonpolar GaN/(Al,Ga)N Heterostructures: A Path toward Low-Cost and Scalable Mid- to Far-Infrared Optoelectronics

We report on the impact of structural defects on mid-infrared intersubband (ISB) properties of $\mathrm{Ga}\mathrm{N}/(\mathrm{Al},\mathrm{Ga})\mathrm{N}$ heterostructures grown by ammonia molecular beam epitaxy (${\mathrm{NH}}_{3}$ MBE). Twenty-period $\mathrm{Ga}\mathrm{N}/(\mathrm{Al},\mathrm{Ga})\mathrm{N}$ multi-quantum-well (MQW) heterostructures are grown on co-loaded a-plane freestanding $\mathrm{Ga}\mathrm{N}$ substrates and heteroepitaxial a-plane $\mathrm{Ga}\mathrm{N}$ on r-plane sapphire templates (a-$\mathrm{Ga}\mathrm{N}$/r-sap) for three different quantum-well (QW) widths (3.0, 3.3, and 3.7 nm). Co-loaded structures grown on freestanding a-plane with no basal-plane stacking faults (BSFs), prismatic stacking faults (PSFs), and partial dislocations (PDs), with low threading dislocation (TD) densities of about ${10}^{5}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ are compared with those grown on a-$\mathrm{Ga}\mathrm{N}$ templates on $(10\overline{1}2)$ r-sapphire with BSF, PSF, PD, and TD densities of about 4 \ifmmode\times\else\texttimes\fi{} ${10}^{5}$ to ${10}^{6}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, 5 \ifmmode\times\else\texttimes\fi{} ${10}^{3}$ to 2 \ifmmode\times\else\texttimes\fi{} ${10}^{4}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, about 9 \ifmmode\times\else\texttimes\fi{} ${10}^{10}$ to 2 \ifmmode\times\else\texttimes\fi{} ${10}^{11}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, and about ${10}^{10}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, respectively. Fourier-transform infrared absorption spectroscopy indicates ISB transition energies in the range of about 250--300 meV (wavelength range 4.1--4.8 \textmu{}m) for MQWs with different QW widths. The ISB absorption spectra indicate about 5% smaller transition energies and only about 10%--20% larger spectral linewidths for structures grown on a-$\mathrm{Ga}\mathrm{N}/$r-sapphire templates compared with those on freestanding $\mathrm{Ga}\mathrm{N}$ substrates. The strong defect tolerance in the nonpolar a-plane ISB structures could be due to the nature of defects and their energy levels with respect to the conduction-band minima, which do not affect the ISB properties. Our results pave the way toward the production of low-cost scalable nonpolar III-nitride MQW heterostructures for a variety of passive and active optical materials and devices based on intersubband transitions.

Mid-wave/long-wave dual-color infrared quantum cascade detector enhanced by antenna-coupled microcavity.

We have demonstrated a mid-wave/long-wave dual-color infrared quantum cascade detector enhanced by antenna-coupled microcavity. By optimizing the size of patches, the coupling wavelength of the antenna-coupled microcavity can be conveniently tuned to match the targeted intersubband transition energy. At 77 K, the peak responsivity of our detector is 4.1 mA/W for long wave (10.4 µm) and 0.6 mA/W for mid wave (5.8 µm), while the detectivity is 1.8×109 cm·Hz1/2/W (Jones) and 2.6×108 cm·Hz1/2/W (Jones), respectively. Compared with a reference device with a 45° multi-pass geometry, the responsivity of our detector has been increased by a factor of 9.1 for the long wave and 2.7 for the mid wave. Our results illustrate how to realize a dual-color infrared detector and improve the optoelectronic performance through the concept of antenna-coupled microcavity.

Mid-infrared intersubband absorption in strain-balanced non-polar (In)AlGaN/InGaN multi-quantum wells

Mid-infrared intersubband transitions in strain-balanced m-plane (In)AlxGa1-xN/In0.16Ga0.84N (0.19≤x≤0.3) multi-quantum wells are reported for the first time in the range of 3.4–5.1 µm (244–360 meV). Direct and attenuated total-reflection infrared absorption measurements are interpreted using structural information revealed by high-resolution x-ray diffraction and transmission electron microanalysis. The experimental intersubband energies are better reproduced by calculations using the local-density approximation than the Hartree-Fock approximation for the exchange-correlation correction. The effect of charge density, quantum well width, and barrier alloy composition on the intersubband transition energy is also investigated to evaluate the potential of this material for practical device applications.

(Invited) Conduction Band Structure of Colloidal Quantum Wells

Similar to epitaxial quantum wells, colloidal quantum wells display electron intersubband transitions when photoexcited or electrochemically charged. Compared to epitaxial quantum wells, the strong confinement effect of thin, monodisperse colloidal quantum wells results in narrow intersubband transitions which reach high energies spanning the near-infrared spectral range. The composition of surface ligand chemistry subtly tunes the strain of the colloidal quantum wells to generate intersubband transitions over nearly continuously tunable energies. The divergent behavior of the band gap energy and intersubband transition energy implicates a substantial role of biaxial (in-plane) strain on the valence band structure. Multiple pump transient spectroscopy was furthermore employed to examine the lifetime of intersubband transitions, which is critical to their implementation in quantum cascade lasers or infrared detectors. In all cases, including core/shell samples, the lifetime of intersubband transitions is limited by surface or LO phonons to 1 ps or less, much shorter than comparable intraband transitions in quantum dots.

Temperature, hydrostatic pression and composition x effects on intersubband energy levels in ZnSe/ZnSxSe1-x core–shell quantum dot

In this paper, we have studied theoretically the effects of the, temperature, hydrostatic pressure and the composition on the intersubband transition energies. We have used a mathematical method, which based on a combination of coordinate transformation and the finite difference method. We have controlled the size and shape of core shell quantum dot CSQD by varying the parameters introduced in the function of coordinate transformation. We have studied the influence of hydrostatic pressure and temperature on the optical properties.

Structural and optical properties of nonpolar m- and a-plane GaN/AlGaN heterostructures for narrow-linewidth mid-infrared intersubband transitions

Mid-infrared intersubband transitions are investigated in nonpolar m-plane and a-plane GaN/AlGaN multi-quantum well heterostructures. Nominally identical heterostructures were grown by ammonia molecular-beam epitaxy on free-standing m-plane and a-plane GaN substrates. A total of 12 well- and barrier-doped samples with intersubband transition energies in the range of 220–320 meV (wavelength range 3.8–5.6 μm) were grown. The intersubband absorption lines of the m-plane samples were 10–40% narrower than those of the a-plane samples, and a very narrow intersubband absorption linewidth of 38 meV (full width at half maximum) at a transition energy of approximately 250 meV (5 μm wavelength) was observed in an m-plane sample. Narrower intersubband absorption linewidths of m-plane samples can be explained by more abrupt heterostructure interfaces revealed by structural characterization, which is attributed to a higher stability of the m-plane compared to the a-plane. No significant difference in the intersubband absorption linewidth was observed between the barrier- and well-doped samples.

Band Structure and Intersubband Transitions of Three-Layer Semiconductor Nanoplatelets.

This paper presents the first general theory of electronic band structure and intersubband transitions in three-layer semiconductor nanoplatelets. We find a dispersion relation and wave functions of the confined electrons and use them to analyze the band structure of core/shell nanoplatelets with equal thicknesses of the shell layers. It is shown that the energies of electrons localized inside the shell layers can be degenerate for certain electron wave vectors and certain core and shell thicknesses. We also show that the energies of intersubband transitions can be nonmonotonic functions of the core and shell thicknesses, exhibiting pronounced local minima and maxima which can be observed in the infrared absorption spectra. Our results will prove useful for the design of photonic devices based on multilayered semiconductor nanoplatelets operating at infrared frequencies.

Computation of intersubband transition energy and absorption spectra of binary capped GaAs/Al0.42Ga0.58As core-shell quantum dot

Intersubband transition energy is computed for both core-shell quantum dot (CSQD) and binary capped core-shell quantum dot (CCSQD) of cubic geometry by solving the time-independent Schrodinger equation using the finite difference method. The discretization of the structures in all three spatial directions generates sparse Hamiltonian matrices, which are diagonalized to obtain energy eigenstates for the conduction band. The transition energy for the lowest three energy eigenstates is compared for different structural parameters considering GaAs / Al0.42Ga0.58As CSQDs. CSQD capped with AlAs (CCSQD) viz GaAs / Al0.42Ga0.58As / AlAs shows higher eigenstates and transition energy, which decreases with the increase in core thickness. Furthermore, the optical properties of these structures have been investigated which are in concurrence with the obtained eigen energy. The broader tuning range and blueshifted higher absorption coefficient of CCSQD support significant application in quantum dot detectors and lasers.

Influence of electric field on direct and indirect exciton in a concentrically coupled quantum ring heterostructure embedded in SiO2 matrix

Abstract Excitonic states of Concentric Double Quantum Ring (CDQR) heterostructures embedded in SiO2 matrix under the influence of electric field has been calculated theoretically using variational technique. CDQR has been confined in a quantum well of suitable width which serve as the height of the quantum ring along which the electric field is applied. Central barrier width, outer ring width and carrier location has been tuned to understand the localization of both direct and indirect excitons. Effect of ring dimension and electric field on the energy states of Inner Ring Exciton (IRE) and Outer Ring Exciton (ORE) has been determined. Tilting of conduction and valence bands due to the application of electric field invokes a triangular confinement to the carriers and the same has been discussed. Diamagnetic susceptibility which is merely an indicator of electron hole separation and intersubband transition energy has also been calculated for both symmetric and asymmetric quantum ring. Height of the QR has also been changed to study its effect on the diamagnetic susceptibility of an exciton.

Many-body effects on intersubband transitions in polar ZnO/ZnMgO multiple quantum wells

ZnO/ZnMgO multiple quantum wells are investigated in terms of intersubband transitions within the conduction band. The Schrödinger's and Poisson's equations are solved self-consistently. The intersubband transition energy is represented as a function of their physical dimension and Mg concentration. The effects of the depolarization and excitonic shifts on the transition energy are considered. A high level of doping, such as 1 × 1025 m−3, is required to screen the built-in electric field, which is around 0.355 MV/cm at most. In case of screening, it is possible to have quantum wells aligned in energy and a homogeneous electron distribution over the quantum wells. The transition energy is between 51 and 403 meV if the width of the constituting quantum wells is between 20 and 100 Å. The contribution of the many-body effects is 29 meV at most. However, this contribution strongly depends on the well width: it can be as low as −1 meV.

Nonlinear optical absorption in wurtzite ZnCdO quantum wells

Nonlinear optical properties of a Zn1−xCdxO/ZnO quantum well structure originating from the transitions between the subbands within its conduction band are studied in terms of its dimension, Cd concentration and doping level. The intersubband transition energies are calculated within the effective mass and envelope function approximations. The Schrödinger and Poisson equations are self-consistently solved by considering the free and the bound charge distributions. The many-body effects, such as depolarization and excitonic shifts, are taken into account. The results are represented as functions the Cd concentration, limited between 0.05 and 0.2, and the well width, changing from 10 to 50 Å. The calculations indicate an intersubband transition energy range from 40 (9.67) to 138 meV (33.4 THz) and a blue-shift in it, as a result of the many-body effects, up to 11 meV in case of a doping level of 1 × 1025 m−3. It is shown that the transition energy increases with the widening quantum well within the range of the parameters as a result of the strong built-in electric field. The nonlinear absorption lineshape is found be to asymmetric and has the peak positions depending on the light intensity and the doping level inside the quantum well.