Search

Theoretical investigation of electro-absorption modulators in the mid-infrared range (>∼2 μm) is performed using asymmetric intra-step quantum wells based on Ge1−η1Snη1/Ge1−η2Snη2 heterostructures with SiGeSn outer barriers. After exploring the parameter space of the Sn content difference and width ratio of the intra-layers, a linear and much larger Stark shift is realized, compared to that of a square quantum well, without an increase of the total structure width. A modulator based on an optimized intra-step quantum well structure with a total well width of 12 nm is theoretically predicted to have both a larger peak shift per unit applied field and a larger absorption change than a 12 nm square quantum well device. By analyzing the device performance based on the two figures of merit: (1) absorption change per applied field and (2) absorption change per applied field squared, and taking 10 dB extinction ratio, a 44% higher bandwidth per volt and 46% lower power consumption per bit are achieved in intra-step than in a square well. Although the swing voltage for a square quantum well can be reduced by using a larger on-set applied field and performance could be improved, we found that the intra-step quantum well using zero on-set still retains its advantages when compared to the square quantum well which uses a 0.5 V on-set voltage.

A direct bandgap can be engineered in Ge-rich group-IV alloys by increasing Sn content and by introducing tensile strain in GeSn. Here, we combine these two routes in quantum well (QW) structures and systematically analyze the properties of SiGeSn/GeSn quantum wells for a range of Sn content, strain, and well width values, within realistic boundaries. Using the k ⋅ p method, and including L-valley within the effective mass method, we find that 13–16 nm is a preferred range of well widths to achieve high gain for tensile-strained SiGeSn/GeSn quantum wells. Within the range of the well widths, a loss ridge caused by inter-valence band absorption and free carrier absorption is found in the region of parameter space where Sn content and strain in the well are related as Sn ( % ) ≈ − 7.71 e x x ( % ) + 17.13. Limited by a practical strain boundary of 1.7%, for a 14 nm quantum well, we find that 7.5 ± 1 % Sn and 1 ± 0.2 % strain is a promising combination to get a good net gain for photon transition energy higher than ∼0.42 eV. A maximum utilization of strain is preferred to obtain the best gain with lower energies (

GeSn is discussed as solution to realize the dream of a group IV light source integrated on a Si chip. Sn added into a Ge lattice decreases the conduction band energies leading to a direct bandgap semiconductor band structure. However, the compressive strain increases the direct band energy imposing a large Sn content in the GeSn bulk. In spite of many difficulties regarding the growth of epitaxial GeSn alloys on Si, several hundred nm thick GeSn layers with various Sn concentrations up to 15% could be realized and used as gain material for lasers. Nowadays research concentrates on increasing the Sn content towards 20 at% as well as structural layout. The challenge here is the decreasing quality at high Sn contents and the isolation of the active layer from the mists formed at the interface with Ge/Si which increase the laser threshold. In this direction we discuss the influence on lasing and threshold of MQW SiGeSn/GeSn heterostructures with different quantum well thicknesses. Other solution proposed is the change of intrinsic strain type from compressive into tensile by introducing Si3N4 stressors and also GeSn on Insulator technology. These methods are well known in CMOS technology and can be applied to very low Sn content GeSn alloys. The discussion on the best way to reach room temperature laser is addressed both theoretical and experimental.

We consider the influence of additional carrier confinement, achieved by application of strong perpendicular magnetic field, on inter Landau levels electron relaxation rates and the optical gain, of two different GaAs quantum cascade laser structures operating in the terahertz spectral range. Breaking of the in-plane energy dispersion and the formation of discrete energy levels is an efficient mechanism for eventual quenching of optical phonon emission and obtaining very long electronic lifetime in the relevant laser state. We employ our detailed model for calculating the electron relaxation rates (due to interface roughness and electron-longitudinal optical phonon scattering), and solve a full set of rate equations to evaluate the carrier distribution over Landau levels. The numerical simulations are performed for three- and four-well (per period) based structures that operate at 3.9 THz and 1.9 THz, respectively, both implemented in GaAs/Al0.15Ga0.85As. Numerical results are presented for magnetic field values from 1.5 T up to 20 T, while the band nonparabolicity is accounted for. (C) 2016 Elsevier B.V. All rights reserved.

The optimal design of asymmetric quantum well structures for generation of entangled photons in the mid-infrared range by spontaneous parametric downconversion is considered, and the efficiency of this process is estimated. Calculations show that a reasonably good degree of entanglement can be obtained, and that the optical interaction length required for optimal conversion is very short, in the few μm range.

The terahertz (THz) frequency range is very important in various practical applications, such as terahertz imaging, chemical sensing, biological sensing, high-speed telecommunications, security, and medical applications. Based on the density functional theory (DFT), this work presents electronic and optical properties of N-doped ZnO/ZnO/N-doped ZnO quantum well and quantum wire nanostructures. The density of states (DOS), the band structures, effective masses, and the band offsets of ZnO and N-doped ZnO were calculated as the input parameters for the subsequent modeling of the ZnO/N-doped ZnO heterojunctions. The results show that the energy gaps of the component materials are different, and the conduction and valence band offsets at the ZnO/N-doped ZnO heterojunction give type-II alignment. Furthermore, the optical characteristics of N-doped ZnO/ZnO/N-doped ZnO quantum well were studied by calculating the absorption coefficient from transitions between the confined states in the conduction band under the applied electric field (Stark effect). The results indicate that N-doped ZnO/ZnO/N-doped ZnO quantum wells, quantum wires, and quantum cascade structures could offer the absorption spectrum tunable in the THz range by varying the electric field and the quantum system size. Therefore, our work indicates the possibility of using ZnO as a promising candidate for infrared and terahertz applications.

GeSn and SiGeSn are promising materials for the fabrication of a group IV laser source offering a number of design options from bulk to heterostructures and quantum wells. Here, we investigate GeSn/SiGeSn multi quantum wells using the optically pumped laser effect. Three complex heterostructures were grown on top of 200 nm thick strain-relaxed Ge0.9Sn0.1 buffers. The lasing is investigated in terms of threshold and maximal lasing operation temperature by comparing multiple quantum well to double heterostructure samples. Pumping under two different wavelengths of 1064 and 1550 nm yields comparable lasing thresholds. The design with multi quantum wells reduces the lasing threshold to 40 ± 5 kW/cm2 at 20 K, almost 10 times lower than for bulk structures. Moreover, 20 K higher maximal lasing temperatures were found for lower energy pumping of 1550 nm.

Energy-efficient integrated circuits for on-chip or chip-to-chip data transfer via photons could be tackled by monolithically grown group IV photonic devices. The major goal here is the realization of fully integrated group IV room temperature electrically driven lasers. An approach beyond the already demonstrated optically-pumped lasers would be the introduction of GeSn/(Si)Ge(Sn) heterostructures and exploitation of quantum mechanical effects by reducing the dimensionality, which affects the density of states. In this contribution we present epitaxial growth, processing and characterization of GeSn/(Si)Ge(Sn) heterostructures, ranging from GeSn/Ge multi quantum wells (MQWs) to GeSn quantum dots (QDs) embedded in a Ge matrix. Light emitting diodes (LEDs) were fabricated based on the MQW structure and structurally analyzed via TEM, XRD and RBS. Moreover, EL measurements were performed to investigate quantum confinement effects in the wells. The GeSn QDs were formed via Sn diffusion /segregation upon thermal annealing of GeSn single quantum wells (SQW) embedded in Ge layers. The evaluation of the experimental results is supported by band structure calculations of GeSn/(Si)Ge(Sn) heterostructures to investigate their applicability for photonic devices.

The experimental demonstration of fundamental direct bandgap, group IV GeSn alloys has constituted an important step towards realization of the last missing ingredient for electronic-photonic integrated circuits, i.e. the efficient group IV laser source. In this contribution, we present electroluminescence studies of reduced-pressure CVD grown, direct bandgap GeSn light emitting diodes (LEDs) with Sn contents up to 11 at.%. Besides homojunction GeSn LEDs, complex heterojunction structures, such as GeSn/Ge multi quantum wells (MQWs) have been studied. Structural and compositional investigations confirm high crystalline quality, abrupt interfaces and tailored strain of the grown structures. While also being suitable for light absorption applications, all devices show light emission in a narrow short-wave infrared (SWIR) range. Temperature dependent electroluminescence (EL) clearly indicates a fundamentally direct bandgap in the 11 at.% Sn sample, with room temperature emission at around 0.55 eV (2.25 µm). We have, however, identified some limitations of the GeSn/Ge MQW approach regarding emission efficiency, which can be overcome by introducing SiGeSn ternary alloys as quantum confinement barriers.

In recent years, SiGeSn alloys have been considered as promising materials for optoelectronic applications because they offer the possibility of a direct bandgap, and are compatible with Si-based technology; therefore, they have prospective applications such as in interband lasers and detectors, solar cells, etc. Here, we consider another possible application of nanostructures based on these materials: to extend the suite of Si-based optoelectronic devices for interband electroabsorption (EA) modulators. Using the 8-band k.p method, we have designed asymmetric double quantum wells that are optimized, by varying the well and barrier widths and material composition, to show large optical transmission sensitivity to the applied bias. Generally, these structures are useful for EA modulators in the midinfrared spectral range.

The results of simulations of Γ-X scattering in GaAs/AlGaAs quantum wells are presented, discussing the importance of the mole fraction, doping density, and lattice and electron temperatures in determining the scattering rates. A systematic study of Γ-X scattering in GaAs/AlxGa1-xAs heterostructures, using a single quantum well to determine the importance of well width, molar concentration x, lattice temperature, and doping density, has been performed. After this we consider a double quantum well to determine the role of intervalley scattering in the transport through single-layer heterostructures, i.e. Γ-X-Γ scattering compared with Γ-Γ scattering. Finally, we estimate the relative importance of intervalley scattering in a GaAs-based quantum-cascade laser device and compare it with other relevant scattering mechanisms important to describe carrier dynamics in the structure. Our simulations suggest that Γ-X scattering can be significant at room temperature but falls off rapidly at lower temperatures.

We report on the results of our simulations of Γ−X scattering in GaAs/AlGaAs heterostructures, discussing the importance of the mole fraction, doping density, and lattice and electron temperature in determining the scattering rates. We consider three systems, a single quantum well (for the investigation of Γ−X scattering), a double quantum well (to compare the Γ−X−G and Γ−Γ scattering rates), and an example of a GaAs/AlGaAs mid-infrared quantum cascade laser. Our simulations suggest that Γ−X scattering can be significant at room temperature but falls off rapidly at lower temperatures. One important factor determining the scattering rate is found to be the energy difference between the Γ and X-states.

The aim of this work is to calculate carrier relaxation rates from the upper laser level due to electron–electron interactions in three and four-level quantum cascade lasers (QCLs) in a strong magnetic field. The comparison between calculated results and previously obtained values for acoustical and optical-phonon scattering processes indicates that carrier–carrier scattering might have noticeable influence on laser output properties, depending on the structural design. Numerical results are presented for two λ ∼ 9 μm GaAs-based QCLs in magnetic fields between 20 T and 60 T and the band nonparabolicity is taken into account.

Experimental results showing room temperature normal incidence mid-infrared detection by AlGaN/GaN quantum well infrared photodetectors are presented. Designed structures have intersubband transitions corresponding to wavelengths in the region of 3 and 4 µm, where strong absorption in a sapphire substrate dominates. The intersubband spectra, therefore, were characterized by electronic Raman scattering and infrared photocurrent spectroscopy. The absorption spectra agree well with theoretical predictions. Details of device fabrication are presented with sensitivity estimates for the devices.

We propose an optically pumped laser based on intersublevel transitions in InAs-GaAs pyramidal self-assembled quantum dots. A theoretical rate equations model of the laser is given in order to predict the dependence of the gain on pumping flux and temperature. The energy levels and wave functions were calculated using the 8-band k/spl middot/p method where the symmetry of the pyramid was exploited to reduce the computational complexity. Carrier dynamics in the laser were modeled by taking both electron-longitudinal optical phonon and electron-longitudinal acoustic phonon interactions into account. The proposed laser emits at 14.6 /spl mu/m with a gain of g/spl ap/ 570 cm/sup -1/ at the pumping flux /spl Phi/=10/sup 24/ cm/sup -2/ s/sup -1/ and a temperature of T=77 K. By varying the size of the investigated dots, laser emission in the spectral range 13-21 /spl mu/m is predicted. In comparison to optically pumped lasers based on quantum wells, an advantage of the proposed type of laser is a lower pumping flux, due to the longer carrier lifetime in quantum dots, and also that both surface and edge emission are possible. The appropriate waveguide and cavity designs are presented, and by comparing the calculated values of the gain with the estimated losses, lasing is predicted even at room temperature for all the quantum dots investigated.

Analysis of hole transport in cascaded p-Si∕SiGe quantum well structures is performed using self-consistent rate equations simulations. The hole subband structure is calculated using the 6×6k·p model, and then used to find carrier relaxation rates due to the alloy disorder, acoustic, and optical phonon scattering, as well as hole-hole scattering. The simulation accounts for the in-plane k-space anisotropy of both the hole subband structure and the scattering rates. Results are presented for prototype THzSi∕SiGe quantum cascade structures.

A systematic design of a GaN/AlGaN active region for a three-level intersubband laser emitting in the near-infrared is presented. The proposed system is based on a double coupled quantum well structure with the laser level energy separation corresponding to 1.55 µm (ΔE32~800 meV), a wavelength important for optical communications. The lower laser level depopulates mostly via longitudinal optical phonon resonance, requiring ΔE21~90 meV. The model for scattering rate calculations, including many-body carrier screening effects, was first tested by comparison with recent experimental measurements in GaN/AlGaN quantum wells, and a very good agreement was obtained. It was then employed to calculate the gain coefficient, a commonly used figure of merit for three level laser systems, and this in turn allowed the optimal structural parameters of GaN/AlGaN quantum well to be found.

The design of ZnCdSe-ZnMnSe-based quantum wells is considered, in order to obtain a large shift of the peak absorption wavelength in the far infrared range, due to a giant Zeeman splitting with magnetic field, while maintaining a reasonably large value of peak absorption. A triple quantum-well structure with a suitable choice of parameters has been found to satisfy such requirements. A maximal tuning range between 14.6 and 34.7 meV is obtained, when the magnetic field varies from zero to 5 T, so the wavelength of the absorbed radiation decreases from 85.2 to 35.7 /spl mu/m with absorption up to 1.25% at low temperatures. These structures might form the basis for magnetic field tunable photodetectors and quantum cascade lasers in the terahertz range.

A method is proposed for finding optimal AlGaN-based quantum well profiles with respect to specified intersubband optical properties, e.g. nonlinear optical susceptibilities at resonance. It is based on applying the isospectral transformations (supersymmetric and coordinate transform) to an initial Hamiltonian in order to generate a family of parameters of controlled potentials, accompanied by position-dependent effective mass. By varying those control parameters, one call change the values of dipole matrix elements relevant for a particular effect, while energy levels, once obtained in the initial potential, remain unchanged. The internal electrostatic field, originating from piezoelectric and spontaneous polarization in the nitride alloy, has been taken into account. The use of the method is illustrated by maximizing second order nonlinear optical susceptibilities at wavelengths in the near infrared spectral range, and optimal smooth structures are eventually discretized to make their fabrication easier.

Hole transport in p-Si/SiGe quantum well cascade structures has been analyzed using a rate equation method with thermal balancing (self-consistent energy balance method). The carrier and energy relaxation due to alloy disorder, acoustic and optical phonon scattering are included. The model includes the in-plane k-space anisotropy. The results are compared to those obtained from Monte Carlo simulations and from the basic particle rate equation method.

A detailed analysis of hole transport in cascaded p-Si/SiGe quantum well structures is performed using the rate equation method with thermal balancing. The hole subband structure is calculated within the 6 × 6 k · p model, and used to find carrier and energy relaxation rates due to the alloy disorder, acoustic and optical phonon scattering. The simulation accounts for the in-plane k-space anisotropy of both the hole subband structure and the scattering rates. Results for prototype Si/SiGe cascade emitter structures are compared to those from both the Monte Carlo and the basic, particle rate equation method.

We present a Raman scattering study of two GaN/AlxGa1−xN multiple quantum well (MQW) structures with x = 0.3 and x = 0.4 and well widths of 6 nm and 4 nm respectively, they were nominally undoped but are expected to contain a low-density, n-type, carrier population due to the residual donors in the barriers. Polarization dependent Raman scattering was performed at room temperature, strong scattering due to intersubband transitions (ISBT's) in the GaN quantum wells was observed from both the e1–e2 and e1–e3 transitions for both samples. The first sample has an e1–e2 transition at 300 meV and e1–e3 at 480 meV whilst the second sample has an e1–e2 transition at 360 meV and e1–e3 at 580 meV. The full width half maximum of the Raman peaks is lower than that reported in infra-red absorption for similar, heavily doped, MQW structures. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A method is proposed for the design and optimization of structural parameters of GaN-AlGaN Bragg-confined structures with respect to peak intersubband absorption from the ground to the first excited state,1 /spl rarr/ 2 electronic transition, in the near infrared spectral range. An above-the-barrier bound state was used to extend the range of transition energies above the values available in conventional quantum wells. Intrinsic polarization fields and nonparabolicity effects were taken into account. The selection of optimal parameters, maximizing the absorption at wavelengths of 1.55 and 1.3 /spl mu/m, was performed by using a simulated annealing algorithm, and optimal structures with infinite superlattices as confinement regions were thus designed. These optimal parameters were then used to set realistic, finite structures with a small number of layers, the performance of which was re-evaluated by solving the Schrodinger-Poisson equation self-consistently for a few different levels and profiles of doping.

The object of this paper is to analyze the electronic and optical properties of semiconductor nanostructures made of semiconductors with more than one significant minimum in the conduction band by applying the single-band envelope-function model in the entire Brillouin zone. We have developed a method of calculating the eigenvalues and eigenfunctions of the corresponding Hamiltonian and explored the possibility of forming a single-zone model for the entire Brillouin zone by taking the exact dependence of the conduction band energy on the wave vector E(k). Since the crystalline potential is periodic, an expansion of the electron energy band can be made in terms of vectors of the direct lattice. This method is applied to GaAs and Si quantum wells and quantum wires with square and rectangular cross sections of [001] and [110] orientation, and its accuracy is examined by comparing it with other numerically more complex and demanding methods.

A procedure is proposed for finding the globally optimal semiconductor quantum well profile in respect to the second order intersubband susceptibilities at resonance. It relies on the combined use of simulated annealing and variational calculus (i.e. the optimal control theory), and effectively performs a free variation of the quantum well (QW) profile, searching for the global optimum. The proposed method does not depend on the choice of input ("seed") potential and includes the variation of a continuous function instead of a set of scalar parameters, so it should lead to the globally optimal QW profile for a particular application, unconstrained to any particular class of functional forms.

A procedure is proposed for finding the optimal profile of a semiconductor quantum well to obtain maximal value of the optical rectification coefficient. It relies on the variational calculus, i.e. the optimal control theory, combined with the method of simulated annealing, and should deliver a globally optimized profile, unconstrained to any particular class of functional forms. For the purpose of illustration, the procedure is applied to the optimized design of AlxGa1−xAs based quantum wells, for rectification of ω = 116 meV (CO2 laser) radiation. The optimal smooth profile may eventually be discretized to make the structure fabrication easier.

A method is described for the design and realization of quantum-well (QW) structures optimized in respect to gain in optically pumped intersubband lasers. It relies on finding the optimal smooth potential (QW profile) by employing supersymmetric quantum mechanics, and then using the simulated annealing method to design a suitable QW structure with a small number of layers of different composition, such that it delivers the closest approximate to the optimal smooth profile after the interdiffusion process. This route towards achieving optimal QW profiles should be much easier to apply than a direct modulation of material composition at time of growing the structure.

A method is described for the optimized design of quantum-well structures, with respect to maximizing the stimulated gain in optically pumped intersubband lasers. It relies on applying the supersymmetric quantum mechanics to an initial Hamiltonian. The use of this procedure is demonstrated by designing smoothly graded and stepwise-constant AlxGa1−xAs ternary alloy quantum wells, with the self-consistent potential taken into account.

The design of rectangular GaN/AlGaN quantum wells for maximal intersubband absorption in the near infrared wavelength range 1.3–2 μm, on the 0→1 electronic transition is considered, taking into account the effects of internal polarization fields and nonparabolicity. The nonlinear optimization method based on solving a system of nonlinear equations is employed in finding the structural parameters which give the maximal dipole matrix element while keeping the absorption peak at the desired wavelength.

The alloy disorder and phonon scattering intersubband relaxation rates of holes in SiGe quantum wells have been calculated using the 6 × 6 k.p method. The relative importance of different branches of non-polar optical and acoustic phonons is discussed, and compared with the alloy disorder contribution. Generally, for low-energy transitions at lower temperatures, the alloy scattering is found to be the dominant relaxation mechanism. The results are relevant for the design of SiGe based quantum cascade lasers operating via valence intersubband transitions.

We report the first detailed ensemble Monte Carlo simulation of hole dynamics in cascaded p-Si/SiGe quantum wells. The hole subband structure is calculated using the 6 × 6 k · p model. The simulation accounts for the in-plane k-space anisotropy of both the hole subband structure and the scattering rates. The scattering mechanisms included are the alloy disorder, acoustic and optical phonon scattering. Results are presented for prototype Si/SiGe cascade structures.

The influence of permanent dipole moments of quantized states on intersubband second harmonic generation based on quantum-interference induced transparency in semiconductor quantum wells is explored using the harmonic balance method. The permanent moments are found to be quite important: they affect the transparency condition, especially at larger pump intensities. Hence, both the conversion efficiency and the optimal interaction path length change significantly when accounting for the permanent moments, and the conversion efficiency is reduced.

A new method is proposed for semiconductor quantum well (QW) shape optimization in respect to the electro-optic coefficients. It relies on iterative solving of the system of coupled Schrodinger equations, which determine the wave functions of relevant states, and of Euler–Lagrange equations derived by applying the variational calculus to the target functional, with the starting potential supplied by the simulated annealing. Due to the combined use of the two methods, one may expect to get the globally optimal QW profile. Numerical example is presented for an AlxGa1−xAs based structure.

Possibilities are discussed for global optimization of quantum wells based on ternary alloys, with respect to third-order intersubband susceptibilities at resonance. The method employed in this work relies on variational calculus, i.e. optimal control theory, combined with the simulated annealing method in the initial phase of optimization. Its use is demonstrated by designing Al x Ga 1− x As quantum wells intended for resonant third harmonic generation with ℏω=116 meV pump photon energy, the objective being the largest susceptibility achievable with the chosen material system.

A method is described for the optimized design of quantum-well (QW) structures, in respect to maximizing the stimulated gain in optically pumped intersubband lasers. It relies on applying supersymmetric quantum mechanics (SUSYQM) to an initial Hamiltonian, in order to both map one bound state below the spectral range of the initial Hamiltonian, and to generate a parameter-controlled family of isospectral Hamiltonians with the desired energy spectrum. By varying the control parameter, one changes the potential shape and, thus, the values of dipole matrix elements and electron-phonon scattering matrix elements. The use of this procedure is demonstrated by designing smoothly graded and stepwise-constant Al/sub x/Ga/sub 1-x/As ternary alloy QWs, with the self-consistent potential taken into account. Finally, the possibility of employing layer interdiffusion to get optimal smooth potentials is discussed.

A method is described for the optimized design of quantum-well structures, with respect to maximizing the second-order susceptibilities relevant for second harmonic generation. The possibility is explored of obtaining resonantly enhanced nonlinear optical susceptibilities in quantum wells with two bound states and a continuum resonance state as the dominant third state. The method relies on applying the isospectral (energy structure preserving) transformations to an initial Hamiltonian in order to generate a parameter-controlled family of Hamiltonians. By changing the values of control parameters one changes the potential shape and thus the values of matrix elements relevant to susceptibility to be maximized. The method was used for the design of Alx Ga1 x As-based QWs. The results indicate the possibility of employing continuum states in resonant second harmonic generation at higher photon energies, !D 200‐300 meV. c 2000 Academic Press

A systematic procedure is described for the maximization of gain in optically pumped intersubband lasers, via optimal tailoring of the quantum well profile. The procedure relies on using the inverse spectral theory, allowing one to shift the bound states as desired, and additionally to make the isospectral reshaping of the well, with the eventual aim of finding the best combination of those optical dipole and electron-phonon scattering matrix elements which are relevant for the laser gain. Example design is presented for a laser based on the AlxGa1-xAs system, and the band nonparabolicity is accounted for in the final design. (C) 2000 American Institute of Physics. [S0021-8979(00)01511-5].

The bound–bound, bound–free and free–free intersubband optical transitions are considered in semiconductor quantum wells derived from the Poschl–Teller potential with a constant electron mass. To make this type of Hamiltonian effectively realizable in ternary (Al x Ga 1− x As) alloys, where the effective electron mass necessarily varies, just as does the potential, the coordinate transform method was employed to design variable-mass variable-potential structures realizable in Al x Ga 1− x As, with their Hamiltonians fully equivalent to the constant-mass Poschl–Teller case. Applications of this special QW profile for photodetectors operating in the infrared are analyzed and the results are compared to those obtained in the simple square wells.

The optimization of Bragg-confined double quantum wells with respect to resonant second harmonic generation and electro-optic coefficients is considered. The method of optimization relies on solving a system of nonlinear equations which involve the geometric parameters of the structure and bound state energies. Advantage is taken of the above-the-barrier bound states in Bragg-confined structures, in order to extend the range of allowable incident photon energies. Numerical results are given for Al0.48In0.52As/Ga0.47In0.53As based structure and the incident photon energy ω = 240 meV.

The possibility of employing Bragg-confined semiconductor structures for resonant second and third order intersubband nonlinearities is explored. The existence of above-the-barrier bound states is used to extend the range of pump photon energies above that available in conventional Al x Ga 1− x As quantum wells. The optimization procedure, developed previously, is applied to design optimally shaped Al x Ga 1− x As based structures for resonant second harmonic (at ℏ ω =240 meV pump) and third harmonic (at ℏ ω =116 meV pump) generation.

A method is described for the optimized design of quantum-well structures, in respect to maximizing the second-order susceptibilities relevant for electro-optic applications. It relies on applying the isospectral (energy structure preserving) transformations to an initial Hamiltonian, in order to generate parameter(s) controlled family of Hamiltonians that (i) are isospectral to the initial one and, (ii) have the potential variation proportional to the effective mass variation, being thus realizable by graded ternary alloys like ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}.$ By changing the values of control parameters one changes the potential shape and thus the values of matrix elements relevant for the susceptibility to be maximized. The use of the method is demonstrated by designing optimal quantum wells for the Stark effect or quantum interference derived electro-optical effects.

The possibility is explored of obtaining resonantly enhanced nonlinear optical susceptibilities in semiconductor quantum wells with two bound states and the resonant state in continuum as the third state. This would significantly extend the range of input radiation photon energies that may be frequency-doubled under resonance conditions in realistic structures. Calculation for the AlGaAs alloy based wells designed for the second harmonic generation of 240meV radiation indicate the usefulness of this approache.

Methods for systematic optimization of step-graded and continuously graded ternary alloy based quantum wells (QW's), in respect to second- or third-order intersubband nonlinear susceptibilities at resonance, are discussed. The use of these methods is examplified on the design of Al/sub x/Ga/sub 1-x/N and Al/sub x/Ga/sub 1-x/As-based QW's intended for resonant second harmonic or third harmonic generation with h/spl omega/=116 meV or h/spl omega/=240 meV pump photon energies, the objective being the largest susceptibility achievable with the chosen material. The obtained results exceed those previously reported in the literature.

A procedure for the design of quantum well structures optimized in respect to the intersubband resonant optical rectification generation is proposed. It relies on the inverse spectral theory, allowing one to generate a family of potentials with prescribed level energies, and additionally vary the potential shape in an isospectral manner, via a scalar parameter. (C) 1998 Elsevier Science B.V.

Using the inverse spectral theory we devise a systematic procedure to generate quantum well profiles optimized in respect to the optical rectification coefficient. Cases that, in course of design, start with an infinite barrier rectangular or with parabolic quantum well allow for a large part of the procedure to be done analytically, and are considered in more detail. Calculations for GaAs based wells indicate somewhat larger values of the rectification coefficient may be obtained than are reported in the current literature. The design procedure may also be extended to optimization of other optical nonlinearities.

A systematic procedure is proposed for the design of ternary alloys based quantum-well structures optimized for double-resonance second-harmonic generation. The method relies on the supersymmetric quantum mechanics as derived here for the case of position-dependent effective mass. Starting from a symmetric, truncated quasiparabolic potential, itself lacking any second-order nonlinearity, we generate a family of asymmetric potentials, fully isospectral with the starting potential, and choose the one which maximizes the product of transition matrix elements relevant for the second-order nonlinearity. Realization of the optimized potential (in an approximate manner) by grading the ternary alloy AlxGa1-xAs is then described. The best value of nonlinear susceptibility obtained exceeds those reported in the literature.

We report a systematic and quantitative study of near-infrared intersubband absorption in strained AlGaN/GaN and lattice-matched AlInN/GaN superlattices grown by plasma-assisted molecular-beam epitaxy as a function of Si-doping profile with and without \ensuremath{\delta} doping. For AlGaN/GaN, we obtained good theoretical agreement with experimental measurements of transition energy, integrated absorbance and linewidth by considering many-body effects, interface roughness, and calculations of the transition lifetime that include dephasing. For the AlInN/GaN system, experimental measurements of the integrated absorbance due to the superlattice transitions produced values more than one order of magnitude lower than AlGaN/GaN heterostructures at similar doping levels. Furthermore, observed transition energies were roughly 150 meV higher than expected. The weak absorption and high transition energies measured in these structures is attributed to columnar alloy inhomogeneity in the AlInN barriers observed in high-angle annular dark-field scanning transmission electron microscopy. We simulated the effect of these inhomogeneities using three-dimensional band-structure calculations. The inhomogeneities were modeled as AlInN nanorods with radially varying In composition embedded in the barrier material of the superlattice. We show that inclusion of the nanorods leads to the depletion of the quantum wells (QWs) due to localization of charge carriers in high-In-containing regions. The higher energy of the intersubband transitions was attributed to the relatively uniform regions of the QWs surrounded by high Al (95$%$) composition barriers. The calculated transition energy assuming Al${}_{0.95}$In${}_{0.05}$N barriers was in good agreement with experimental results.

We report on the effects of growth variations on the computed electroluminescence of Ge/SiGe chirped superlattice quantum cascade structures. The chirped structures investigated comprise 50 periods, each of which has a sequence of 5-7 Ge quantum wells and SiGe barriers. We used an extended density matrix carrier transport model to calculate the electron population of each subband in a QW structure via solution of the Liouville equation. Our modelling results have shown that a 5% increase of all layers of designed structures implies 10% decrease of the electroluminescence intensity and a 15% increase implies a 40% reduction in electroluminescence. Structures with 25% increase of layer thickness lose up to 60% of their electroluminescence intensity.