Search

Recent research shows that terahertz quantum cascade lasers are well-suited to high speed free space communication. The results of both theoretical and laboratory work indicate the devices are able to deliver bandwidths in the gigahertz to tens of gigahertz range without the burden of relaxation oscillations found in diode lasers. Using a novel rate equation model we explore the frequency response characteristics of a real device and report on the finding of a strongly peaked bias current-dependent response.

We developed an accurate model accounting for electron-phonon interaction in colloidal\ud quantum dot supercrystals that allowed us to identify the nature of charge carriers and the\ud electrical transport regime. We find that in experimentally analyzed CdSe nanocrystal solids the electron-phonon interaction is sufficiently strong that small polarons localized to single dots are formed. Charge-carrier transport occurs by small polaron hopping between the dots, with mobility that decreases with increasing temperature. While such a temperature dependence of mobility is usually considered as a proof of band transport, we show that the same type of dependence occurs in the system where transport is dominated by small polaron hopping.

Lasing under optical pumping is shown in suspended GeSn microdisks fabricated on a Ge virtual substrate with a lasing threshold below 1 mW at 20K.

The recent observation of a fundamental direct bandgap for GeSn group IV alloys and the demonstration of low temperature lasing provide new perspectives on the fabrication of Si photonic circuits. This work addresses the progress in GeSn alloy epitaxy aiming at room temperature GeSn lasing. Chemical vapor deposition of direct bandgap GeSn alloys with a high Γ- to L-valley energy separation and large thicknesses for efficient optical mode confinement is presented and discussed. Up to 1 μm thick GeSn layers with Sn contents up to 14 at. % were grown on thick relaxed Ge buffers, using Ge2H6 and SnCl4 precursors. Strong strain relaxation (up to 81%) at 12.5 at. % Sn concentration, translating into an increased separation between Γ- and L-valleys of about 60 meV, have been obtained without crystalline structure degradation, as revealed by Rutherford backscattering spectroscopy/ion channeling and transmission electron microscopy. Room temperature reflectance and photoluminescence measurements were performed to probe the optical properties of these alloys. The emission/absorption limit of GeSn alloys can be extended up to 3.5 μm (0.35 eV), making those alloys ideal candidates for optoelectronics in the mid-infrared region. Theoretical net gain calculations indicate that large room temperature laser gains should be reachable even without additional doping. (Figure Presented).

Terahertz-frequency quantum cascade lasers (THz QCLs) based on bound-to-continuum active regions are difficult to model owing to their large number of quantum states. We present a computationally efficient reduced rate equation (RE) model that reproduces the experimentally observed variation of THz power with respect to drive current and heat-sink temperature. We also present dynamic (time-domain) simulations under a range of drive currents and predict an increase in modulation bandwidth as the current approaches the peak of the light-current curve, as observed experimentally in mid-infrared QCLs. We account for temperature and bias dependence of the carrier lifetimes, gain, and injection efficiency, calculated from a full rate equation model. The temperature dependence of the simulated threshold current, emitted power, and cut-off current are thus all reproduced accurately with only one fitting parameter, the interface roughness, in the full REs. We propose that the model could therefore be used for rapid dynamical simulation of QCL designs. (C) 2015 AIP Publishing LLC

Large-scale optoelectronics integration is limited by the inability of Si to emit light efficiently1, because Si and the chemically well-matched Ge are indirect-bandgap semiconductors. To overcome this drawback, several routes have been pursued, such as the all-optical Si Raman laser2 and the heterogeneous integration of direct-bandgap III–V lasers on Si3,4,5,6,7. Here, we report lasing in a direct-bandgap group IV system created by alloying Ge with Sn8 without mechanically introducing strain9,10. Strong enhancement of photoluminescence emerging from the direct transition with decreasing temperature is the signature of a fundamental direct-bandgap semiconductor. For T ≤ 90 K, the observation of a threshold in emitted intensity with increasing incident optical power, together with strong linewidth narrowing and a consistent longitudinal cavity mode pattern, highlight unambiguous laser action11. Direct-bandgap group IV materials may thus represent a pathway towards the monolithic integration of Si-photonic circuitry and complementary metal–oxide–semiconductor (CMOS) technology. ISSN:1749-4885 ISSN:1749-4893

A quantum-cascade laser (QCL) thermal model is presented. On the basis of a finite-difference approach, the model is used in conjunction with a self-consistent carrier transport model to calculate the temperature distribution in a near-infrared InGaAs/AlAsSb QCL. The presented model is used to investigate the effects of driving conditions and device geometries on the active-region temperature, which has a major influence on the device performance. A buried heterostructure combined with epilayer-down mounting is found to offer the best performance compared with alternative structures and has thermal time constants up to eight times smaller. The presented model provides a valuable tool for understanding the thermal dynamics inside a QCL and will help to improve operating temperatures.

In this work we report on modeling the electron transport in n-Si/SiGe structures. The electronic structure is calculated within the effective-mass complex-energy framework, separately for perpendicular (Xz) and in-plane (Xxy) valleys, the degeneracy of which is lifted by strain, and additionally by size quantization. The transport is described via scattering between quantized states, using the rate equations approach and tight-binding expansion, taking the coupling with two nearest-neighbour periods. The acoustic phonon, optical phonon, alloy and interface roughness scattering are taken in the model. The calculated U/I dependence and gain profiles are presented for a couple of QC structures.

We investigate theoretically the possibility of lasing in the terahertz in a suitably designed quantum cascade structure comprising dilute magnetic semiconductors. quantum cascade laser. Tunability is based on the spin-dependent potential induced by a magnetic field. The Boltzmann equation approach was used in order to predict the dependence of the gain and current on magnetic and electric fields. The electron dynamics were modeled by taking electron-longitudinal optical phonon, electron-longitudinal acoustic phonon and electron-electron scattering into account. Tunability of the emission energy between 10 meV and 26 meV, and 38 meV and 42 meV, for transitions of spin-down and spin-up electrons respectively, may be achieved by varying a magnetic field up to 5 T, at a temperature of 1.5 K. Comparison of the calculated optical gain, and losses in the appropriate waveguide, indicates the prospect of achieving laser operation. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A symmetry based method for the efficient calculation of single-particle states in hexagonally symmetric GaN/AlN quantum dots within the framework of a k · p model is presented. The envelope functions are expanded into a plane wave basis on a hexagonal lattice and the group projector method is used to adapt the basis to exploit the symmetry, resulting in block diagonalization of the corresponding Hamiltonian matrix into six matrices and classification of the states by the quantum number of total quasi-angular momentum. The method is applied to the calculation of the electron and hole energy levels in a quantum dot superlattice and the intraband optical absorption matrix elements. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A theoretical model describing the electron transport in InAs/GaAs quantum dot infrared photodetectors, modelled as ideal quantum dot superlattices, is presented. The carrier wave functions and energy levels were evaluated using the strain dependent 8-band k · p Hamiltonian and used to calculate all intra- and inter-period transition rates due to interaction with phonons and electromagnetic radiation. The interaction with longitudinal acoustic phonons and electromagnetic radiation was treated perturbatively within the framework of Fermi's golden rule, while the interaction with longitudinal optical phonons was considered taking into account their strong coupling to electrons. The populations of energy levels were then found from a system of rate equations, and the electron current in the superlattice was subsequently extracted. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Theoretical modelling of the intraband absorption spectrum in InAs/GaAs quantum dot infrared photodetectors is performed for several typical structures reported in the literature. The calculations are performed within the framework of the two methods: a simple and so far widely used effective mass method with the values of conduction band offset and the effective mass modified to take account of the effects of strain and band mixing on average and the more realistic eight-band k × p method with the strain distribution taken into account via the continuum mechanical model. Both methods give qualitatively the same results; however, the peak positions obtained within the effective mass approach are blue shifted and the absorption cross sections are overestimated, compared to the more accurate k × p approach.

We have analyzed the spin-filtering effects of the electron current in asymmetric ZnSe/Zn1-xMnxSe multilayer structures, under the influence of both an external magnetic field and a bias voltage. In this type of semiconductor systems, conduction band electrons interact with 3d electrons of the magnetic Mn2+ ions. Because of this sp-d exchange interaction, an external magnetic field modulates the effective potential profile seen by spin-up and spin-down electrons, giving rise to a large Zeeman effect. It is found that the degree of spin polarization changes significantly when the electrical bias is switched from forward to reverse, thus the proposed structure displays obvious behavior of spin-filter diode. This originates from the effective “lifting” of the potential for spin-up electrons, which tunnel through actual potential barriers. Structural parameters optimization, with the goal of maximizing the spin-filtering coefficient, was performed by using simulated annealing algorithm. The described effect may be important for designing new tunable spin-based multifunctional semiconductor devices.

We present a symmetry-based method for the efficient calculation of energy levels in hexagonal GaN/AlN quantum dots within the framework of a kp model. The envelope functions are expanded into a plane wave basis on a hexagonal lattice and the group projector method is used to adapt the basis to exploit the symmetry, resulting in block diagonalization of the corresponding Hamiltonian matrix into six matrices and classification of the states by the quantum number of total quasi-angular momentum. The method is applied to the calculation of the electron and hole single-particle states in a quantum dot superlattice. The selection rules for absorption of electromagnetic waves in the dipole approximation are established and the intraband optical absorption matrix elements are found. Good agreement with the available experimental data on intraband optical absorption is found.

A first principles theory of electronic transport in quantum well infrared photodetectors (QWIPs) is presented. The model is based on the cascade scattering rate equations approach and uses the translation symmetry to reduce the computational cost of the problem. The model delivers a non-equilibrium electron distribution in both dark conditions and under illumination. The model was tested by simulating a non-conventional broadband InGaAs/AlGaAs QWIP and good agreement with experimental data available in the literature was achieved. The model can be applied to any QWIP structure and can be used for extending the range of materials employed.

We report on a comparative study of GaAs/Al0.45Ga0.55As mid-infrared quantum cascade lasers. Devices differ in the mechanisms for depopulating the lower laser level. The depopulation mechanisms rely on longitudinal optical (LO) phonon emission into a single subband, into two subbands or into a set of subbands forming a miniband. The influence of injector doping density and electron confinement on device characteristics is explored experimentally and theoretically. The mechanisms limiting high temperature operation and the dynamic range are investigated. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The formation of domains in quantum cascade structures is one of the mechanisms strongly affecting the operation of quantum cascade lasers, quantum-well infrared detectors, and other devices. In this paper, we consider the problem of domain formation in p-doped Si/SiGe quantum cascades, using a carrier scattering transport framework. In effect, the hole flow along the cascade is described via scattering between quantized states belonging to neighboring periods, caused by phonons, alloy disorder, and carrier-carrier interactions. The generation of either periodic or of nonperiodic domains is studied in uniformly doped cascades, as well as the influence of modulation doping of cascades on the domain formation.

We describe the application of our computational model, developed for finding the optical gain in a mid-infrared quantum cascade laser subjected to a strong magnetic field, to two distinct λ ~ 9 µm GaAs-based structures. The additional carrier confinement induced by the field alters the transition rates for the optical- and acoustic-phonon scattering processes from the upper laser level, thus affecting the laser output properties, in particular the optical gain. Within this model, the gain is found by solving the system of rate equations, from which the carrier densities in each level are calculated. Numerical results are presented for magnetic fields between 10 and 60 T, and the band nonparabolicity is taken into account.

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.

A microscopic model of quantum well infrared photodetectors (QWIPs) is presented, based on quantum scattering rate equation approach. The macroscopic parameters and figures of merit such as current density, responsivity, capture probability and drift velocity, can be estimated directly from a first principles calculation. For conventional GaAs/AlGaAs QWIP device, a very good agreement was found between the calculated and the measured data taken from the literature. The model is general and can be applied to any other material system or QWIP structure.

The optical gain in the active region of quantum cascade laser in an external magnetic field is analyzed. When the magnetic field is applied in the direction perpendicular to the plane of the layers, electron dispersion is broken into series of discrete Landau levels. This additional confinement strongly modifies the lifetime of electrons in the upper state of the laser transition, which is controlled by electron-phonon scattering. Landau levels are magnetically tuneable and, depending on their configuration, phonon emission is either inhibited or resonantly enhanced. This translates into a strong modulation of the population inversion, and consequently of the optical gain by varying the magnetic field. Numerical results are presented for a structure previously considered by Smirnov et al. [Phys. Rev B 66 (2002) 125317] which is designed to emit radiation at λ~11.4µm, with the magnetic field varied in the range 10-60T. The effects of band nonparabolicity are taken into account in this model.

Properly designed waveguides are of great importance for efficient operation of mid-infrared and tera-hertz quantum cascade semiconductor laser. Such waveguides comprise layers with suitably chosen doping, which determines refractive index, which enables bound modes to exist. It is of great interest to tailor the structure so to get minimal losses and maximal modal overlap with the active (gain) layer. In this paper we use the analogy between the Schrodinger and Helmholtz equations, and employ supersymmetric quantum mechanics, extended into the complex domain, in order to vary refractive index profile in search for better waveguide design. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Analysis of carrier transport in cascaded GaAs/AlGaAs, p-Si/SiGe and GaN/AlGaN terahertz structures is performed using self-consistent rate equations with kinetic energy-balance simulations. The electron transport in n-type structures is simulated using standard envelope function and nonparabolic effective mass approximations with all relevant electron–electron and electron–optical phonon scattering. In the case of GaAs-based structures the model is extended to include ionized impurity and interface roughness scattering as well. The hole subband structure in the p-type device is calculated using the 6 × 6 k 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 the original GaAs/AlGaAs THz quantum cascade laser and prototypes of GaN/AlGaN and Si/SiGe THz quantum cascade structures.

The formation of domains in p Si/SiGe quantum cascade structures is considered, because this is one of mechanisms affecting the operation of quantum cascade lasers and other devics. Hole transport is described via scattering between quantized states belonging to neighbouring periods, caused by phonons, alloy disorder, and carrier-carrier interactions. The conditions for domain formation are found for examples of homogeneously and modulation doped cascades.

The use of buried tungsten silicide layers for confinement of terahertz optical modes is described. Silicon-on-silicide substrates are prepared using a bond and etch-back technique, and the successful growth of extremely long (600 period) strain-balanced p-Si/SiGe quantum cascade heterostructures on these substrates is demonstrated. THz electroluminescence is observed from these structures at low temperature, when the structure is biased so as to obtain interwell (‘diagonal’) transitions between heavy and light hole subbands. The emission shows a strong polarisation dependence, indicating the efficacy of the silicide layer in confining long wavelength TM modes.

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 fully quantum mechanical model for electron transport in quantum well infrared photodetectors is\ud presented, based on a self-consistent solution of the coupled rate equations. The important macroscopic\ud parameters like current density, responsivity and capture probability can be estimated directly from this\ud first principles calculation. The applicability of the model was tested by comparison with experimental\ud measurements from a GaAs/AlGaAs device, and good agreement was found. The model is general and can\ud be applied to any other material system or QWIP design.

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)

Design considerations are presented for waveguides to be used in p-Si/SiGe based quantum cascade lasers operating in the mid- and far-infrared wavelength ranges. Modal losses and confinement factors are calculated for both TM and TE modes in conventional double metal clad structures, metal–highly doped semiconductor layer structures and also in novel metal–metal silicide structures. Guidelines for choosing the confinement and contact layer parameters are given.

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.

Improved designs of surface plasmon waveguides for use in GaAs/AlGaAs terahertz quantum cascade lasers are presented. Modal losses and confinement factors are calculated for TM modes in metal-variably doped multilayer semiconductor and metal-intermetallic compound layer clad structures and compared with those obtained in recently realized metal-highly doped semiconductor clad layer structures. Considerable improvements of the mode confinement factors are predicted, and guidelines for choosing the confinement layer parameters are given.

The band structure of Ge 1 − x − y Si x Sn y ternary alloys, which are easier to grow than binary Ge 1-x Sn x alloys, and clearly offer a wider tunability of their direct band-gap and other properties, was calculated and investigated by using the empirical pseudo-potential plane wave method with modified Falicov pseudo-potential formfunction. The virtual crystal approximation (VCA) and 2 × 2 × 2 super-cell (mixed atoms) method were adopted to model the alloy. In order to calculate all of these properties, the empirical pseudo-potential code was developed. The lattice constant of the alloy varies between 0.543 to 0.649 nm. The regions in the parameter space that corresponds to a direct or indirect band gap semiconductor are identified. The Ge 1 − x − y Si x Sn y ternary alloy shows the direct band gap for appropriate composition of Si, Ge and Sn. The direct energy gap is in the range 0–1.4 eV (from the VCA calculation), and 0–0.8 eV (from the super-cell calculation), depending on the alloy composition. Therefore, this alloy is a promising material for optoelectronic applications in both visible and infrared range, such as interband lasers or, solar cells. Furthermore, strain-free heterostructures based on such alloys are designed and, using the effective-mass Hamiltonian model, the electronic structure of GeSiSn quantum wells with arbitrary composition is investigated, in order to understand their properties and the potential of their use in devices.

We present the epitaxial growth of Ge and Ge_(0.94)Sn_(0.06) layers with 1.4% and 0.4% tensile strain, respectively, by reduced pressure chemical vapor deposition on relaxed GeSn buffers and the formation of high-k/metal gate stacks thereon. Annealing experiments reveal that process temperatures are limited to 350°C to avoid Sn diffusion. Particular emphasis is placed on the electrical characterization of various high-k dielectrics, as 5nm Al_(2)O_(3), 5nm HfO_(2) or 1nm Al_(2)O_(3)/4nm HfO_(2), on strained Ge and strained Ge_(0.94)Sn_(0.06). Experimental capacitance-voltage characteristics are presented and the effect of the small bandgap, like strong response of minority carriers at applied field, are discussed via simulations.

Efforts towards development of monolithically integrated silicon-compatible lasers have been revitalized since the demonstration of optically pumped GeSn waveguide lasers. Here we investigate the laser emission of GeSn alloys by means of waveguide and microdisk photoluminescence.

Progress of Si based devices and technology is in strong interdependence with the development of new materials. Presently the list of group IV semiconductors is being extended beyond Ge to the semimetal Sn. Investigation in group IV semiconductors has been extended above strain engineering and Ge to the next element in group IV, the semimetal Sn. Significant efforts have been directed toward the epitaxial growth of GeSn binary and SiGeSn ternary alloys. In this work we discuss the growth and material properties of GeSn alloys and their application possibilities in photonic and nanoelectronic devices.

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.

It is shown that bound states in continuum can be generated by applying supersymmetric quantum mechanics to real potentials in such a way that the transformed potential becomes complex. Three cases of the initial potential are considered in more detail: flat potential, step potential and rectangular quantum well. The conditions are found for the existence of bound state in continuum in these cases, as well as the expressions for the wavefunction of this state and the corresponding complex potential in analytic form.

We explore the single-band envelope-function model in the entire Brillouin zone for electronic structure calculation of nanostructures that involve semiconductors with one or more significant valleys in the conduction band. Comparison with the numerically more demanding empirical pseudopotential calculation shows reasonably good accuracy of the method described here, which correctly predicts the quantized state energies, splittings in multivalley semiconductor nanostructures, as well as the intersubband transition derived optical properties. This makes it potentially useful for modeling nanostructures and nanodevices based on Si and other materials.

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.