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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.

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.

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)

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.

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.

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.

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.

Designs of GaN/AlGaN quantum-cascade lasers emitting at 34 and 38 μm (ΔE∼36 and 34 meV) are presented, assuming either a- or c-plane crystal growth orientation. In the calculation of the quasibound state energies and wave functions, we account for the intrinsic electric field induced by piezoelectric and (in case of c-plane growth) the spontaneous polarization. The quantum-cascade structures were simulated, and their output characteristics extracted, using a fully self-consistent rate equation model with all relevant intra- and interperiod scatterings included. Both electron–LO-phonon and electron–electron scattering mechanisms are taken into account. Maximal population inversions between active laser states of up to 19% for the a-plane, and up to 40% for the c-plane design, are predicted and, based on estimated modal gain and waveguide/mirror losses in suitably designed structures, these indicate the feasibility of laser action in GaN/AlGaN cascades.

This paper compares and contrasts different theoretical approaches based on incoherent electron scattering transport with experimental measurements of optoelectronic devices formed from semiconductor heterostructures. The Monte Carlo method which makes no a priori assumptions about the carrier distribution in momentum or phase space is compared with less computationally demanding energy-balance rate equation models which assume thermalised carrier distributions. It is shown that the two approaches produce qualitatively similar results for hole transport in p-type Si1-xGex/Si superlattices designed for terahertz emission. The good agreement of the predictions of rate equation calculations with experimental measurements of mid- and far-infrared quantum cascade lasers, quantum well infrared photodetectors and quantum dot infrared photodetectors substantiate the assumption of incoherent scattering dominating the transport in these quantum well based devices. However, the paper goes on to consider the possibility of coherent transport through the density matrix method and suggests an experiment that could allow coherent and incoherent transport to be distinguished from each other.

The aim of this work is to report a detailed theoretical investigation, supported by the experimental measurements, of the influence of the injector doping level on the output characteristics and the working range limits in mid-infrared GaAs/AlGaAs quantum cascade lasers. A fully self-consistent Schrodinger - Poisson analysis based on the scattering rate equation approach was employed to simulate the above-threshold electron transport in the device. An onset of v-shaped local band edge bowing has been observed, preventing the resonant subband level alignment in the high pump current regime. The observed saturation of the maximal current, together with an increase of threshold current, limits the dynamic working range for higher doping levels.

A quantum cascade laser (QCL) thermal model is presented. The model is used to calculate the active region temperature of an InGaAs/AlAsSb QCL under a range of operating conditions and device geometries. The active region temperature is a limiting factor of QCL performance and knowledge of the thermal dynamics inside these devices is crucial in order to improve their temperature performance. Buried heterostructures (BH) are found to offer the best thermal performance, allowing higher heat sink temperatures than alternative device geometries under both pulsed and continuous‐wave (CW) operating conditions.

This paper compares and contrasts different theoretical approaches with experimental measurements of transport in optoelectronic devices based on semiconductor heterostructures. The Monte Carlo method which makes no a priori assumptions about the carrier distribution in momentum or phase space is compared with less computationally demanding energy-balance rate equation models which assume thermalised carrier distributions. It is shown that the two approaches produce qualitatively similar results for the specific case of hole transport in p-type Si1-xGex/Si superlattices designed for terahertz emission though there are significant differences which originate in the absolute values of the carrier kinetic energy in the plane of the quantum wells. In contrast to this, simulations of electron transport in n-type GaAs/Ga1-xAlxAs terahertz quantum cascade lasers show a more similar behaviour between both theoretical approaches. In addition, the very good comparison with experiment substantiates the incoherent scattering approach which under-pins both methods. Further evidence to support the applicability of carrier transport in semiconductor heterostructures at finite temperature being dominated by incoherent scattering, which can be modelled by self-consistent energy-balance rate equations, is given by further comparisons with experimental measurements of two other categories of devices namely quantum well infrared photodetectors and quantum dot infrared photodetectors

The development of a thermal model for quantum cascade lasers (QCLs) is presented. The model is used in conjunction with a self-consistent scattering rate calculation of the electron dynamics of an InGaAs-AlAsSb QCL to calculate the temperature distribution throughout the device which can be a limiting factor for high temperature operation. The model is used to investigate the effects of various driving conditions and device geometries, such as epilayer down bonding and buried heterostructures, on the active region temperature. It is found that buried heterostructures have a factor of eight decrease in thermal time constants compared to standard ridge waveguide structures in pulsed mode and allow a /spl sim/78% increase in heat sink temperature compared to epilayer down mounted devices in continuous-wave mode. The model presented provides a valuable tool for understanding the thermal dynamics inside a quantum cascade laser and will help to improve their operating temperatures.

A theoretical model of electron transport in quantum cascade lasers subjected to a magnetic field is developed. The Landau level electronic structure was calculated from the envelope-function Schr\"odinger equation within the effective-mass approximation. The electron transport in a magnetic field was modeled using the self-consistent rate-equation description for the full period of the cascade and its interaction with adjacent periods. The scattering processes included in the model are electron\char21{}longitudinal-optical-phonon, electron\char21{}longitudinal-acoustic-phonon, and electron-electron scattering. All these processes show oscillatory behavior with magnetic field, and their interplay determines the electron transport and the output characteristics of quantum cascade lasers in magnetic field. The model was applied to investigate the influence of magnetic field on the performance of a $\mathrm{GaAs}∕\mathrm{AlGaAs}$ quantum cascade laser emitting at $\ensuremath{\lambda}\ensuremath{\approx}11.4\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{m}$ [P. Kruck et al., Appl. Phys. Lett. 76, 3340 (2000)]. The calculated results show good overall agreement with the available experimental data.

A fully quantum-mechanical model for electron transport in quantum devices, based on a self-consistent solution of the coupled rate equations, was applied to design of novel intersubband optically pumped or quantum cascade lasers and quantum-well infrared photodetectors. The important macroscopic parameters like I-V curves, laser gain, threshold current density, responsivity and capture probability can be estimated directly from this first-principles calculation. The presentation will review the method and describe its application to infrared and terahertz devices based on various semiconductor heterostructure systems

A method for the calculation of the electronic structure of pyramidal self-assembled InAs/GaAs quantum dots is presented. The method is based on exploiting the C-4 symmetry of the 8-band k·p Hamiltonian with the strain taken into account via the continuum mechanical model. The operators representing symmetry group elements were represented in the plane wave basis and the group projectors were used to find the symmetry adapted basis in which the corresponding Hamiltonian matrix is block diagonal with four blocks of approximately equal size. The quantum number of total quasiangular momentum is introduced and the states are classified according to its value. Selection rules for interaction with electromagnetic field in the dipole approximation are derived. The method was applied to calculate electron and hole quasibound states in a periodic array of vertically stacked pyramidal self-assembled InAs/GaAs quantum dots for different values of the distance between the dots and external axial magnetic field. As the distance between the dots in an array is varied, an interesting effect of simultaneous change of ground hole state symmetry, type, and the sign of miniband effective mass is predicted. This effect is explained in terms of the change of biaxial strain. It is also found that the magnetic field splitting of Kramer's double degenerate states is most prominent for the first and second excited state in the conduction band and that the magnetic field can both separate otherwise overlapping minibands and concatenate otherwise nonoverlapping minibands.

In this paper we show that the idea of a mid‐infrared quantum‐cascade laser with gain regions based on a double‐phonon resonance can be implemented in the GaAs/AlGaAs system. In contrast to the usual GaAs/AlGaAs laser active region design, which involves a triple quantum well active region, here we identify an optimal heterostructure design by using a simulated annealing algorithm which is programmed to maximize the laser gain for a given wavelength and for subband spacings prescribed to satisfy the double‐phonon resonance condition. The output characteristics are calculated using a full self‐consistent rate equation model of the intersubband electron transport. Initial devices grown according to this design show laser action up to 240K in pulsed mode with good agreement between the calculated and measured characteristics.

We report on a detailed experimental and theoretical analysis of the role of injector doping density on both the threshold current density (Jth) and the saturation current density (Jsat), determining the dynamic range of the quantum cascade lasers. The experimental results were obtained from two growth series of λ≈9μm GaAs∕Al0.45Ga0.55As quantum-cascade lasers based on single and double phonon resonance depopulation mechanisms. We derive a quasilinear dependence of Jth on the injector doping density of both designs for doping range as wide as (4–10)×1011cm−2. Despite threshold current increase the faster rise of saturation current with doping results in an enhanced dynamic range for injector doping up to ∼8×1011cm−2. For higher doping levels, Jsat itself saturates. Furthermore, our investigations yield that single phonon resonance devices exhibit clear current saturation simultaneously with a decrease of the optical power, whereas two phonon resonance devices show only power saturation, which we attribute...

A detailed theoretical and experimental study of the influence of injector doping on the output characteristics and electron heating in midinfrared GaAs∕AlGaAs quantum cascade lasers is presented. The employed theoretical model of electron transport was based on a fully nonequilibrium self-consistent Schrodinger-Poisson analysis of the scattering rate and energy balance equations. Three different devices with injector sheet doping densities in the range of (4–6.5)×1011cm–2 have been grown and experimentally characterized. Optimized arsenic fluxes were used for the growth, resulting in high-quality layers with smooth surfaces and low defect densities. A quasilinear increase of the threshold current with sheet injector doping has been observed both theoretically and experimentally. The experimental and calculated current-voltage characteristics are in a very good agreement. A decrease of the calculated coupling constant of average electron temperature versus the pumping current with doping level was found.

The design and simulation of an In0.53Ga0.47As∕Al0.56As0.44Sb quantum-cascade laser emitting in the near infrared is presented. Designed using a self-consistent rate equation solver coupled with an energy balance rate equation, the proposed laser has a calculated population inversion of ∼20% at 77K and sufficient gain to achieve room-temperature laser emission at λ∼2.8μm. Threshold currents in the range 4–8kA∕cm2 are estimated as the temperature increases from 77Kto300K. The output characteristics of the proposed laser are compared to an existing λ∼3.1μm In0.53Ga0.47As∕Al0.56As0.44Sb quantum-cascade structure presented in the literature.

The influence of doping density on the performance of GaAs∕AlGaAs quantum-cascade lasers is presented. A fully self-consistent Schrodinger–Poisson analysis, based on a scattering rate equation approach, was employed to simulate the above threshold electron transport in laser devices. V-shaped local field domain formation was observed, preventing resonant subband level alignment in the high pumping-current regime. The resulting saturation of the maximal current, together with an increase of the threshold current, limits the dynamic working range under higher doping. Experimental measurements are in good agreement with the theoretical predictions.

A design for a GaN∕AlGaN optically pumped terahertz laser emitting at 34 μm (ΔE∼36meV) is presented. This laser uses a simple three-level scheme where the depopulation of the lower laser level is achieved via resonant longitudinal-optical-phonon emission. The quasibound energies and associated wave functions are calculated with the intrinsic electric field induced by the piezoelectric and the spontaneous polarizations. The structures based on a double quantum well were simulated and the output characteristics extracted using a fully self-consistent rate equation model with all relevant scattering processes included. Both electron-longitudinal-optical phonon and electron-acoustic-phonon interactions were taken into account. The carrier distribution in subbands was assumed to be Fermi–Dirac-like, with electron temperature equal to the lattice temperature, but with different Fermi levels for each subband. A population inversion of 12% for a pumping flux Φ=1027cm−2s−1 at room temperature was calculated for th...

We report the growth and characterization of AlGaN/GaN multiple quantum well structures designed to have intersubband transitions in the mid-infrared region of the spectrum. The samples were nominally undoped but were found to contain a high electron population in the wells induced by the local polarization fields. The sample was characterized by the use of the Raman spectroscopy and photocurrent spectroscopy. The Raman spectroscopy shows electronic Raman scattering from intersubband transitions in the AlGaN/GaN quantum wells. The e1-e2 and e1-e3 transitions of the confined 2d electron population in the wells can clearly be observed. A sample designed to absorb at 4μm was fabricated into mesa structures and the vertical photocurrent measured under normal incidence illumination from the free-electron laser FELIX. A wavelength and bias dependent photocurrent was observed in the mid-IR region of spectrum. The peak responsivity was of the order of 50μA/W at 4 K, the photocurrent still being measurable at room temperature.

A fully quantum-mechanical model for carrier scattering transport in semiconductor intersubband devices was applied to modelling of carrier dynamics in quantum cascade lasers. The standard model uses the envelope function and effective mass approximations to solve electron band structure under an applied bias. The k(.)p model has been employed in p-type systems where the more complex band structure requires it. The resulting wave functions are then used to evaluate all relevant carrier-phonon, carrier-carrier and alloy scattering rates from each quantised state to all others within the same and the neighbouring period. This piece of information is then used to construct a rate equation for the equilibrium carrier density in each sub-band and this set of coupled rate equations are solved self-consistently to obtain the carrier density in each eigenstate. The latter is a fundamental description of the device and can be used to calculate the current density and gain as a function of the applied bias and temperature, which in turn yields the threshold current and expected temperature dependence of the device characteristics