Second harmonic generation at the quantum-interference induced transparency in semiconductor quantum wells: The influence of permanent dipole moments

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

Electric field domains in p-Si/SiGe quantum cascade structures

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

n-Type Si/SiGe quantum cascade structures

Detail, looking up at figure of Mickiewicz; Adam Bernard Mickiewicz (1798-1855) was a Polish poet, publisher and political writer of the Romantic period. Mickiewicz was active in the struggle to achieve independence for Poland (from Russia) and so lived in exile. He settled first in Rome, later in Paris, where he became professor of Slavic literature at the Collège de France. Mickiewicz is depicted on top of the column as a pilgrim, with his left arm raised. Source: Wikipedia; http://en.wikipedia.org/wiki/Main_Page (accessed 5/7/2011

Intervalley mixing and intersubband transitions in n-type Si/SiGe quantum wells

The Si/SiGe materials system offers the prospect of excellent integration between CMOS technology and optoelectronics, employing well-established fabrication technology at low cost. Whilst the indirect bandgap means that interband lasing is challenging, stimulated emission from intersubband transitions offers a route to long wavelength Si based lasers. In bulk silicon, the conduction band minima are located in six degenerate valleys near the Brillouin zone edge in the directions. In a two-dimensional system however, uniaxial strain effects split the degeneracy of the valleys into two sets — two z valleys perpendicular to the heterostructure interfaces and four xy valleys in the growth plane. Atomistic simulation methods have shown that the two degenerate valley sets are sufficiently separated from each other to be considered independently within an effective mass approximation (EMA) model. Electrons emanating transversely from each of the xy valleys contribute identically to the z-varying component of the wavefunction, resulting in four degenerate states. In the case of the z valleys however, the electrons have different wavevector components in the z-direction. Quantum confinement yields interference between these basis functions and two distinct solutions to Schr¨odinger’s equation exist at separate energies, i.e. the degeneracy of z states is split. The effect has been observed experimentally in Schubnikov-de Haas oscillations in high magnetic fields.[1] It is therefore important to consider the mixing effect between the z valleys when determining states in a quantum confined system. Whilst atomistic simulation methods such as the tightbinding approximation implicitly take intervalley mixing into account,[2] the computation is considerably slower than the effective mass approximation — particularly in the case of large complicated structures such as a quantum cascade laser (QCL). A Double Valley Effective Mass Approximation (DVEMA) is therefore desirable as it offers the rapid computation of the EMA whilst including intervalley mixing effects explicitly. Such a model was derived for a square quantum well by Ting and Chang.[3] The energy splitting in the lowest states is shown to be a decaying oscillatory function of well width. The present work details the expansion of the DVEMA model to a general symmetric envelope potential. In SiGe molecular beam epitaxy (MBE), interdiffusion of Ge between heterolayers prevents abrupt interfaces from existing in the envelope potential. By considering a number of structures with more realistic interfaces than previous studies, the surface segregation effect is shown to reduce valley splitting slightly. Although the DVEMA applies only to symmetric structures, the present studies show that the model often remains reliable for slightly asymmetric structures. Using the DVEMA model, the effect of valley splitting upon realistic Si/SiGe intersubband optical devices has been investigated. The optical matrix elements for valley split intersubband transitions are shown to be almost identical, whilst their energies may differ by up to 10 meV. The emission spectrum is therefore expected to exhibit transition doublets when the valley splitting becomes large. It is shown that through careful design, the valley splitting may be minimized; although there is scope for exploiting intervalley scattering effects to achieve population inversion in an intersubband laser

Self-consistent scattering model of carrier dynamics in GaAs-AlGaAs terahertz quantum-cascade lasers

Intersubband electron scattering transport in terahertz GaAs–AlGaAs quantum cascade lasers is analyzed, using a full 13-level self-consistent rate equation model. The approach includes all relevant scattering mechanisms between injector–collector and active region states in the cascade structures. Employing an energy balance equation which includes the influence of both electron longitudinal optical phonon and electron–electron scattering, the method also enables evaluation of the average electron temperature of the nonequilibrium carrier distributions in the device. The electron temperature is found to give a strong influence on the output characteristics, particularly at very low temperatures. The threshold currents and electric field-current density characteristics are in very good agreement with experiment, implying that the model has a strong predictive capability

Electron transport and terahertz gain in quantum-dot cascades

Electron transport through quantum-dot (QD) cascades was investigated using the formalism of nonequilibrium Green's functions within the self-consistent Born approximation. Polar coupling to optical phonons, deformation potential coupling to acoustic phonons, as well as anharmonic decay of longitudinal optical phonons were included in the simulation. A QD cascade laser structure comprising two QDs per period was designed and its characteristics were simulated. Significant values of population inversion enabling lasing in the terahertz frequency range were predicted, with operating current densities being more than an order of magnitude smaller than in existing terahertz quantum-well-based quantum-cascade lasers

On the interdiffusion-based quantum cascade laser

Design procedure for the active region of current pumped quantum cascade laser is proposed, so to achieve maximal gain. Starting with an arbitrary smooth potential, a family of isospectral Hamiltonians with predefined energy spectrum is generated using the inverse spectral theory. By varying the relevant control parameter the potential shape is varied, inducing changes in transition dipole moments and electron–phonon scattering times, and the optimal potential which gives the largest gain is thus found. For purpose of realization, a simple step quantum-well structure with just a few layers is then designed so that in the post-growth heating-induced layer interdiffusion, it will acquire a shape as close as possible to the optimal smooth potentia

Thermal analysis of mid-infrared quantum-cascade lasers

We present a theoretical thermal analysis of mid-infrared quantum-cascade lasers (QCLs) using a two-dimensional anisotropic heat diffusion model. Several InP-based devices are simulated over a range of operating conditions in order to extract temperature-dependent thermal resistances. These thermal resistances are used to compare the effectiveness of various heat management techniques. Finally, heat flow analysis is performed in order to understand the internal thermal dynamics of these devices

Physical model of quantum-well infrared photodetectors

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

Simulated [111] Si-SiGe terahertz quantum cascade laser

The prospect of developing a silicon laser has long been an elusive goal, mainly due to the indirect band gap and large effective carrier masses. We present a design for a terahertz intersubband laser grown on the [111] crystal plane and simulate performance using a rate equation method including scattering due to alloy disorder, interface roughness, carrier-phonon and Coulombic interactions. We predict gain greater than 40 cm-1 and a threshold current density of 70 A/cm2