Iso-tolerance - A hypothesis

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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

The importance of electron temperature in silicon-based terahertz quantum cascade lasers

Quantum cascade lasers (QCLs) are compact sources of coherent terahertz radiation. Although all existing QCLs use III-V compound semiconductors, silicon-based devices are highly desirable due to the high thermal conductivity and mature processing technology. We use a semiclassical rate-equation model to show that Ge/SiGe THz QCL active region gain is strongly enhanced by reducing the electron temperature. We present a bound-to-continuum QCL design employing L-valley intersubband transitions, using high Ge fraction barriers to reduce interface roughness scattering, and a low electric field to reduce the electron temperature. We predict a gain of similar to 50 cm(-1), which exceeds the calculated waveguide losses. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3237177

Substrate orientation and alloy composition effects in n-type SiGe quantum cascade structures

We show using a theoretical self-consistent effective mass/rate equation approach that n-type SiGe-based quantum cascade lasers are potentially made viable by either using the (111) orientation or a Ge-rich substrate

Design of Ge/SiGe quantum-confined Stark effect electroabsorption heterostructures for CMOS compatible photonics

We describe a combined 6×6 k.p and one-band effective mass modelling tool to calculate absorption spectra in Ge–SiGe multiple quantum well (MQW) heterostructures. We find good agreement with experimentally measured absorption spectra of Ge–SiGe MQW structures described previously in the literature, proving its predictive capability, and the simulation tool is used for the analysis and design of electroabsorption modulators. We employ strain-engineering in Ge–SiGe MQW systems to design structures for modulation at 1310 nm and 1550 nm

Density matrix modelling of Ge/GeSi bound-to-continuum terahertz quantum cascade lasers

In addition to the mainstream III-V quantum cascade lasers (QCLs), Si-based QCLs have attracted considerable research interest in recent years, due to their significant potential advantages including a mature Si processing technology, prospect of integration with Si microelectronics, superior thermal performance to that of III–V devices and absence of optical absorption in the Reststrahlen band [1–3]. Amongst various proposed designs, (001) oriented n-type Ge/GeSi structures utilising L-valley intersubband transitions appear to be the most promising due to a small quantisation effective mass, and hence large optical matrix elements and practically feasible layer widths [4]. All previous simulations for group IV-based QCLs followed the rate equation approach, which is considered not to be accurate enough for predicting the performance of terahertz QCLs, due to its limitation in describing coherent transport [5, 6]. Therefore, a quantum-mechanics approach such as non-equilibrium Green’s function or density matrix is required. Although the former is more accurate, its complexity and computational burden make it difficult to be implemented as a simulation tool. In this work, a density matrix (DM) model for Ge/SiGe QCL simulation has been developed. The existing models have used a reduced set of basis states, leaving out some coherences, which was justified for the particular structures they were used for, but potentially limits their generality and accuracy. In this work, we present an extended DM model, which considers all basis states involved in transport between periods of a QCL. The simulator based on it is sufficiently general to be able to simulate a QCL with any number of states and tight-binding modules per period. This is useful for investigating various QCL structures without modifying the code. It also includes multiple scattering mechanisms existing in Si and Ge quantum wells [4], i.e. intravalley scattering due to interface roughness, alloy disorder, ionized impurities, electron-electron, electron-acoustic phonon and optical phonon interactions, and intervalley phonon scattering. Since the simulator is still quite fast, it was used, in conjunction with a semi-automated optimization algorithm, to improve the predicted performance of bound-to-continuum QCLs, and to compensate for the gain-reduction associated with diffuse Ge/GeSi interfaces

Density matrix modelling of Ge/GeSi quantum cascade terahertz lasers

The prospect of making silicon-based quantum cascade lasers (QCLs) has attracted considerable research interest in recent years, due to their significant potential advantages including a mature Si processing technology, the prospect of integration with Si microelectronics, and superior thermal performance to that of III–V devices. Amongst various proposed designs, with different material compositions and substrate orientations, (001)-oriented n-type Ge/GeSi structures utilising L-valley intersubband transitions appear to be the most promising due to a small quantisation effective mass, and hence large optical matrix elements, and practically realisable layer widths. While all the previous simulations for group IV-based QCLs used the rate equation model, this neglects the coherence effects and is of limited usefulness for predicting QCL performance, particularly in the terahertz range. In this work, a quantum-mechanics transport model for Ge/SiGe QCL simulation has been developed, using the density matrix (DM) approach. In contrast to the existing DM formulations which have been used to simulate III-V based QCLs, the present model accounts for the role of all the QCL states in coherent transport, or in optical transitions, or both. The simulator includes all the principal scattering mechanisms in Ge/SiGe heterostructures: intravalley scattering due to interface roughness, alloy disorder, ionized impurities, electron-acoustic phonon and optical phonon interactions, and intervalley phonon scattering. It was used in conjunction with a semi-automated optimization algorithm to identify heterostructure designs for bound-to-continuum Ge/GeSi QCLs, and to compensate for the gain-reduction associated with diffuse Ge/GeSi interfaces