R. W. Kelsall

Self-consistent scattering theory of transport and output characteristics of quantum cascade lasers

Electron transport in GaAs/AlGaAs quantum cascade lasers operating in midinfrared is calculated self–consistently using an intersubband scattering model. Subband populations and carrier transition rates are calculated and all relevant electron-LO phonon and electron–electron scatterings between injector/collector, active region, and continuum resonance levels are included. The calculated carrier lifetimes and subband populations are then used to evaluate scattering current densities, injection efficiencies, and carrier backflow into the active region for a range of operating temperatures. From the calculated modal gain versus total current density dependencies the output characteristics, in particular the gain coefficient and threshold current, are extracted. For the original GaAs/Al0.33Ga0.67As quantum cascade structure [C. Sirtori et al., Appl. Phys. Lett. 73, 3486 (1998)] these are found to be g=11.3 cm/kA and Jth=6±1 kA/cm2 (at T=77 K), and g=7.9 cm/kA and Jth=10±1 kA/cm2 (at T=200 K), in good agreeme...

Intersubband electroluminescence from Si/SiGe cascade emitters at terahertz frequencies

The quantum cascade laser provides one possible method of realizing high efficiency light emitters in indirect band gap materials such as silicon. Electroluminescence results from Si/SiGe quantum cascade emitters are presented demonstrating edge emission from heavy-hole to heavy-hole transitions and light-hole to heavy-hole transitions. In surface-normal emission, only light-hole to heavy-hole electroluminescence is observed as predicted by theory. Intersubband emission is demonstrated at 2.9 THz (103 μm wavelength), 8.9 THz (33.7 μm), and 16.2 THz (18.5 μm) from the Si/SiGe quantum cascade heterostructures.

Interwell intersubband electroluminescence from Si/SiGe quantum cascade emitters

The quantum cascade laser provides one potential method for the efficient generation of light from indirect materials such as silicon. While to date electroluminescence results from THz Si/SiGe quantum cascade emitters have shown higher output powers than equivalent III–V emitters, the absence of population inversion within these structures has undermined their potential use for the creation of a laser. Electroluminescence results from Si/SiGe quantum cascade emitters are presented demonstrating intersubband emission from heavy to light holes interwell (diagonal) transitions between 1.2 THz (250 μm) and 1.9 THz (156 μm). Theoretical modeling of the transitions suggests the existence of population inversion within the system.

Mechanisms of temperature performance degradation in terahertz quantum-cascade lasers

Electron transport in a terahertz GaAs/AlGaAs quantum-cascade laser is calculated using a fully self-consistent intersubband scattering model. Subband populations, carrier transition rates, and current densities are calculated and all relevant intra- and interperiod electron–electron and electron–LO-phonon scattering mechanisms are included. Employing an energy balance equation that includes the influence of both electron–LO-phonon and electron–electron scattering, the method also enables evaluation of the average electron temperature of the nonequilibrium carrier distributions in the device. In particular, the influence of the lattice temperature on the degradation of population inversion and device performance is investigated. The threshold currents, electric-field-current-density characteristics, and temperature-dependent performance are in good qualitative and quantitative agreement with measurement in a recent experimental realization [Kohler et al., Nature (London) 417, 156 (2002)]. Calculations ind...

A rectangular dielectric waveguide technique for determination of permittivity of materials at W-band

The rectangular dielectric waveguide (RDWG) technique has been previously described for the determination of complex permittivity of a wide class of dielectric materials of various thicknesses and cross sections. This paper presents a unified RDWG technique for the determination of the dielectric constant of materials. Some measurement results are reported at W-band frequencies to demonstrate the usefulness of the RDWG technique where other techniques are usually constrained by the sample dimensions.

Investigation of Self-Heating Effects in Submicrometer GaN/AlGaN HEMTs Using an Electrothermal Monte Carlo Method

An electrothermal Monte Carlo (MC) method is applied in this paper to investigate electron transport in submicrometer wurtzite GaN/AlGaN high-electron mobility transistors (HEMTs) grown on various substrate materials including SiC, Si, GaN, and sapphire. The simulation method is an iterative technique that alternately runs an MC electronic simulation and solves the heat diffusion equation using an analytical thermal resistance matrix method. Results demonstrate how the extent of the thermal droop in the Id-Vds characteristics and the device peak temperature depend upon both the biasing conditions and the substrate material type. Polarization effects are considered in the simulations, as they greatly influence electron transport in GaN/AlGaN HEMTs by creating a highly concentrated two-dimensional electron gas (2DEG) at the GaN/AlGaN interface. It is shown that a higher 2DEG density provides the devices with a better current handling capability but also increases the importance of the thermal effects

Electron temperature and mechanisms of hot carrier generation in quantum cascade lasers

A technique for calculating the temperature of the nonequilibrium electron distribution functions in general quantum well intersubband devices is presented. Two recent GaAs/Ga1−xAlxAs quantum cascade laser designs are considered as illustrative examples of the kinetic energy balance method. It is shown that at low current densities the electron temperature recovers the expected physical limit of the lattice temperature, and that it is also a function of current density and the quantised energy level structure of the device. The results of the calculations show that the electron temperature Te can be approximated as a linear function of the lattice temperature Tl and current density J, of the form Te=Tl+αe−lJ, where αe−l is a coupling constant (∼6–7 K/kA cm−2 for the devices studied here) which is fixed for a particular device.

Self-consistent solutions to the intersubband rate equations in quantum cascade lasers: Analysis of a GaAs/AlxGa1-xAs device

The carrier transition rates and subband populations for a GaAs/AlGaAs quantum cascade laser operating in the mid-infrared frequency range are calculated by solving the rate equations describing the electron densities in each subband self-consistently. These calculations are repeated for a range of temperatures from 20 to 300 K. The lifetime of the upper laser level found by this self-consistent method is then used to calculate the gain for this range of temperatures. At a temperature of 77 K, the gain of the laser is found to be 34 cm−1/(kA/cm−2), when only electron–longitudinal-optical phonon transitions are considered in the calculation. The calculated gain decreases to 19.6 cm−1/(kA/cm−2) when electron–electron transition rates are included, thus showing their importance in physical models of these devices. Further analysis shows that thermionic emission could be occurring in real devices.

Influence of leakage current on temperature performance of GaAs/AlGaAs quantum cascade lasers

A detailed analysis of intersubband electron scattering transport in GaAs/AlGaAs quantum cascade lasers (QCLs) is presented, using a full self-consistent rate equation model. Our approach includes all relevant scattering mechanisms between injector/collector, active region and continuumlike states in the cascade structures. In particular, the influence of the Al mole fraction in the quantum barriers on QCLs performance is investigated, by studying GaAs/AlGaAs structures with 33% and 45% Al barrier compositions, respectively. Excellent qualitative and quantitative agreement with recent experimental results at cryogenic and room temperatures is obtained. The model reproduces the gain saturation reported for the 33% Al device, which precludes laser operation at room temperature, and also the much improved room-temperature performance of the 45% Al device, with calculated 300 K threshold current of 17 kA/cm2, and confirms that the superior performance of the 45% Al device is due to suppression of parasitic co...

Population inversion in optically pumped asymmetric quantum well terahertz lasers

Intersubband carrier lifetimes and population ratios are calculated for three- and four-level optically pumped terahertz laser structures. Laser operation is based on intersubband transitions between the conduction band states of asymmetric GaAs-Ga1−xAlxAs quantum wells. It is shown that the carrier lifetimes in three-level systems fulfill the necessary conditions for stimulated emission only at temperatures below 200 K. The addition of a fourth level, however, enables fast depopulation of the lower laser level by resonant longitudinal optical phonon emission and thus offers potential for room temperature laser operation.