53 research outputs found
Modeling techniques for quantum cascade lasers
Quantum cascade lasers are unipolar semiconductor lasers covering a wide
range of the infrared and terahertz spectrum. Lasing action is achieved by
using optical intersubband transitions between quantized states in specifically
designed multiple-quantum-well heterostructures. A systematic improvement of
quantum cascade lasers with respect to operating temperature, efficiency and
spectral range requires detailed modeling of the underlying physical processes
in these structures. Moreover, the quantum cascade laser constitutes a
versatile model device for the development and improvement of simulation
techniques in nano- and optoelectronics. This review provides a comprehensive
survey and discussion of the modeling techniques used for the simulation of
quantum cascade lasers. The main focus is on the modeling of carrier transport
in the nanostructured gain medium, while the simulation of the optical cavity
is covered at a more basic level. Specifically, the transfer matrix and finite
difference methods for solving the one-dimensional Schr\"odinger equation and
Schr\"odinger-Poisson system are discussed, providing the quantized states in
the multiple-quantum-well active region. The modeling of the optical cavity is
covered with a focus on basic waveguide resonator structures. Furthermore,
various carrier transport simulation methods are discussed, ranging from basic
empirical approaches to advanced self-consistent techniques. The methods
include empirical rate equation and related Maxwell-Bloch equation approaches,
self-consistent rate equation and ensemble Monte Carlo methods, as well as
quantum transport approaches, in particular the density matrix and
non-equilibrium Green's function (NEGF) formalism. The derived scattering rates
and self-energies are generally valid for n-type devices based on
one-dimensional quantum confinement, such as quantum well structures
Non-equilibrium Green's function predictions of band tails and band gap narrowing in III-V semiconductors and nanodevices
High-doping induced Urbach tails and band gap narrowing play a significant
role in determining the performance of tunneling devices and optoelectronic
devices such as tunnel field-effect transistors (TFETs), Esaki diodes and
light-emitting diodes. In this work, Urbach tails and band gap narrowing values
are calculated explicitly for GaAs, InAs, GaSb and GaN as well as ultra-thin
bodies and nanowires of the same. Electrons are solved in the non-equilibrium
Green's function method in multi-band atomistic tight binding. Scattering on
polar optical phonons and charged impurities is solved in the self-consistent
Born approximation. The corresponding nonlocal scattering self-energies as well
as their numerically efficient formulations are introduced for ultra-thin
bodies and nanowires. Predicted Urbach band tails and conduction band gap
narrowing agree well with experimental literature for a range of temperatures
and doping concentrations. Polynomial fits of the Urbach tail and band gap
narrowing as a function of doping are tabulated for quick reference
Engineering Nanowire n-MOSFETs at Lg < 8 nm
As metal-oxide-semiconductor field-effect transistors (MOSFET) channel
lengths (Lg) are scaled to lengths shorter than Lg<8 nm source-drain tunneling
starts to become a major performance limiting factor. In this scenario a
heavier transport mass can be used to limit source-drain (S-D) tunneling.
Taking InAs and Si as examples, it is shown that different heavier transport
masses can be engineered using strain and crystal orientation engineering.
Full-band extended device atomistic quantum transport simulations are performed
for nanowire MOSFETs at Lg<8 nm in both ballistic and incoherent scattering
regimes. In conclusion, a heavier transport mass can indeed be advantageous in
improving ON state currents in ultra scaled nanowire MOSFETs.Comment: 6 pages, 7 figures, journa
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