
Atomistic Simulation of Realistically Sized Nanodevices Using NEMO 3D—Part II: Applications
14 Jan 2008  Papers  Contributor(s): Gerhard Klimeck, Shaikh S. Ahmed, Neerav Kharche, Marek Korkusinski, Muhammad Usman, marta prada, Timothy Boykin
In part I, the development and deployment of a general nanoelectronic modeling tool (NEMO 3D) has been discussed. Based on the atomistic valenceforce field and the sp3d5s* nearest neighbor tightbinding models, NEMO 3D enables the computation of strain and electronic structure in nanostructures consisting of more than 64 and 52 million atoms, corresponding to volumes of (110 nm)3 and (101 nm)3, respectively. In this part, successful applications of NEMO 3D are demonstrated in the atomistic ...

Bismide Semiconductors: Revolutionising Telecom Lasers
19 Oct 2015  Papers  Contributor(s): Muhammad Usman, Christopher A Broderick, Eoin P O\'reilly
Today’s telecomm lasers are plagued with Augerrelated losses, which significantly reduce their efficiency and make device cooling essential. We are proposing a radical change in the laser technology by developing a new class of materials, bismide semiconductors. These novel nanomaterials can efficiently suppress the Augerrelated loss mechanisms and therefore can lead to the realization of highly efficient and temperature stable GaAsbased photonic devices operating at telecomm and...

Electronic Structure Theory of Dilute Impurity Alloys: GaBiP and GaBiAs
18 Jan 2013  Online Presentations  Contributor(s): M. Usman
We report an atomistic model established for electronic structure calculations of GaBiAs (0 < Bi < 12%) alloys based on empirical tight binding parameters. Alloy supercells consisting of 1000 and 8000 atoms are relaxed using valence force field (VFF) method, including anharmonic corrections to the Keating potential. Nearest neighbor tight binding Hamiltonian is solved for electronic spectra by representing each atomic sight with sp3s* parameters including spin orbit coupling (10band model). …

Excited State Spectroscopy of a Quantum Dot Molecule
11 Jan 2013  Online Presentations  Contributor(s): Muhammad Usman
Atomistic electronic structure calculations are performed to study the coherent interdot couplings of the electronic states in a single InGaAs quantum dot molecule. The experimentally observed excitonic spectrum by Krenner et al (Phys. Rev. Lett. 94 057402, 2005) is quantitatively reproduced, and the correct energy states are identified based on a previously validated atomistic tight binding model. The extended devices are represented explicitly in space with 15millionatom structures. An …

Nanoelectronic Modeling Lecture 32: Strain Layer Design through Quantum Dot TCAD
04 Aug 2010  Online Presentations  Contributor(s): Gerhard Klimeck, Muhammad Usman
This presentation demonstrates the utilization of NEMO3D to understand complex experimental data of embedded InAs quantum dots that are selectively overgrown with a strain reducing InGaAs layer. Different alloy concentrations of the strain layer tune the optical emission and absorption wavelength of the quantum dots. The role of the nonlinear strain behavior ovserved in the experimental data is explored in NEMO3D. The simulation engine serves as a virtual microscope to understand the interplay of disorder, strain, and quantum dot shape.
Learning Objectives:
 Objective:
 Optical emission at 1.5μm without GaN
 Understand experimental data on QD spectra in selective overgrowth
 Approach:
 Model large structure
 60nm x 60nm x 60nm
 9 million atoms
 No changes to the published tight binding parameters
 Result:
 Match experiment remarkably well
 Strain
 change in quantum dot aspect ratio
 Quantitative model of complex system
 Studied sensitivity to experimental imperfections – small variations
 Effective mass theories provided the wrong guidance

NEMO3D User Guide for Quantum Dot Simulations
29 Nov 2011  Papers  Contributor(s): M. Usman, Gerhard Klimeck
NEMO 3D is a large and complex simulator; and understanding of its source code requires considerable knowledge of quantum mechanics, condensed matter theory, and parallel programming.

Polarization Response of Multilayer InAs Quantum Dot Stacks
25 Oct 2011  Online Presentations  Contributor(s): Muhammad Usman
Recent experimental measurements, without any theoretical guidance, showed that isotropic polarization response can be achieved by increasing the number of QD layers in a QD stack. In this work, we analyse the polarization response of multilayer quantum dot stacks containing up to nine quantum dot layers by linearly polarized PL measurements and by carrying out a systematic set of multimillion atom simulations. The atomistic modeling and simulations allow us to include correct symmetry …

Quantitative Modeling and Simulation of Quantum Dots
18 Apr 2011  Presentation Materials  Contributor(s): Muhammad Usman
Quantum dots grown by selfassembly process are typically constructed by 50,000 to 5,000,000 structural atoms which confine a small, countable number of extra electrons or holes in a space that is comparable in size to the electron wavelength. Under such conditions quantum dots can be interpreted as artificial atoms with the potential to be custom tailored to new functionality. In the past decade or so, these nanostructures have attracted significant experimental and theoretical attention in …

Quantum Dot based Photonic Devices
01 Apr 2012  Online Presentations  Contributor(s): Muhammad Usman
Deployment of nanometersized semiconductor quantum dots (QDs) in the active region of
photonic devices such as lasers, semiconductor optical amplifiers (SOA's), photodetectors etc.
for the next generation communication systems offers unique characteristics such as
temperatureinsensitivity, high optical output power, high speed operation, wideband
characteristics etc.; not commonly attainable in the conventional design approaches. Despite
more than twenty years of research on the electronic …

SelfAssembled Quantum Dot Structure (pyramid)
02 Feb 2011  Animations  Contributor(s): Gerhard Klimeck, Insoo Woo, Muhammad Usman, David S. Ebert
Pyramidal InAs Quantum dot. The quantum dot is 27 atomic monolayers wide at the base and 15 atomic monolayers tall.

SelfAssembled Quantum Dot Wave Structure
31 Jan 2011  Animations  Contributor(s): Gerhard Klimeck, Insoo Woo, Muhammad Usman, David S. Ebert
A 20nm wide and 5nm high dome shaped InAs quantum dot grown on GaAs and embedded in InAlAs is visualized.

TE/TM polarisation response of InAs/GaAs quantum dot bilayers
22 Oct 2015  Presentation Materials  Contributor(s): Muhammad Usman
Quantum dot bilayers are strong candidates for the design of telecom devices working at 1300 nm wavelength range. Here we analyse  both experimentally and theoretically  their optical and polarisation properties.

Why quantum dot simulation domain must contain multimillion atoms?
11 Jan 2013  Online Presentations  Contributor(s): Muhammad Usman
The InGaAs quantum dots obtained from the selfassembly growth process are heavily strained. The longrange strain and piezoelectric fields significantly modifies the electronic structure of the quantum dots. This imposes a critical constraint on the minimum size of the simulation domain to study the electronic and optical spectra of these nanostructures. The aim of this presentation is to carry out a very simple numerical experiment by performing systematic simulations of an experimentally …