Moving Toward Nano-TCAD Through Multimillion-Atom Quantum-Dot Simulations Matching Experimental Data

Abstract

Abstract-Low-loss optical communication requires light sources at 1.5um wavelengths. Experiments showed without much theoretical guidance that InAs/GaAs quantum dots (QDs) may be tuned to such wavelengths by adjusting the In fraction in an In x Ga 1-x As strain-reducing capping layer (SRCL). In this work systematic multimillion atom electronic structure calculations qualitatively and quantitatively explain for the first time available experimental data. The NEMO 3-D simulations treat strain in a 15 million atom system and electronic structure in a subset of ~9 million atoms using the experimentally given nominal geometries and without any further parameter adjustments the simulations match the nonlinear behavior of experimental data very closely. With the match to experimental data and the availability of internal model quantities significant insight can be gained through mapping to reduced order models and their relative importance. We can also demonstrate that starting from simple models has in the past led to the wrong conclusions. The critical new insight presented here is that the QD changes its shape. The quantitative simulation agreement with experiment without any material or geometry parameter adjustment in a general atomistic tool leads us to believe that the era of nano Technology Computer Aided Design (nano-TCAD) is approaching. NEMO 3-D will be released on nanoHUB.org where the community Manuscript received July 13, 2008. This work was carried out in part at the Jet Propulsion Laboratory (JPL), California Institute of Technology, under a contract with the National Aeronautics and Space Administration (NASA). Muhammad Usman is funded through Fulbright USAID (Grant ID # 15054783). nanoHUB.org computational resources operated by the Network for Computational Nanotechnology (NCN) funded by the National Science Foundation were used in this work.Copyright (c) 2008 IEEE. Personal use of this material is permitted. However, permission to use this material for any other purposes must be obtained from the IEEE by sending a request to pubs-permissions@ieee.org .Muhammad Usman is with the Network for Computational Nanotechnology, Electrical Engineering Department, Purdue University West Lafayette, Indiana 47907 USA can duplicate and expand on the results presented here through interactive simulations.

In-plane polarization anisotropy of ground state optical intensity in InAs/GaAs quantum dots

“…The atomistic calculations of strain and electronic structure allows us to incorporate the symmetry-lowering effects caused by QD/GaAs interface roughness and the inequivalence of the [110] and [À110] directions in the underlying zincblende crystal structure. 1,32 Continuum modeling techniques such as the effective mass approximation and kp approach cannot include these effects and hence fail to incorporate some of the essential physics. A single InAs QD is simulated for this purpose because the separation between the QD layers in the experimental bilayer samples 2,25 is such that there is negligible hybridization of electronic states outside the QDs.…”

“…The height, "H," of the QD is varied from 2 to 7 nm, corresponding to aspect ratios (AR ¼ H/B) varying from 0.1 to 0.35, respectively. Since the electronic structure of QDs is much more sensitive to changes in their height when compared to the changes in their base diameter, 1,45 we therefore choose to fix the base diameter and increase the aspect ratio by increasing the height of the QD. The size of GaAs buffer for the strain relaxation is 60 Â 60 Â 66 nm 3 ($ 15 million atoms) and for the electronic structure calculations is 50 Â 50 Â 56 nm 3 ($ 9 million atoms).…”

“…Thus, a directional degree of polarization (DOP) should be measured (or calculated) to fully characterize the polarization response of quantum dot stacks. Previous theoretical and experimental studies have considered only a single value of DOP: either [110] The electronic structure of single and stacked InAs quantum dots (QDs) has been extensively studied in the last couple of decades for the design of optical devices [1][2][3][4][5][6][7][8] and devices suited to quantum information processing. [9][10][11][12] Recent efforts are focused to achieve isotropic polarization response of ground state optical intensity (GSOI) at telecommunication wavelengths (1300-1500 nm).…”

“…[14][15][16][17][18] In such QDs, the highest few valence band states possess dominant heavy hole character due to the large magnitude of biaxial strain, which splits the heavy hole (HH) and light hole (LH) bands inside the QD region. 1 As a result, the TE-mode becomes much stronger than TM-mode and the DOP > þ 0.9. Several methods have been explored to reduce the value of DOP.…”

On the Validity of the Top of the Barrier Quantum Transport Model for Ballistic Nanowire MOSFETs

“…A critical element in the model is the representation of the device in an atomistic TightBinding (TB) model [6], which understands the finite number of atoms in the structure, their local arrangement with details such as strain distribution and disorder [7,8]. A full 3D atomistic quantum transport model [9,10,11] can provide the device characteristics, however, this model is computationally time consuming [12].…”