  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
InxGa1-xAs 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 can duplicate and expand on the
results presented here through interactive simulations.
