  Built-in electrostatic fields in Zincblende quantum dots originate mainly
from - (1) the fundamental crystal atomicity and the interfaces between two
dissimilar materials, (2) the strain relaxation, and (3) the piezoelectric
polarization. In this paper, using the atomistic NEMO 3-D simulator, we study
the origin and nature of the internal fields in InAs/GaAs quantum dots with
three different geometries, namely, box, dome, and pyramid. We then calculate
and delineate the impact of the internal fields in the one-particle electronic
states in terms of shift in the conduction band energy states, anisotropy and
non-degeneracy in the P level, and formation of mixed excited bound states.
Models and approaches used in this study are as follow: (1) Valence force field
(VFF) with strain-dependent Keating potentials for atomistic strain relaxation;
(2) 20-band nearest-neighbor sp3d5s* tight-binding model for the calculation of
single-particle energy states; and (3) For piezoelectricity, for the first time
within the framework of sp3d5s* tight-binding theory, four different
recently-proposed polarization models (linear and non-linear) have been
considered in conjunction with an atomistic 3-D Poisson solver that also takes
into account the image charge effects. Specifically, in contrast to recent
studies on similar quantum dots, our calculations yield a non-vanishing net
piezoelectric contribution to the built-in electrostatic field. Demonstrated
also is the importance of full three-dimensional (3-D) atomistic material
representation and the need for using realistically-extended substrate and cap
layers (systems containing ~2 million atoms) in the numerical modeling of these
reduced-dimensional quantum dots.
