Density Functional Simulation of Quantum Dots in InAs-InP Nanowires: SPM Imaging and Surface Effects

We have recently carried out self-consistent electronic structure calculations within the effective mass approximation to density functional theory (DFT) for quantum dots and double quantum dots formed through atomic layer epitaxy in semiconductor (specifically InAs) nanowires. The simulations employ the full 3D structure of the device with a buried backgate and an idealistic geometry for the source and drain leads. In addition, the influence of a scanning probe microscope (SPM) tip in modulating the Coulomb blockade of electrons in the dot(s) is included in the simulation. This geometric fidelity is essential for evaluating the effectiveness of using such nanowire dots for electronic components and chemical sensors. The principle physical uncertainty in the model arises from the incomplete understanding of the surface states in InAs (and InP). In particular, a variety of experimental studies have shown that the nature of the surface states and the resulting band-bending and Schottky barrier (or lack thereof) is sensitively dependent on both surface preparation and surface orientation. This makes a first principles treatment of a round surface (i.e. the wire) impossible, even if the surface preparation is well-defined. We have developed a method which employs a separate, room temperature calculation with the surface pinned at a chosen potential to determine the surface charge distribution and band-bending which gets frozen in at low temperature. The results of the simulations are compared with experiments carried out recently in the Westervelt group on imaging quantum wire bound dots.

The simulation, along with a variety of other software packages for materials calculations, is available for use on the National Nanotechnology Infrastructure Network Computation Project (NNIN/C) site. I will briefly discuss this NSF-supported program and how (free) access to the NNIN/C resources can be achieved.
Dr. Stopa received his Ph.D in condensed matter theory from the University of Maryland in 1990. He has since served as senior theorist at several research institutions including NTT Basic Research Laboratories, RIKEN Frontier Research Program, Walter Schottky Institute, ERATO-JST. He joined the Center for Nanoscale Systems at Harvard University in 2004 and is currently the Director of NNIN’s Computation Project.