Electronic states of Ge/Si nanocrystals with crescent-shaped Ge-cores

Abstract

Ge/Si nanocrystals can serve as charge storage sites in a nanocrystal memory by providing a hole quantum-well in the Ge region. The electronic states of realistically shaped Ge/Si nanocrystals with crescent-shaped Ge-cores are calculated to determine the hole confinement energies, effective masses, barrier heights, and thermionic lifetimes. As the Ge crescent thickness increases from 1 nm to 3.5 nm, the hole confinement energy decreases from 0.52 to 0.28 eV, the barrier height to escape into the Si valence band increases from 0.25 to 0.51 eV, and the resulting thermionic hole lifetime increases from 10−9 to 10−5 s. The nanocrystals are modeled with an atomistic, 20-band sp3d5s* tight-binding model including spin-orbit coupling as implemented in NEMO3D. Geometry relaxation and strain are included using the valence-force-field model with Keating potentials.