SiGe nanocrystals core and surface electronic structure from ab initio large unit cell calculations

Abstract

Oxidised surface and core parts of SiGe nanocrystals are investigated using first-principle techniques that incorporate self-consistent Hartree–Fock method coupled with large unit cell (LUC) method to obtain the electronic structure of Bravais and primitive LUC multiples-based nanocrystals. Results show that lattice constant and ionicity of the core part show decreasing order as nanocrystals grow up in size. The smallest investigated nanocrystal is 2.5% larger in lattice constant and 15% larger in ionicity than the converged value of largest investigated nanocrystal. The lattice constant of both the surface and core has nearly the same value indicating good adherence of oxidised surface. Surface oxygen atoms absorb some of the electronic cloud of germanium and silicon surface atoms. Contrary to what one can expect, Si is the cation in this compound. The core energy gap increases and fluctuates as nanocrystals increase in size. The oxidised surface part on the other side has a dramatically lower gap. Oxygenated Ge- and Si-terminated nanocrystals have entirely different behaviour and properties. This includes a wider gap at the Si-terminated surface and highly ionised atoms. A bilayer with positively charged Ge and Si atoms are located at the Ge-terminated surface behind the oxygen layer. The present work suggests the addition of ionicity and lattice constant to the quantities that are affected by quantum confinement phenomenon. Some quantities, such as ionisation potential and affinity, continue to fluctuate indefinitely as nanocrystals grow up in size because of the generation of new shapes and surfaces.