Mechanics and quantum dots: Strain size effects and 'reverse coupling'.

摘要

Quantum dots are nanoscale dusters of (typically) semiconductor materials and due to quantum mechanical size effects exhibit unusual mechanical, electronic and optical behavior. While several technological barriers remain, they are often considered as basis for several revolutionary nanoelectronic devices and applications. From a scientific viewpoint, they are a fertile laboratory to test fundamental quantum and solid mechanics effects and associated theories.; In this dissertation we explore two issues related to the coupling of mechanical strain and quantum mechanical behavior: (1) Size-dependency of strain at the nanoscale and coupling to electronic structure: Firstly, in the isotropic case, based on the physical mechanisms of nonlocal interactions, we herein derive new (closed-form) scaling formulae for strain in embedded lattice mismatched spherical quantum dots. We finally extend our results to cubic anisotropy and arbitrary shape. We attempt to assess the qualitative and quantitative effects on the electronic band structure of InAs-GaAs quantum dot system. (2) Quantum Field Induced Strain in Nanostructures : The second issue leads us to some tantalizing conclusions. We investigate an unusual size-dependent physical phenomenon wherein mechanical strain can be induced in small quantum dots (1-3 nm) in the complete absence of external stress, predicated purely on quantum mechanical confinement effects. While the phenomenon of strain impact on electronic structure in quantum dots is well known and studied (and is the focus of the first few chapters of this dissertation), this "reverse coupling" appears to be largely ignored or buried in all-numerical ab initio calculations. A modified multiband envelope function approach is used to model this phenomenon thus providing a transparent scheme to identify its occurrence. Further, we have used our developed model to quantitatively predict the electronic structure and the induced strain field for colloidal Si and GaAs quantum dots. After due interpretation of the "quantum confinement induced strain" in terms of acoustic polarons, we confirm our model and predictions by detailed ab initio calculations. The present work may potentially provide a basis for next generation Quantum Electromechanical Systems (QEMs).