自組式量子點在史塔克效應下的光學異向性

Abstract

在本論文中使用k□p六能帶理論計算InAs/GaAs 自組式量子點在外加電場下的電子結構與光學性質。首先使用有限差分法計算量子盒、金字塔與截角金字塔形狀量子點內單一粒子(導電帶的電子與價電帶的電洞)的電子結構。而量子點內外的應變分布則使用有限元素法Comsol multiphysics®。費米黃金定律與電子結構的計算可得到量子點中激子所產生的光子偏振。應變的計算發現垂直方向的應變主導重電洞侷限位能，平行方向的應變則主導輕電洞侷限位能的變化。比較量子盒與截角金字塔的形狀發現金字塔狹窄的上半部區域為較大的體應變分布(較小的雙軸應變分布)，導致截角金字塔尖端效應越強則輕重電洞耦合越強。所以較高的截角金字塔量子點，輕電洞成分較多，則導致偏振越大。此外，外加電場於量子點的成長方向導致基態的輕電洞成分改變。研究結果發現，在較高的量子點外加電場200 KeV/cm，史塔克效應大約增加偏振5%。

This thesis theoretically investigates the electronic structures and optical properties of InAs/GaAs self-assembled quantum dots under external electric fields by using six-band Luttinger-Kohn k□p theory. First, the single-particle (conduction electron or valence hole) spectra of box, pyramid, and truncated-pyramid shaped quantum dots are calculated by using finite difference method. The strain distributions in and out of dots are computed by using finite element software package Comsol multiphysics®. Based on the single particle spectra, optical polarization degree of the ground states of an exciton photo-generated in a quantum dot is calculated by using Fermi’s golden rule. The strain calculations show that the effective confining potential for heavy-hole in a strained dot is governed by out-of-plane strain while the one for light-hole by in-plane strain. The theoretical results show that, compared with box and truncated-pyramid shaped dots, heavy- and light-hole coupling is significantly increased in a pyramid-shaped dot because of larger hydrostatic strain (smaller bi-axial strain) in the narrow upper region of the nano-pyramid. With more light-hole component, larger optical polarization degree is observed for high pyramid-shped dots. Furthermore, the light-hole component in a hole ground state can be significantly changed by applying an external electric field along the growth-axis. As a result, it is found that the stark effect could increases the polarization degree of a high pyramid dot by 5% by applying an electric field up to 200 KeV/cm.