Self-assembled quantum dot molecules by molecular beam epitaxy and their potential applications

The aim of this thesis is to investigate the formation mechanism of nanostructures, especially lateral quantum dot molecules (QDMs) using partial-capping-and-regrowth technique with both solid-source molecular beam epitaxy (MBE) and gas-source MBE. The physical and optical properties of all nanostructures are measured by ex-situ atomic force microscopy (AFM) and photoluminescence (PL). With solid-source MBE under an As[subscript4] ambient, as-grown quantum dots (QDs) change to a camel-like nanostructure after being partially overgrown with GaAs (partial capping). When additional InAs is deposited (regrowth), QDMs with 10-12 dots per QDM are obtained. During thepartial GaAs capping of InAs QDs, if the capping temperature is varied, the length of the nanopropellers can be controlled. The lowering of the capping temperature leads to smaller dot per QDM after regrowth process. With gas-source MBE where As[subscript2] is generated by thermally cracked arsine, as-grown QDs are transformed into quantum rings after partial capping with GaAs. After the regrowth process, QD pairs or double QDs are obtained. At higher regrowth temperature, QD rings with 5-7 dots per ring are formed. Surface morphologies of self-assembled (Ga) InAs nanostructures and QDMs grown by partial-capping-and-regrowth technique using gas-source MBE and solid-source MBE are compared. The difference in the migration length of group III adatoms under different arsenic species, As[subscript2] in gas-source MBE and As[subscript4] in solid-source MBE, is the origin of respective outcomes of nanostructures and QDMs.

本论文旨在研究纳米结构的形成机理，重点针对采用部分覆盖-再生长工艺（partial-capping-and-regrowth technique），结合固相分子束外延（solid-source molecular beam epitaxy, MBE）与气相分子束外延（gas-source MBE）两种外延技术制备的侧向量子点分子（lateral quantum dot molecules, QDMs）展开探究。所有纳米结构的物理与光学特性均通过非原位原子力显微镜（ex-situ atomic force microscopy, AFM）与光致发光（photoluminescence, PL）进行表征。
在As₄气氛下采用固相MBE生长时，生长态量子点（quantum dots, QDs）经GaAs部分覆盖后，会转变为骆驼状纳米结构。当继续沉积额外InAs（即开展再生长工艺）后，可得到每个QDM包含10~12个量子点的侧向量子点分子。在对InAs QDs进行GaAs部分覆盖的过程中，若改变覆盖温度，可调控纳米螺旋桨（nanopropellers）的长度；覆盖温度降低会导致再生长后每个QDM所包含的量子点尺寸更小。
在通过热裂解胂（thermally cracked arsine）制备As₂气氛的气相MBE体系中，生长态QDs经GaAs部分覆盖后会转变为量子环（quantum rings）结构。再生长工艺完成后，可得到量子点对或双量子点（double QDs）结构；当再生长温度较高时，会形成每个量子环包含5~7个量子点的量子环结构。
本研究对比了采用气相MBE与固相MBE、通过部分覆盖-再生长工艺制备的自组装(Ga)InAs纳米结构与QDMs的表面形貌。气相MBE中的As₂与固相MBE中的As₄这两种不同砷物种（arsenic species）环境下，III族吸附原子（group III adatoms）的迁移长度存在差异，这正是两类工艺最终得到不同纳米结构与QDMs的根本原因。