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

Abstract

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.