Date of Graduation

5-2009

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Microelectronics-Photonics (PhD)

Degree Level

Graduate

Department

Graduate School

Advisor

Gregory J. Salamo

Committee Member

Min Zou

Second Committee Member

David Storm

Third Committee Member

Hameed Naseem

Fourth Committee Member

Kenneth Vickers

Keywords

Applied sciences; Droplet epitaxy; Molecular beam epitaxy; Quantam dots; Self-assembly

Abstract

There is increasing interest in quantum dot (QD) structures for a plethora of applications, including optoelectronic devices, quantum computing and energy harvesting. While strain driven surface diffusion via stranski-krastanow (SK) method has been commonly used to fabricate these structures, a more recent technique, droplet epitaxy (DE) does not require mismatch strain and is therefore much more flexible in the combination of materials utilized for the formation of QDs.

As reported in this work, a hybrid approach that combines DE and SK techniques for realizing lateral ordering of QDs was explored. First, the droplet formation of various materials was discussed and two different growths approaches, simultaneous and sequential, were investigated. With an in depth understanding of droplet formation, in-situ templates were developed via the droplet homo-epitaxy. Ga droplets were manipulated with varied growth conditions such as surface temperature, As flux and growth interruption (GI) time for fabrication of three types of templates: 1) nanoholes, 2) shallow-elongated GaAs nanomounds and 3) tall-rounded GaAs nanomounds.

With the help of these templates, various laterally-ordered QDs such as QD pairs (QDPs), QD clusters (QDCs) and QD molecules (QDMs) were fabricated. The GaAs QDPs were spontaneously formed on an AlGaAs surface due to anisotropy of surface diffusion at high temperatures and the overall structure of the QDPs were revealed by cross-section transmission electron microscopy (XTEM). XTEM revealed that the QDPs were immersed under the plane of the AlGaAs due to redistribution of the underlying AlGaAs layers. The overall shape and configuration of the QDPs can be attributed to the corrugated sidewalls that resulted from the redistribution of materials and the substrate etching caused by the "nanodrill" effect at high temperatures. The nanodrill effect was investigated on an AlAs/GaAs superlattice (SL) and the resulting nanohole formation as well as the hole refill process was explored via XTEM.

On the other hand, investigations of the annealing effect of crystallized GaAs at both high and low temperatures showed a clear distinction in the type of resulting GaAs nanomound template and subsequent QD alignment. The elongated-shallow template was a direct consequence of Ga atoms' diffusion leading to a misoriented GaAs surface, which QDs favored energetically. The tall-rounded templates resulted at low temperatures because the rate at which diffusion slowed down. This template type led to the formation of QDMs. The results showed a strong correlation between the configurations and the size of the droplets. The bigger droplets resulted in hexa-QDMs and the smaller droplets resulted in quad-QDMs. The proposed evolution of QDMs was based on the transfer of materials, adatom diffusion and resulting diffused shape of the droplets. Configuration control for achieving quad-QDMs resulted in two different types of structures: 1) quad-QDMs elongated along the [011] and 2) quantum rod pairs (QRPs) due to high anisotropy of surface diffusion along the [01-1].

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