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Title: Fabrication of InAs/GaAs single quantum dots by molecular beam epitaxy
Author: Lin, Jacob Che-Chen
ISNI:       0000 0001 3610 3122
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2007
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Quantum dots (QDs) are a class of semiconductor structure widely studied for their unique electronic and opto-electronic properties. Its extreme narrow line width property has been long predicted before first successful fabrication, and currently it is one of the main nano-structures used in light emitting devices. The QDs Idescribed here are engineered to form in a self-assembled manner where large quantities ofthem are produced at once. In the last decade, huge research efforts have been pumped controlling the property of the QD ensembles, such as areal density, wavelength, homogeneity and defect density. Towards the other extreme of the research interest, single QD fabrication is desirable for single electron and single photon devices, which are crucial tools both for quantum phenomenon observation and for the development ofquantum cryptography and quantum information processing. Two approaches are used to fabricate single QDs. In the micro-mesa approach areas on the dielectric mask are 'lectively opened so that the crystal structure is allowed to grow on the opened area. Usually the structures grown have a pyramidal shape. Choosing the base orientation correctly can result in pyramids with reducing top area and non-QD-growing sidewalls. Only a single QD would form at the apex of the pyramid if the pyramid size and maturity are well-controlled. The other approach described here is the micro-dimple approach. Since the formation of self-assembled QDs is a strain driven process, pin-point disturbance on a plain growth surface such as a dimple promotes QD nucleation on that point. Thus, a deposition of QD material with less than critical thickness may result in QD nucleation only on the dimples. For the micro-mesa approach, successful single QD growth has been performed in metal organic vapour phase epitaxy (MOVPE) and chemical beam epitaxy (CBE), and is demonstrated here in solid source molecular beam epitaxy (MBE). Performing lnAs/GaAs selective area QD growth in MBE appears to be problematic. The low GaAs growth selectivity between the dielectric mask and the GaAs epilayer in MBE destroys the shape of the pyramid grown. A combination of high growth temperature and low growth rate by periodic supply epitaxy (PSE) is used to reduce the polycrystalline fonnation on the mask. It is discovered that the polycrystals fonned on the dielectric mask reduce the neighbouring InAs QD areal density, whereas in MOVPE the dielectric mask increases the InAs QD areal density and/or height. Growing single QDs with the micro-dimple approach by MBE without atomic . hydrogen cleaning (ARC) is also investigated. During oxygen desorption in MBE, the native oxide on the processed sample fonns pits with a density as high as the selfassembled QDs and size comparable to the purposely made dimples. In order to realise a plain surface with only the dimples made during the fabrication processing, atomic hydrogen cleaning is usually employed to stop the pit fonnation during the oxygen desorption step. The alternative Ga beam supplement to reduce the pits is investigated and used in single QD fabrication by the micro-dimple approach in MBE.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available