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Title: Regrowth applied to N-type modulation-doped Si/SiGe heterostructures
Author: Ahmed, Ataf
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2001
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Conventional silicon/silicon-germanium (Si/SiGe) two dimensional electron gas (2DEG) heterostructures formed by modulation-doping, require a SiGe virtual substrate grown on a Si substrate. Active heterolayers are grown on this virtual substrate, usually in a single step process. This dissertation describes the growth of Si/SiGe wafers in a two stage process. The virtual substrate is removed from growth system and is cleaned ex-situ. The wafer is then returned to the growth chamber for the deposition of the active heterolayers, introducing a regrowth interface. The successful incorporation of this regrowth interface is shown along with an investigation into the effects of the interface on transport properties. A temperature gradient across the substrate heater, in the growth system that has been used, leads to a variation in the wafer structure. One useful consequence of this thermal gradient is that it is possible to use a single wafer to study the effect on the transport properties as a function of the Ge fraction, dopant concentration, quantum well depth and width. The introduction of arsine as an n-type dopant into the growth chamber results in all subsequent material being n-type due to surface segregation. An ex-situ ion implantation technique has been developed which can be used to form a modulation-doped region in an inverted Si/SiGe 2DEG. Forming a doped region ex-situ circumvents the need to introduce a dopant species into the growth system, preventing surface segregation. Low temperature measurements of devices fabricated using this technique are presented, showing that it is possible to form inverted Si/SiGe 2DEGs and that ex-situ ion implantation can be used to form dopant supply regions. This technique is subsequently developed to show selective area ex-situ ion implantation technique. The successful production of samples shows that selective doping in exact areas of a wafer during growth is feasible and this has important consequences in device production.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral