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Title: Fractional quantum phenomena of 2DHGs within strained germanium quantum well heterostructures
Author: Newell, Oliver
ISNI:       0000 0004 8497 7272
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2018
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Strained Ge modulation doped quantum well (MODQW) heterostructures facilitate a high mobility channel layer. Spatial separation of mobile carriers from the ionised dopants that supply them is key to increasing mobilities many orders of magnitude above bulk values. Materials characterisation techniques are employed to asses and improve the buffer layers within the MODQW heterostructures. The efficacy of dislocation filter layers (DFLs) is investigated, along with annealing trials and suspended structures. A 4% enhancement in SiGe relaxation is reported utilising a Ge DFL, while optimum SiGe relaxation is obtained through balancing the opposing thermal and lattice mismatch within the Gebuff layer. Reverse linearly graded buffers demonstrate a grading rate of 350% μm-1, while achieving a 4 nm RMS roughness. This represents an order of magnitude improvement on the quoted limits of forward linear grading rates. Suspended microwires are presented, providing isolation of channel layers from buffer layers. Strain mapping, using Micro-XRD at the Diamond Light Source, is employed to record strain development during microwire fabrication, resulting in a 0.9% increase in out of plane tensile strain upon suspension. The 150 × 15 μm microwires represent the first demonstration of suspending a Ge QW heterostructure. Low temperature (300 mK), high magnetic field (37.5 T) Hall and resistivity measurements return the effective mass and mobility of composite fermions (CFs) within a 2DHG, along with well-developed FQHE oscillations. CF mobility is shown to contain greater thermal sensitivity compared to its bare electron counterpart. Fractional filling ν = 4/11 is of great interest for its unknown origins and non-Abelian statistics, a potential observation is presented using a differentiated Hall signal. Inverted doping geometry returns the highest Ge MODQW room temperature mobility of 4,900 cm2V-1s-1, assisted by a new contact process comprising Ar milling and Al/Ti/Au deposition, ideal for inverted structures.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics ; QD Chemistry ; TK Electrical engineering. Electronics Nuclear engineering