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Characterization of InAs Nanowires by Electron Transport Measurements
Master thesis defence by Peter Dahl Nissen, Nanoscale Quantum Electronics group, Nano-Science Center.
Abstract
With the idea of the spin of the electron as the carrier of information
in spintronics and quantum computing, it has become important to be
able to manipulate the spin of the electron in a coherent way. As
important is it that the spin does not spontaneously relax, if, for
instance, the spin should be used to store information.
In the search for suitable structures for such spin based electronics,
inherently one-dimensional systems such as semiconductor nanowires and
carbon nanotubes are being investigated as an bottom-up alternative to
the traditional top-down heterostructure systems. Predictions, that
spin relaxation mechanisms responsible for spin relaxation in bulk
systems should be suppressed in one- and zero-dimensional systems, make
the investigation of spin related phenomena in such low-dimensional
structures particulary interesting; understanding the dependence of
spin relaxation on various factors, e.g. size of a system, is a
prerequisite for designing systems with desired properties. Here we
report on measurements on InAs nanowires both as open systems and as
quantum dots, depending on the voltage applied to a back gate. In the
open regime we measured the magnetoconductance, showing both weak
localization and weak antilocalization effects. By fitting theoretical
expressions for one-dimensional systems to the measured data, values
for the spin relaxation length and the phase coherence length were
found. The extracted values showed a dependence on gate voltage, and
were shorter than previously reported values gained from
magnetoconductance measurements. For the nanowires as quantum dots, we
measured the shift in energy of the charge states of the dot in a
magnetic field. The g-factors determined from these measurements showed
large scale fluctuations between subsequent charge states and the
maximum values were lager than the bulk value. Behaviour like this was
recently reported for a similar InAs nanowire quantum dot device. We
observed the mentioned g-factor fluctuations for both a multi-electron
and a few-electron nanowire quantum dot. We believe, this is the first
observation of such behaviour in a few-electron semiconductor quantum
dot. For the multi-electron dot, the measured distribution of
g-factors could not be described by a theoretical expression found to
describe similar behaviour measured by others. The cause of this
discrepancy is not clear to us.
Supervisor: Jesper Nygård, Nanoscale Quantum Electronics