Characterization of InAs Nanowires by Electron Transport Measurements – Niels Bohr Institute - University of Copenhagen

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Characterization of InAs Nanowires by Electron Transport Measurements

Master Thesis Defence by Peter Dahl Nissen 

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.