Injection and detection of spin polarised current in a metal/semiconductor device
and the measurement of the degree of injected spin polarisation are two key issues
in the development of hybrid spintronics. This thesis touches on both themes as
it details the development of planar Andreev spectroscopy as a tool to measure
injected spin and the electrical characterisation of MgO tunnel barriers for efficient
spin injection and detection.
Point contact Andreev reflection spectroscopy has been widely used tomeasure
transport spin polarisation in magnetic materials. Planar Andreev structures have
an advantage over point contacts as they offer greater control over interface quality
and the possibility of spatially resolved information about the spin polarisation
using nanojunction arrays. We find that planar junctions compare favourably to
point contacts in that they can yield low interface barriers and minimal nonthermal
smearing. We show that a low interface barrier is critical for accurate detection
of spin polarisation, particularly in semiconductorswhere large Fermi velocitymismatch
contributes to the barrier. Furthermore, the fabrication method strongly affects
all parameter values. For Pb/InAs planar junctions we demonstrate that the
most feasible way to obtain interfaces suitable for spin detection is an “etch-back”
processing strategy. The processing routes are shown to be scalable to nanoarray
fabrication to allow measurement of spin accumulation.
We also examine the electrical properties of ultrathin MgO barriers grown on
InAs epilayers and the dependence of barrier characteristics on InAs surface pretreatment
and growth conditions. Chemical pretreatment improves the yield of
tunnel junctions and changes the roughness of the interface between the oxide and
the semiconductor. Electrical characterisation confirms that tunnel barriers with
appropriate values of interface resistance for efficient spin injection/detection have
been achieved. Using the Rowell criteria and various tunnelling models we show
that single step tunnelling occurs above 150 K and a thermal smearing model suggests
that tunnelling is the dominant transport process down to 10 K.