Biomedical Engineering Reference
In-Depth Information
micrograph ending in finger-like features represent metallic gates, which are
utilized to drive away electrons residing 90 nm below the top surface, at the
GaAs/Al
x
Ga
1
x
As heterojunction, with the application of negative gate
voltages. The 'pincher' gates, V1 and V5 in Fig. 2.8(a), and V1 and V3 in
Fig. 2.8(b), control the coupling to the leads, setting the scale for the level
broadening,
, due to coupling to the leads, while the pincher gates V3 (V5
for the parallel case) control the inter-dot tunneling, t. Plunger gates V2 and
V4 control separately the number of electrons on each dot.
The real difficulty in the operation of such complex devices arises from the
close proximity of all the gates. Each quantum dot has a lithographic size of
~
G
180 nm. The close proximity gives rise to sizable capacitive coupling
between all gates, as well as to the electron gas puddles, which make up the
quantum dots. Changing one gate voltage will inevitably affect the charge
on all other gates and in the puddles at the same time. To be able to tune the
device parameters over a significant range (e.g. the inter-dot tunneling
matrix t) without affecting other characteristics (e.g. electron number), it is
necessary to experimentally 'diagonalize' the capacitance matrix. Practically
speaking, this means that in order to tune t, in addition to varying the main
gate voltage at V5, it is necessary to map out other gates, some of which
need to be adjusted in an amount proportional to the change in V3 (V5),
which in effect means that it is necessary to tune a linear combination of
different gates, with V3 (V5) being the dominant one.
2.6.4 Spin-dependent single-electron tunneling effects in
epitaxial Fe nanoparticles
Fe/MgO/Fe nanoparticle /MgO/Co double-tunnel junctions have been
prepared by molecular beam epitaxy for current perpendicular-to-plane
transport measurements on submicrometer-sized pillars. Microstructural
observations indicate that the samples exhibit a fully epitaxial layered
structure with sharp and flat interfaces including well-defined separated Fe
nanoparticles between the barriers. The introduction of asymmetric MgO
tunnel barriers (i.e. with different thicknesses), in the double junction leads
to a clear observation of a Coulomb staircase and associated tunnel
magnetoresistance oscillations. Estimation of the capacitance of the system
indicates that these transport phenomena are due to charging effects of the
magnetic particles.
2.6.5 Coulomb promotion of spin-dependent tunneling
An important phenomenon is the transport of spin-polarized electrons
through a magnetic single-electron transistor (SET) in the presence of an
