A Comprehensive Study of Dilute Magnetic ... - OPUS Würzburg

6 Summary
of DMS quantum well RTDs. Counterintuitively, a remanent zero field splitting is also
observed for two dimensional RTDs, as BMP states form at the interface of the (Zn,Mn)Se
quantum well.
To further test the assumption that imperfections at the quantum well interface significantly
alter the transport properties of an RTD, we investigate the tensile strain that
builds up in our II-VI heterostructures due to the lattice mismatch to the GaAs substrate.
We remove the GaAs substrate from an RTD which shows the aforementioned splitting
of the spin-up and down channels without an applied external magnetic field. The tensile
strain built up during growth relaxes as the substrate is removed and the interface is
flattened, weakening the BMP states. By comparing fits to the I-V characteristics of the
device before and after removing the substrate, the device properties are altered according
to what is expected within our model for a smoother interface. The sharpening of the
resonance, the reduction of the zero field splitting as well as an increase in tunnel barrier
quality are clearly observed.
Equipped with a complete picture of the transport physics of a II-VI DMS RTD, the
next logical step is to combine two such spin sensitive RTDs in a injector-detector scheme.
We investigate such a spin-valve device and discuss why no clear off-state is observed when
the two RTDs are set to transmit opposite spin species. While the middle contact between
the two RTDs is necessary to independently operate the building blocks of the spin-valve,
it also hinders spin-valve operation as it connects the region between the two RTDs to
an unpolarized reservoir. The model developed for the single DMS RTDs is extended
for the compound device and is again in full qualitative and quantitative agreement with
the experiment. We show that spin is mostly conserved as it is transferred from the
injector to the detector and extract the spin valve efficiencies. As each of the DMS RTDs
exhibit remanent zero field splitting, the possibility of using the spin-valve at B= 0 T is
subsequently assessed.
Measurements on RTDs showing remanent zero field splitting reflect simultaneous
transport through the ensemble of low dimensional interface states in the device. RTDs
employing self assembled CdSe quantum dots as resonant tunneling states however allow
for the observation of resonant transport through a single zero dimensional state. The
reduced dimensionality of the resonant tunneling state also gives rise to many particle
effects such as the Fermi-edge singularity (FES).
First we investigate an RTD containing self-assembled CdSe quantum dots without
magnetic doping. A singularity in the I-V characteristic arises due to a FES when the
quantum dot state is aligned with the emitter Fermi energy. Previous experimental studies
of FES have been limited to III-V devices, often at high magnetic fields. The feature we
report here is already clearly observed in the absence of external magnetic field for temperatures
up to 45 K, and shows magnetic field dependence reflective of two dimensional
character in the injector.
A second many particle effect is observed in these zero dimensional RTDs as we add
magnetic dopants to the tunnel barriers. Resonant tunneling transport through a single