2,560 research outputs found
The impacts of surface conditions on the vapor-liquid-solid growth of germanium nanowires on Si (100) substrate
The impacts of surface conditions on the growth of Ge nanowires on a Si (100) substrate are discussed in detail. On SiO2-terminated Si substrates, high-density Ge nanowires can be easily grown. However, on H-terminated Si substrates, growing Ge nanowires is more complex. The silicon migration and the formation of a native SiO2 overlayer on a catalyst surface retard the growth of Ge nanowires. After removing this overlayer in the HF solution, high-density and well-ordered Ge nanowires are grown. Ge nanowires cross vertically and form two sets of parallel nanowires. It is found that nanowires grew along ?110? direction
Nonpolar resistive switching in Cu/SiC/Au non-volatile resistive memory devices
Amorphous silicon carbide (a-SiC) based resistive memory (RM) Cu/a-SiC/Au devices were fabricated and their resistive switching characteristics investigated. All four possible modes of nonpolar resistive switching were achieved with ON/OFF ratio in the range 10 6-10 8. Detailed current-voltage I-V characteristics analysis suggests that the conduction mechanism in low resistance state is due to the formation of metallic filaments. Schottky emission is proven to be the dominant conduction mechanism in high resistance state which results from the Schottky contacts between the metal electrodes and SiC. ON/OFF ratios exceeding 10 7 over 10 years were also predicted from state retention characterizations. These results suggest promising application potentials for Cu/a-SiC/Au RM
Unification of bulk and interface electroresistive switching in oxide systems
We demonstrate that the physical mechanism behind electroresistive switching
in oxide Schottky systems is electroformation, as in insulating oxides.
Negative resistance shown by the hysteretic current-voltage curves proves that
impact ionization is at the origin of the switching. Analyses of the
capacitance-voltage and conductance-voltage curves through a simple model show
that an atomic rearrangement is involved in the process. Switching in these
systems is a bulk effect, not strictly confined at the interface but at the
charge space region.Comment: 4 pages, 3 figures, accepted in PR
Ga-induced atom wire formation and passivation of stepped Si(112)
We present an in-depth analysis of the atomic and electronic structure of the
quasi one-dimensional (1D) surface reconstruction of Ga on Si(112) based on
Scanning Tunneling Microscopy and Spectroscopy (STM and STS), Rutherford
Backscattering Spectrometry (RBS) and Density Functional Theory (DFT)
calculations. A new structural model of the Si(112)6 x 1-Ga surface is
inferred. It consists of Ga zig-zag chains that are intersected by
quasi-periodic vacancy lines or misfit dislocations. The experimentally
observed meandering of the vacancy lines is caused by the co-existence of
competing 6 x 1 and 5 x 1 unit cells and by the orientational disorder of
symmetry breaking Si-Ga dimers inside the vacancy lines. The Ga atoms are fully
coordinated, and the surface is chemically passivated. STS data reveal a
semiconducting surface and show excellent agreement with calculated Local
Density of States (LDOS) and STS curves. The energy gain obtained by fully
passivating the surface calls the idea of step-edge decoration as a viable
growth method toward 1D metallic structures into question.Comment: Submitted, 13 pages, accepted in Phys. Rev. B, notational change in
Fig.
Polarity control of carrier injection at ferroelectric/metal interfaces for electrically switchable diode and photovoltaic effects
We investigated a switchable ferroelectric diode effect and its physical
mechanism in Pt/BiFeO3/SrRuO3 thin-film capacitors. Our results of electrical
measurements support that, near the Pt/BiFeO3 interface of as-grown samples, a
defective layer (possibly, an oxygen-vacancy-rich layer) becomes formed and
disturbs carrier injection. We therefore used an electrical training process to
obtain ferroelectric control of the diode polarity where, by changing the
polarization direction using an external bias, we could switch the transport
characteristics between forward and reverse diodes. Our system is characterized
with a rectangular polarization hysteresis loop, with which we confirmed that
the diode polarity switching occurred at the ferroelectric coercive voltage.
Moreover, we observed a simultaneous switching of the diode polarity and the
associated photovoltaic response dependent on the ferroelectric domain
configurations. Our detailed study suggests that the polarization charge can
affect the Schottky barrier at the ferroelectric/metal interfaces, resulting in
a modulation of the interfacial carrier injection. The amount of
polarization-modulated carrier injection can affect the transition voltage
value at which a space-charge-limited bulk current-voltage (J-V) behavior is
changed from Ohmic (i.e., J ~ V) to nonlinear (i.e., J ~ V^n with n \geq 2).
This combination of bulk conduction and polarization-modulated carrier
injection explains the detailed physical mechanism underlying the switchable
diode effect in ferroelectric capacitors.Comment: Accepted for publication in Phys. Rev.
Self-consistent model of unipolar transport in organic semiconductor diodes: accounting for a realistic density-of-states distribution
A self-consistent, mean-field model of charge-carrier injection and unipolar
transport in an organic semiconductor diode is developed utilizing the
effective transport energy concept and taking into account a realistic
density-of-states distribution as well as the presence of trap states in an
organic material. The consequences resulting from the model are discussed
exemplarily on the basis of an indium tin oxide/organic semiconductor/metallic
conductor structure. A comparison of the theory to experimental data of a
unipolar indium tin oxide/poly-3-hexyl-thiophene/Al device is presented.Comment: 6 pages, 2 figures; to be published in Journal of Applied Physic
Self consistent theory of unipolar charge-carrier injection in metal/insulator/metal systems
A consistent device model to describe current-voltage characteristics of
metal/insulator/metal systems is developed. In this model the insulator and the
metal electrodes are described within the same theoretical framework by using
density of states distributions. This approach leads to differential equations
for the electric field which have to be solved in a self consistent manner by
considering the continuity of the electric displacement and the electrochemical
potential in the complete system. The model is capable of describing the
current-voltage characteristics of the metal/insulator/metal system in forward
and reverse bias for arbitrary values of the metal/ insulator injection
barriers. In the case of high injection barriers, approximations are provided
offering a tool for comparison with experiments. Numerical calculations are
performed exemplary using a simplified model of an organic semiconductor.Comment: 21 pages, 8 figure
Surface dissipation in nanoelectromechanical systems: Unified description with the standard tunneling model and effects of metallic electrodes
By modifying and extending recent ideas [C. Seoanez et al., Europhys. Lett.
78, 60002 (2007)], a theoretical framework to describe dissipation processes in
the surfaces of vibrating micro- and nanoelectromechanical devices, thought to
be the main source of friction at low temperatures, is presented. Quality
factors as well as frequency shifts of flexural and torsional modes in doubly
clamped beams and cantilevers are given, showing the scaling with dimensions,
temperature, and other relevant parameters of these systems. Full agreement
with experimental observations is not obtained, leading to a discussion of
limitations and possible modifications of the scheme to reach a quantitative
fitting to experiments. For nanoelectromechanical systems covered with metallic
electrodes, the friction due to electrostatic interaction between the flowing
electrons and static charges in the device and substrate is also studied.Comment: 17 pages, 7 figure
Space-charge mechanism of aging in ferroelectrics: an exactly solvable two-dimensional model
A mechanism of point defect migration triggered by local depolarization
fields is shown to explain some still inexplicable features of aging in
acceptor doped ferroelectrics. A drift-diffusion model of the coupled charged
defect transport and electrostatic field relaxation within a two-dimensional
domain configuration is treated numerically and analytically. Numerical results
are given for the emerging internal bias field of about 1 kV/mm which levels
off at dopant concentrations well below 1 mol%; the fact, long ago known
experimentally but still not explained. For higher defect concentrations a
closed solution of the model equations in the drift approximation as well as an
explicit formula for the internal bias field is derived revealing the plausible
time, temperature and concentration dependencies of aging. The results are
compared to those due to the mechanism of orientational reordering of defect
dipoles.Comment: 8 pages, 4 figures. accepted to Physical Review
Gate Coupling to Nanoscale Electronics
The realization of single-molecule electronic devices, in which a
nanometer-scale molecule is connected to macroscopic leads, requires the
reproducible production of highly ordered nanoscale gaps in which a molecule of
interest is electrostatically coupled to nearby gate electrodes. Understanding
how the molecule-gate coupling depends on key parameters is crucial for the
development of high-performance devices. Here we directly address this,
presenting two- and three-dimensional finite-element electrostatic simulations
of the electrode geometries formed using emerging fabrication techniques. We
quantify the gate coupling intrinsic to these devices, exploring the roles of
parameters believed to be relevant to such devices. These include the thickness
and nature of the dielectric used, and the gate screening due to different
device geometries. On the single-molecule (~1nm) scale, we find that device
geometry plays a greater role in the gate coupling than the dielectric constant
or the thickness of the insulator. Compared to the typical uniform nanogap
electrode geometry envisioned, we find that non-uniform tapered electrodes
yield a significant three orders of magnitude improvement in gate coupling. We
also find that in the tapered geometry the polarizability of a molecular
channel works to enhance the gate coupling
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