50 research outputs found
Optical Phonon Limited High Field Transport in Layered Materials
An optical phonon limited velocity model has been employed to investigate
high-field transport in a selection of layered 2D materials for both, low-power
logic switches with scaled supply voltages, and high-power, high-frequency
transistors. Drain currents, effective electron velocities and intrinsic
cut-off frequencies as a function of carrier density have been predicted thus
providing a benchmark for the optical phonon limited high-field performance
limits of these materials. The optical phonon limited carrier velocities of a
selection of transition metal dichalcogenides and black phosphorus are found to
be modest as compared to their n-channel silicon counterparts, questioning the
utility of these devices in the source-injection dominated regime. h-BN, at the
other end of the spectrum, is shown to be a very promising material for
high-frequency high-power devices, subject to experimental realization of high
carrier densities, primarily due to its large optical phonon energy.
Experimentally extracted saturation velocities from few-layer MoS2 devices show
reasonable qualitative and quantitative agreement with predicted values.
Temperature dependence of measured vsat is discussed and found to fit a
velocity saturation model with a single material dependent fit parameter.Comment: 8 pages, 6 figure
Electrical Properties of Atomic Layer Deposited Aluminum Oxide on Gallium Nitride
We report on our investigation of the electrical properties of
metal/Al2O3/GaN metal-insulator-semiconductor (MIS) capacitors. We determined
the conduction band offset and interface charge density of the alumina/GaN
interface by analyzing capacitance-voltage characteristics of atomic layer
deposited Al2O3 films on GaN substrates. The conduction band offset at the
Al2O3/GaN interface was calculated to be 2.13 eV, in agreement with theoretical
predications. A non-zero field of 0.93 MV/cm in the oxide under flat-band
conditions in the GaN was inferred, which we attribute to a fixed net positive
charge density of magnitude 4.60x1012 cm-2 at the Al2O3/GaN interface. We
provide hypotheses to explain the origin of this charge by analyzing the energy
band line-up.Comment: 8 pages, 4 figures, Applied Physics Letter
Surface States Engineering of Metal/MoS2 Contacts Using Sulfur Treatment for Reduced Contact Resistance and Variability
Variability and lack of control in the nature of contacts between metal/MoS2
interface is a major bottleneck in the realisation of high-performance devices
based on layered materials for several applications. In this letter, we report
on the reduction in Schottky barrier height at metal/MoS2 interface by
engineering the surface states through sulphur treatment. Electrical
characteristics for back-gated MoS2 field effect transistor structures were
investigated for two high work-function metal contacts Ni and Pd. Contacts on
MoS2 treated with sulphur exhibited significant improvements in Ohmic nature
with concomitant reduction in variability compared to those on untreated MoS2
films leading to a 2x increase in extracted mobility. X-ray Photoelectron
Spectroscopy (XPS) measurements, Raman Spectroscopy and comparison of threshold
voltages indicated absence of additional doping or structural changes due to
sulphur treatment. The Schottky barrier heights were extracted from
temperature-dependent transfer characteristics based on the thermionic current
model. A reduction in barrier height of 80 and 135 meV extracted for Ni/MoS2
and Pd/MoS2 contacts respectively is hence attributed to the increase in
surface states (or stronger Fermi level pinning) due to sulphur treatment. The
corresponding charge neutrality levels at metal/MoS2 interface, were extracted
to be 0.16 eV (0.17 eV) below the conduction band before (after) Sulphur
treatment. This first report of surface states engineering in MoS2 leading to
superior contacts is expected to significantly benefit the entire class of
devices based on layered 2D materials.Comment: 13 pages, 5 figure