161 research outputs found
Electron Mobility and Magneto Transport Study of Ultra-Thin Channel Double-Gate Si MOSFETs
We report on detailed room temperature and low temperature transport
properties of double-gate Si MOSFETs with the Si well thickness in the range
7-17 nm. The devices were fabricated on silicon-on-insulator wafers utilizing
wafer bonding, which enabled us to use heavily doped metallic back gate. We
observe mobility enhancement effects at symmetric gate bias at room
temperature, which is the finger print of the volume inversion/accumulation
effect. An asymmetry in the mobility is detected at 300 K and at 1.6 K between
the top and back interfaces of the Si well, which is interpreted to arise from
different surface roughnesses of the interfaces. Low temperature peak
mobilities of the reported devices scale monotonically with Si well thickness
and the maximum low temperature mobility was 1.9 m2/Vs, which was measured from
a 16.5 nm thick device. In the magneto transport data we observe single and two
sub-band Landau level filling factor behavior depending on the well thickness
and gate biasing
Intervalley-Scattering Induced Electron-Phonon Energy Relaxation in Many-Valley Semiconductors at Low Temperatures
We report on the effect of elastic intervalley scattering on the energy
transport between electrons and phonons in many-valley semiconductors. We
derive a general expression for the electron-phonon energy flow rate at the
limit where elastic intervalley scattering dominates over diffusion. Electron
heating experiments on heavily doped n-type Si samples with electron
concentration in the range m are performed at
sub-1 K temperatures. We find a good agreement between the theory and the
experiment.Comment: v2: Notations changed: --> ,
removed. Eq. (1) changed, Eq. (2) added and complete derivation of Eq. (3)
included. Some further discussion about single vs. many valley added [3rd
paragraph after Eq. (7)]. End notes removed and new reference added [Kragler
and Thomas]. Typos in references correcte
Double Gate Bias Dependency of Low Temperature Conductivity of SiO2-Si-SiO2 Quantum Wells
The gate bias dependency of conductivity is examined in two Si quantum wells
with well thickness tw = 7 nm and tw = 14 nm. The conductivity of the thinner
device behaves smoothly whereas the thicker device shows strong non-monotonic
features as a function of gate voltages. We show that a strong minimum in
conductivity occurs close to the threshold of second sub-band. Another minimum
is seen at high electron density at symmetric well potential. This feature is
addressed to sub-band wave function delocalization in the quantization
direction.Comment: To appear in the Proceedings of ICPS28th (Vienna, Austria, July
24-28, 2006
Superconducting MoSi nanowires
We have fabricated disordered superconducting nanowires of molybdenium
silicide. A molybdenium nanowire is first deposited on top of silicon, and the
alloy is formed by rapid thermal annealing. The method allows tuning of the
crystal growth to optimise, e.g., the resistivity of the alloy for potential
applications in quantum phase slip devices and superconducting nanowire
single-photon detectors. The wires have effective diameters from 42 to 79 nm,
enabling the observation of crossover from conventional superconductivity to
regimes affected by thermal and quantum fluctuations. In the smallest diameter
wire and at temperatures well below the superconducting critical temperature,
we observe residual resistance and negative magnetoresistance, which can be
considered as fingerprints of quantum phase slips
Ex-situ Tunnel Junction Process Technique Characterized by Coulomb Blockade Thermometry
We investigate a wafer scale tunnel junction fabrication method, where a
plasma etched via through a dielectric layer covering bottom Al electrode
defines the tunnel junction area. The ex-situ tunnel barrier is formed by
oxidation of the bottom electrode in the junction area. Room temperature
resistance mapping over a 150 mm wafer give local deviation values of the
tunnel junction resistance that fall below 7.5 % with an average of 1.3 %. The
deviation is further investigated by sub-1 K measurements of a device, which
has one tunnel junction connected to four arrays consisting of N junctions (N =
41, junction diameter 700 nm). The differential conductance is measured in
single-junction and array Coulomb blockade thermometer operation modes. By
fitting the experimental data to the theoretical models we found an upper limit
for the local tunnel junction resistance deviation of ~5 % for the array of
2N+1 junctions. This value is of the same order as the minimum detectable
deviation defined by the accuracy of our experimental setup
Electron-Acoustic Phonon Energy Loss Rate in Multi-Component Electron Systems with Symmetric and Asymmetric Coupling Constants
We consider electron-phonon (\textit{e-ph}) energy loss rate in 3D and 2D
multi-component electron systems in semiconductors. We allow general asymmetry
in the \textit{e-ph} coupling constants (matrix elements), i.e., we allow that
the coupling depends on the electron sub-system index. We derive a
multi-component \textit{e-ph}power loss formula, which takes into account the
asymmetric coupling and links the total \textit{e-ph} energy loss rate to the
density response matrix of the total electron system. We write the density
response matrix within mean field approximation, which leads to coexistence of\
symmetric energy loss rate and asymmetric energy loss rate
with total energy loss rate at temperature
. The symmetric component F_{S}(T) F_{S}(T)\propto T^{n_{S}}n_{S}F_{A}(T). Screening strongly
reduces the symmetric coupling, but the asymmetric coupling is unscreened,
provided that the inter-sub-system Coulomb interactions are strong. The lack of
screening enhances and the total energy loss rate .
Especially, in the strong screening limit we find . A
canonical example of strongly asymmetric \textit{e-ph} matrix elements is the
deformation potential coupling in many-valley semiconductors.Comment: v2: Typos corrected. Some notations changed. Section III.C is
embedded in Section III.B. Paper accepted to PR
Diffusion-emission theory of photon enhanced thermionic emission solar energy harvesters
Numerical and semi-analytical models are presented for
photon-enhanced-thermionic-emission (PETE) devices. The models take diffusion
of electrons, inhomogeneous photogeneration, and bulk and surface recombination
into account. The efficiencies of PETE devices with silicon cathodes are
calculated. Our model predicts significantly different electron affinity and
temperature dependence for the device than the earlier model based on a
rate-equation description of the cathode. We show that surface recombination
can reduce the efficiency below 10% at the cathode temperature of 800 K and the
concentration of 1000 suns, but operating the device at high injection levels
can increase the efficiency to 15%.Comment: 5 pages, 4 figure
Lifetimes of Confined Acoustic Phonons in Ultra-Thin Silicon Membranes
We study the relaxation of coherent acoustic phonon modes with frequencies up
to 500 GHz in ultra-thin free-standing silicon membranes. Using an ultrafast
pump-probe technique of asynchronous optical sampling, we observe that the
decay time of the first-order dilatational mode decreases significantly from
\sim 4.7 ns to 5 ps with decreasing membrane thickness from \sim 194 to 8 nm.
The experimental results are compared with theories considering both intrinsic
phonon-phonon interactions and extrinsic surface roughness scattering including
a wavelength-dependent specularity. Our results provide insight to understand
some of the limits of nanomechanical resonators and thermal transport in
nanostructures
Primary thermometry in the intermediate Coulomb blockade regime
We investigate Coulomb blockade thermometers (CBT) in an intermediate
temperature regime, where measurements with enhanced accuracy are possible due
to the increased magnitude of the differential conductance dip. Previous
theoretical results show that corrections to the half width and to the depth of
the measured conductance dip of a sensor are needed, when leaving the regime of
weak Coulomb blockade towards lower temperatures. In the present work, we
demonstrate experimentally that the temperature range of a CBT sensor can be
extended by employing these corrections without compromising the primary nature
or the accuracy of the thermometer.Comment: 8 pages, 4 figure
Electrons and holes in Si quantum well: a room-temperature transport and drag resistance study
We investigate carrier transport in a single 22 nm-thick double-gated Si
quantum well device, which has independent contacts to electrons and holes.
Conductance, Hall density and Hall mobility are mapped in a broad double-gate
voltage window. When the gate voltage asymmetry is not too large only either
electrons or holes occupy the Si well and the Hall mobility shows the
fingerprints of volume inversion/accumulation. At strongly asymmetric
double-gate voltage an electric field induced electron-hole (EH) bi-layer is
formed inside the well. The EH drag resistance R_{he} is explored at balanced
carrier densities: R_{he} decreases monotonically from 860 to 37 Ohms when the
electron and hole density is varied between ~0.4-1.7x10^{16} m^{-2}
- …