124 research outputs found
Influence of cross-section geometry and wire orientation on the phonon shifts in ultra-scaled Si nanowires
Engineering of the cross-section shape and size of ultra-scaled Si nanowires
(SiNWs) provides an attractive way for tuning their structural properties. The
acoustic and optical phonon shifts of the free-standing circular, hexagonal,
square and triangular SiNWs are calculated using a Modified Valence Force Field
(MVFF) model. The acoustic phonon blue shift (acoustic hardening) and the
optical phonon red shift (optical softening) show a strong dependence on the
cross-section shape and size of the SiNWs. The triangular SiNWs have the least
structural symmetry as revealed by the splitting of the degenerate flexural
phonon modes and The show the minimum acoustic hardening and the maximum
optical hardening. The acoustic hardening, in all SiNWs, is attributed to the
decreasing difference in the vibrational energy distribution between the inner
and the surface atoms with decreasing cross-section size. The optical softening
is attributed to the reduced phonon group velocity and the localization of the
vibrational energy density on the inner atoms. While the acoustic phonon shift
shows a strong wire orientation dependence, the optical phonon softening is
independent of wire orientation.Comment: 10 figures, 4 Tables, submitted to JAP for revie
Full 3D Quantum Transport Simulation of Atomistic Interface Roughness in Silicon Nanowire FETs
The influence of interface roughness scattering (IRS) on the performances of
silicon nanowire field-effect transistors (NWFETs) is numerically investigated
using a full 3D quantum transport simulator based on the atomistic sp3d5s*
tight-binding model. The interface between the silicon and the silicon dioxide
layers is generated in a real-space atomistic representation using an
experimentally derived autocovariance function (ACVF). The oxide layer is
modeled in the virtual crystal approximation (VCA) using fictitious SiO2 atoms.
-oriented nanowires with different diameters and randomly generated
surface configurations are studied. The experimentally observed ON-current and
the threshold voltage is quantitatively captured by the simulation model. The
mobility reduction due to IRS is studied through a qualitative comparison of
the simulation results with the experimental results
Atomistic study of electronic structure of PbSe nanowires
Lead Selenide (PbSe) is an attractive `IV-VI' semiconductor material to
design optical sensors, lasers and thermoelectric devices. Improved fabrication
of PbSe nanowires (NWs) enables the utilization of low dimensional quantum
effects. The effect of cross-section size (W) and channel orientation on the
bandstructure of PbSe NWs is studied using an 18 band tight-binding
theory. The bandgap increases almost with the inverse of the W for all the
orientations indicating a weak symmetry dependence. [111] and [110] NWs show
higher ballistic conductance for the conduction and valence band compared to
[100] NWs due to the significant splitting of the projected L-valleys in [100]
NWs.Comment: 4 figures, Prepared for AP
Equatorial scintillations in relation to the development of ionization anomaly
International audienceThe irregularities in the electron density distribution of the ionosphere over the equatorial region frequently disrupt space-based communication and navigation links by causing severe amplitude and phase scintillations of signals. Development of a specification and forecast system for scintillations is needed in view of the increased reliance on space-based communication and navigation systems, which are vulnerable to ionospheric scintillations. It has been suggested in recent years that a developed equatorial anomaly in the afternoon hours, with a steep gradient of the F-region ionization or Total Electron Content (TEC) in the region between the crest and the trough, may be taken as a precursor to scintillations on transionospheric links. Latitudinal gradient of TEC measured using Faraday Rotation technique from LEO NOAA 12/14 transmissions during the afternoon hours at Calcutta shows a highly significant association with L-band scintillations recorded on the INMARSAT link, also from Calcutta, during the equinoxes, August through October 2000, and February through April 2001. The daytime equatorial electrojet is believed to control the development of the equatorial anomaly and plays a crucial role in the subsequent development of F-region irregularities in the post-sunset hours. The diurnal maximum and integrated value (integrated from the time of onset of plasma influx to off-equatorial latitudes till local sunset) of the strength of the electrojet in the Indian longitude sector shows a significant association with post-sunset L-band scintillations recorded at Calcutta during the two equinoxes mentioned earlier. Generation of equatorial irregularities over the magnetic equator in the post-sunset hours is intimately related to the variation of the height of the F-layer around sunset. Ionosonde data from Kodaikanal, a station situated close to the magnetic equator, has been utilized to calculate the vertical drift of the F-layer over the magnetic equator for the period August through October 2000. The post-sunset F-region height rise over the magnetic equator shows a remarkable correspondence with the occurrence of scintillations at Calcutta located near the northern crest of the equatorial anomaly. Existence of a flat-topped ionization distribution over the magnetic equator around sunset has been suggested as a possible indication of occurrence of post-sunset scintillations. Width of the latitudinal distribution of ionization obtained from DMSP satellite shows some correspondence with post-sunset L-band scintillations. During the period of observation of the present study (August through October 2000, and February through April 2001), it has been observed that although the probability of occurrence of scintillations is high on days with flat-topped ion density variation over the equator, there are cases when no scintillations were observed even when a pronounced flat top variation was recorded
Bandstructure Effects in Silicon Nanowire Hole Transport
Bandstructure effects in PMOS transport of strongly quantized silicon
nanowire field-effect-transistors (FET) in various transport orientations are
examined. A 20-band sp3d5s* spin-orbit-coupled (SO) atomistic tight-binding
model coupled to a self consistent Poisson solver is used for the valence band
dispersion calculation. A ballistic FET model is used to evaluate the
capacitance and current-voltage characteristics. The dispersion shapes and
curvatures are strong functions of device size, lattice orientation, and bias,
and cannot be described within the effective mass approximation. The anisotropy
of the confinement mass in the different quantization directions can cause the
charge to preferably accumulate in the (110) and secondly on the (112) rather
than (100) surfaces, leading to significant charge distributions for different
wire orientations. The total gate capacitance of the nanowire FET devices is,
however, very similar for all wires in all the transport orientations
investigated ([100], [110], [111]), and is degraded from the oxide capacitance
by ~30%. The [111] and secondly the [110] oriented nanowires indicate highest
carrier velocities and better ON-current performance compared to [100] wires.
The dispersion features and quantization behavior, although a complicated
function of physical and electrostatic confinement, can be explained at first
order by looking at the anisotropic shape of the heavy-hole valence band.Comment: 30 pages, 7 figures, to be published in IEEE Transactions on
Nanotechnolog
Atomistic Approach to Alloy Scattering in Si(1-x)Ge(x)
SiGe alloy scattering is of significant importance with the introduction of strained layers and SiGe channels into complementary metal-oxide semiconductor technology. However, alloy scattering has till now been treated in an empirical fashion with a fitting parameter. We present a theoretical model within the atomistic tight-binding representation for treating alloy scattering in SiGe. This approach puts the scattering model on a solid atomistic footing with physical insights. The approach is shown to inherently capture the bulk alloy scattering potential parameters for both n-type and p-typecarriers and matches experimental mobility data
Tuning Lattice Thermal Conductance in Ultra-Scaled Hollow SiNW: Role of Porosity Size, Density and Distribution
Porous crystalline Si nanowires (PC-SiNW) represent an attractive solution for enhancing the thermoelectric efficiency (ZT) of SiNWs by reducing the lattice thermal conductance (Îșl). A modified valence force field (MVFF) phonon model along with Landauerâs approach is used to analyze the ballistic Îșl in PC-SiNWs. A systematic study focusing on the influence of pore size, density, and distribution on the ballistic Îșl of PC-SiNWs is presented. The model predicts a maximum reduction of ~19%, ~23% and ~30% for 1, 2 and 3 pores, respectively with a constant removal of ~12% of the atoms in all the cases. The model also predicts a higher reduction of the ballistic Îșl as the pore separation increases, in the case of 2, 3 and 4 pores, for the same percentage of atoms removed (~12%) in all the cases. Thus, the presence of a high number of small, well-separated pores suppress Îșl strongly. This reduction in ballistic Îșl, in the coherent limit, is attributed to the reduction of the total number of phonon modes and smaller participation of phonon modes (in Îșl) with increasing number of pores
Bandstructure Effects in Silicon Nanowire Electron Transport
Bandstructure effects in the electronic transport of strongly quantized
silicon nanowire field-effect-transistors (FET) in various transport
orientations are examined. A 10-band sp3d5s* semi-empirical atomistic
tight-binding model coupled to a self consistent Poisson solver is used for the
dispersion calculation. A semi-classical, ballistic FET model is used to
evaluate the current-voltage characteristics. It is found that the total gate
capacitance is degraded from the oxide capacitance value by 30% for wires in
all the considered transport orientations ([100], [110], [111]). Different wire
directions primarily influence the carrier velocities, which mainly determine
the relative performance differences, while the total charge difference is
weakly affected. The velocities depend on the effective mass and degeneracy of
the dispersions. The [110] and secondly the [100] oriented 3nm thick nanowires
examined, indicate the best ON-current performance compared to [111] wires. The
dispersion features are strong functions of quantization. Effects such as
valley splitting can lift the degeneracies especially for wires with cross
section sides below 3nm. The effective masses also change significantly with
quantization, and change differently for different transport orientations. For
the cases of [100] and [111] wires the masses increase with quantization,
however, in the [110] case, the mass decreases. The mass variations can be
explained from the non-parabolicities and anisotropies that reside in the first
Brillouin zone of silicon.Comment: 35 pages, 7 figures, submitted to IEEE TE
Intrinsic Reliability improvement in Biaxially Strained SiGe p-MOSFETs
In this letter we not only show improvement in the performance but also in
the reliability of 30nm thick biaxially strained SiGe (20%Ge) channel on Si
p-MOSFETs. Compared to Si channel, strained SiGe channel allows larger hole
mobility ({\mu}h) in the transport direction and alleviates charge flow towards
the gate oxide. {\mu}h enhancement by 40% in SiGe and 100% in Si-cap SiGe is
observed compared to the Si hole universal mobility. A ~40% reduction in NBTI
degradation, gate leakage and flicker noise (1/f) is observed which is
attributed to a 4% increase in the hole-oxide barrier height ({\phi}) in SiGe.
Similar field acceleration factor ({\Gamma}) for threshold voltage shift
({\Delta}VT) and increase in noise ({\Delta}SVG) in Si and SiGe suggests
identical degradation mechanisms.Comment: 4 figures, 3 pages, accepted for publication in IEEE ED
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