8,130 research outputs found
Carrier Transport in High Mobility InAs Nanowire Junctionless Transistors
Ability to understand and model the performance limits of nanowire
transistors is the key to design of next generation devices. Here, we report
studies on high-mobility junction-less gate-all-around nanowire field effect
transistor with carrier mobility reaching 2000 cm2/V.s at room temperature.
Temperature-dependent transport measurements reveal activated transport at low
temperatures due to surface donors, while at room temperature the transport
shows a diffusive behavior. From the conductivity data, the extracted value of
sound velocity in InAs nanowires is found to be an order less than the bulk.
This low sound velocity is attributed to the extended crystal defects that
ubiquitously appear in these nanowires. Analyzing the temperature-dependent
mobility data, we identify the key scattering mechanisms limiting the carrier
transport in these nanowires. Finally, using these scattering models, we
perform drift-diffusion based transport simulations of a nanowire field-effect
transistor and compare the device performances with experimental measurements.
Our device modeling provides insight into performance limits of InAs nanowire
transistors and can be used as a predictive methodology for nanowire-based
integrated circuits.Comment: 22 pages, 5 Figures, Nano Letter
Composition Modulation of Ag_2Te Nanowires for Tunable Electrical and Thermal Properties
In this article, we demonstrated that composition modulation of Ag_2Te nanowires can be achieved during the self-templated transformation of Te nanowires into Ag_2Te nanowires during solution phase synthesis, which provides a mean to tune the carrier density of the Ag_2Te nanowires. Both nearly stoichiometric and Ag-rich nanowires have been synthesized, which give rise to p-type and n-type Ag_2Te nanocomposites after hot press, respectively. The electrical and thermal properties of the two kinds of samples have been measured. Theoretical modeling based on the near-equilibrium Boltzmann transport equations has been used to understand the experimental results. We found that ZT of the heavily doped n-type sample reaches 0.55 at 400 K, which is the highest ZT value reported for Ag_2Te at the same temperature mainly due to the reduced thermal conductivity by the nanostructures. Theoretical analysis on the carrier transport shows that the power factor is also very well optimized in the doped Ag_2Te sample considering the reduced carrier mobility by the nanostructures
Determining the electronic performance limitations in top-down fabricated Si nanowires with mean widths down to 4 nm
Silicon nanowires have been patterned with mean widths down to 4 nm using top-down lithography and dry etching. Performance-limiting scattering processes have been measured directly which provide new insight into the electronic conduction mechanisms within the nanowires. Results demonstrate a transition from 3-dimensional (3D) to 2D and then 1D as the nanowire mean widths are reduced from 12 to 4 nm. The importance of high quality surface passivation is demonstrated by a lack of significant donor deactivation, resulting in neutral impurity scattering ultimately limiting the electronic performance. The results indicate the important parameters requiring optimization when fabricating nanowires with atomic dimensions
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Giant Light-Emission Enhancement in Lead Halide Perovskites by Surface Oxygen Passivation.
Surface condition plays an important role in the optical performance of semiconductor materials. As new types of semiconductors, the emerging metal-halide perovskites are promising for next-generation optoelectronic devices. We discover significantly improved light-emission efficiencies in lead halide perovskites due to surface oxygen passivation. The enhancement manifests close to 3 orders of magnitude as the perovskite dimensions decrease to the nanoscale, improving external quantum efficiencies from <0.02% to over 12%. Along with about a 4-fold increase in spontaneous carrier recombination lifetimes, we show that oxygen exposure enhances light emission by reducing the nonradiative recombination channel. Supported by X-ray surface characterization and theoretical modeling, we propose that excess lead atoms on the perovskite surface create deep-level trap states that can be passivated by oxygen adsorption
Design space for low sensitivity to size variations in [110] PMOS nanowire devices: The implications of anisotropy in the quantization mass
A 20-band sp3d5s* spin-orbit-coupled, semi-empirical, atomistic tight-binding
model is used with a semi-classical, ballistic, field-effect-transistor (FET)
model, to examine the ON-current variations to size variations of [110]
oriented PMOS nanowire devices. Infinitely long, uniform, rectangular nanowires
of side dimensions from 3nm to 12nm are examined and significantly different
behavior in width vs. height variations are identified and explained. Design
regions are identified, which show minor ON-current variations to significant
width variations that might occur due to lack of line width control. Regions
which show large ON-current variations to small height variations are also
identified. The considerations of the full band model here show that ON-current
doubling can be observed in the ON-state at the onset of volume inversion to
surface inversion transport caused by structural side size variations. Strain
engineering can smooth out or tune such sensitivities to size variations. The
cause of variations described is the structural quantization behavior of the
nanowires, which provide an additional variation mechanism to any other
ON-current variations such as surface roughness, phonon scattering etc.Comment: 24 pages, 5 figure
Non-equilibrium Green's function predictions of band tails and band gap narrowing in III-V semiconductors and nanodevices
High-doping induced Urbach tails and band gap narrowing play a significant
role in determining the performance of tunneling devices and optoelectronic
devices such as tunnel field-effect transistors (TFETs), Esaki diodes and
light-emitting diodes. In this work, Urbach tails and band gap narrowing values
are calculated explicitly for GaAs, InAs, GaSb and GaN as well as ultra-thin
bodies and nanowires of the same. Electrons are solved in the non-equilibrium
Green's function method in multi-band atomistic tight binding. Scattering on
polar optical phonons and charged impurities is solved in the self-consistent
Born approximation. The corresponding nonlocal scattering self-energies as well
as their numerically efficient formulations are introduced for ultra-thin
bodies and nanowires. Predicted Urbach band tails and conduction band gap
narrowing agree well with experimental literature for a range of temperatures
and doping concentrations. Polynomial fits of the Urbach tail and band gap
narrowing as a function of doping are tabulated for quick reference
An efficient algorithm to calculate intrinsic thermoelectric parameters based on Landauer approach
The Landauer approach provides a conceptually simple way to calculate the
intrinsic thermoelectric (TE) parameters of materials from the ballistic to the
diffusive transport regime. This method relies on the calculation of the number
of propagating modes and the scattering rate for each mode. The modes are
calculated from the energy dispersion (E(k)) of the materials which require
heavy computation and often supply energy relation on sparse momentum (k)
grids. Here an efficient method to calculate the distribution of modes (DOM)
from a given E(k) relationship is presented. The main features of this
algorithm are, (i) its ability to work on sparse dispersion data, and (ii)
creation of an energy grid for the DOM that is almost independent of the
dispersion data therefore allowing for efficient and fast calculation of TE
parameters. The inclusion of scattering effects is also straight forward. The
effect of k-grid sparsity on the compute time for DOM and on the sensitivity of
the calculated TE results are provided. The algorithm calculates the TE
parameters within 5% accuracy when the K-grid sparsity is increased up to 60%
for all the dimensions (3D, 2D and 1D). The time taken for the DOM calculation
is strongly influenced by the transverse K density (K perpendicular to
transport direction) but is almost independent of the transport K density
(along the transport direction). The DOM and TE results from the algorithm are
bench-marked with, (i) analytical calculations for parabolic bands, and (ii)
realistic electronic and phonon results for .Comment: 16 Figures, 3 Tables, submitted to Journal of Computational
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