arXiv subjects

Neophytos Neophytou

Publications and source records attributed to Neophytos Neophytou.

At least 73 records · Page 4Linked to original sources

arXiv2013

On the interplay between electrical conductivity and Seebeck coefficient in ultra-narrow silicon nanowires

We analyze the effect of low dimensionality on the electrical conductivity (σ) and Seebeck coefficient (S) in ultra-narrow Si nanowires (NWs) by employing atomistic considerations for the electronic structures and linearized Boltzmann transport theory. We show that changes in the geometrical features of the NWs such as diameter and orientation, mostly affect σ and S in two ways: i) the distance of the band edges from the Fermi level (ηF) changes, and ii) quantum confinement in some cases strongly affect the effective mass of the subbands, which influences the conductivity of the NWs and ηF. Changes in eta_F cause exponential changes in σ, but linear changes in S. S seems to be only weakly dependent on the curvature of the bands, the strength of the scattering mechanisms, and the shape of the DOS(E) function, contrary to current view. Our results indicate that low dimensionality has a stronger influence on σ than on S due to the stronger sensitivity of σ on ηF. We identify cases where bandstructure engineering through confinement can improve σ without significantly affecting S, which can result in power factor improvements.

cond-mat.mtrl-sci↗

arXiv2013

Optimizing Thermoelectric Power Factor by Means of a Potential Barrier

Large efforts in improving thermoelectric energy conversion are devoted to energy filtering by nanometer size potential barriers. In this work we perform an analysis and optimization of such barriers for improved energy filtering. We merge semiclassical with quantum mechanical simulations to capture tunneling and reflections due to the barrier, and analyze the influence of the width W, the height Vb, and the shape of the barrier, and the position of the Fermi level (EF) above the band edge, ηF. We show that for an optimized design, approx. 40 per cent improvement in the thermoelectric power factor can be achieved if the following conditions are met: ηF is large; the different of Vb from EF is somewhat higher but comparable to kBT; and W is large enough to suppress tunneling. Finally, we show that a smooth energy barrier is beneficial compared to a sharp (square) barrier for increasing the thermoelectric power factor.

cond-mat.mtrl-sci↗

arXiv2013

Ballistic Phonon Transport in Ultra-Thin Silicon Layers: Effects of Confinement and Orientation

We investigate the effect of confinement and orientation on the phonon transport properties of ultra-thin silicon layers of thicknesses between 1 nm-16 nm. We employ the modified valence force field method to model the lattice dynamics and the ballistic Landauer transport formalism to calculate the thermal conductance. We consider the major thin layer surface orientations {100}, {110}, {111}, and {112}. For every surface orientation, we study thermal conductance as a function of the transport direction within the corresponding surface plane. We find that the ballistic thermal conductance in the thin layers is anisotropic, with the {110}/<110> channels exhibiting the highest and the {112}/<111> channels the lowest thermal conductance with a ratio of about two. We find that in the case of the {110} and {112} surfaces, different transport orientations can result in ~50% anisotropy in thermal conductance. The thermal conductance of different transport orientations in the {100} and {111} layers, on the other hand, is mostly isotropic. These observations are invariant under different temperatures and layer thicknesses. We show that this behavior originates from the differences in the phonon group velocities, whereas the phonon density of states is very similar for all the thin layers examined. We finally show how the phonon velocities can be understood from the phonon spectrum of each channel. Our findings could be useful in the design of the thermal properties of ultra-thin Si layers for thermoelectric and thermal management applications.

cond-mat.mes-hall↗

arXiv2013

Calculation of Confined Phonon Spectrum in Narrow Silicon Nanowires using the Valence Force Field Method

We study the effect of confinement on the phonon properties of ultra-narrow silicon nanowires of side sizes of 1-10nm . We use the modified valence force field method to compute the phononic dispersion, and extract the density of states, the transmission function, the sound velocity, the ballistic thermal conductance and boundary scattering-limited diffusive thermal conductivity. We find that the phononic dispersion and the ballistic thermal conductance are functions of the geometrical features of the structures, i.e. the transport orientation and confinement dimension. The phonon group velocity and thermal conductance can vary by a factor of two depending on the geometrical features of the channel. The <110> nanowire has the highest group velocity and thermal conductance, whereas the <111> the lowest. The <111> channel is thus the most suitable orientation for thermoelectric devices based on Si nanowires since it also has a large power factor. Our findings could be useful in the thermal transport design of silicon-based devices for thermoelectric and thermal management applications.

cond-mat.mes-hall↗

arXiv2012

Large Thermoelectric Power Factor in P-type Si (110)/[110] Ultra-Thin-Layers Compared to Differently Oriented Channels

Using atomistic electronic structure calculations and Boltzmann semi-classical transport we compute the thermoelectric power factor of ultra-thin-body p-type Si layers of thicknesses from W=3nm up to 10nm. We show that the power factor for channels in [110] transport orientation and strong (110) surface confinement largely outperforms all differently oriented channels by more than 2X. Furthermore, the power factor in this channel increases by ~40% with layer thickness reduction. This increase, together with the large confinement effective mass of the (110) surface, make this particular channel less affected by the detrimental effects of enhanced surface roughness scattering and distortion at the nanoscale. Our results, therefore, point towards the optimal geometrical features regarding orientation and length scale for power factor improvement in 2D thin-layers of zincblende semiconductors.

cond-mat.mtrl-sci↗

arXiv2012

Engineering Enhanced Thermoelectric Properties in Zigzag Graphene Nanoribbons

We theoretically investigate the thermoelectric properties of zigzag graphene nanoribbons in the presence of extended line defects, substrate impurities and edge roughness along the nanoribbon's length. A nearest-neighbor tight-binding model for the electronic structure and a fourth nearest- neighbor force constant model for the phonon bandstructure are used. For transport we employ quantum mechanical non-equilibrium Green's function simulations. Starting from the pristine zigzag nanoribbon structure that exhibits very poor thermoelectric performance, we demonstrate how after a series of engineering design steps the performance can be largely enhanced. Our results could be useful in the design of highly efficient nanostructured graphene nanoribbon based thermoelectric devices.

cond-mat.mtrl-sci↗

arXiv2012

Study of Thermal Properties of Graphene-Based Structures Using the Force Constant Method

The thermal properties of graphene-based materials are theoretically investigated. The fourth-nearest neighbor force constant method for phonon properties is used in conjunction with both the Landauer ballistic and the non-equilibrium Green's function techniques for transport. Ballistic phonon transport is investigated for different structures including graphene, graphene antidot lattices, and graphene nanoribbons. We demonstrate that this particular methodology is suitable for robust and efficient investigation of phonon transport in graphene-based devices. This methodology is especially useful for investigations of thermoelectric and heat transport applications.

cond-mat.mtrl-sci↗

arXiv2012

Numerical study of the thermoelectric power factor in ultra-thin Si nanowires

Low dimensional structures have demonstrated improved thermoelectric (TE) performance because of a drastic reduction in their thermal conductivity, κl. This has been observed for a variety of materials, even for traditionally poor thermoelectrics such as silicon. Other than the reduction in κl, further improvements in the TE figure of merit ZT could potentially originate from the thermoelectric power factor. In this work, we couple the ballistic (Landauer) and diffusive linearized Boltzmann electron transport theory to the atomistic sp3d5s*-spin-orbit-coupled tight-binding (TB) electronic structure model. We calculate the room temperature electrical conductivity, Seebeck coefficient, and power factor of narrow 1D Si nanowires (NWs). We describe the numerical formulation of coupling TB to those transport formalisms, the approximations involved, and explain the differences in the conclusions obtained from each model. We investigate the effects of cross section size, transport orientation and confinement orientation, and the influence of the different scattering mechanisms. We show that such methodology can provide robust results for structures including thousands of atoms in the simulation domain and extending to length scales beyond 10nm, and point towards insightful design directions using the length scale and geometry as a design degree of freedom. We find that the effect of low dimensionality on the thermoelectric power factor of Si NWs can be observed at diameters below ~7nm, and that quantum confinement and different transport orientations offer the possibility for power factor optimization.

cond-mat.mtrl-sci↗

arXiv2011

Hole mobility increase in ultra-narrow Si channels under strong (110) surface confinement

We report on the hole mobility of ultra-narrow [110] Si channels as a function of the confinement length scale. We employing atomistic bandstructure calculations and linearized Boltzmann transport approach. The phonon-limited mobility of holes in thin [110] channels can be improved by more than 3X as the thickness of the (110) confining surface is reduced down to 3nm. This behavior originates from confinement induced bandstructure changes that decrease the hole effective mass and the scattering rates. Our results provide explanations for recent mobility measurements in nanobelts of similar dimensions.

cond-mat.mtrl-sci↗

arXiv2011

Atomistic simulations of low-field mobility in Si nanowires: Influence of confinement and orientation

A simulation framework that couples atomistic electronic structures to Boltzmann transport formalism is developed and applied to calculate the transport characteristics of thin silicon nanowires (NWs) up to 12nm in diameter. The sp3d5s*-spin-orbit-coupled atomistic tight-binding (TB) model is used for the electronic structure calculation. Linearized Boltzmann transport theory is applied, including carrier scattering by phonons, surface roughness (SRS), and impurities. We present a comprehensive investigation of the low-field mobility in silicon NWs considering: i) n- and p-type NWs, ii) [100], [110], and [111] transport orientations, and iii) diameters from D=12nm (electronically almost bulk-like) down to D=3nm (ultra-scaled). The simulation results display strong variations in the characteristics of the different NW types. For n-type NWs, phonon scattering and SRS become stronger as the diameter is reduced and drastically degrade the mobility by up to an order of magnitude depending on the orientation. For the [111] and [110] p-type NWs, on the other hand, large mobility enhancements (of the order of ~4X) can be achieved as the diameter scales down to D=3nm. This enhancement originates from the increase in the subband curvatures as the diameter is scaled. It overcompensates for the mobility reduction caused by SRS in narrow NWs and offers an advantage with diameter scaling. Our results may provide understanding of recent experimental measurements, as well as guidance in the design of NW channel devices with improved transport properties.

cond-mat.mtrl-sci↗

arXiv2011

Effects of Confinement and Orientation on the Thermoelectric Power Factor of Silicon Nanowires

It is suggested that low dimensionality can improve the thermoelectric (TE) power factor of a device, offering an enhancement of the ZT figure of merit. In this work the atomistic sp3d5s*-spin-orbit-coupled tight-binding model and the linearized Boltzmann transport theory is applied to calculate the room temperature electrical conductivity, Seebeck coefficient, and power factor of narrow 1D silicon nanowires (NWs). We present a comprehensive analysis of the thermoelectric coefficients of n-type and p-type NWs of diameters from 12nm down to 3nm, in [100], [110], and [111] transport orientations at different carrier concentrations. We find that the length scale at which the influence of confinement on the power factor can be observed is at diameters below 7nm. We show that contrary to the current view, the effect of confinement and geometry on the power factor originates mostly from changes in the conductivity which is strongly affected, rather than the Seebeck coefficient. In general, enhanced scattering at these diameter scales strongly degrades the conductivity and power factor of the device. We identify cases, however, for which confinement largely improves the channel's conductivity, resulting in ~2-3X power factor improvements. Our results may provide guidance in the design of efficient low dimensional thermoelectric devices.

cond-mat.mes-hall↗

arXiv2011

Thermoelectric Properties of Scaled Silicon Nanowires Using the sp3d5s*-SO Atomistic Tight-Binding Model and Boltzmann Transport

As a result of suppressed phonon conduction, large improvements of the thermoelectric figure of merit, ZT, have been recently reported for nanostructures compared to the raw materials' ZT values. It has also been suggested that low dimensionality can improve a device's power factor as well, offering a further enhancement. In this work the atomistic sp3d5s*-spin-orbit-coupled tight-binding model is used to calculate the electronic structure of silicon nanowires (NWs). The linearized Boltzmann transport theory is applied, including all relevant scattering mechanisms, to calculate the electrical conductivity, the Seebeck coefficient, and the thermoelectric power factor. We examine n-type nanowires of diameters of 3nm and 12nm, in [100], [110], and [111] transport orientations at different carrier concentrations. Using experimental values for the lattice thermal conductivity in nanowires, the expected ZT value is computed. We find that at room temperature, although scaling the diameter below 7nm can be beneficial to the power factor due to banstructure changes alone, at those dimensions enhanced phonon and surface roughness scattering degrades the conductivity and reduces the power factor.

cond-mat.mes-hall↗

arXiv2011

Subband engineering for p-type silicon ultra-thin layers for increased carrier velocities: An atomistic analysis

Ultra-thin-body (UTB) channel materials of a few nanometers in thickness are currently considered as candidates for future electronic, thermoelectric, and optoelectronic applications. Among the features that they possess, which make them attractive for such applications, their confinement length scale, transport direction, and confining surface orientation serve as degrees of freedom for engineering their electronic properties. This work presents a comprehensive study of hole velocities in p-type UTB films of widths from 15nm down to 3nm. Various transport and surface orientations are considered. The atomistic sp3d5s*-spin-orbit-coupled tight-binding model is used for the electronic structure, and a semiclassical ballistic model for the carrier velocity calculation. We find that the carrier velocity is a strong function of orientation and layer thickness. The (110) and (112) surfaces provide the highest hole velocities, whereas the (100) surfaces the lowest velocities, almost 30% lower than the best performers. Additionally, up to 35% velocity enhancements can be achieved as the thickness of the (110) or (112) surfaces is scaled down to 3nm. This originates from strong increase in the curvature of the p-type UTB film subbands with confinement, unlike the case of n-type UTB channels. The velocity behavior directly translates to ballistic on-current trends, and correlates with trends in experimental mobility measurements.

cond-mat.mes-hall↗

arXiv2010

Large enhancement in hole velocity and mobility in p-type [110] and [111] silicon nanowires by cross section scaling: An atomistic analysis

The mobility of p-type nanowires (NWs) of diameters of D=12nm down to D=3nm, in [100], [110], and [111] transport orientations is calculated. An atomistic tight-binding model is used to calculate the NW electronic structure. Linearized Boltzmann transport theory is applied, including phonon and surface roughness scattering (SRS) mechanisms, for the mobility calculation. We find that large mobility enhancements (of the order of 4X) can be achieved as the diameter of the [110] and even more that of the [111] NWs scales down to D=3nm. This enhancement originates from the increase in the dispersion curvatures and consequently the hole velocities as the diameter is scaled. This benefit over compensates the mobility reduction caused by SRS as the diameter reduces. The mobility of the [100] NWs, on the other hand, is the lowest compared the other two NW orientations, and additionally suffers as the diameter scales. The bandstructure engineering techniques we describe are a generic feature of anisotropic bulk bands, and can be also applied to 2D thin-body-layers as well as other channel materials.

cond-mat.mes-hall↗

arXiv2010

Analysis of Thermoelectric Properties of Scaled Silicon Nanowires Using an Atomistic Tight-Binding Model

Low dimensional materials provide the possibility of improved thermoelectric performance due to the additional length scale degree of freedom for engineering their electronic and thermal properties. As a result of suppressed phonon conduction, large improvements on the thermoelectric figure of merit, ZT, have been recently reported in nanostructures, compared to the raw materials' ZT values. In addition, low dimensionality can improve a device's power factor, offering an additional enhancement in ZT. In this work the atomistic sp3d5s*-spin-orbit-coupled tight-binding model is used to calculate the electronic structure of silicon nanowires (NWs). The Landauer formalism is applied to calculate an upper limit for the electrical conductivity, the Seebeck coefficient, and the power factor. We examine n-type and p-type nanowires of diameters from 3nm to 12nm, in [100], [110], and [111] transport orientations at different doping concentrations. Using experimental values for the lattice thermal conductivity in nanowires, an upper limit for ZT is computed. We find that at room temperature, scaling the diameter below 7nm can at most double the power factor and enhance ZT. In some cases, however, scaling does not enhance the performance at all. Orientations, geometries, and subband engineering techniques for optimized designs are discussed.

cond-mat.mes-hall↗

arXiv2010

On the Bandstructure Velocity and Ballistic Current of Ultra Narrow Silicon Nanowire Transistors as a Function of Cross Section Size, Orientation and Bias

A 20 band sp3d5s* spin-orbit-coupled, semi-empirical, atomistic tight-binding (TB) model is used with a semi-classical, ballistic, field-effect-transistor (FET) model, to theoretically examine the bandstructure carrier velocity and ballistic current in silicon nanowire (NW) transistors. Infinitely long, uniform, cylindrical and rectangular NWs, of cross sectional diameters/sides ranging from 3nm to 12nm are considered. For a comprehensive analysis, n-type and p-type metal-oxide-semiconductor (NMOS and PMOS) NWs in [100], [110] and [111] transport orientations are examined. In general, physical cross section reduction increases velocities, either by lifting the heavy mass valleys, or significantly changing the curvature of the bands. The carrier velocities of PMOS [110] and [111] NWs are a strong function of diameter, with the narrower D=3nm wires having twice the velocities of the D=12nm NWs. The velocity in the rest of the NW categories shows only minor diameter dependence. This behavior is explained through features in the electronic structure of the silicon host material. The ballistic current, on the other hand, shows the least sensitivity with cross section in the cases where the velocity has large variations. Since the carrier velocity is a measure of the effective mass and reflects on the channel mobility, these results can provide insight into the design of NW devices with enhanced performance and performance tolerant to structure geometry variations. In the case of ballistic transport in high performance devices, the [110] NWs are the ones with both high NMOS and PMOS performance, as well as low on-current variations with cross section geometry variations.

cond-mat.mes-hall↗

arXiv2009

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.

cond-mat.mes-hall↗

arXiv2008

Performance Analysis of 60nm gate length III-V InGaAs HEMTs: Simulations vs. experiments

An analysis of recent experimental data for high-performance In0.7Ga0.3As high electron mobility transistors (HEMTs) is presented. Using a fully quantum mechanical, ballistic model, we simulate In0.7Ga0.3As HEMTs with gate lengths of LG = 60nm, 85, and 135 nm and compare the result to the measured I-V characteristics including draininduced barrier lowering, sub-threshold swing, and threshold voltage variation with gate insulator thickness, as well as on-current performance. To first order, devices with three different oxide thicknesses and channel lengths can all be described by our ballistic model with appropriate values of parasitic series resistance. For high gate voltages, however, the ballistic simulations consistently overestimate the measured on-current, and they do not show the experimentally observed decrease in on-current with increasing gate length. With no parasitic series resistance at all, the simulated on-current of the LG = 60 nm device is about twice the measured current. According to the simulation, the estimated ballistic carrier injection velocity for this device is about 2.7 x 10^7 cm/s. Because of the importance of the semiconductor capacitance, the simulated gate capacitance is about 2.5 times less than the insulator capacitance. Possible causes of the transconductance degradation observed under high gate voltages in these devices are also explored. In addition to a possible gate-voltage dependent scattering mechanism, the limited ability of the source to supply carriers to the channel, and the effect of nonparabolicity are likely to play a role. The drop in on-current with increasing gate length is an indication that the devices operate below the ballistic limit.

cond-mat.mes-hall↗