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Papers by Edwin Bosco Ramayya
Journal of Computational Electronics
Journal of Computational Electronics, 2010
Quantum-conflned semiconductor structures are the cornerstone of modern-day electronics. Spa- tia... more Quantum-conflned semiconductor structures are the cornerstone of modern-day electronics. Spa- tial conflnement in these structures leads to formation of discrete low-dimensional subbands. At room temperature, carriers transfer among difierent states due to e-cient scattering with phonons, charged impurities, surface roughness and other electrons, so transport is scattering-limited (dif- fusive) and well described by the Boltzmann transport equation. In this review,
2006 Sixth IEEE Conference on Nanotechnology, 2006
2008 8th IEEE Conference on Nanotechnology, 2008
The thermoelectric figure of merit (ZT) of gated silicon nanowires with square cross section has ... more The thermoelectric figure of merit (ZT) of gated silicon nanowires with square cross section has been calculated by solving the Boltzmann transport equations for electrons and phonons. The electrical conductivity, electronic Seebeck coefficient, and electronic thermal conductivity were calculated by including electron scattering from confined acoustic phonons, intervalley phonons and imperfections at the Si/SiO2 interface. Three-phonon scattering and boundary scattering were included in the calculation of the lattice thermal conductivity. The Seebeck coefficient, electrical conductivity, and thermal conductivity vary non-monotonically with the variation of the wire cross section mainly because of the non-uniform variation of the electron mobility with the wire cross section. In particular, when the wire cross section is decreased from 8 x 8 nm2 to 3 x 3 nm2, ZT increases initially, reaches a maximum value ZT = 0.11 for a 5 x 5 nm2 wire, and then decreases with further reduction of the wire cross section.
2009 13th International Workshop on Computational Electronics, 2009
ABSTRACT
Thermoelectric (TE) refrigeration using Si-based nanostructures is an attractive approach for on-... more Thermoelectric (TE) refrigeration using Si-based nanostructures is an attractive approach for on-chip thermal energy harvesting and targeted cooling of local hotspots [1] due to the ease of on-chip integration and the nanowires' enhanced TE figure of merit [2] ZT=S2σ/(κl+κe) [3]. Silicon-on-insulator (SOI) membranes [4] and membrane-based nanowires [3] and ribbons (Fig. 1) show promise for application as efficient thermoelectrics, which
Physical Review B, 2012
We calculate the room-temperature thermoelectric properties of highly doped ultrathin silicon nan... more We calculate the room-temperature thermoelectric properties of highly doped ultrathin silicon nanowires (SiNW) of square cross section (3 × 3 to 8 × 8 nm 2) by solving the Boltzmann transport equations for electrons and phonons on an equal footing, using the ensemble Monte Carlo technique for each. We account for the two-dimensional confinement of both electrons and phonons and all the relevant scattering mechanisms, and present data for the dependence of electrical conductivity, the electronic and phononic thermal conductivities, the electronic and phonon-drag Seebeck coefficients, as well as the thermoelectric figure of merit (ZT) on the SiNW rms roughness and thickness. ZT in ultrascaled SiNWs does not increase as drastically with decreasing wire cross section as suggested by earlier studies. The reason is surface roughness, which (beneficially) degrades thermal conductivity, but also (adversely) degrades electrical conductivity and offsets the Seebeck coefficient enhancement that comes from confinement. Overall, room-temperature ZT of ultrathin SiNWs varies slowly with thickness, having a soft maximum of about 0.4 at the nanowire thickness of 4 nm.
Journal of Physics: Conference Series, 2006
We investigate electron transport in silicon nanowires taking into account acoustic, non-polar op... more We investigate electron transport in silicon nanowires taking into account acoustic, non-polar optical phonons and surface/interface roughness scattering. We find that at very high transverse fields the reduced density of final states to which the carriers can scatter into gives rise to a reduced influence of interface-roughness scattering, which is promising result from a fabrication point of view.
Journal of Computational and Theoretical Nanoscience, 2009
Quantum-confined semiconductor structures are the cornerstone of modern-day electronics. Spatial ... more Quantum-confined semiconductor structures are the cornerstone of modern-day electronics. Spatial confinement in these structures leads to formation of discrete low-dimensional subbands. At room temperature, carriers transfer among different states due to efficient scattering with phonons, charged impurities, surface roughness and other electrons, so transport is scattering-limited (diffusive) and well described by the Boltzmann transport equation. In this review, we present the theoretical framework used for the description and simulation of diffusive electron transport in quasi-two-dimensional and quasi-one-dimensional semiconductor structures. Transport in silicon MOSFETs and nanowires is presented in detail.
Journal of Applied Physics, 2008
IEEE Transactions On Nanotechnology, 2000
The low-field electron mobility in rectangular silicon nanowire (SiNW) transistors was computed u... more The low-field electron mobility in rectangular silicon nanowire (SiNW) transistors was computed using a self-consistent Poisson-Schrödinger-Monte Carlo solver. The behavior of the phonon-limited and surface-roughness-limited components of the mobility was investigated by decreasing the wire width from 30 nm to 8 nm, the width range capturing a crossover between twodimensional (2D) and one-dimensional (1D) electron transport. The phonon-limited mobility, which characterizes transport at low and moderate transverse fields, is found to decrease with decreasing wire width due to an increase in the electron-phonon wavefunction overlap. In contrast, the mobility at very high transverse fields, which is limited by surface roughness scattering, increases with decreasing wire width due to volume inversion. The importance of acoustic phonon confinement is also discussed briefly.
Applied Physics Letters, 2009
The thermoelectric figure of merit (ZT) of silicon nanowires (SiNWs) can be almost two orders of ... more The thermoelectric figure of merit (ZT) of silicon nanowires (SiNWs) can be almost two orders of magnitude higher than that in bulk silicon, holding promise for all-silicon on-chip cooling. However, the physics behind the increase in ZT is not clear. We calculate the ZT using a detailed Monte Carlo technique that accounts for the localization of phonons at the rough
ABSTRACT Surface roughness has a significant impact on the thermal conductivity and thermoelectri... more ABSTRACT Surface roughness has a significant impact on the thermal conductivity and thermoelectric properties of nanowires. We investigate the effect of surface roughness on thermal transport using a phonon Monte Carlo simulation. In addition to allowing us to simulate a wide range of wire dimensions and surface topographies, Monte Carlo enables us to investigate different models for surface scattering: constant specularity parameters, momentum-dependent specularity parameters, and specular scattering from randomly generated rough surfaces. We investigate the relative merits of different surface scattering models and the limitations on their validity.
ACS Nano, 2010
We report direct measurements of changes in the conduction-band structure of ultrathin silicon na... more We report direct measurements of changes in the conduction-band structure of ultrathin silicon nanomembranes with quantum confinement. Confinement lifts the 6-fold-degeneracy of the bulk-silicon conduction-band minimum (CBM), ⌬, and two inequivalent sub-band ladders, ⌬ 2 and ⌬ 4 , form. We show that even very small surface roughness smears the nominally steplike features in the density of states (DOS) due to these sub-bands. We obtain the energy splitting between ⌬ 2 and ⌬ 4 and their shift with respect to the bulk value directly from the 2p 3/2 ¡⌬ transition in X-ray absorption. The measured dependence of the sub-band splitting and the shift of their weighted average on degree of confinement is in excellent agreement with theory, for both Si(001) and Si(110).
Journal of Computational Electronics
Journal of Computational Electronics, 2010
Quantum-conflned semiconductor structures are the cornerstone of modern-day electronics. Spa- tia... more Quantum-conflned semiconductor structures are the cornerstone of modern-day electronics. Spa- tial conflnement in these structures leads to formation of discrete low-dimensional subbands. At room temperature, carriers transfer among difierent states due to e-cient scattering with phonons, charged impurities, surface roughness and other electrons, so transport is scattering-limited (dif- fusive) and well described by the Boltzmann transport equation. In this review,
2006 Sixth IEEE Conference on Nanotechnology, 2006
2008 8th IEEE Conference on Nanotechnology, 2008
The thermoelectric figure of merit (ZT) of gated silicon nanowires with square cross section has ... more The thermoelectric figure of merit (ZT) of gated silicon nanowires with square cross section has been calculated by solving the Boltzmann transport equations for electrons and phonons. The electrical conductivity, electronic Seebeck coefficient, and electronic thermal conductivity were calculated by including electron scattering from confined acoustic phonons, intervalley phonons and imperfections at the Si/SiO2 interface. Three-phonon scattering and boundary scattering were included in the calculation of the lattice thermal conductivity. The Seebeck coefficient, electrical conductivity, and thermal conductivity vary non-monotonically with the variation of the wire cross section mainly because of the non-uniform variation of the electron mobility with the wire cross section. In particular, when the wire cross section is decreased from 8 x 8 nm2 to 3 x 3 nm2, ZT increases initially, reaches a maximum value ZT = 0.11 for a 5 x 5 nm2 wire, and then decreases with further reduction of the wire cross section.
2009 13th International Workshop on Computational Electronics, 2009
ABSTRACT
Thermoelectric (TE) refrigeration using Si-based nanostructures is an attractive approach for on-... more Thermoelectric (TE) refrigeration using Si-based nanostructures is an attractive approach for on-chip thermal energy harvesting and targeted cooling of local hotspots [1] due to the ease of on-chip integration and the nanowires' enhanced TE figure of merit [2] ZT=S2σ/(κl+κe) [3]. Silicon-on-insulator (SOI) membranes [4] and membrane-based nanowires [3] and ribbons (Fig. 1) show promise for application as efficient thermoelectrics, which
Physical Review B, 2012
We calculate the room-temperature thermoelectric properties of highly doped ultrathin silicon nan... more We calculate the room-temperature thermoelectric properties of highly doped ultrathin silicon nanowires (SiNW) of square cross section (3 × 3 to 8 × 8 nm 2) by solving the Boltzmann transport equations for electrons and phonons on an equal footing, using the ensemble Monte Carlo technique for each. We account for the two-dimensional confinement of both electrons and phonons and all the relevant scattering mechanisms, and present data for the dependence of electrical conductivity, the electronic and phononic thermal conductivities, the electronic and phonon-drag Seebeck coefficients, as well as the thermoelectric figure of merit (ZT) on the SiNW rms roughness and thickness. ZT in ultrascaled SiNWs does not increase as drastically with decreasing wire cross section as suggested by earlier studies. The reason is surface roughness, which (beneficially) degrades thermal conductivity, but also (adversely) degrades electrical conductivity and offsets the Seebeck coefficient enhancement that comes from confinement. Overall, room-temperature ZT of ultrathin SiNWs varies slowly with thickness, having a soft maximum of about 0.4 at the nanowire thickness of 4 nm.
Journal of Physics: Conference Series, 2006
We investigate electron transport in silicon nanowires taking into account acoustic, non-polar op... more We investigate electron transport in silicon nanowires taking into account acoustic, non-polar optical phonons and surface/interface roughness scattering. We find that at very high transverse fields the reduced density of final states to which the carriers can scatter into gives rise to a reduced influence of interface-roughness scattering, which is promising result from a fabrication point of view.
Journal of Computational and Theoretical Nanoscience, 2009
Quantum-confined semiconductor structures are the cornerstone of modern-day electronics. Spatial ... more Quantum-confined semiconductor structures are the cornerstone of modern-day electronics. Spatial confinement in these structures leads to formation of discrete low-dimensional subbands. At room temperature, carriers transfer among different states due to efficient scattering with phonons, charged impurities, surface roughness and other electrons, so transport is scattering-limited (diffusive) and well described by the Boltzmann transport equation. In this review, we present the theoretical framework used for the description and simulation of diffusive electron transport in quasi-two-dimensional and quasi-one-dimensional semiconductor structures. Transport in silicon MOSFETs and nanowires is presented in detail.
Journal of Applied Physics, 2008
IEEE Transactions On Nanotechnology, 2000
The low-field electron mobility in rectangular silicon nanowire (SiNW) transistors was computed u... more The low-field electron mobility in rectangular silicon nanowire (SiNW) transistors was computed using a self-consistent Poisson-Schrödinger-Monte Carlo solver. The behavior of the phonon-limited and surface-roughness-limited components of the mobility was investigated by decreasing the wire width from 30 nm to 8 nm, the width range capturing a crossover between twodimensional (2D) and one-dimensional (1D) electron transport. The phonon-limited mobility, which characterizes transport at low and moderate transverse fields, is found to decrease with decreasing wire width due to an increase in the electron-phonon wavefunction overlap. In contrast, the mobility at very high transverse fields, which is limited by surface roughness scattering, increases with decreasing wire width due to volume inversion. The importance of acoustic phonon confinement is also discussed briefly.
Applied Physics Letters, 2009
The thermoelectric figure of merit (ZT) of silicon nanowires (SiNWs) can be almost two orders of ... more The thermoelectric figure of merit (ZT) of silicon nanowires (SiNWs) can be almost two orders of magnitude higher than that in bulk silicon, holding promise for all-silicon on-chip cooling. However, the physics behind the increase in ZT is not clear. We calculate the ZT using a detailed Monte Carlo technique that accounts for the localization of phonons at the rough
ABSTRACT Surface roughness has a significant impact on the thermal conductivity and thermoelectri... more ABSTRACT Surface roughness has a significant impact on the thermal conductivity and thermoelectric properties of nanowires. We investigate the effect of surface roughness on thermal transport using a phonon Monte Carlo simulation. In addition to allowing us to simulate a wide range of wire dimensions and surface topographies, Monte Carlo enables us to investigate different models for surface scattering: constant specularity parameters, momentum-dependent specularity parameters, and specular scattering from randomly generated rough surfaces. We investigate the relative merits of different surface scattering models and the limitations on their validity.
ACS Nano, 2010
We report direct measurements of changes in the conduction-band structure of ultrathin silicon na... more We report direct measurements of changes in the conduction-band structure of ultrathin silicon nanomembranes with quantum confinement. Confinement lifts the 6-fold-degeneracy of the bulk-silicon conduction-band minimum (CBM), ⌬, and two inequivalent sub-band ladders, ⌬ 2 and ⌬ 4 , form. We show that even very small surface roughness smears the nominally steplike features in the density of states (DOS) due to these sub-bands. We obtain the energy splitting between ⌬ 2 and ⌬ 4 and their shift with respect to the bulk value directly from the 2p 3/2 ¡⌬ transition in X-ray absorption. The measured dependence of the sub-band splitting and the shift of their weighted average on degree of confinement is in excellent agreement with theory, for both Si(001) and Si(110).