High-frequency surface acoustic waves (SAWs) in the GHz range can be generated using absorption f... more High-frequency surface acoustic waves (SAWs) in the GHz range can be generated using absorption from an ultrafast laser in a patterned metallic grating on a substrate. Reducing the attenuation at these frequencies can yield better sensors as well as enable them to better probe phonon and electron-phonon interactions near surfaces. It is not clear from existing experiments which mechanisms dominate damping at high frequencies. We calculate damping times of SAWs due to various mechanism in the 1-100 GHz range to find that mechanical loading of the grating on the substrate dominates dissipation by radiating energy from the surface into the bulk. To overcome this and enable future measurements to probe intrinsic damping, we propose incorporating distributed acoustic Bragg reflectors (DABRs) in the experimental structure. Layers of alternating materials with contrasting acoustic impedances embedded a wavelength away from the surface serve to reflect energy back to the surface. Using numerical simulations, we show that a single Bragg reflector is sufficient to increase the energy density at the surface by more than five times. We quantify the resulting damping time to find that it is longer than the intrinsic damping time. The proposed structure can enable future measurements of intrinsic damping in SAWs at ∼100 GHz.
The dramatic reduction in the thermal conductivity of rough silicon nanowires is due to phonon lo... more The dramatic reduction in the thermal conductivity of rough silicon nanowires is due to phonon localization in the wire resulting from multiple scattering of phonons from the rough walls. We report the dependence of thermal conductivity of the nanowires as a function of the surface roughness and the diameter of the wire by modeling the nanowire as a waveguide. In addition, we estimate the impact of boundary condition, dimensionality and cross section of rough wire on the thermal conductivity. This theoretical model gives insights for tailoring thermal conductivity and enhancing the ZT of silicon to 1 for its use in thermoelectrics
We report the fabrication of degenerately doped silicon (Si) nanowires of different aspect ratios... more We report the fabrication of degenerately doped silicon (Si) nanowires of different aspect ratios using a simple, low-cost and effective technique that involves metal-assisted chemical etching (MacEtch) combined with soft lithography or thermal dewetting metal patterning. We demonstrate sub-micron diameter Si nanowire arrays with aspect ratios as high as 180:1, and present the challenges in producing solid nanowires using MacEtch as the doping level increases in both p-and n-type Si. We report a systematic reduction in the porosity of these nanowires by adjusting the etching solution composition and temperature. We found that the porosity decreases from top to bottom along the axial direction and increases with etching time. With a MacEtch solution that has a high [HF]:[H 2 O 2 ] ratio and low temperature, it is possible to form completely solid nanowires with aspect ratios of less than approximately 10:1. However, further etching to produce longer wires renders the top portion of the nanowires porous.
We report measurements and modeling of thermal conductivity in periodic three-dimensional dielect... more We report measurements and modeling of thermal conductivity in periodic three-dimensional dielectric nanostructures, silicon inverse opals. Such structures represent a three-dimensional "phononic crystal" but affect heat flow instead of acoustics. Employing the Stober method, we fabricate high quality silica opal templates that on filling with amorphous silicon, etching and recrystallizing produce silicon inverse opals. The periodicities and shell thicknesses are in the range 420−900 and 18−38 nm, respectively. The thermal conductivity of inverse opal films are relatively low, ∼0.6−1.4 W/mK at 300 K and arise due to macroscopic bending of heat flow lines in the structure. The corresponding material thermal conductivity is in the range 5−12 W/mK and has an anomalous ∼T 1.8 dependence at low temperatures, distinct from the typical ∼T 3 behavior of bulk polycrystalline silicon. Using phonon scattering theory, we show such dependence arising from coherent phonon reflections in the intergrain region. This is consistent with an unconfirmed theory proposed in 1955. The low thermal conductivity is significant for applications in photonics where they imply significant temperature rise at relatively low absorption and in thermoelectrics, where they suggest the possibility of enhancement in the figure of merit for polysilicon with optimal doping.
A two-step metal assisted chemical etching technique is used to systematically vary the sidewall ... more A two-step metal assisted chemical etching technique is used to systematically vary the sidewall roughness of Si nanowires in vertically aligned arrays. The thermal conductivities of nanowire arrays are studied using time domain thermoreflectance and compared to their high-resolution transmission electron microscopy determined roughness. The thermal conductivity of nanowires with small roughness is close to a theoretical prediction based on an upper limit of the mean-free-paths of phonons given by the nanowire diameter. The thermal conductivity of nanowires with large roughness is found to be significantly below this prediction. Raman spectroscopy reveals that nanowires with large roughness also display significant broadening of the one-phonon peak; the broadening correlates well with the reduction in thermal conductivity. The origin of this broadening is not yet understood, as it is inconsistent with phonon confinement models, but could derive from microstructural changes that affect both the optical phonons observed in Raman scattering and the acoustic phonons that are important for heat conduction. V
ABSTRACT Using aluminum gratings of varying duty cycles, we report picoseconds acoustics measurem... more ABSTRACT Using aluminum gratings of varying duty cycles, we report picoseconds acoustics measurements of frequency shift and attenuation in surface acoustic phonons in silicon at ∼ 15 GHz. We observe that the frequency shifts nonlinearly with the duty cycle, particularly in the range of 0.3 to 0.5. The data deviate from the perturbation model as a sinusoidal function of the duty cycle. The attenuation peaks at 0.5 duty cycle which is in good agreement with an eigenmode analysis of the composite structure. This work elucidates the mechanism of surface acoustic phonon scattering at periodic interfaces.
Thermal conductivity data for rough surface silicon nanowires suggest the breakdown of the Casimi... more Thermal conductivity data for rough surface silicon nanowires suggest the breakdown of the Casimir limit which assumes completely diffuse phonon boundary scattering. We show that coherent effects in phonon transport at room temperature indeed lead to such breakdown. Correlated multiple scattering of phonons off the rough surface lead to a reduced thermal conductivity that is dependent not just on the roughness amplitude but more importantly on the roughness correlation length. A correlation length less than diameter of wire is typically necessary for lowering the thermal conductivity below the Casimir limit. Our model explains seeming anomalies in data reported for electrolessly etched and electron beam lithography defined nanowires.
Capacitive thermal management in electronics requires an enhancement in the volumetric storage de... more Capacitive thermal management in electronics requires an enhancement in the volumetric storage density of compatible materials to approximately 1 GJ/m3. We investigate potential enhancement in sensible storage density arising from size effects in thin polymer films. We design and fabricate a micro-calorimeter to measure the volumetric heat capacity of nanometer scale thin fluorocarbon polymer films. The storage density increases approximately three times as the film thickness decreases from 27 nm to 12 nm. Using the Debye theory of phonons, we relate this behavior to an increase in the Grüneisen parameter with decreasing film thickness. This work advances the understanding of size dependence in the heat capacity of amorphous polymers as well as highlights their potential for capacitive thermal management. ii To my parents and sister, for their love and support. iii ACKNOWLEDGMENTS I would like to thank everyone who made their contributions to this thesis by providing their support a...
Photoinduced strain gradients in nanometer length scales using periodically patterned grating on ... more Photoinduced strain gradients in nanometer length scales using periodically patterned grating on a substrate is an attractive way of producing coherent surface acoustic waves (SAW). Ultrafast laser pulses incident on this structure produce constrasting thermal strain in the grating and the substrate thus launching surface acoustic waves. The frequency of the SAW is inversely dependant on the period of the grating and thus the generation of high frequency SAW (ν ∼1-100 GHz) requires sub-micron scale gratings. Hypersonic SAW are scientifically significant for photoacoustic spectroscopy and metrology of nanostructures in addition to investigations of phonon mediated heat transport. In this work, we study the propagation of hypersonic surface acoustic waves (ν ∼10-20 GHz) in silicon with aluminum gratings fabricated on them for varying periods and duty cycles (η) of the grating (coverage ratio of Al on Si). Modeling the optical absorption of the laser and the resultant thermoelastic str...
Porosity control in metal-assisted chemical etching of degenerately doped silicon nanowires This ... more Porosity control in metal-assisted chemical etching of degenerately doped silicon nanowires This article has been downloaded from IOPscience. Please scroll down to see the full text article.
ABSTRACT Patterning silicon as nanowires with roughened sidewalls enhances the thermoelectric fig... more ABSTRACT Patterning silicon as nanowires with roughened sidewalls enhances the thermoelectric figure-of-merit ZT by order of magnitude compared to the bulk at 300 K [1]. The enhancement is mainly achieved by the remarkable reduction in the thermal conductivity below 5 W/mK at 300 K with only a negligible effect on the power factor of these nanowires. While the focus remained on understanding the implications of surface disorder on the thermal conductivity, the phonon transport effects on the Seebeck coefficient of these wires remains largely unexplored. We developed an electroless etching technique to generate nanowire arrays (NWAs) with controlled surface roughness, morphology, porosity and doping [2]. We conduct the simultaneous device-level measurements of the Seebeck coefficient and thermal conductivity of the NWAs using frequency domain techniques. We observe that nano-structuring quenches the phonon drag [3] in NWAs thereby reducing the Seebeck coefficient by ˜25% compared to the bulk at degenerate doping levels. Further, we observe that the sidewall roughness greater than 3 nm roughness height lowers the thermal conductivity 75% below the Casimir limit [4] with 10% - 15% increase in Seebeck coefficient. The porous NWAs show thermal conductivity close to the amorphous limit of Si with enhancement in the Seebeck coefficient primarily due to the carrier depletion. References: [1] A. I. Hochbaum et al, Nature 451, 163-167 (2008). [2] K. Balasundaram et. al., Nanotechnology 23, 305304 (2012). [3] C. Herring, Phys. Rev. 96, 1163 (1954). [4] H. G. B. Casimir, Physica 5, 495 (1938).
ABSTRACT Nanostructured materials [1-3] have lower thermal conductivities than the bulk and are p... more ABSTRACT Nanostructured materials [1-3] have lower thermal conductivities than the bulk and are promising candidates for thermoelectric applications. In particular, measurements on single silicon nanowires show a reduction in thermal conductivity below the Casimir limit. This reduction increases with surface roughness [4] but the trend and its connection to phonon boundary scattering are still elusive. Here, we measure the thermal conductivity of single silicon nanowires fabricated using metal-assisted chemical etching. High resolution TEM imaging shows crystalline wires with slightly rough surfaces. Their statistical correlation lengths (5-15 nm) and RMS heights (0.8-1.5 nm) are in a range where perturbation-based wave scattering theory is still applicable. We use the thermal conductivity data to extract the frequency dependence of phonon boundary scattering at low temperatures (10-40 K) and show agreement with multiple scattering theory. This work provides insight into enhancing the thermoelectric performance of nanostructures. 1-A. I. Hochbaum et al, Nature Lett. 451, 163-167 (2008). 2-A. J. Minnich et al, Energy Environ. Sci. 2, 466-479 (2009). 3-L. Shi, Nanoscale Microscale Thermophys. Eng. 16, 79--116 (2012). 4-J. Lim et al, Nano Lett. 12, 2475-2482 (2012).
We report simultaneous measurements of thermal conductivity and Seebeck coefficient on array-scal... more We report simultaneous measurements of thermal conductivity and Seebeck coefficient on array-scale silicon nanowires fabricated by metal assisted chemical etching. The measurements are conducted on the solid and the mesoporous nanowire arrays (NWAs) obtained from etching 1 ohm-cm and 0.002 ohm-cm Si substrates respectively. We demonstrate control on sidewall morphology and doping of the arrays that have an aspect ratio up to 20 and 30 % areal coverage. We employ differential frequency-domain measurements, separately on the array and the corresponding substrate to obtain the temperature drop and Seebeck voltage contribution of the nanowire array. The technique is validated by measurements on bulk silicon across the resistivity 0.002-1 ohm-cm. The Seebeck measurements reveal quenching of the phonon drag in the nanowires in comparison to the bulk in the measured temperature range of 300 K- 500 K. The Seebeck coefficient shows a ~18 % decrease in the solid NWAs and ~22 % increase in the...
Thermoelectrics enable solid-state conversion of heat to electricity by the Seebeck effect, but m... more Thermoelectrics enable solid-state conversion of heat to electricity by the Seebeck effect, but must provide scalable and cost-effective technology for practical waste heat harvesting. This dissertation explores the thermoelectric properties of electrochemically etched silicon nanowires through experiments, complemented by charge and thermal transport theories. Electrolessly etched silicon nanowires show anomalously low thermal conductivity that has been attributed to the increased scattering of heat conducting phonons from the surface disorder introduced by etching. The reduction is below the incoherent limit for phonon scattering at the boundary, the so-called Casimir limit. A new model of partially coherent phonon transport shows that correlated multiple scattering of phonons off resonantly matched rough surfaces can indeed lead to thermal conductivity below the Casimir limit. Using design guidelines from the theory, silicon nanowires of controllable surface roughness are fabrica...
2017 IEEE International Ultrasonics Symposium (IUS)
The production and the characterization of high-quality ScAlN films, whose electromechanical coup... more The production and the characterization of high-quality ScAlN films, whose electromechanical coupling and bandwidth are larger than those of traditional AlN films, is attracting interest to develop the next generation of bulk acoustic wave resonators for telecommunication devices operating above 5 GHz.
ABSTRACT Nanostructured materials [1-3] have lower thermal conductivities than the bulk and are p... more ABSTRACT Nanostructured materials [1-3] have lower thermal conductivities than the bulk and are promising candidates for thermoelectric applications. In particular, measurements on single silicon nanowires show a reduction in thermal conductivity below the Casimir limit. This reduction increases with surface roughness [4] but the trend and its connection to phonon boundary scattering are still elusive. Here, we measure the thermal conductivity of single silicon nanowires fabricated using metal-assisted chemical etching. High resolution TEM imaging shows crystalline wires with slightly rough surfaces. Their statistical correlation lengths (5-15 nm) and RMS heights (0.8-1.5 nm) are in a range where perturbation-based wave scattering theory is still applicable. We use the thermal conductivity data to extract the frequency dependence of phonon boundary scattering at low temperatures (10-40 K) and show agreement with multiple scattering theory. This work provides insight into enhancing the thermoelectric performance of nanostructures. 1-A. I. Hochbaum et al, Nature Lett. 451, 163-167 (2008). 2-A. J. Minnich et al, Energy Environ. Sci. 2, 466-479 (2009). 3-L. Shi, Nanoscale Microscale Thermophys. Eng. 16, 79--116 (2012). 4-J. Lim et al, Nano Lett. 12, 2475-2482 (2012).
Understanding thermal transport in two-and three-dimensional periodic "holey" nanostructures is i... more Understanding thermal transport in two-and three-dimensional periodic "holey" nanostructures is important for realizing applications of these structures in thermoelectrics, photonics and batteries. In terms of continuum heat diffusion physics, the effective medium theory provides the framework for obtaining the effective thermal conductivity of such structures. However, recently measured nanostructures possess thermal conductivities well below these continuum predictions. In some cases, their thermal conductivities are even lower than predictions that account for sub-continuum phonon transport. We analyze current understanding of thermal transport in such structures, discussing the various theories, the measurements and the insights gained from comparing the two.
Top-down electroless chemical etching enables non-lithographic patterning of wafer-scale nanostru... more Top-down electroless chemical etching enables non-lithographic patterning of wafer-scale nanostructured arrays, but the etching on highly doped wafers produces porous structures. The lack of defect-free nanostructures at desired doping and the difficulties in forming reliable electrical side-contacts to the nanostructure arrays limits their integration into high performance nanoelectronics. We developed a barrier layer diffusion technique to controllably dope waferscale silicon nanowire arrays (10 17-10 20 cm −3) produced by chemically etching lightly doped silicon wafers. In order to achieve low resistance top-side electrical contacts to the arrays, we developed a two step tip-doping procedure to locally dope the tips (∼10 20 cm −3) to metallic levels. The dopant concentration is characterized by depth profiling using secondary ion mass spectroscopy and four-point probe electrical measurements. Further, array scale electrical measurements show that the tip-doping lowers the specific contact resistivity (∼10 −5 Ω cm 2) since the metallic tips enable direct tunneling of electrons across the nickel silicide contacts to the nanowire arrays. This work provides a scalable and cost-effective doping approach to control charge injection and charge conduction in nanowire arrays, thus advancing their integration into various device applications.
High-frequency surface acoustic waves (SAWs) in the GHz range can be generated using absorption f... more High-frequency surface acoustic waves (SAWs) in the GHz range can be generated using absorption from an ultrafast laser in a patterned metallic grating on a substrate. Reducing the attenuation at these frequencies can yield better sensors as well as enable them to better probe phonon and electron-phonon interactions near surfaces. It is not clear from existing experiments which mechanisms dominate damping at high frequencies. We calculate damping times of SAWs due to various mechanism in the 1-100 GHz range to find that mechanical loading of the grating on the substrate dominates dissipation by radiating energy from the surface into the bulk. To overcome this and enable future measurements to probe intrinsic damping, we propose incorporating distributed acoustic Bragg reflectors (DABRs) in the experimental structure. Layers of alternating materials with contrasting acoustic impedances embedded a wavelength away from the surface serve to reflect energy back to the surface. Using numerical simulations, we show that a single Bragg reflector is sufficient to increase the energy density at the surface by more than five times. We quantify the resulting damping time to find that it is longer than the intrinsic damping time. The proposed structure can enable future measurements of intrinsic damping in SAWs at ∼100 GHz.
The dramatic reduction in the thermal conductivity of rough silicon nanowires is due to phonon lo... more The dramatic reduction in the thermal conductivity of rough silicon nanowires is due to phonon localization in the wire resulting from multiple scattering of phonons from the rough walls. We report the dependence of thermal conductivity of the nanowires as a function of the surface roughness and the diameter of the wire by modeling the nanowire as a waveguide. In addition, we estimate the impact of boundary condition, dimensionality and cross section of rough wire on the thermal conductivity. This theoretical model gives insights for tailoring thermal conductivity and enhancing the ZT of silicon to 1 for its use in thermoelectrics
We report the fabrication of degenerately doped silicon (Si) nanowires of different aspect ratios... more We report the fabrication of degenerately doped silicon (Si) nanowires of different aspect ratios using a simple, low-cost and effective technique that involves metal-assisted chemical etching (MacEtch) combined with soft lithography or thermal dewetting metal patterning. We demonstrate sub-micron diameter Si nanowire arrays with aspect ratios as high as 180:1, and present the challenges in producing solid nanowires using MacEtch as the doping level increases in both p-and n-type Si. We report a systematic reduction in the porosity of these nanowires by adjusting the etching solution composition and temperature. We found that the porosity decreases from top to bottom along the axial direction and increases with etching time. With a MacEtch solution that has a high [HF]:[H 2 O 2 ] ratio and low temperature, it is possible to form completely solid nanowires with aspect ratios of less than approximately 10:1. However, further etching to produce longer wires renders the top portion of the nanowires porous.
We report measurements and modeling of thermal conductivity in periodic three-dimensional dielect... more We report measurements and modeling of thermal conductivity in periodic three-dimensional dielectric nanostructures, silicon inverse opals. Such structures represent a three-dimensional "phononic crystal" but affect heat flow instead of acoustics. Employing the Stober method, we fabricate high quality silica opal templates that on filling with amorphous silicon, etching and recrystallizing produce silicon inverse opals. The periodicities and shell thicknesses are in the range 420−900 and 18−38 nm, respectively. The thermal conductivity of inverse opal films are relatively low, ∼0.6−1.4 W/mK at 300 K and arise due to macroscopic bending of heat flow lines in the structure. The corresponding material thermal conductivity is in the range 5−12 W/mK and has an anomalous ∼T 1.8 dependence at low temperatures, distinct from the typical ∼T 3 behavior of bulk polycrystalline silicon. Using phonon scattering theory, we show such dependence arising from coherent phonon reflections in the intergrain region. This is consistent with an unconfirmed theory proposed in 1955. The low thermal conductivity is significant for applications in photonics where they imply significant temperature rise at relatively low absorption and in thermoelectrics, where they suggest the possibility of enhancement in the figure of merit for polysilicon with optimal doping.
A two-step metal assisted chemical etching technique is used to systematically vary the sidewall ... more A two-step metal assisted chemical etching technique is used to systematically vary the sidewall roughness of Si nanowires in vertically aligned arrays. The thermal conductivities of nanowire arrays are studied using time domain thermoreflectance and compared to their high-resolution transmission electron microscopy determined roughness. The thermal conductivity of nanowires with small roughness is close to a theoretical prediction based on an upper limit of the mean-free-paths of phonons given by the nanowire diameter. The thermal conductivity of nanowires with large roughness is found to be significantly below this prediction. Raman spectroscopy reveals that nanowires with large roughness also display significant broadening of the one-phonon peak; the broadening correlates well with the reduction in thermal conductivity. The origin of this broadening is not yet understood, as it is inconsistent with phonon confinement models, but could derive from microstructural changes that affect both the optical phonons observed in Raman scattering and the acoustic phonons that are important for heat conduction. V
ABSTRACT Using aluminum gratings of varying duty cycles, we report picoseconds acoustics measurem... more ABSTRACT Using aluminum gratings of varying duty cycles, we report picoseconds acoustics measurements of frequency shift and attenuation in surface acoustic phonons in silicon at ∼ 15 GHz. We observe that the frequency shifts nonlinearly with the duty cycle, particularly in the range of 0.3 to 0.5. The data deviate from the perturbation model as a sinusoidal function of the duty cycle. The attenuation peaks at 0.5 duty cycle which is in good agreement with an eigenmode analysis of the composite structure. This work elucidates the mechanism of surface acoustic phonon scattering at periodic interfaces.
Thermal conductivity data for rough surface silicon nanowires suggest the breakdown of the Casimi... more Thermal conductivity data for rough surface silicon nanowires suggest the breakdown of the Casimir limit which assumes completely diffuse phonon boundary scattering. We show that coherent effects in phonon transport at room temperature indeed lead to such breakdown. Correlated multiple scattering of phonons off the rough surface lead to a reduced thermal conductivity that is dependent not just on the roughness amplitude but more importantly on the roughness correlation length. A correlation length less than diameter of wire is typically necessary for lowering the thermal conductivity below the Casimir limit. Our model explains seeming anomalies in data reported for electrolessly etched and electron beam lithography defined nanowires.
Capacitive thermal management in electronics requires an enhancement in the volumetric storage de... more Capacitive thermal management in electronics requires an enhancement in the volumetric storage density of compatible materials to approximately 1 GJ/m3. We investigate potential enhancement in sensible storage density arising from size effects in thin polymer films. We design and fabricate a micro-calorimeter to measure the volumetric heat capacity of nanometer scale thin fluorocarbon polymer films. The storage density increases approximately three times as the film thickness decreases from 27 nm to 12 nm. Using the Debye theory of phonons, we relate this behavior to an increase in the Grüneisen parameter with decreasing film thickness. This work advances the understanding of size dependence in the heat capacity of amorphous polymers as well as highlights their potential for capacitive thermal management. ii To my parents and sister, for their love and support. iii ACKNOWLEDGMENTS I would like to thank everyone who made their contributions to this thesis by providing their support a...
Photoinduced strain gradients in nanometer length scales using periodically patterned grating on ... more Photoinduced strain gradients in nanometer length scales using periodically patterned grating on a substrate is an attractive way of producing coherent surface acoustic waves (SAW). Ultrafast laser pulses incident on this structure produce constrasting thermal strain in the grating and the substrate thus launching surface acoustic waves. The frequency of the SAW is inversely dependant on the period of the grating and thus the generation of high frequency SAW (ν ∼1-100 GHz) requires sub-micron scale gratings. Hypersonic SAW are scientifically significant for photoacoustic spectroscopy and metrology of nanostructures in addition to investigations of phonon mediated heat transport. In this work, we study the propagation of hypersonic surface acoustic waves (ν ∼10-20 GHz) in silicon with aluminum gratings fabricated on them for varying periods and duty cycles (η) of the grating (coverage ratio of Al on Si). Modeling the optical absorption of the laser and the resultant thermoelastic str...
Porosity control in metal-assisted chemical etching of degenerately doped silicon nanowires This ... more Porosity control in metal-assisted chemical etching of degenerately doped silicon nanowires This article has been downloaded from IOPscience. Please scroll down to see the full text article.
ABSTRACT Patterning silicon as nanowires with roughened sidewalls enhances the thermoelectric fig... more ABSTRACT Patterning silicon as nanowires with roughened sidewalls enhances the thermoelectric figure-of-merit ZT by order of magnitude compared to the bulk at 300 K [1]. The enhancement is mainly achieved by the remarkable reduction in the thermal conductivity below 5 W/mK at 300 K with only a negligible effect on the power factor of these nanowires. While the focus remained on understanding the implications of surface disorder on the thermal conductivity, the phonon transport effects on the Seebeck coefficient of these wires remains largely unexplored. We developed an electroless etching technique to generate nanowire arrays (NWAs) with controlled surface roughness, morphology, porosity and doping [2]. We conduct the simultaneous device-level measurements of the Seebeck coefficient and thermal conductivity of the NWAs using frequency domain techniques. We observe that nano-structuring quenches the phonon drag [3] in NWAs thereby reducing the Seebeck coefficient by ˜25% compared to the bulk at degenerate doping levels. Further, we observe that the sidewall roughness greater than 3 nm roughness height lowers the thermal conductivity 75% below the Casimir limit [4] with 10% - 15% increase in Seebeck coefficient. The porous NWAs show thermal conductivity close to the amorphous limit of Si with enhancement in the Seebeck coefficient primarily due to the carrier depletion. References: [1] A. I. Hochbaum et al, Nature 451, 163-167 (2008). [2] K. Balasundaram et. al., Nanotechnology 23, 305304 (2012). [3] C. Herring, Phys. Rev. 96, 1163 (1954). [4] H. G. B. Casimir, Physica 5, 495 (1938).
ABSTRACT Nanostructured materials [1-3] have lower thermal conductivities than the bulk and are p... more ABSTRACT Nanostructured materials [1-3] have lower thermal conductivities than the bulk and are promising candidates for thermoelectric applications. In particular, measurements on single silicon nanowires show a reduction in thermal conductivity below the Casimir limit. This reduction increases with surface roughness [4] but the trend and its connection to phonon boundary scattering are still elusive. Here, we measure the thermal conductivity of single silicon nanowires fabricated using metal-assisted chemical etching. High resolution TEM imaging shows crystalline wires with slightly rough surfaces. Their statistical correlation lengths (5-15 nm) and RMS heights (0.8-1.5 nm) are in a range where perturbation-based wave scattering theory is still applicable. We use the thermal conductivity data to extract the frequency dependence of phonon boundary scattering at low temperatures (10-40 K) and show agreement with multiple scattering theory. This work provides insight into enhancing the thermoelectric performance of nanostructures. 1-A. I. Hochbaum et al, Nature Lett. 451, 163-167 (2008). 2-A. J. Minnich et al, Energy Environ. Sci. 2, 466-479 (2009). 3-L. Shi, Nanoscale Microscale Thermophys. Eng. 16, 79--116 (2012). 4-J. Lim et al, Nano Lett. 12, 2475-2482 (2012).
We report simultaneous measurements of thermal conductivity and Seebeck coefficient on array-scal... more We report simultaneous measurements of thermal conductivity and Seebeck coefficient on array-scale silicon nanowires fabricated by metal assisted chemical etching. The measurements are conducted on the solid and the mesoporous nanowire arrays (NWAs) obtained from etching 1 ohm-cm and 0.002 ohm-cm Si substrates respectively. We demonstrate control on sidewall morphology and doping of the arrays that have an aspect ratio up to 20 and 30 % areal coverage. We employ differential frequency-domain measurements, separately on the array and the corresponding substrate to obtain the temperature drop and Seebeck voltage contribution of the nanowire array. The technique is validated by measurements on bulk silicon across the resistivity 0.002-1 ohm-cm. The Seebeck measurements reveal quenching of the phonon drag in the nanowires in comparison to the bulk in the measured temperature range of 300 K- 500 K. The Seebeck coefficient shows a ~18 % decrease in the solid NWAs and ~22 % increase in the...
Thermoelectrics enable solid-state conversion of heat to electricity by the Seebeck effect, but m... more Thermoelectrics enable solid-state conversion of heat to electricity by the Seebeck effect, but must provide scalable and cost-effective technology for practical waste heat harvesting. This dissertation explores the thermoelectric properties of electrochemically etched silicon nanowires through experiments, complemented by charge and thermal transport theories. Electrolessly etched silicon nanowires show anomalously low thermal conductivity that has been attributed to the increased scattering of heat conducting phonons from the surface disorder introduced by etching. The reduction is below the incoherent limit for phonon scattering at the boundary, the so-called Casimir limit. A new model of partially coherent phonon transport shows that correlated multiple scattering of phonons off resonantly matched rough surfaces can indeed lead to thermal conductivity below the Casimir limit. Using design guidelines from the theory, silicon nanowires of controllable surface roughness are fabrica...
2017 IEEE International Ultrasonics Symposium (IUS)
The production and the characterization of high-quality ScAlN films, whose electromechanical coup... more The production and the characterization of high-quality ScAlN films, whose electromechanical coupling and bandwidth are larger than those of traditional AlN films, is attracting interest to develop the next generation of bulk acoustic wave resonators for telecommunication devices operating above 5 GHz.
ABSTRACT Nanostructured materials [1-3] have lower thermal conductivities than the bulk and are p... more ABSTRACT Nanostructured materials [1-3] have lower thermal conductivities than the bulk and are promising candidates for thermoelectric applications. In particular, measurements on single silicon nanowires show a reduction in thermal conductivity below the Casimir limit. This reduction increases with surface roughness [4] but the trend and its connection to phonon boundary scattering are still elusive. Here, we measure the thermal conductivity of single silicon nanowires fabricated using metal-assisted chemical etching. High resolution TEM imaging shows crystalline wires with slightly rough surfaces. Their statistical correlation lengths (5-15 nm) and RMS heights (0.8-1.5 nm) are in a range where perturbation-based wave scattering theory is still applicable. We use the thermal conductivity data to extract the frequency dependence of phonon boundary scattering at low temperatures (10-40 K) and show agreement with multiple scattering theory. This work provides insight into enhancing the thermoelectric performance of nanostructures. 1-A. I. Hochbaum et al, Nature Lett. 451, 163-167 (2008). 2-A. J. Minnich et al, Energy Environ. Sci. 2, 466-479 (2009). 3-L. Shi, Nanoscale Microscale Thermophys. Eng. 16, 79--116 (2012). 4-J. Lim et al, Nano Lett. 12, 2475-2482 (2012).
Understanding thermal transport in two-and three-dimensional periodic "holey" nanostructures is i... more Understanding thermal transport in two-and three-dimensional periodic "holey" nanostructures is important for realizing applications of these structures in thermoelectrics, photonics and batteries. In terms of continuum heat diffusion physics, the effective medium theory provides the framework for obtaining the effective thermal conductivity of such structures. However, recently measured nanostructures possess thermal conductivities well below these continuum predictions. In some cases, their thermal conductivities are even lower than predictions that account for sub-continuum phonon transport. We analyze current understanding of thermal transport in such structures, discussing the various theories, the measurements and the insights gained from comparing the two.
Top-down electroless chemical etching enables non-lithographic patterning of wafer-scale nanostru... more Top-down electroless chemical etching enables non-lithographic patterning of wafer-scale nanostructured arrays, but the etching on highly doped wafers produces porous structures. The lack of defect-free nanostructures at desired doping and the difficulties in forming reliable electrical side-contacts to the nanostructure arrays limits their integration into high performance nanoelectronics. We developed a barrier layer diffusion technique to controllably dope waferscale silicon nanowire arrays (10 17-10 20 cm −3) produced by chemically etching lightly doped silicon wafers. In order to achieve low resistance top-side electrical contacts to the arrays, we developed a two step tip-doping procedure to locally dope the tips (∼10 20 cm −3) to metallic levels. The dopant concentration is characterized by depth profiling using secondary ion mass spectroscopy and four-point probe electrical measurements. Further, array scale electrical measurements show that the tip-doping lowers the specific contact resistivity (∼10 −5 Ω cm 2) since the metallic tips enable direct tunneling of electrons across the nickel silicide contacts to the nanowire arrays. This work provides a scalable and cost-effective doping approach to control charge injection and charge conduction in nanowire arrays, thus advancing their integration into various device applications.
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Papers by jyothi sadhu