Journal Articles by Brian Daly
Journal of Applied Physics, 2008
In this paper, we report the measurements of the thermal conductivity and longitudinal sound velo... more In this paper, we report the measurements of the thermal conductivity and longitudinal sound velocity of four types of hydrogenated amorphous carbon films. The measurements were made with an ultrafast optical pump-probe apparatus. The films were deposited at various temperatures by plasma enhanced chemical vapor deposition. The films are of a low density ͑1.0-1.4 g cm −3 ͒ and are found to have sound velocities that range from as low as 3 km/s to as high as 10 km/s. The thermal conductivities are between 0.6 and 1.4 W/mK which are low compared to amorphous SiO 2 but are larger than published measurements of thin films of similar composition and density by a factor of 2-3.
Papers by Brian Daly
Applied Physics Letters, Jun 10, 2004
We demonstrate submicron resolution imaging using picosecond acoustic phonon pulses. High-frequen... more We demonstrate submicron resolution imaging using picosecond acoustic phonon pulses. High-frequency acoustic pulses are generated by impulsive thermoelastic excitation of a patterned 15-nm-thick metal film on a crystalline substrate using ultrafast optical pulses. The spatiotemporal diffracted acoustic strain field is measured on the opposite side of the substrate, and this field is used in a time-reversal algorithm to reconstruct the object. The image resolution is characterized using lithographically defined 1-micron-period Al structures on Si. Straightforward technical improvements should lead to resolution approaching 45 nm, extending the resolution of acoustic microscopy into the nanoscale regime.
Journal of physics, Dec 1, 2007
In this paper we present a series of experiments which show that 2-D and possibly 3-D imaging wit... more In this paper we present a series of experiments which show that 2-D and possibly 3-D imaging with sub-micron resolution is possible by means of ultrafast acoustic techniques. Optical pulses from a Ti:sapphire laser are used to generate picosecond acoustic pulses on one side of a ~1 mm thick Si wafer. The 1 mm distance is sufficient for the acoustic waves to diffract to the far field before they are detected by time-delayed probe pulses from the Ti:sapphire laser. The acoustic waves are either generated by a surface nanostructure or scattered from a buried nanostructure, and an image of that nanostructure is reconstructed through an analysis of the detected acoustic waves.
Physical Review B, 2009
We report ultrafast optical measurements of the attenuation of 50 and 100 GHz longitudinal acoust... more We report ultrafast optical measurements of the attenuation of 50 and 100 GHz longitudinal acoustic-phonon pulses in Si. Picosecond acoustic measurements were made at temperatures 50Ͻ T Ͻ 300 K on thinned ͑50-m-thick͒ wafers. The measured phonon lifetimes at 300 K, Ϸ5-7 ns, are an order of magnitude less than expected based on three-phonon scattering rates derived from thermal conductivity data. We find instead that relaxational damping is the dominant mechanism in this frequency and temperature range. This attenuation sets an intrinsic limit on the quality factor of nanomechanical resonators that operate near room temperature.
Bulletin of the American Physical Society, Mar 8, 2018
Bulletin of the American Physical Society, Mar 3, 2015
2 / Si (100) substrate. The TiN film was etched with lines of rectangular cross-section with pitc... more 2 / Si (100) substrate. The TiN film was etched with lines of rectangular cross-section with pitch = 168 to 420 nm. Ultrafast optical experiments on these samples have detected high frequency surface waves in the range of 10's of GHz. The MD simulation demonstrates the presence of strongly excited modes at frequencies that closely match those found in the experiments. Moreover, the simulation predicts that the type of surface wave mode detected should change depending on the pitch. For larger pitch, Rayleigh-like waves are predicted, but for smaller pitch, Sezawa waves (surface waves with properties similar to free plate modes of the thin films) are predicted. The MD simulation also demonstrates the cutoff wavelength for the Sezawa modes, as is reflected in the experimental results and as is also predicted by isotropic elastic calculations of the surface modes of a thin film on an infinite substrate.
Journal of Applied Physics, Mar 6, 2015
We have measured surface acoustic waves on nanostructured TiN wires overlaid on multiple thin fil... more We have measured surface acoustic waves on nanostructured TiN wires overlaid on multiple thin films on a silicon substrate using the ultrafast pump-probe technique known as picosecond ultrasonics. We find a prominent oscillation in the range of 11-54 GHz for samples with varying pitch ranging from 420 nm down to 168 nm. We find that the observed oscillation increases monotonically in frequency with decrease in pitch, but that the increase is not linear. By comparing our data to twodimensional mechanical simulations of the nanostructures, we find that the type of surface oscillation to which we are sensitive changes depending on the pitch of the sample. Surface waves on substrates that are loaded by thin films can take multiple forms, including Rayleigh-like waves, Sezawa waves, and radiative (leaky) surface waves. We describe evidence for detection of modes that display characteristics of these three surface wave types. V
Bulletin of the American Physical Society, 2017
2 / Si (100) substrate. The Al was etched to form lines of rectangular cross section with pitches... more 2 / Si (100) substrate. The Al was etched to form lines of rectangular cross section with pitches ranging from 1000 nm down to 140 nm and the lines were oriented parallel to the [110] direction on the wafer surface. The absorption of ultrafast pulses from a Ti:sapphire oscillator operating at 800 nm generated SAWs that were detected by time-delayed probe pulses from the same oscillator via a reflectivity change (∆R). The SAW frequency increased with decreasing pitch in a non-linear fashion due to dispersion of the wave caused by the presence of the oxide layer. We also experimentally demonstrate the traveling of the SAW's by separating the focused pump and probe laser spots by several microns. We compare the results to coarse-grained molecular dynamics simulations and simplified calculations using isotropic elasticity theory.
We present ultrafast optical measurements of the attenuation of 50 -- 100 GHz ultrasound in hydro... more We present ultrafast optical measurements of the attenuation of 50 -- 100 GHz ultrasound in hydrogenated amorphous silicon (a-Si:H) thin films. The films were grown using a modified very high frequency glow discharge method on steel substrates. The deposition conditions were similar to those used for high efficiency solar cells. The measurements were performed at 300 K using the picosecond ultrasonics technique. Films of varying thickness were measured so that the effect of intrinsic acoustic loss within the a-Si:H could be determined. We find that the ultrasonic attenuation in a-Si:H at 100 GHz is more than an order of magnitude lower than is found in other amorphous materials. Our results may impact theoretical models of thermal transport in amorphous materials, and could provide a new avenue for studying voids in a-Si:H and nanocrystalline Si films.
Journal of Applied Physics, 2015
We have measured surface acoustic waves on nanostructured TiN wires overlaid on multiple thin fil... more We have measured surface acoustic waves on nanostructured TiN wires overlaid on multiple thin films on a silicon substrate using the ultrafast pump-probe technique known as picosecond ultrasonics. We find a prominent oscillation in the range of 11–54 GHz for samples with varying pitch ranging from 420 nm down to 168 nm. We find that the observed oscillation increases monotonically in frequency with decrease in pitch, but that the increase is not linear. By comparing our data to two-dimensional mechanical simulations of the nanostructures, we find that the type of surface oscillation to which we are sensitive changes depending on the pitch of the sample. Surface waves on substrates that are loaded by thin films can take multiple forms, including Rayleigh-like waves, Sezawa waves, and radiative (leaky) surface waves. We describe evidence for detection of modes that display characteristics of these three surface wave types.
2 / Si (100) substrate. The TiN layer was dry plasma etched to form lines of rectangular cross se... more 2 / Si (100) substrate. The TiN layer was dry plasma etched to form lines of rectangular cross section with pitches of 420 nm, 250 nm, 180 nm, and 168 nm and the lines were oriented parallel to the [110] direction on the wafer surface. The absorption of ultrafast pulses from a Ti:sapphire oscillator operating at 800 nm generated SAWs that were detected by time-delayed probe pulses from the same oscillator via a reflectivity change (∆R). In each of the four cases the SAW frequency increased with decreasing pitch, but not in a linear way as had been seen in previous experiments of this sort. By comparing the results with mechanical simulations, we present evidence for the detection of different types of SAWs in each case, including Rayleigh-like waves, Sezawa waves, and leaky or radiative waves.
Physical Review B, 2003
We report on molecular dynamics calculations of the thermal conductivity of superlattices in the ... more We report on molecular dynamics calculations of the thermal conductivity of superlattices in the direction parallel to the layers. We employ a simple, classical, fcc model that is designed to model the GaAs/AlAs system. Both rough and perfect interfaces are considered. The results are compared to experimental data from the literature.
We present ultrafast optical pump-probe measurements of thermal transport in a series of amorphou... more We present ultrafast optical pump-probe measurements of thermal transport in a series of amorphous SiC samples. The samples were grown on Si wafers by plasma enhanced chemical vapor deposition utilizing various combinations of methylsilanes and H2 and He diluent gases. The sample films were well characterized and found to have densities (1.3 -- 2.3 g cm-3) and dielectric constants (4.0
Journal of Nanophotonics, 2013
Brillouin light scattering (BLS) and picosecond laser ultrasonics (PLU) are two noncontact optica... more Brillouin light scattering (BLS) and picosecond laser ultrasonics (PLU) are two noncontact optical techniques that have garnered significant interest for thin film elastic constant measurements. PLU and BLS measurements were utilized to determine the elastic constants of 100 to 500 nm thick nanoporous low-k dielectric materials of significant interest for reducing capacitive delays in nanoelectronic interconnect circuits. PLU measurements with and without a metal acousto-optic transducer are described in detail and compared to previously reported BLS measurements. The values of Young's modulus determined by both BLS and PLU were found to be in excellent agreement and consistent with nanoindentation measurements on thicker 2 micrometer films. While successful BLS measurements were achieved for films as thin as 100 nm, PLU measurements were limited to > ∼ 200 nm thick films due to experimental constraints on observing acoustic pulses in thinner films. However, these results clearly demonstrate the capability of both BLS and PLU to determine the elastic constants of low-k dielectric materials at the desired thickness targets for future nanoelectronic interconnect technologies.
Plasma Enhanced Chemically Vapor Deposited a-SiC:H thin films are compelling materials for both s... more Plasma Enhanced Chemically Vapor Deposited a-SiC:H thin films are compelling materials for both semiconductor nano-electronic and MEMS/NEMS technologies due to the extreme chemical inertness of this material and the ability to tune a variety of material properties across an extreme range of values. As one example of the latter, we demonstrate that using PECVD the Young's modulus of a-SiC:H thin
Physical Review B, 2004
We report experimental and theoretical investigations of the behavior of coherent acoustic phonon... more We report experimental and theoretical investigations of the behavior of coherent acoustic phonon pulses after propagation through millimeter-scale distances in crystalline Si. An ultrafast optical pump and probe technique is used to generate and detect the phonon pulses. The geometry of our experiment is such that we can make this measurement in either the far field or the near field of the acoustic source, allowing studies of diffraction effects in addition to dispersion and nonlinearity in the Si. We also present a rigorous derivation of the Korteweg-de Vries equation, which describes the behavior of these acoustic pulses in the nonlinear regime in one dimension. This one-dimensional (1D) model is combined with a 3D analysis of diffraction effects in anisotropic media in order to analyze far-field data.
Physical Review B, 2011
We have measured the attenuation of longitudinal acoustic waves in a series of amorphous and nano... more We have measured the attenuation of longitudinal acoustic waves in a series of amorphous and nanocrystalline silicon films using picosecond ultrasonics. The films were grown using a modified very high frequency glow discharge method on steel substrates. The deposition conditions were similar to that used in the fabrication of high efficiency solar cells. The film thicknesses were varied so that we could distinguish between interface losses and intrinsic losses within the silicon films. We determine the attenuation of amorphous Si to be 780 ± 160 cm-1 at 100 GHz and 340 ± 120 cm-1 at 50 GHz, values that are lower than predicted by theories based on anharmonic interactions of the sound wave with localized phonons or extended resonant modes. We determine the attenuation of nanocrystalline Si at 50 GHz to be nearly an order of magnitude higher than amorphous Si (2600 ± 660 cm-1) and compare that value to a simple Rayleigh scattering prediction.
We present ultrafast optical measurements of longitudinal sound velocity and thermal transport in... more We present ultrafast optical measurements of longitudinal sound velocity and thermal transport in hydrogenated amorphous carbon (a-SiC:H) films. The films were grown on Si wafers by PECVD using combinations of methylsilanes and H2 and He diluent gases. The films were well characterized and found to have densities (1.0 -- 2.5 g cm-3) and dielectric constants (2.8 -- 7.2) that spanned a wide range of values. Prior to their measurement, the a-SiC:H films were coated with 40-70 nm of polycrystalline Al. The pump-probe measurements were performed at room temperature using a modelocked Ti:sapphire laser. Transient reflectivity changes that are associated with very high frequency sound waves (picosecond ultrasonics) and the cooling rate of the SiC sample (Time Domain Thermorerflectance (TDTR)) were measured. We extract values for the thermal conductivity and sound velocity of the SiC films, and analyze the results in terms of rigidity percolation effects within the SiC layers.
Journal of Non-Crystalline Solids, 2013
As demand for lower power and higher performance nano-electronic products increases, the semicond... more As demand for lower power and higher performance nano-electronic products increases, the semiconductor industry must adopt insulating materials with progressively lower dielectric constants (i.e. low-k) in order to minimize capacitive related power losses in integrated circuits. However in addition to a lower dielectric constant, low-k materials typically exhibit many other reduced material properties that have limited the ability of the semiconductor industry to implement them. In this article, we demonstrate that the reduced material properties exhibited by low-k materials can be understood based on bond constraint and percolation theory. Using a-SiC:H as a case study material, we utilize nuclear reaction analysis, Rutherford backscattering, nuclear magnetic resonance and transmission Fourier transform infra-red spectroscopy measurements to determine the average coordination (〈r〉) for these materials. Correlations of 〈r〉 to Young's modulus, hardness, thermal conductivity, resistivity, refractive index, intrinsic stress, mass density and porosity show that an extremely wide range in material properties (in some cases several orders of magnitude) can be achieved through reducing 〈r〉 via the controlled incorporation of terminal Si\H x and C\H x groups. We also demonstrate that the critical point at 〈r〉 ≤ 2.4 predicted by constraint theory exists in this material system and places limitations on the range of properties that can be achieved for future low-k a-SiC:H materials.
Journal of Applied Physics, 2002
We report on measurements of the thermal conductivity of epitaxially grown nitride thin films. Th... more We report on measurements of the thermal conductivity of epitaxially grown nitride thin films. These semiconductor materials are of considerable technological importance for applications such as blue-light emitters and high-power, high frequency electronic devices. Measurements were made in the temperature range of 150–400 K using an optical technique in which the sample is heated with an ultrashort (∼150 fs) light pulse, and a time-delayed probe light pulse is used to measure the temperature of the sample as a function of time. The conductivity of the polycrystalline sample and the alloys are found to be significantly reduced compared to published values for bulk GaN.
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Journal Articles by Brian Daly
Papers by Brian Daly