We studied physical properties of titanium hafnium oxide (Ti x Hf 1-x O 2) alloy thin films depos... more We studied physical properties of titanium hafnium oxide (Ti x Hf 1-x O 2) alloy thin films deposited by pulsed DC reactive magnetron sputtering with AC substrate bias. Thin films of two end oxides, hafnium oxides (HfO 2) and titanium oxides (TiO 2), and their alloys Ti x Hf 1-x O 2 with a range of compositions deposited with and without the substrate bias were compared to study the dependence of physical properties of the thin films on the substrate bias. Structural, chemical and optical properties of the thin films were analyzed to assess interrelationship among these properties. Thin films deposited with the AC substrate bias consistently show much higher refractive index and significantly lower optical extinction coefficient than those of thin films deposited without the substrate bias suggesting that characteristic microstructures developed in these thin films are responsible for the differences in the optical properties.
Langmuir : the ACS journal of surfaces and colloids, Jan 21, 2015
The structural properties of optically thin (15 nm) silver (Ag) films deposited on SiO2/Si(100) s... more The structural properties of optically thin (15 nm) silver (Ag) films deposited on SiO2/Si(100) substrates with a germanium (Ge) nucleation layer were studied. The morphological and crystallographical characteristics of Ag thin films with different Ge nucleation layer thicknesses were assessed by cross-sectional transmission electron microscopy (XTEM), reflection high-energy electron diffraction (RHEED), X-ray diffractometry (XRD), grazing incidence X-ray diffractometry (GIXRD), X-ray reflection (XRR), and Fourier transform infrared spectroscopy (FTIR). The surface roughness of Ag thin films was found to decrease significantly by inserting a Ge nucleation layer with a thickness in the range of 1 to 2 nm (i.e., smoothing mode). However, as the Ge nucleation layer thickness increased beyond 2 nm, the surface roughness increased concomitantly (i.e., roughing mode). For the smoothing mode, the role of the Ge nucleation layer in the Ag film deposition is discussed by invoking the surface...
Plasmonics: Metallic Nanostructures and Their Optical Properties XI, 2013
ABSTRACT Chemical sensing applications utilizing surface enhanced Raman spectroscopy (SERS) have ... more ABSTRACT Chemical sensing applications utilizing surface enhanced Raman spectroscopy (SERS) have drawn significant attention recently. However, developing a reliable, high performance SERS platform remains a challenge. A novel SERS substrate based on nanofingers was successfully demonstrated to provide large enhancement reliably and showed great promise for practical applications. Capillary forces bring the gold caps on the nanofingers into close proximity upon exposure to a solution containing molecules of interest, trapping molecules within the gaps and producing greatly enhanced Raman signals. Transmission electron microscopy (TEM) was used to characterize the structure of the nanofingers, in particular the gaps between finger tips to improve the fundamental understanding of the structural-performance relationship.
Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation, 2014
ABSTRACT We describe progress in the on-going effort at the University of California Observatorie... more ABSTRACT We describe progress in the on-going effort at the University of California Observatories Advanced Coatings Lab to develop efficient, durable silver-based coatings for telescope mirrors. We have continued to improve previously identified recipes produced with e-beam ion-assisted deposition (IAD). We have started exploring nitride adhesion and barrier layers added to or replacing layers in promising recipes. Our coating chamber now has one magnetron installed, and two more will be added shortly so we can perform direct comparisons of e-beam IAD and sputtering processes for the same recipes. We report on recent tests and findings relevant to protected-Ag coatings, including e-beam vs sputter deposited silver; our current work with nitrides; and a comparison of certain fluorides. While focused on telescope mirror coatings, we have also developed and tested two Ag-based coatings suitable for AO and for CCD-range instruments. We also report on field-testing of earlier samples that have been exposed in the dome of the 3-m telescope at Lick Observatory for a period of 2 years. Finally, we describe results of a pilot study using atomic-layer deposition (ALD), a chemical vapor deposition technique, to produce barrier layers over silver. Optical quality ALD films are smooth, conformal and have excellent uniformity and thickness control, and their barrier properties look extremely promising for protecting silver from corrosion.
ABSTRACT Our nation discards more than 50% of the total input energy as waste heat in various ind... more ABSTRACT Our nation discards more than 50% of the total input energy as waste heat in various industrial processes such as metal refining, heat engines, and cooling. If we could harness a small fraction of the waste heat through the use of thermoelectric (TE) devices while satisfying the economic demands of cost versus performance, then TE power generation could bring substantial positive impacts to our society in the forms of reduced carbon emissions and additional energy. To increase the unit-less figure of merit, ZT, single-crystal semiconductor nanowires have been extensively studied as a building block for advanced TE devices because of their predicted large reduction in thermal conductivity and large increase in power factor. In contrast, polycrystalline bulk semiconductors also indicate their potential in improving overall efficiency of thermal-to-electric conversion despite their large number of grain boundaries. To further our goal of developing practical and economical TE devices, we designed a material platform that combines nanowires and polycrystalline semiconductors which are integrated on a metallic surface. We will assess the potential of polycrystalline group III-V compound semiconductor nanowires grown on low-cost copper sheets that have ideal electrical/thermal properties for TE devices. We chose indium phosphide (InP) from group III-V compound semiconductors because of its inherent characteristics of having low surface states density in comparison to others, which is expected to be important for polycrystalline nanowires that contain numerous grain boundaries. Using metal organic chemical vapor deposition (MOCVD) polycrystalline InP nanowires were grown in three-dimensional networks in which electrical charges and heat travel under the influence of their characteristic scattering mechanisms over a distance much longer than the mean length of the constituent nanowires. We studied the growth mechanisms of polycrystalline InP nanowires on copper surfaces by analyzing their chemical, optical, and structural properties in comparison to those of single-crystal InP nanowires formed on single-crystal surfaces. We also assessed the potential of polycrystalline InP nanowires on copper surfaces as a TE material by modeling based on finite-element analysis to obtain physical insights of three-dimensional networks made of polycrystalline InP nanowires. Our discussion will focus on the synthesis of polycrystalline InP nanowires on copper surfaces and structural properties of the nanowires analyzed by transmission electron microscopy that provides insight into possible nucleation mechanisms, growth mechanisms, and the nature of grain boundaries of the nanowires.
ABSTRACT An urgent demand remains in astronomy for high-reflectivity silver mirrors that can with... more ABSTRACT An urgent demand remains in astronomy for high-reflectivity silver mirrors that can withstand years of exposure in observatory environments. The University of California Observatories Astronomical Coatings Lab has undertaken development of protected silver coatings suitable for telescope mirrors that maintain high reflectivity at wavelengths from 340 nm through the mid-infrared spectrum. We present initial results of an investigation into whether plasma-enhanced atomic layer deposition (PEALD) can produce superior protective layers of transparent dielectrics. Several novel coating recipes have been developed with ion-assisted electron beam deposition (IAEBD) of materials including yttrium fluoride, and oxides of yttrium, hafnium, and titanium. Samples of these mirror coatings were covered with conformal layers of aluminum oxide (AlOx) deposited by PEALD using trimethylaluminum as a metal precursor and oxygen as an oxidant gas activated by remote plasma. Samples of coating recipes with and without PEALD oxide undergo aggressive environmental testing, including high temperature/high humidity (HTHH), in which samples were exposed to an environment of 80% humidity at 80°C for ten days in a simple test set-up. HTHH testing show visible results suggesting that the PEALD oxide offers enhanced robust protection against chemical corrosion and moisture from an accelerated aging environment. Mirror samples are further characterized by reflectivity/absorption and atomic force microscopy before and after deposition of oxide coatings. AlOx is suitable for many applications and has been the initial material choice for this study, although we also tried TiOx and HfOx. Further experimentation based on these initial results is on-going.
ABSTRACT A range of optical and optoelectronic applications would benefit from high refractive in... more ABSTRACT A range of optical and optoelectronic applications would benefit from high refractive index ( n ), dense and transparent films that guide, concentrate and couple light. However, materials with high n usually have a high optical extinction coefficient (κ) which keeps these materials from being suitable for optical components that require long optical paths. We studied titanium hafnium oxide alloy films to obtain high refractive index ( n >2) with minimum optical extinction coefficients (κ < 10 −5 ) over the visible and near IR spectrum (380-930 nm). Titanium hafnium oxide alloys were deposited using pulsed DC reactive magnetron sputtering with and without RF substrate bias on silicon dioxide. For a given deposition condition intended for a specific titanium/hafnium molar fraction ratio, the ion energy of deposition species was explicitly controlled by varying the RF substrate bias. Spectroscopic ellipsometry, transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS) and atomic force microscopy (AFM) were used to characterize the films. It appears that applying RF substrate bias reduces the nanocrystalline size, changes the surface morphology and increases the refractive index while maintaining comparable titanium/hafnium cation molar fraction. Precise control of the nanostructure of ternary metal oxides can alter their macroscopic properties, resulting in improved optical films.
ABSTRACT The growth of indium phosphide (InP) nanowires on transparent conductive aluminum-doped ... more ABSTRACT The growth of indium phosphide (InP) nanowires on transparent conductive aluminum-doped zinc oxide (AZO) thin films on polycrystalline copper (Cu) foils was proposed and demonstrated. AZO thin films and zinc oxide (ZnO) thin films, as comparison, were deposited on Cu foils by radio frequency magnetron sputtering. Subsequently, InP was grown by metal organic chemical vapor deposition with gold catalysts. InP nanowire networks formed on the AZO thin films, while no InP nanowires grew on the ZnO thin films. Morphological, crystalline, and optical properties of the InP nanowires on AZO thin films were compared with those of InP nanowires grown on silicon (Si) substrates. Zinc diffusion from AZO thin films into InP nanowire networks was suggested as the cause of substantial modifications on the optical properties of the InP nanowires on AZO thin films; redshift in photoluminescence spectra and a larger relative TO/LO intensity ratio in Raman spectra were observed, in comparison to those of the InP nanowires grown on Si substrates. In this paper, we proposed and demonstrated a new route to grow semiconductor nanowires on metals that potentially provide low-cost and mechanically flexible substrates and establish a reliable electrical contact by utilizing conductive oxide thin films as a template, which could offer a new material platform for such applications as sensors and thermoelectric devices.
Journal of Materials Science: Materials in Electronics, 2014
ABSTRACT Enhanced Raman signal of the longitudinal optical phonon mode in indium phosphide nanowi... more ABSTRACT Enhanced Raman signal of the longitudinal optical phonon mode in indium phosphide nanowire networks with gold coating of up to 5 nm thickness was observed experimentally to further study the phonon spectrum of nanowire networks. Indium phosphide nanowire networks coated with different nominal thicknesses of gold were prepared and optically studied. Scanning electron microscopy, photoluminescence spectroscopy and Raman spectroscopy were used to study the dependence of surface morphology and phonon modes of the nanowire networks on the nominal thickness of the gold coating. The Raman peak of longitudinal optical phonon mode for as grown sample was negligible, while the peak intensity for 1 and 5 nm gold coated sample reached to 1,379 and 792 a.u. respectively. Electromagnetic enhancement and extinction coefficient are discussed to qualitatively assess the role of the gold coating on indium phosphide nanowire networks.
ABSTRACT Indium phosphide (InP) nanowires were grown by metal organic chemical vapor deposition (... more ABSTRACT Indium phosphide (InP) nanowires were grown by metal organic chemical vapor deposition (MOCVD). InP nanowires grew in the structure of three-dimensional networks in which electrical charges and heat can travel over distances much longer than the mean length of the constituent nanowires. We studied the dependence of thermoelectric properties on geometrical factors within the InP nanowire networks. The InP nanowire networks show Seebeck coefficients comparable with that of bulk InP. Rather than studying single nanowires, we chose networks of nanowires formed densely across large areas required for large scale production. We also studied the role played by intersections where multiple nanowires were fused to form the nanowire networks. Modeling based on finite-element analysis, structural analysis, and transport measurements were carried out to obtain insights of physical properties at the intersections. Understanding these physical properties of three-dimensional nanowire networks will advance the development of thermoelectric devices.
We studied physical properties of titanium hafnium oxide (Ti x Hf 1-x O 2) alloy thin films depos... more We studied physical properties of titanium hafnium oxide (Ti x Hf 1-x O 2) alloy thin films deposited by pulsed DC reactive magnetron sputtering with AC substrate bias. Thin films of two end oxides, hafnium oxides (HfO 2) and titanium oxides (TiO 2), and their alloys Ti x Hf 1-x O 2 with a range of compositions deposited with and without the substrate bias were compared to study the dependence of physical properties of the thin films on the substrate bias. Structural, chemical and optical properties of the thin films were analyzed to assess interrelationship among these properties. Thin films deposited with the AC substrate bias consistently show much higher refractive index and significantly lower optical extinction coefficient than those of thin films deposited without the substrate bias suggesting that characteristic microstructures developed in these thin films are responsible for the differences in the optical properties.
Langmuir : the ACS journal of surfaces and colloids, Jan 21, 2015
The structural properties of optically thin (15 nm) silver (Ag) films deposited on SiO2/Si(100) s... more The structural properties of optically thin (15 nm) silver (Ag) films deposited on SiO2/Si(100) substrates with a germanium (Ge) nucleation layer were studied. The morphological and crystallographical characteristics of Ag thin films with different Ge nucleation layer thicknesses were assessed by cross-sectional transmission electron microscopy (XTEM), reflection high-energy electron diffraction (RHEED), X-ray diffractometry (XRD), grazing incidence X-ray diffractometry (GIXRD), X-ray reflection (XRR), and Fourier transform infrared spectroscopy (FTIR). The surface roughness of Ag thin films was found to decrease significantly by inserting a Ge nucleation layer with a thickness in the range of 1 to 2 nm (i.e., smoothing mode). However, as the Ge nucleation layer thickness increased beyond 2 nm, the surface roughness increased concomitantly (i.e., roughing mode). For the smoothing mode, the role of the Ge nucleation layer in the Ag film deposition is discussed by invoking the surface...
Plasmonics: Metallic Nanostructures and Their Optical Properties XI, 2013
ABSTRACT Chemical sensing applications utilizing surface enhanced Raman spectroscopy (SERS) have ... more ABSTRACT Chemical sensing applications utilizing surface enhanced Raman spectroscopy (SERS) have drawn significant attention recently. However, developing a reliable, high performance SERS platform remains a challenge. A novel SERS substrate based on nanofingers was successfully demonstrated to provide large enhancement reliably and showed great promise for practical applications. Capillary forces bring the gold caps on the nanofingers into close proximity upon exposure to a solution containing molecules of interest, trapping molecules within the gaps and producing greatly enhanced Raman signals. Transmission electron microscopy (TEM) was used to characterize the structure of the nanofingers, in particular the gaps between finger tips to improve the fundamental understanding of the structural-performance relationship.
Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation, 2014
ABSTRACT We describe progress in the on-going effort at the University of California Observatorie... more ABSTRACT We describe progress in the on-going effort at the University of California Observatories Advanced Coatings Lab to develop efficient, durable silver-based coatings for telescope mirrors. We have continued to improve previously identified recipes produced with e-beam ion-assisted deposition (IAD). We have started exploring nitride adhesion and barrier layers added to or replacing layers in promising recipes. Our coating chamber now has one magnetron installed, and two more will be added shortly so we can perform direct comparisons of e-beam IAD and sputtering processes for the same recipes. We report on recent tests and findings relevant to protected-Ag coatings, including e-beam vs sputter deposited silver; our current work with nitrides; and a comparison of certain fluorides. While focused on telescope mirror coatings, we have also developed and tested two Ag-based coatings suitable for AO and for CCD-range instruments. We also report on field-testing of earlier samples that have been exposed in the dome of the 3-m telescope at Lick Observatory for a period of 2 years. Finally, we describe results of a pilot study using atomic-layer deposition (ALD), a chemical vapor deposition technique, to produce barrier layers over silver. Optical quality ALD films are smooth, conformal and have excellent uniformity and thickness control, and their barrier properties look extremely promising for protecting silver from corrosion.
ABSTRACT Our nation discards more than 50% of the total input energy as waste heat in various ind... more ABSTRACT Our nation discards more than 50% of the total input energy as waste heat in various industrial processes such as metal refining, heat engines, and cooling. If we could harness a small fraction of the waste heat through the use of thermoelectric (TE) devices while satisfying the economic demands of cost versus performance, then TE power generation could bring substantial positive impacts to our society in the forms of reduced carbon emissions and additional energy. To increase the unit-less figure of merit, ZT, single-crystal semiconductor nanowires have been extensively studied as a building block for advanced TE devices because of their predicted large reduction in thermal conductivity and large increase in power factor. In contrast, polycrystalline bulk semiconductors also indicate their potential in improving overall efficiency of thermal-to-electric conversion despite their large number of grain boundaries. To further our goal of developing practical and economical TE devices, we designed a material platform that combines nanowires and polycrystalline semiconductors which are integrated on a metallic surface. We will assess the potential of polycrystalline group III-V compound semiconductor nanowires grown on low-cost copper sheets that have ideal electrical/thermal properties for TE devices. We chose indium phosphide (InP) from group III-V compound semiconductors because of its inherent characteristics of having low surface states density in comparison to others, which is expected to be important for polycrystalline nanowires that contain numerous grain boundaries. Using metal organic chemical vapor deposition (MOCVD) polycrystalline InP nanowires were grown in three-dimensional networks in which electrical charges and heat travel under the influence of their characteristic scattering mechanisms over a distance much longer than the mean length of the constituent nanowires. We studied the growth mechanisms of polycrystalline InP nanowires on copper surfaces by analyzing their chemical, optical, and structural properties in comparison to those of single-crystal InP nanowires formed on single-crystal surfaces. We also assessed the potential of polycrystalline InP nanowires on copper surfaces as a TE material by modeling based on finite-element analysis to obtain physical insights of three-dimensional networks made of polycrystalline InP nanowires. Our discussion will focus on the synthesis of polycrystalline InP nanowires on copper surfaces and structural properties of the nanowires analyzed by transmission electron microscopy that provides insight into possible nucleation mechanisms, growth mechanisms, and the nature of grain boundaries of the nanowires.
ABSTRACT An urgent demand remains in astronomy for high-reflectivity silver mirrors that can with... more ABSTRACT An urgent demand remains in astronomy for high-reflectivity silver mirrors that can withstand years of exposure in observatory environments. The University of California Observatories Astronomical Coatings Lab has undertaken development of protected silver coatings suitable for telescope mirrors that maintain high reflectivity at wavelengths from 340 nm through the mid-infrared spectrum. We present initial results of an investigation into whether plasma-enhanced atomic layer deposition (PEALD) can produce superior protective layers of transparent dielectrics. Several novel coating recipes have been developed with ion-assisted electron beam deposition (IAEBD) of materials including yttrium fluoride, and oxides of yttrium, hafnium, and titanium. Samples of these mirror coatings were covered with conformal layers of aluminum oxide (AlOx) deposited by PEALD using trimethylaluminum as a metal precursor and oxygen as an oxidant gas activated by remote plasma. Samples of coating recipes with and without PEALD oxide undergo aggressive environmental testing, including high temperature/high humidity (HTHH), in which samples were exposed to an environment of 80% humidity at 80°C for ten days in a simple test set-up. HTHH testing show visible results suggesting that the PEALD oxide offers enhanced robust protection against chemical corrosion and moisture from an accelerated aging environment. Mirror samples are further characterized by reflectivity/absorption and atomic force microscopy before and after deposition of oxide coatings. AlOx is suitable for many applications and has been the initial material choice for this study, although we also tried TiOx and HfOx. Further experimentation based on these initial results is on-going.
ABSTRACT A range of optical and optoelectronic applications would benefit from high refractive in... more ABSTRACT A range of optical and optoelectronic applications would benefit from high refractive index ( n ), dense and transparent films that guide, concentrate and couple light. However, materials with high n usually have a high optical extinction coefficient (κ) which keeps these materials from being suitable for optical components that require long optical paths. We studied titanium hafnium oxide alloy films to obtain high refractive index ( n >2) with minimum optical extinction coefficients (κ < 10 −5 ) over the visible and near IR spectrum (380-930 nm). Titanium hafnium oxide alloys were deposited using pulsed DC reactive magnetron sputtering with and without RF substrate bias on silicon dioxide. For a given deposition condition intended for a specific titanium/hafnium molar fraction ratio, the ion energy of deposition species was explicitly controlled by varying the RF substrate bias. Spectroscopic ellipsometry, transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS) and atomic force microscopy (AFM) were used to characterize the films. It appears that applying RF substrate bias reduces the nanocrystalline size, changes the surface morphology and increases the refractive index while maintaining comparable titanium/hafnium cation molar fraction. Precise control of the nanostructure of ternary metal oxides can alter their macroscopic properties, resulting in improved optical films.
ABSTRACT The growth of indium phosphide (InP) nanowires on transparent conductive aluminum-doped ... more ABSTRACT The growth of indium phosphide (InP) nanowires on transparent conductive aluminum-doped zinc oxide (AZO) thin films on polycrystalline copper (Cu) foils was proposed and demonstrated. AZO thin films and zinc oxide (ZnO) thin films, as comparison, were deposited on Cu foils by radio frequency magnetron sputtering. Subsequently, InP was grown by metal organic chemical vapor deposition with gold catalysts. InP nanowire networks formed on the AZO thin films, while no InP nanowires grew on the ZnO thin films. Morphological, crystalline, and optical properties of the InP nanowires on AZO thin films were compared with those of InP nanowires grown on silicon (Si) substrates. Zinc diffusion from AZO thin films into InP nanowire networks was suggested as the cause of substantial modifications on the optical properties of the InP nanowires on AZO thin films; redshift in photoluminescence spectra and a larger relative TO/LO intensity ratio in Raman spectra were observed, in comparison to those of the InP nanowires grown on Si substrates. In this paper, we proposed and demonstrated a new route to grow semiconductor nanowires on metals that potentially provide low-cost and mechanically flexible substrates and establish a reliable electrical contact by utilizing conductive oxide thin films as a template, which could offer a new material platform for such applications as sensors and thermoelectric devices.
Journal of Materials Science: Materials in Electronics, 2014
ABSTRACT Enhanced Raman signal of the longitudinal optical phonon mode in indium phosphide nanowi... more ABSTRACT Enhanced Raman signal of the longitudinal optical phonon mode in indium phosphide nanowire networks with gold coating of up to 5 nm thickness was observed experimentally to further study the phonon spectrum of nanowire networks. Indium phosphide nanowire networks coated with different nominal thicknesses of gold were prepared and optically studied. Scanning electron microscopy, photoluminescence spectroscopy and Raman spectroscopy were used to study the dependence of surface morphology and phonon modes of the nanowire networks on the nominal thickness of the gold coating. The Raman peak of longitudinal optical phonon mode for as grown sample was negligible, while the peak intensity for 1 and 5 nm gold coated sample reached to 1,379 and 792 a.u. respectively. Electromagnetic enhancement and extinction coefficient are discussed to qualitatively assess the role of the gold coating on indium phosphide nanowire networks.
ABSTRACT Indium phosphide (InP) nanowires were grown by metal organic chemical vapor deposition (... more ABSTRACT Indium phosphide (InP) nanowires were grown by metal organic chemical vapor deposition (MOCVD). InP nanowires grew in the structure of three-dimensional networks in which electrical charges and heat can travel over distances much longer than the mean length of the constituent nanowires. We studied the dependence of thermoelectric properties on geometrical factors within the InP nanowire networks. The InP nanowire networks show Seebeck coefficients comparable with that of bulk InP. Rather than studying single nanowires, we chose networks of nanowires formed densely across large areas required for large scale production. We also studied the role played by intersections where multiple nanowires were fused to form the nanowire networks. Modeling based on finite-element analysis, structural analysis, and transport measurements were carried out to obtain insights of physical properties at the intersections. Understanding these physical properties of three-dimensional nanowire networks will advance the development of thermoelectric devices.
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Papers by David Fryauf