Materials used or considered for plasma-side applications are not entirely satisfactory, particul... more Materials used or considered for plasma-side applications are not entirely satisfactory, particularly in the medium edge temperature (100 to 300 eV) regime. An approach to impurity control based on self-sustaining surface segregated low-Z layers with high secondary ion fractions was tested in laboratory experiments. A crucial requirement is that substrate sputtering yields be reduced about an order of magnitude by monolayer adsorbate coverages. Theoretical and experimental evidence is adduced to support the contention that overlayer coverages (a monolayer of Li on Cu) result in profound reductions of substrate (e.g., Cu) sputtering yields. The conclusion that a material such as a dilute alloy of Li in Cu could function as a limiter or a divertor plate material is, in part, based on the fact that more than 85% of the sputtered flux originates in the first atomic layer (e.g., Li) of the target.
Because of the practical importance of sputtering, numerous theories and computer simulations are... more Because of the practical importance of sputtering, numerous theories and computer simulations are used for predicting many aspects of the sputtering process. Unfortunately, many of the calculated sputtering results are untested by experiment. Until recently, most sputtering experiments required either very high ion fluences or the detection of only minor constituents of the sputtered flux, i.e., ions. These techniques may miss the subtleties involved in the sputtering process. High-detection-efficiency mass spectrometry, coupled with the laser ionization of neutral atoms, allows the detection of the major sputtered species with very low incident ion fluences. The depth-of-origin of sputtered atoms is one example of an important but poorly understood aspect of the sputtering process. By following the sputtering yield of a substrate atom with various coverages of an adsorbed overlayer, the depth of origin of sputtered atoms has been determined. Our results indicate that two-thirds of the sputtered flux originates in the topmost atomic layer. The ion-dose dependence of sputtering yields has long been assumed to be quite minor for low- to-moderate primary ion fluences. We have observed a two-fold decrease in the sputtering yield of the Ru(0001) surface for very low primary ion fluences. Data analysis results in a cross section for damage of 2.7 {plus minus} 1.0 à 10â»Â¹âµcm². 40 refs., 3 figs., 2 tabs.
Velocity distributions and relative populations in the fine structure levels of the a(5)D/sub J/ ... more Velocity distributions and relative populations in the fine structure levels of the a(5)D/sub J/ ground state of Fe atoms, produced by sputtering with a 3 keV argon ions, were investigated by Doppler shifted laser induced fluorescence. The laser system employs a single mode, scanning ring dye laser, amplified by a sequence of three excimer pumped flowing dye cells. Frequency doubling in a KD*P crystal was used to produce high energy ( .5 mJ) pulses of narrowband tunable UV output near 300 nm. Laser power influence on effective velocity bandwidth was investigated. Favorable light collection geometry minimized distortion of the velocity spectra from apparatus averaging effects. In impurity flux diagnostic applications in fusion devices, substantial spatial averaging may occur. In the latter case, the narrow velocity bandwidth (70 m/s, transform limit) of the present laser system is particularly useful.
Electron-stimulated desorption of neutral aluminum from the system CH3O/Al(111) has been directly... more Electron-stimulated desorption of neutral aluminum from the system CH3O/Al(111) has been directly monitored via quasiresonant photoionization with 193 nm excimer laser light and confirmed by two-step resonant ionization, utilizing the Al 3D (sup 2)D manifold. Velocity distribution measurements for the neutral Al peak at approximately 800 m/s for 1 keV incident electron energy. An absolute yield of 3.2 x 10(exp -6) Al atoms/electron was determined by comparison with sputtering measurements in the same apparatus. This is the first observation of electron-stimulated metal desorption from adsorbate-covered metallic surfaces.
ABSTRACT We report the use of atomic layer deposition (ALD) to synthesize thin superconducting fi... more ABSTRACT We report the use of atomic layer deposition (ALD) to synthesize thin superconducting films and multilayer superconductor-insulator (S-I) heterostructures. The ALD technique applied to superconducting films opens the way for a variety of applications, including improving the performance and decreasing the cost of high energy particle accelerators, superconducting wires for energy storage, and bolometers for radiation detection. Furthermore, the atomic-scale thickness control afforded by ALD enables the study of superconductivity and associated phenomena in homogeneous layers in the ultra-thin film limit. In this respect, we will present results of ALD-grown transition metal-based superconductors, including nitrides, carbides, and silicides of niobium, nitrides of molybdenum and titanium, and Nb1-xTixN/AlN-based S-I heterostructures. Transport measurement for various composition and film thicknesses will be presented.
We present evidence of extinct Tc in presolar SiC grains in the form of an anomalous Ru isotopic ... more We present evidence of extinct Tc in presolar SiC grains in the form of an anomalous Ru isotopic composition compared to the one expected from the AGB stars that produced the grains. We show that AGB stars do not produce enough Tc to leave a detectable Ru anomaly in early solar system materials.
Ieee Transactions on Applied Superconductivity, Jun 1, 2011
The presence of magnetic impurities in native niobium oxides have been confirmed by Point contact... more The presence of magnetic impurities in native niobium oxides have been confirmed by Point contact spectroscopy (PCT), SQUID magnetometry and Electron paramagnetic resonance (EPR). All niobium (Nb) samples displayed a small impurity contribution to the magnetic susceptibility at low temperatures which exhibited Curie-Weiss behavior, indicative of weakly coupled localized paramagnetic moments. By examining Nb samples with widely varying surface-to-volume ratios (rods, foils, wires, powders) it was found that the impurity contribution is correlated with surface area. Tunneling measurements which use the native oxide layers as barriers exhibit a zero-bias conductance peak which splits in a magnetic field 4T, consistent with the Appelbaum-Anderson model [1] for spin flip tunneling. Viewed together the experiments strongly suggest that the native oxides of Nb are intrinsically defective, and consistently exhibit localized paramagnetic moments caused by oxygen vacancies in Nb 2 O 5 . The computation of the surface impedance (R s ) in presence of magnetic impurities in the Shiba approximation reveals the saturation at low temperature of R s , suggesting that magnetic impurities are responsible for the so-called residual resistance. These properties may have an impact on Nb based superconducting devices and shine a new light on the origin of the paramagnetic Meissner effect (PME).
The samples returned to Earth by the NASA's Genesis Mission contain a record of the elemental... more The samples returned to Earth by the NASA's Genesis Mission contain a record of the elemental and isotopic abundances of the Solar Wind (SW). This record is formed by the SW ions implanted in the near-surface regions of the Genesis sample collectors, so that the SW material can be distinguished from a terrestrial contamination, which occurred due to the crash
A new secondary neutral mass spectrometry (SNMS) instrument implementing laser post ionization (L... more A new secondary neutral mass spectrometry (SNMS) instrument implementing laser post ionization (LPI) of ion sputtered and laser desorbed neutral species has been developed and constructed for the specific purpose of quantitative analysis of metallic elements at ultra trace levels in solar wind collector samples returned to Earth by the Genesis Discovery mission. The first LPI SNMS measurements are focusing on determining Al, Ca, Cr, and Mg in these samples. These measurements provide the first concentration and isotopic abundances determinations for several key metallic elements and also elucidate possible fractionation effects between the photosphere and the solar wind compositions. It is now documented that Genesis samples suffered surface contamination both during flight and during the breach of the Sample Return Capsule when it crashed. Since accurate quantitative analysis is compromised by sample contamination, several features have been built into the new LPI SNMS instrument to mitigate this difficulty. A normally-incident, low-energy (<500 eV) ion beam combined with a keV energy ion beam and a desorbing laser beam (both microfocused) enables dual beam analyses. The low-energy ion beam can be used to remove surface contaminant by sputtering with minimum ion beam mixing. This low-energy beam also will be used to perform ion beam milling, while either the microfocused ion or laser beam probes the solar wind elemental compositions as a function of sample depth. Because of the high depth resolution of dual beam analyses, such depth profiles clearly distinguish between surface contaminants and solar wind implanted atoms. In addition, in-situ optical and electron beam imaging for observing and avoiding particulates and scratches on solar wind sample surfaces is incorporated in the new LPI SNMS instrument to further reduce quantification problems. The current status of instrument tests and analyses will be presented. This work is supported by the U. S. Department of Energy, BES-Materials Sciences, under Contract W-31-109-ENG-38, and by NASA under Work Orders W-19,895 and W-10,091.
Improvements to our instrument and analytical protocols now enable the first ever high-precision,... more Improvements to our instrument and analytical protocols now enable the first ever high-precision, low-interference isotopic measurements of trace elements in individual presolar grains.
The samples returned to Earth by the Genesis Mission of NASA's Discovery Program contain a record... more The samples returned to Earth by the Genesis Mission of NASA's Discovery Program contain a record of the elemental and isotopic abundances of the solar wind. This record is implanted in the near-surface region of the sample collectors allowing the solar wind material to be distinguished from terrestrial contamination, which occurred due to the abrupt landing of the Genesis spacecraft. At Argonne National Laboratory, we have recently developed a new Laser Post-Ionization Secondary Neutral Mass Spectrometer (LPI SNMS) called SARISA, which is capable of accurate measurements of ultra-trace concentrations of many metallic elements implanted in Genesis solar wind collectors. In this work, we will report results of our measurements of abundances of Mg in two types of such collectors, silicon and diamond-like carbon (film on silicon). These depth profiling measurements were conducted in Resonance-Enhanced Multi-Photon Ionization (REMPI) regime, in two-color scheme with two Ti-sapphire post-ionization lasers tuned to 285.30 nm and 375.66 nm wavelengths, with the repetition rate of 1 kHz.
Early transition metal carbides are of interest in a wide variety of applications ranging from re... more Early transition metal carbides are of interest in a wide variety of applications ranging from refractory coatings 1,2 to catalysis. 3 Thin amorphous films of transition metal carbides are of particular interest due to reduced brittleness and higher resistance to corrosion and laser damage in comparison to crystalline films. 4 Niobium carbide is a refractory ceramic with a high melting point (3600°C), excellent mechanical properties, and low electrical resistance (35 μΩ 3 cm). 2 Additionally, cubic NbC is a superconductor below 12 K. 5 Thin films of NbC x have been grown by a variety of techniques, including reactive 6,7 and compound-target 8 magnetron sputtering, cathodic arc deposition, 9 and pulsed laser ablation. 10 Atomic layer deposition (ALD) 11 is a synthesis technique wherein film growth is achieved by alternating exposure to two or more vaporous chemical precursors with each dose separated by an inert gas purge. Film growth proceeds in an atomic layer-bylayer fashion from sequential self-saturating chemical reactions between the precursor vapors and the solid film surface. ALD has been used previously to deposit a wide range of materials including metals, metal oxides, and metal nitrides. The atomic-layer control and excellent conformality characteristic of ALD has the potential to impact a variety of low-temperature superconductor-based devices and applications, including superconducting radiofrequency (SRF) resonator cavities used in high-energy particle accelerators 12 and bolometers used for ultrasensitive radiation detection, 13 which are currently limited by line-of-sight deposition techniques. However, there have been only two previously reported studies of superconductivity in ALD materials. 14,15 Likewise, while considerable effort has been concentrated on the synthesis of transition metal nitride films by ALD for use as diffusion barriers in metal/semiconductor-based devices, there are comparably few examples of carbide film growth by atomic layer deposition. Elers et al. proposed in a 2002 patent the use of metal halides together with a boron-, silicon-, or phosphorus-containing carbon source (e.g., triethylborane) as route to ALD of numerous transition metal carbide films. 16 However, to date, published reports of transition metal carbide ALD have examined the synthesis of a limited selection of materials: tungsten carbide (WC x ), 17 tungsten carbo-nitride (WN x C y ), 18 and tantalum carbonitride (TaN x C y ). 19À21 We previously reported the use of niobium pentafluoride (NbF 5 ) as a precursor for ALD of niobium silicide (NbSi) using disilane (Si 2 H 6 ) and niobium oxide (Nb 2 O 5 ) using either H 2 O or H 2 O 2 . Here, we demonstrate the ALD of superconducting ABSTRACT: Niobium carbide thin films were synthesized by atomic layer deposition (ALD) using trimethylaluminum (TMA), NbF 5 , and NbCl 5 precursors. In situ quartz crystal microbalance (QCM) measurements performed at 200 and 290°C revealed controlled, linear deposition with a high growth rate of 5.7 and 4.5 Å/cycle, respectively. The chemical composition, growth rate, structure, and electronic properties of the films were studied over the deposition temperature range 125À350°C. Varying amounts of impurities, including amorphous carbon (a-C), AlF 3 , NbF x , and NbCl x , were found in all samples. A strong growth temperature dependence of film composition, growth rate, and room temperature DC resistivity was observed. Increasing film density, decreasing total impurity concentration, and decreasing resistivity were observed as a function of increasing deposition temperature for films grown with either NbF 5 or NbCl 5 . Superconducting quantum interference device (SQUID) magnetometry measurements down to 1.2 K revealed a superconducting transition at T c = 1.8 K in a 75 nm thick film grown at 350°C with TMA and NbF 5 . The superconducting critical temperature could be increased up to 3.8 K with additional use of NH 3 during ALD film growth.
Materials used or considered for plasma-side applications are not entirely satisfactory, particul... more Materials used or considered for plasma-side applications are not entirely satisfactory, particularly in the medium edge temperature (100 to 300 eV) regime. An approach to impurity control based on self-sustaining surface segregated low-Z layers with high secondary ion fractions was tested in laboratory experiments. A crucial requirement is that substrate sputtering yields be reduced about an order of magnitude by monolayer adsorbate coverages. Theoretical and experimental evidence is adduced to support the contention that overlayer coverages (a monolayer of Li on Cu) result in profound reductions of substrate (e.g., Cu) sputtering yields. The conclusion that a material such as a dilute alloy of Li in Cu could function as a limiter or a divertor plate material is, in part, based on the fact that more than 85% of the sputtered flux originates in the first atomic layer (e.g., Li) of the target.
Because of the practical importance of sputtering, numerous theories and computer simulations are... more Because of the practical importance of sputtering, numerous theories and computer simulations are used for predicting many aspects of the sputtering process. Unfortunately, many of the calculated sputtering results are untested by experiment. Until recently, most sputtering experiments required either very high ion fluences or the detection of only minor constituents of the sputtered flux, i.e., ions. These techniques may miss the subtleties involved in the sputtering process. High-detection-efficiency mass spectrometry, coupled with the laser ionization of neutral atoms, allows the detection of the major sputtered species with very low incident ion fluences. The depth-of-origin of sputtered atoms is one example of an important but poorly understood aspect of the sputtering process. By following the sputtering yield of a substrate atom with various coverages of an adsorbed overlayer, the depth of origin of sputtered atoms has been determined. Our results indicate that two-thirds of the sputtered flux originates in the topmost atomic layer. The ion-dose dependence of sputtering yields has long been assumed to be quite minor for low- to-moderate primary ion fluences. We have observed a two-fold decrease in the sputtering yield of the Ru(0001) surface for very low primary ion fluences. Data analysis results in a cross section for damage of 2.7 {plus minus} 1.0 à 10â»Â¹âµcm². 40 refs., 3 figs., 2 tabs.
Velocity distributions and relative populations in the fine structure levels of the a(5)D/sub J/ ... more Velocity distributions and relative populations in the fine structure levels of the a(5)D/sub J/ ground state of Fe atoms, produced by sputtering with a 3 keV argon ions, were investigated by Doppler shifted laser induced fluorescence. The laser system employs a single mode, scanning ring dye laser, amplified by a sequence of three excimer pumped flowing dye cells. Frequency doubling in a KD*P crystal was used to produce high energy ( .5 mJ) pulses of narrowband tunable UV output near 300 nm. Laser power influence on effective velocity bandwidth was investigated. Favorable light collection geometry minimized distortion of the velocity spectra from apparatus averaging effects. In impurity flux diagnostic applications in fusion devices, substantial spatial averaging may occur. In the latter case, the narrow velocity bandwidth (70 m/s, transform limit) of the present laser system is particularly useful.
Electron-stimulated desorption of neutral aluminum from the system CH3O/Al(111) has been directly... more Electron-stimulated desorption of neutral aluminum from the system CH3O/Al(111) has been directly monitored via quasiresonant photoionization with 193 nm excimer laser light and confirmed by two-step resonant ionization, utilizing the Al 3D (sup 2)D manifold. Velocity distribution measurements for the neutral Al peak at approximately 800 m/s for 1 keV incident electron energy. An absolute yield of 3.2 x 10(exp -6) Al atoms/electron was determined by comparison with sputtering measurements in the same apparatus. This is the first observation of electron-stimulated metal desorption from adsorbate-covered metallic surfaces.
ABSTRACT We report the use of atomic layer deposition (ALD) to synthesize thin superconducting fi... more ABSTRACT We report the use of atomic layer deposition (ALD) to synthesize thin superconducting films and multilayer superconductor-insulator (S-I) heterostructures. The ALD technique applied to superconducting films opens the way for a variety of applications, including improving the performance and decreasing the cost of high energy particle accelerators, superconducting wires for energy storage, and bolometers for radiation detection. Furthermore, the atomic-scale thickness control afforded by ALD enables the study of superconductivity and associated phenomena in homogeneous layers in the ultra-thin film limit. In this respect, we will present results of ALD-grown transition metal-based superconductors, including nitrides, carbides, and silicides of niobium, nitrides of molybdenum and titanium, and Nb1-xTixN/AlN-based S-I heterostructures. Transport measurement for various composition and film thicknesses will be presented.
We present evidence of extinct Tc in presolar SiC grains in the form of an anomalous Ru isotopic ... more We present evidence of extinct Tc in presolar SiC grains in the form of an anomalous Ru isotopic composition compared to the one expected from the AGB stars that produced the grains. We show that AGB stars do not produce enough Tc to leave a detectable Ru anomaly in early solar system materials.
Ieee Transactions on Applied Superconductivity, Jun 1, 2011
The presence of magnetic impurities in native niobium oxides have been confirmed by Point contact... more The presence of magnetic impurities in native niobium oxides have been confirmed by Point contact spectroscopy (PCT), SQUID magnetometry and Electron paramagnetic resonance (EPR). All niobium (Nb) samples displayed a small impurity contribution to the magnetic susceptibility at low temperatures which exhibited Curie-Weiss behavior, indicative of weakly coupled localized paramagnetic moments. By examining Nb samples with widely varying surface-to-volume ratios (rods, foils, wires, powders) it was found that the impurity contribution is correlated with surface area. Tunneling measurements which use the native oxide layers as barriers exhibit a zero-bias conductance peak which splits in a magnetic field 4T, consistent with the Appelbaum-Anderson model [1] for spin flip tunneling. Viewed together the experiments strongly suggest that the native oxides of Nb are intrinsically defective, and consistently exhibit localized paramagnetic moments caused by oxygen vacancies in Nb 2 O 5 . The computation of the surface impedance (R s ) in presence of magnetic impurities in the Shiba approximation reveals the saturation at low temperature of R s , suggesting that magnetic impurities are responsible for the so-called residual resistance. These properties may have an impact on Nb based superconducting devices and shine a new light on the origin of the paramagnetic Meissner effect (PME).
The samples returned to Earth by the NASA's Genesis Mission contain a record of the elemental... more The samples returned to Earth by the NASA's Genesis Mission contain a record of the elemental and isotopic abundances of the Solar Wind (SW). This record is formed by the SW ions implanted in the near-surface regions of the Genesis sample collectors, so that the SW material can be distinguished from a terrestrial contamination, which occurred due to the crash
A new secondary neutral mass spectrometry (SNMS) instrument implementing laser post ionization (L... more A new secondary neutral mass spectrometry (SNMS) instrument implementing laser post ionization (LPI) of ion sputtered and laser desorbed neutral species has been developed and constructed for the specific purpose of quantitative analysis of metallic elements at ultra trace levels in solar wind collector samples returned to Earth by the Genesis Discovery mission. The first LPI SNMS measurements are focusing on determining Al, Ca, Cr, and Mg in these samples. These measurements provide the first concentration and isotopic abundances determinations for several key metallic elements and also elucidate possible fractionation effects between the photosphere and the solar wind compositions. It is now documented that Genesis samples suffered surface contamination both during flight and during the breach of the Sample Return Capsule when it crashed. Since accurate quantitative analysis is compromised by sample contamination, several features have been built into the new LPI SNMS instrument to mitigate this difficulty. A normally-incident, low-energy (<500 eV) ion beam combined with a keV energy ion beam and a desorbing laser beam (both microfocused) enables dual beam analyses. The low-energy ion beam can be used to remove surface contaminant by sputtering with minimum ion beam mixing. This low-energy beam also will be used to perform ion beam milling, while either the microfocused ion or laser beam probes the solar wind elemental compositions as a function of sample depth. Because of the high depth resolution of dual beam analyses, such depth profiles clearly distinguish between surface contaminants and solar wind implanted atoms. In addition, in-situ optical and electron beam imaging for observing and avoiding particulates and scratches on solar wind sample surfaces is incorporated in the new LPI SNMS instrument to further reduce quantification problems. The current status of instrument tests and analyses will be presented. This work is supported by the U. S. Department of Energy, BES-Materials Sciences, under Contract W-31-109-ENG-38, and by NASA under Work Orders W-19,895 and W-10,091.
Improvements to our instrument and analytical protocols now enable the first ever high-precision,... more Improvements to our instrument and analytical protocols now enable the first ever high-precision, low-interference isotopic measurements of trace elements in individual presolar grains.
The samples returned to Earth by the Genesis Mission of NASA's Discovery Program contain a record... more The samples returned to Earth by the Genesis Mission of NASA's Discovery Program contain a record of the elemental and isotopic abundances of the solar wind. This record is implanted in the near-surface region of the sample collectors allowing the solar wind material to be distinguished from terrestrial contamination, which occurred due to the abrupt landing of the Genesis spacecraft. At Argonne National Laboratory, we have recently developed a new Laser Post-Ionization Secondary Neutral Mass Spectrometer (LPI SNMS) called SARISA, which is capable of accurate measurements of ultra-trace concentrations of many metallic elements implanted in Genesis solar wind collectors. In this work, we will report results of our measurements of abundances of Mg in two types of such collectors, silicon and diamond-like carbon (film on silicon). These depth profiling measurements were conducted in Resonance-Enhanced Multi-Photon Ionization (REMPI) regime, in two-color scheme with two Ti-sapphire post-ionization lasers tuned to 285.30 nm and 375.66 nm wavelengths, with the repetition rate of 1 kHz.
Early transition metal carbides are of interest in a wide variety of applications ranging from re... more Early transition metal carbides are of interest in a wide variety of applications ranging from refractory coatings 1,2 to catalysis. 3 Thin amorphous films of transition metal carbides are of particular interest due to reduced brittleness and higher resistance to corrosion and laser damage in comparison to crystalline films. 4 Niobium carbide is a refractory ceramic with a high melting point (3600°C), excellent mechanical properties, and low electrical resistance (35 μΩ 3 cm). 2 Additionally, cubic NbC is a superconductor below 12 K. 5 Thin films of NbC x have been grown by a variety of techniques, including reactive 6,7 and compound-target 8 magnetron sputtering, cathodic arc deposition, 9 and pulsed laser ablation. 10 Atomic layer deposition (ALD) 11 is a synthesis technique wherein film growth is achieved by alternating exposure to two or more vaporous chemical precursors with each dose separated by an inert gas purge. Film growth proceeds in an atomic layer-bylayer fashion from sequential self-saturating chemical reactions between the precursor vapors and the solid film surface. ALD has been used previously to deposit a wide range of materials including metals, metal oxides, and metal nitrides. The atomic-layer control and excellent conformality characteristic of ALD has the potential to impact a variety of low-temperature superconductor-based devices and applications, including superconducting radiofrequency (SRF) resonator cavities used in high-energy particle accelerators 12 and bolometers used for ultrasensitive radiation detection, 13 which are currently limited by line-of-sight deposition techniques. However, there have been only two previously reported studies of superconductivity in ALD materials. 14,15 Likewise, while considerable effort has been concentrated on the synthesis of transition metal nitride films by ALD for use as diffusion barriers in metal/semiconductor-based devices, there are comparably few examples of carbide film growth by atomic layer deposition. Elers et al. proposed in a 2002 patent the use of metal halides together with a boron-, silicon-, or phosphorus-containing carbon source (e.g., triethylborane) as route to ALD of numerous transition metal carbide films. 16 However, to date, published reports of transition metal carbide ALD have examined the synthesis of a limited selection of materials: tungsten carbide (WC x ), 17 tungsten carbo-nitride (WN x C y ), 18 and tantalum carbonitride (TaN x C y ). 19À21 We previously reported the use of niobium pentafluoride (NbF 5 ) as a precursor for ALD of niobium silicide (NbSi) using disilane (Si 2 H 6 ) and niobium oxide (Nb 2 O 5 ) using either H 2 O or H 2 O 2 . Here, we demonstrate the ALD of superconducting ABSTRACT: Niobium carbide thin films were synthesized by atomic layer deposition (ALD) using trimethylaluminum (TMA), NbF 5 , and NbCl 5 precursors. In situ quartz crystal microbalance (QCM) measurements performed at 200 and 290°C revealed controlled, linear deposition with a high growth rate of 5.7 and 4.5 Å/cycle, respectively. The chemical composition, growth rate, structure, and electronic properties of the films were studied over the deposition temperature range 125À350°C. Varying amounts of impurities, including amorphous carbon (a-C), AlF 3 , NbF x , and NbCl x , were found in all samples. A strong growth temperature dependence of film composition, growth rate, and room temperature DC resistivity was observed. Increasing film density, decreasing total impurity concentration, and decreasing resistivity were observed as a function of increasing deposition temperature for films grown with either NbF 5 or NbCl 5 . Superconducting quantum interference device (SQUID) magnetometry measurements down to 1.2 K revealed a superconducting transition at T c = 1.8 K in a 75 nm thick film grown at 350°C with TMA and NbF 5 . The superconducting critical temperature could be increased up to 3.8 K with additional use of NH 3 during ALD film growth.
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Papers by Michael Pellin