Matching Items (7)
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- All Subjects: Josephson junctions
- All Subjects: Pyrites
- Creators: Newman, Nathan
Description
In this research, our goal was to fabricate Josephson junctions that can be stably processed at 300°C or higher. With the purpose of integrating Josephson junction fabrication with the current semiconductor circuit fabrication process, back-end process temperatures (>350 °C) will be a key for producing large scale junction circuits reliably, which requires the junctions to be more thermally stable than current Nb/Al-AlOx/Nb junctions. Based on thermodynamics, Hf was chosen to produce thermally stable Nb/Hf-HfOx/Nb superconductor tunnel Josephson junctions that can be grown or processed at elevated temperatures. Also elevated synthesis temperatures improve the structural and electrical properties of Nb electrode layers that could potentially improve junction device performance. The refractory nature of Hf, HfO2 and Nb allow for the formation of flat, abrupt and thermally-stable interfaces. But the current Al-based barrier will have problems when using with high-temperature grown and high-quality Nb. So our work is aimed at using Nb grown at elevated temperatures to fabricate thermally stable Josephson tunnel junctions. As a junction barrier metal, Hf was studied and compared with the traditional Al-barrier material. We have proved that Hf-HfOx is a good barrier candidate for high-temperature synthesized Josephson junction. Hf deposited at 500 °C on Nb forms flat and chemically abrupt interfaces. Nb/Hf-HfOx/Nb Josephson junctions were synthesized, fabricated and characterized with different oxidizing conditions. The results of materials characterization and junction electrical measurements are reported and analyzed. We have improved the annealing stability of Nb junctions and also used high-quality Nb grown at 500 °C as the bottom electrode successfully. Adding a buffer layer or multiple oxidation steps improves the annealing stability of Josephson junctions. We also have attempted to use the Atomic Layer Deposition (ALD) method for the growth of Hf oxide as the junction barrier and got tunneling results.
ContributorsHuang, Mengchu, 1987- (Author) / Newman, Nathan (Thesis advisor) / Rowell, John M. (Committee member) / Singh, Rakesh K. (Committee member) / Chamberlin, Ralph (Committee member) / Wang, Robert (Committee member) / Arizona State University (Publisher)
Created2013
Description
A series of pyrite thin films were synthesized using a novel sequential evaporation
technique to study the effects of substrate temperature on deposition rate and micro-structure of
the deposited material. Pyrite was deposited in a monolayer-by-monolayer fashion using
sequential evaporation of Fe under high vacuum, followed by sulfidation at high S pressures
(typically > 1 mTorr to 1 Torr). Thin films were synthesized using two different growth processes; a
one-step process in which a constant growth temperature is maintained throughout growth, and a
three-step process in which an initial low temperature seed layer is deposited, followed by a high
temperature layer, and then finished with a low temperature capping layer. Analysis methods to
analyze the properties of the films included Glancing Angle X-Ray Diffraction (GAXRD),
Rutherford Back-scattering Spectroscopy (RBS), Transmission Electron Microscopy (TEM),
Secondary Ion Mass Spectroscopy (SIMS), 2-point IV measurements, and Hall effect
measurements. Our results show that crystallinity of the pyrite thin film improves and grain size
increases with increasing substrate temperature. The sticking coefficient of Fe was found to
increase with increasing growth temperature, indicating that the Fe incorporation into the growing
film is a thermally activated process.
technique to study the effects of substrate temperature on deposition rate and micro-structure of
the deposited material. Pyrite was deposited in a monolayer-by-monolayer fashion using
sequential evaporation of Fe under high vacuum, followed by sulfidation at high S pressures
(typically > 1 mTorr to 1 Torr). Thin films were synthesized using two different growth processes; a
one-step process in which a constant growth temperature is maintained throughout growth, and a
three-step process in which an initial low temperature seed layer is deposited, followed by a high
temperature layer, and then finished with a low temperature capping layer. Analysis methods to
analyze the properties of the films included Glancing Angle X-Ray Diffraction (GAXRD),
Rutherford Back-scattering Spectroscopy (RBS), Transmission Electron Microscopy (TEM),
Secondary Ion Mass Spectroscopy (SIMS), 2-point IV measurements, and Hall effect
measurements. Our results show that crystallinity of the pyrite thin film improves and grain size
increases with increasing substrate temperature. The sticking coefficient of Fe was found to
increase with increasing growth temperature, indicating that the Fe incorporation into the growing
film is a thermally activated process.
ContributorsWertheim, Alex (Author) / Newman, Nathan (Thesis advisor) / Singh, Rakesh (Committee member) / Bertoni, Mariana (Committee member) / Arizona State University (Publisher)
Created2014
Description
The study of high energy particle irradiation effect on Josephson junction tri-layers is relevant to applications in space and radioactive environments. It also allows us to investigate the influence of defects and interfacial intermixing on the junction electrical characteristics. In this work, we studied the influence of 2MeV Helium ion irradiation with doses up to 5.2×1016 ions/cm2 on the tunneling behavior of Nb/Al/AlOx/Nb Josephson junctions. Structural and analytical TEM characterization, combined with SRIM modeling, indicates that over 4nm of intermixing occurred at the interfaces. EDX analysis after irradiation, suggests that the Al and O compositions from the barrier are collectively distributed together over a few nanometers. Surprisingly, the IV characteristics were largely unchanged. The normal resistance, Rn, increased slightly (<20%) after the initial dose of 3.5×1015 ions/cm2 and remained constant after that. This suggests that tunnel barrier electrical properties were not affected much, despite the significant changes in the chemical distribution of the barrier's Al and O shown in SRIM modeling and TEM pictures. The onset of quasi-particle current, sum of energy gaps (2Δ), dropped systematically from 2.8meV to 2.6meV with increasing dosage. Similarly, the temperature onset of the Josephson current dropped from 9.2K to 9.0K. This suggests that the order parameter at the barrier interface has decreased as a result of a reduced mean free path in the Al proximity layer and a reduction in the transition temperature of the Nb electrode near the barrier. The dependence of Josephson current on the magnetic field and temperature does not change significantly with irradiation, suggesting that intermixing into the Nb electrode is significantly less than the penetration depth.
ContributorsZhang, Tiantian (Author) / Newman, Nathan (Thesis advisor) / Rowell, John M (Committee member) / Singh, Rakesh K. (Committee member) / Chamberlin, Ralph (Committee member) / Wang, Robert (Committee member) / Arizona State University (Publisher)
Created2012
Description
Attaining a sufficiently large critical current density (Jc) in magnetic-barrier Josephson junctions has been one of the greatest challenges to the development of dense low-power superconductor memories. Many experimentalists have used various combinations of superconductor (S) and ferromagnetic (F) materials, with limited success towards the goal of attaining a useful Jc. This trial-and-error process is expensive and time consuming. An improvement in the fundamental understanding of transport through the ferromagnetic layers and across the superconductor-ferromagnetic interface could potentially give fast, accurate predictions of the transport properties in devices and help guide the experimental studies.
In this thesis, parameters calculated using density functional methods are used to model transport across Nb/0.8 nm Fe/Nb/Nb and Nb/3.8 nm Ni /Nb/Nb Josephson junctions. The model simulates the following transport processes using realistic parameters from density functional theory within the generalized gradient approximation: (a) For the first electron of the Cooper pair in the superconductor to cross the interface- conservation of energy and crystal momentum parallel to the interface (kll). (b) For the second electron to be transmitted coherently- satisfying the Andreev reflection interfacial boundary conditions and crossing within a coherence time, (c) For transmission of the coherent pair through the ferromagnetic layer- the influence of the exchange field on the electrons’ wavefunction and (d) For transport through the bulk and across the interfaces- the role of pair-breaking from spin-flip scattering of the electrons. Our model shows the utility of using realistic electronic-structure band properties of the materials used, rather the mean-field exchange energy and empirical bulk and interfacial material parameters used by earlier workers. [Kontos et al. Phys. Rev Lett, 93(13), 137001. (2004); Demler et al. Phys. Rev. B, 55(22), 15174. (1997)].
The critical current densities obtained from out model for Nb/0.8 nm Fe/Nb is 104 A/cm2 and for Nb/3.8 nm Ni/Nb is 7.1*104 A/cm2. These values fall very close to those observed experimentally- i.e. for Nb/0.8 nm Fe/Nb is 8*103 A/cm2 [Robinson et al" Phys. Rev. B 76, no. 9, 094522. (2007)] and for Nb/3.8 nm of Ni/Nb is 3*104 A/cm2 [Blum et al Physical review letters 89, no. 18, 187004. (2002). This indicates that our approach could potentially be useful in optimizing the properties of ferromagnetic-barrier structures for use in low-energy superconducting memories.
In this thesis, parameters calculated using density functional methods are used to model transport across Nb/0.8 nm Fe/Nb/Nb and Nb/3.8 nm Ni /Nb/Nb Josephson junctions. The model simulates the following transport processes using realistic parameters from density functional theory within the generalized gradient approximation: (a) For the first electron of the Cooper pair in the superconductor to cross the interface- conservation of energy and crystal momentum parallel to the interface (kll). (b) For the second electron to be transmitted coherently- satisfying the Andreev reflection interfacial boundary conditions and crossing within a coherence time, (c) For transmission of the coherent pair through the ferromagnetic layer- the influence of the exchange field on the electrons’ wavefunction and (d) For transport through the bulk and across the interfaces- the role of pair-breaking from spin-flip scattering of the electrons. Our model shows the utility of using realistic electronic-structure band properties of the materials used, rather the mean-field exchange energy and empirical bulk and interfacial material parameters used by earlier workers. [Kontos et al. Phys. Rev Lett, 93(13), 137001. (2004); Demler et al. Phys. Rev. B, 55(22), 15174. (1997)].
The critical current densities obtained from out model for Nb/0.8 nm Fe/Nb is 104 A/cm2 and for Nb/3.8 nm Ni/Nb is 7.1*104 A/cm2. These values fall very close to those observed experimentally- i.e. for Nb/0.8 nm Fe/Nb is 8*103 A/cm2 [Robinson et al" Phys. Rev. B 76, no. 9, 094522. (2007)] and for Nb/3.8 nm of Ni/Nb is 3*104 A/cm2 [Blum et al Physical review letters 89, no. 18, 187004. (2002). This indicates that our approach could potentially be useful in optimizing the properties of ferromagnetic-barrier structures for use in low-energy superconducting memories.
ContributorsKalyana Raman, Dheepak Surya (Author) / Newman, Nathan (Thesis advisor) / Muhich, Christopher L (Committee member) / Ferry, David K. (Committee member) / Arizona State University (Publisher)
Created2018
Description
Pyrite is a 0.95 eV bandgap semiconductor which is purported to have great potential in widespread, low–cost photovoltaic cells. A thorough material selection process was used in the design of a pyrite sequential vapor deposition chamber aimed at reducing and possibly eliminating contamination during thin film growth. The design process focused on identifying materials that do not produce volatile components when exposed to high temperatures and high sulfur pressures. Once the materials were identified and design was completed, the ultra–high vacuum growth system was constructed and tested.
Pyrite thin films were deposited using the upgraded sequential vapor deposition chamber by varying the substrate temperature from 250°C to 420°C during deposition, keeping sulfur pressure constant at 1 Torr. Secondary Ion Mass Spectrometry (SIMS) results showed that all contaminants in the films were reduced in concentration by orders of magnitude from those grown with the previous system. Characterization techniques of Rutherford Back–scattering Spectrometry (RBS), X–Ray Diffraction (XRD), Raman Spectroscopy, Optical Profilometry and UV/Vis/Near–IR Spectroscopy were performed on the deposited thin films. The results indicate that stoichiometric ratio of S:Fe, structural–quality (epitaxy), optical roughness and percentage of pyrite in the deposited thin films improve with increase in deposition temperature. A Tauc plot of the optical measurements indicates that the pyrite thin films have a bandgap of 0.94 eV.
Pyrite thin films were deposited using the upgraded sequential vapor deposition chamber by varying the substrate temperature from 250°C to 420°C during deposition, keeping sulfur pressure constant at 1 Torr. Secondary Ion Mass Spectrometry (SIMS) results showed that all contaminants in the films were reduced in concentration by orders of magnitude from those grown with the previous system. Characterization techniques of Rutherford Back–scattering Spectrometry (RBS), X–Ray Diffraction (XRD), Raman Spectroscopy, Optical Profilometry and UV/Vis/Near–IR Spectroscopy were performed on the deposited thin films. The results indicate that stoichiometric ratio of S:Fe, structural–quality (epitaxy), optical roughness and percentage of pyrite in the deposited thin films improve with increase in deposition temperature. A Tauc plot of the optical measurements indicates that the pyrite thin films have a bandgap of 0.94 eV.
ContributorsWalimbe, Aditya (Author) / Newman, Nathan (Thesis advisor) / Alford, Terry (Committee member) / Singh, Rakesh (Committee member) / Arizona State University (Publisher)
Created2016
Description
Measurements of the geometrical magnetoresistance of a conventional semiconductor, gallium arsenide (GaAs), and a more recently developed semiconductor, iron pyrite (FeS2) were measured in the Corbino disc geometry as a function of magnetic field to determine the carrier mobility (μm). These results were compared with measurements of the Hall mobility (μH) made in the Van der Pauw configuration. The scattering coefficient (ξ), defined as the ratio between magnetoresistance and Hall mobility (μm/μH), was determined experimentally for GaAs and natural pyrite from 300 K to 4.2 K. The effect of contact resistance and heating on the measurement accuracy is discussed.
ContributorsRavi, Aditya (Author) / Newman, Nathan (Thesis advisor) / Singh, Rakesh (Committee member) / Ferry, David K. (Committee member) / Arizona State University (Publisher)
Created2016
Description
The coexistence of superconductivity and ferromagnetic orders has been the subject of study for many years. It well known that these materials possess two competing order parameters; however the two order parameters can coexist under special circumstances inducing interesting physical phenomena. In recent years the demand of ultra-low-power, high density cryogenic memories has brought considerable interest to integrate superconducting and magnetic thin films in one structure to produce novel memory elements. The operation of the device depends on the unusual electronic properties associated with the Superconductor (S) /Ferromagnetic (F) proximity effect.
Niobium (Nb) based Josephson junction devices were fabricated with barriers containing two ferromagnetic layers separated by a normal metal space layer. In device operation, electrons in the superconductor are injected into the ferromagnets, causing the superconductor wavefunction to shift its phase and decay in amplitude. Such devices have two different states that depend on the relative magnetization of their ferromagnetic barrier layers, parallel or antiparallel. In these different states, the junctions have different phase shifts and critical currents. Superconducting circuits containing these devices can be designed to operate as memory cells using either one of these outputs.
To quantify the shift in phase and amplitude decay of the wavefunction through a common ferromagnet, permalloy, a series of Nb/permalloy/Nb Josephson junctions with varying ferromagnetic layer thicknesses were fabricated. Data have shown that the optimal thickness of a fixed layer composed of permalloy is 2.4 nm, as it shifts the wavefunction phase to π/2, its “pivot point.” If set to precisely this value, the free layer in SFNF'S junctions will switch the junction into either the 0 or π state depending on its magnetic orientation. To minimize the free-layer switching energy dilute Cu-permalloy alloy [Cu0.7(Ni80Fe20)0.3] with a low magnetic saturation (Ms of ~80 emu/cm3) was used as the free layer. These devices exhibit switching energies at small magnetic fields, demonstrating their potential use for low power non-volatile memory for superconductor circuits.
Lastly, to study the proximity effect using other potentially-useful ferromagnetic layers, measurements were performed on Nb/F bilayers and Nb/F/AlOx/Al tunnel junctions with ferromagnets Ni8Fe19, Ni65Fe15Co20, and Pd1-xNix. The dependence of the critical temperature of the bilayers and density of states that propagated through the ferromagnetic layer were studied as a function of thickness. From this study, crucial magnetic and electrical parameters like magnetic coherence lengths (ξF), exchange energy (Eex), and the rate of shift in the wavefunction’s phase and amplitude as a function of thickness were determined.
Niobium (Nb) based Josephson junction devices were fabricated with barriers containing two ferromagnetic layers separated by a normal metal space layer. In device operation, electrons in the superconductor are injected into the ferromagnets, causing the superconductor wavefunction to shift its phase and decay in amplitude. Such devices have two different states that depend on the relative magnetization of their ferromagnetic barrier layers, parallel or antiparallel. In these different states, the junctions have different phase shifts and critical currents. Superconducting circuits containing these devices can be designed to operate as memory cells using either one of these outputs.
To quantify the shift in phase and amplitude decay of the wavefunction through a common ferromagnet, permalloy, a series of Nb/permalloy/Nb Josephson junctions with varying ferromagnetic layer thicknesses were fabricated. Data have shown that the optimal thickness of a fixed layer composed of permalloy is 2.4 nm, as it shifts the wavefunction phase to π/2, its “pivot point.” If set to precisely this value, the free layer in SFNF'S junctions will switch the junction into either the 0 or π state depending on its magnetic orientation. To minimize the free-layer switching energy dilute Cu-permalloy alloy [Cu0.7(Ni80Fe20)0.3] with a low magnetic saturation (Ms of ~80 emu/cm3) was used as the free layer. These devices exhibit switching energies at small magnetic fields, demonstrating their potential use for low power non-volatile memory for superconductor circuits.
Lastly, to study the proximity effect using other potentially-useful ferromagnetic layers, measurements were performed on Nb/F bilayers and Nb/F/AlOx/Al tunnel junctions with ferromagnets Ni8Fe19, Ni65Fe15Co20, and Pd1-xNix. The dependence of the critical temperature of the bilayers and density of states that propagated through the ferromagnetic layer were studied as a function of thickness. From this study, crucial magnetic and electrical parameters like magnetic coherence lengths (ξF), exchange energy (Eex), and the rate of shift in the wavefunction’s phase and amplitude as a function of thickness were determined.
ContributorsAbd El Qader, Makram (Author) / Newman, Nathan (Thesis advisor) / Rowell, John (Committee member) / Rizzo, Nick (Committee member) / Yu, Hongbin (Committee member) / Arizona State University (Publisher)
Created2016