Matching Items (5)
Filtering by
- All Subjects: Pyrites
- All Subjects: Applied physics
- Creators: Newman, Nathan
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
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 availability of bulk gallium nitride (GaN) substrates has generated great interest in the development of vertical GaN-on-GaN power devices. The vertical devices made of GaN have not been able to reach their true potential due to material growth related issues. Power devices typically have patterned p-n, and p-i junctions in lateral, and vertical direction relative to the substrate. Identifying the variations from the intended layer design is crucial for failure analysis of the devices. A most commonly used dopant profiling technique, secondary ion mass spectroscopy (SIMS), does not have the spatial resolution to identify the dopant distribution in patterned devices. The possibility of quantitative dopant profiling at a sub-micron scale for GaN in a scanning electron microscope (SEM) is discussed. The total electron yield in an SEM is shown to be a function of dopant concentration which can potentially be used for quantitative dopant profiling.
Etch-and-regrowth is a commonly employed strategy to generate the desired patterned p-n and p-i junctions. The devices involving etch-and-regrowth have poor performance characteristics like high leakage currents, and lower breakdown voltages. This is due to damage induced by the dry etching process, and the nature of the regrowth interface, which is important to understand in order to address the key issue of leakage currents in etched and regrown devices. Electron holography is used for electrostatic potential profiling across the regrowth interfaces to identify the charges introduced by the etching process. SIMS is used to identify the impurities introduced at the interfaces due to etch-and-regrowth process.
Etch-and-regrowth is a commonly employed strategy to generate the desired patterned p-n and p-i junctions. The devices involving etch-and-regrowth have poor performance characteristics like high leakage currents, and lower breakdown voltages. This is due to damage induced by the dry etching process, and the nature of the regrowth interface, which is important to understand in order to address the key issue of leakage currents in etched and regrown devices. Electron holography is used for electrostatic potential profiling across the regrowth interfaces to identify the charges introduced by the etching process. SIMS is used to identify the impurities introduced at the interfaces due to etch-and-regrowth process.
ContributorsAlugubelli, Shanthan Reddy (Author) / Ponce, Fernando A. (Thesis advisor) / McCartney, Martha (Committee member) / Newman, Nathan (Committee member) / Zhao, Yuji (Committee member) / Arizona State University (Publisher)
Created2019
Description
The chemical, structural, and electrical properties of niobium-silicon, niobium-germanium, and YBCO-dielectric interfaces are characterized. Reduction in the concentration of interfacial defects in these structures can improve the performance of (i) many devices including low-loss coplanar, microstrip, and stripline microwave resonators used in next-generation cryogenic communication, sensor, and quantum information technologies and (ii) layers used in device isolation, inter-wiring dielectrics, and passivation in microwave and Josephson junction circuit fabrication.
Methods were developed to synthesize amorphous-Ge (a-Ge) and homoepitaxial-Si dielectric thin-films with loss tangents of 1–2×10 -6 and 0.6–2×10 -5 at near single-photon powers and sub-Kelvin temperatures (≈40 mK), making them potentially a better choice over undoped silicon and sapphire substrates used in quantum devices. The Nb/Ge interface has 20 nm of chemical intermixing, which is reduced by a factor of four using 10 nm Ta diffusion layers. Niobium coplanar resonators using this structure exhibit reduced microwave losses.
The nature and concentration of defects near Nb-Si interfaces prepared with commonly-used Si surface treatments were characterized. All samples have H, C, O, F, and Cl in the Si within 50 nm of the interface, and electrically active defects with activation energies of 0.147, 0.194, 0.247, 0.339, and 0.556 eV above the valence band maximum (E vbm ), with concentrations dominated by a hole trap at E vbm +0.556 eV (presumably Nb Si ). The optimum surface treatment is an HF etch followed by an in-situ 100 eV Ar ion mill. RCA etches, and higher energy ion milling processes increase the concentration of electrically active defects.
A thin SrTiO 3 buffer layer used in YBa 2 Cu 3 O 7-δ superconductor/high-performance Ba(Zn 1/3 Ta 2/3 )O 3 and Ba(Cd 1/3 Ta 2/3 )O 3 microwave dielectric trilayers improves the structural quality of the layers and results in 90 K superconductor critical temperatures. This advance enables the production of more compact high-temperature superconductor capacitors, inductors, and microwave microstrip and stripline devices.
Methods were developed to synthesize amorphous-Ge (a-Ge) and homoepitaxial-Si dielectric thin-films with loss tangents of 1–2×10 -6 and 0.6–2×10 -5 at near single-photon powers and sub-Kelvin temperatures (≈40 mK), making them potentially a better choice over undoped silicon and sapphire substrates used in quantum devices. The Nb/Ge interface has 20 nm of chemical intermixing, which is reduced by a factor of four using 10 nm Ta diffusion layers. Niobium coplanar resonators using this structure exhibit reduced microwave losses.
The nature and concentration of defects near Nb-Si interfaces prepared with commonly-used Si surface treatments were characterized. All samples have H, C, O, F, and Cl in the Si within 50 nm of the interface, and electrically active defects with activation energies of 0.147, 0.194, 0.247, 0.339, and 0.556 eV above the valence band maximum (E vbm ), with concentrations dominated by a hole trap at E vbm +0.556 eV (presumably Nb Si ). The optimum surface treatment is an HF etch followed by an in-situ 100 eV Ar ion mill. RCA etches, and higher energy ion milling processes increase the concentration of electrically active defects.
A thin SrTiO 3 buffer layer used in YBa 2 Cu 3 O 7-δ superconductor/high-performance Ba(Zn 1/3 Ta 2/3 )O 3 and Ba(Cd 1/3 Ta 2/3 )O 3 microwave dielectric trilayers improves the structural quality of the layers and results in 90 K superconductor critical temperatures. This advance enables the production of more compact high-temperature superconductor capacitors, inductors, and microwave microstrip and stripline devices.
ContributorsKopas, Cameron Joseph (Author) / Newman, Nathan (Thesis advisor) / Alford, Terry L. (Committee member) / Carpenter, Ray W (Committee member) / Williams, Peter (Committee member) / Arizona State University (Publisher)
Created2020