Toward the Fabrication of Magnesium Diboride-based Quantum Devices
Restricted (Penn State Only)
- Author:
- Rondomanski, Patrick
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 31, 2024
- Committee Members:
- John Mauro, Program Head/Chair
Joshua Robinson, Major Field Member
Qi Li, Co-Chair & Dissertation Advisor
Morteza Kayyalha, Outside Unit & Field Member
Joan Redwing, Co-Chair & Dissertation Advisor - Keywords:
- superconductors
graphene
hybrid physical vapor deposition
intercalation
magnesium diboride
nanowires - Abstract:
- Magnesium diboride (MgB2) is a s-wave superconductor with a transition temperature (Tc) of approximately 40 K at atmospheric pressure, making it the highest temperature Bardeen-Cooper-Schrieffer (BCS) superconductor at ambient pressure yet to be realized. Moreover, this hexagonal structure layered material has achieved upper critical fields (Hc2) up to 51 T in carbon doped MgB2 films and critical current densities (Jc) up to 106 A/cm2. These critical values surpass those of commonly used superconductors utilized in the study of quantum phenomena. Hybrid physical-chemical vapor deposition (HPCVD)-grown films produce the highest quality MgB2 films with, often, the highest critical values. Yet, to integrate MgB2 into the study of quantum devices, new methodologies are required for it to be competitive as a material in the quantum technologies race. Reactivity of the elements that make-up the superconductor makes it difficult to integrate into current device platforms or pair with materials of interest for investigating new physical phenomena. Additionally, minimal research has been done to engineer films of different crystal planes of MgB2, besides C-plane oriented. Some of these other crystal planes, such as M-plane, of the material have recently been shown to possess topological surface states, a proposed pathway toward topological superconductivity. Thus, the focus of this dissertation is on exploring methods that can be used toward fabricating MgB2 heterostructures and manipulation of the MgB2 crystal orientation in films. The first part of the dissertation focuses on investigating the intercalation of Mg underneath epitaxially grown graphene to form n-type doped graphene. The study focuses on comparing the metal intercalation using physical vapor deposition (PVD) via evaporation and chemical vapor deposition (CVD) via a metalorganic source. Surface imaging techniques revealed that PVD results in significantly higher metal deposition and surface roughness compared to CVD. Formation of Mg-intercalated quasi-freestanding epitaxial graphene is inferred from the shifting of peaks in the C 1s spectra using x-ray photoelectron spectroscopy. This is further supported by scanning tunneling electron microscopy micrographs of multiple layers of Mg seen underneath the epitaxial graphene. In addition, the stability of the electron-doped epitaxial graphene was insinuated by a week-long invariance of Raman spectra under ambient conditions. This work suggests that CVD provides a cleaner and smoother surface due to enhanced control over Mg exposure, but PVD allows for higher Mg concentrations per minute, potentially increasing the amount of intercalated metal. Besides graphene modification, this is the first step toward encapsulation of MgB2 with graphene, via intercalation, that could be utilized as a substrate for topological insulators and the study of topological superconductivity. The second part of the dissertation explores the use of graphene as a masking layer for selective area epitaxial (SAE) growth of MgB2. Initial investigations focused on film deposition directly onto graphene that was grown epitaxially on SiC or made by chemical vapor deposition and transferred to various substrates. Results showed that graphene on Al2O3 and SiC inhibited MgB2 growth regardless of substrate doping or crystal orientation. However, graphene on Si, SiO2, and Si3N4 allowed for amorphous MgB2 film formation and indicates substrate influence on film growth. This knowledge was applied toward the fabrication of selectively grown thin film MgB2 nanowires using epitaxially grown monolayer graphene on semi-insulating 6H-SiC as a mask. 300 nm wide by 60 nm thick nanowires exhibited critical temperatures of 37.9 K and critical current densities up to 6.1 x 107 A/cm2 at 2 K. The thin film nature of the nanowires resulted in a display of anisotropic properties when exposed to a magnetic field. Lastly, factors affecting selectivity of film growth using graphene as a mask were explored. A key factor determined was the importance of clean, defect-free graphene to prevent undesirable deposits during MgB2 deposition. This body of work sets the stage for further fabrication of directly grown MgB2 devices, for applications in detectors and sensors, and for the exploration of novel physics in lateral MgB2/graphene structures. The final part of the dissertation details efforts in the synthesis and analysis of MgB2 thin films grown on M-plane (101 ̅0) and R-plane (11 ̅02) sapphire using hybrid physical-chemical vapor deposition (HPCVD). MgB2 films on M-plane sapphire display (101 ̅2) orientation that is comprised of bidirectional grains that tilt the MgB2 (0001) ~37° relative to the substrate surface, exposing the a-axes of the MgB2 grains. Films demonstrated a critical temperature of ~39 K, like bulk crystals. However, contributions from the crystallographic orientation and twinning of the grains in the film present a unique anisotropy of the angular dependence for the upper critical field. The result is two symmetric Hc2 maxima observed when the magnetic field is applied parallel to the a-axes of the opposing grains, at angles equidistant from when the field is in-plane with the substrate. The observed phenomena can largely be explained by the bidirectional orientation of the tilted grains. This result demonstrates the importance of crystallographic orientation of thin films on the physical properties of MgB2. R-plane sapphire did not result in any notable film epitaxy. However, further investigation revealed an oxide at the interface between the MgB2 film and Al2O3 substrate. Thus, this infers an important consideration for future engineering of MgB2 films to obtain a thin film with a desired planar orientation. This material is based upon work supported by the National Science Foundation under the Graduate Research Fellowship Program (Grant No. DGE1255832) and DMR-1808900 and DMR-1905833 and the Department of Energy under Grant No. DE-FG02-08ER46531. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.