Doctor of Philosophy
Miniaturization and performance improvements are driving the electronic industry to shrink the feature size of superconductor devices. Several important questions arise from this development. For example, how small can some devices be fabricated? How will it affect the device performance? Can new operating principles be implemented in nanoscale devices? What other applications exist for nanofabrication techniques?
Hence the goals of this Ph.D. thesis research were three-fold. First, using a scanning electron microscope (SEM) for electron beam lithography (EBL) to optimize, utilize/implement, characterize, and extend existing fabrication techniques and develop new ones (as described in Chapter 3). Second, establishing baseline fabrication processes for graphene-based Josephson junction devices and studying the electronic properties of few-layer graphene with superconducting electrodes. Finally, for superconducting single photon detector (SSPD) applications, exploring the properties of YBa2Cu3O7-x (YBCO) superconducting thin films with various buffers and substrates. Thorough theoretical and experimental studies were conducted based on existing designs and available instruments.
This Ph.D. thesis addresses nanostructure fabrication techniques based on EBL and their application to the creation and study of superconducting material for electron-transport studies. In this dissertation, I will first review the prior literature on superconductivity and the working principles of the Josephson junction and SSPD devices, along with the EBL fabrication technique. One of the aims of this thesis is to optimize the nanofabrication processes, such as the EBL and pulsed laser deposition (PLD) techniques. Electron beam lithography is one of the best methods to produce nanometer-scale patterns due to its resolution without a mask and its focus capability. The optimization and characterization of the minimum achievable feature sizes as small as 100 nm have been produced using the EBL system. It was achieved by modified a SEM and developing with a lift-off process which uses a novel (polymethyl-methacrylate (PMMA)/methyl methacrylate (MMA)) bilayer e- iv beam resist. Also, a hybrid focussed ion beam (FIB) and reactive ion-etching (RIE) process has been used to fabricate complete devices.
During the three and half years of my Ph.D. study, I started two projects that each lasted for about half of the total the time. The objective of the first research project was to develop, design, and fabricate advanced superconducting Josephson junctions on high quality graphene samples for application in the most sensitive devices known for sensing magnetic flux, superconducting quantum interference devices (SQUIDs), as well as superconducting oscillators and superconducting mixers in the high frequency regime. Graphene was investigated as an alternate barrier material (due to its tuneable electronic behaviour) for Josephson junctions by an electron-transport study at ISEM, UoW, Australia. This work presents experiments on the electrical properties and morphology of graphene. It includes an overview of the basic physical concepts relevant to the experimental results presented. In the experimental section, I fabricate a device by electron beam lithography (EBL) for making electrode contacts for electrical transport measurements on graphite flakes. I found that electrons in mesoscopic graphite pieces are delocalized over nearly the whole graphite piece down to low temperatures. Experimental studies of the I-V characteristics of Nb/Ti-carbon-Ti/Nb junctions describe the device characteristics at various temperatures.
High temperature superconducting thin films (YBa2Cu3O7-x) are emerging in Superconducting Single Photon Detector (SSPD) research as a novel replacement for conventional and semiconductor detectors. The final part of thesis is devoted to designing a fabrication process for building Superconducting Single Photon Detector (SSPD) devices. This study compares the performance of the YBCO film layer with optimized thickness, which constitutes the heart of the SSPD, with the best combination of substrate and buffer layer.
The major hindrance to this is the degradation of the superconducting properties of YBa2Cu3O7-x (YBCO) thin film with reduction of its lateral and longitudinal dimensions (i.e. film thickness and width of the stripe).
v Furthermore, the surface of the film should be smooth to enable fabrication of the SSPD device. I have noticed that by optimizing the thickness of the buffer layer, the thickness of the YBCO ultra-thin film can be effectively reduced without significant degradation of its functional properties. In order to improve the quality of YBCO thin films, I exploited various buffer layers (i.e. SrTiO3 (STO), CeO2, and PrBa2Cu3O7 (PBCO)) with thickness of 30 ± 5 nm prior to the YBCO deposition. These combinations of high quality films will allow the design a new class of SSPDs that is capable of the high sensitivity required of a single-photon detector.
Masilamani, Nandhagopal, Graphene and Superconducting (YBa2Cu3O7-x) Thin Film – A Study on Electronic and Magnetic Transport Properties for Detectors Applications, Doctor of Philosophy thesis, , University of Wollongong, 2017. https://ro.uow.edu.au/theses1/124