Enabling technologies for scalable graphene electronics

Doctoral thesis, 2023

This thesis addresses the challenges in scaling up graphene-based technologies, with a focus on the epitaxial growth of graphene (epigraphene) over SiC substrates and the establishment of reliable low-contact resistance electrical contacts. The scalable graphene growth and contact fabrication have enabled the development of three epigraphene-based devices, including large-scale Quantum Hall Arrays, highly sensitive Hall effect magnetometers with minimal noise, and quantum-limited ultraviolet (UV) detectors.

    Hall sensors utilizing epigraphene with their carrier density tuned close to neutrality, outperform the most advanced graphene-based Hall sensors documented in existing literature. Given the exceptionally high epitaxial growth temperatures (∼1850 C) in SiC, crystal imperfections are significantly minimized, leading to remarkably low noise performance. Consequently, SiC-based magnetic field sensors achieve sensitivities of BMIN = 27 nT/sqrt(Hz) at room temperature. By connecting them in parallel, it is possible to increase the sensitivity even further to BMIN = 9 nT/sqrt(Hz), setting a new record for the lowest magnetic field detection in epigraphene-based Hall sensors.

    This thesis demonstrates the largest ever functional quantum Hall arrays with 236-element Hall bars for practical precision resistance metrology. This results in RK/236 ≈ 109 Ω with 0.2 part-per-billion (nΩ/Ω) accuracy and critical current IC ≥ 5 mA (RK the von Klitzing constant). An essential aspect of the study involves investigating the long-term stability of molecularly doped epigraphene Quantum Hall Resistance Standard (QHRS) and analyzing the influence of storage conditions and dopants on device stability and performance over a period of 3 years. With a relative deviation of 0.01 %/day in carrier density, the estimated life of these samples is more than 20 years. Molecular dopants are utilized in every epigraphene-based application that requires charge carrier modulation. Hence, this study is essential to understand the long-term stability of real-life applications based on epigraphene.

    Furthermore, the technologies developed in this work allowed the exploration of epigraphene as solar-blind UV detectors, using SiC as absorber and graphene as transparent contact, leading to a peak external quantum efficiency (η) of approximately 85 % (limited by reflection losses) for wavelengths (λ) in the range of 250 to 280 nm.

    The refinements in epitaxial growth of graphene on SiC and device fabrication strategies have been tested and verified in three applications of epigraphene. Each of them reflects the true scalability of the epigraphene technologies developed in this work and led to devices that surpass conventional material technologies in specific domains. Altogether, the advancements demonstrated in this thesis open up new avenues for the use of epigraphene in multiple practical applications.