Selective Area Epitaxial Growth of Magnesium Diboride on SiC using Epitaxial Graphene

Magnesium diboride (MgB₂) has a bulk transition temperature (<i>T_c</i>) of 39 K, the highest temperature BCS s-wave superconductor at ambient pressure, and it has been shown to achieve an upper critical field (<i>H_c2</i>) larger than 60 T. Furthermore, MgB₂ possesses two superconducting energy gaps, a small gap of ~2.3 meV due to the π band and a larger gap of ~7.1 meV due to the σ band which contributes to the high <i>T_c</i>. Access to the desirable σ band carriers for proximity coupled superconductivity are largely confined to the <i>ab</i>-axes and is not readily available in standard <i>C</i>-plane films. Selective area growth of <i>C</i>-plane films would allow not only direct growth of superconducting devices, but it also affords the opportunity to fabricate high quality thin film-based lateral devices. To selectively deposit MgB₂, a chemically inert masking material is preferred to isolate growth regions to the substrate. Graphene is a unique 2D material that is highly stable and is chemically inert due to its strong sp² bonding. Moreover, graphene possesses high mobility, Dirac bands, and has been observed to have a room temperature quantum Hall effect in relatively low magnetic fields. The Josephson effect across graphene has been a subject of intense research recently due to the predictions of exotic states due to its Dirac bands, gate tunability, and even in low Landau level quantum Hall states. However, current studies on this matter mostly utilize low <i>T_c</i> superconductors, such as Nb, and it is therefore desirable to employ higher <i>T_c</i> materials to increase the operational temperatures. The high <i>T_c</i> and <i>H_c2</i> of MgB₂ make it an ideal candidate to pair with the quantum Hall or fractional quantum Hall effect found in graphene to study the predicted chiral superconductivity.<br/><br/>In this work we present selective area epitaxial (SAE) growth of <i>C</i>-plane MgB₂ by hybrid physical-chemical vapor deposition (HPCVD) using patterned epitaxially grown graphene on semi-insulating 6H-SiC (EG). We find that EG patterned using positive resist photolithography and etched with a N₂ plasma restricts MgB₂ deposition predominantly to the exposed SiC and allows for fabrication of lateral MgB₂/graphene heterostructures. Low surface energy combined with minimal available oxygen bonds prevents Mg from sticking to the graphene and allows high surface mobility of adatoms to the exposed SiC. Thus, this makes graphene a suitable candidate to mask MgB₂ growth. The SAE technique also allows for simple and direct synthesis of MgB₂ devices, such as thin film nanoribbons for applications as single photon detectors or as other quantum sensors, on a semi-insulating substrate without the contamination and degradation of the superconductor from standard lithography processing after deposition. We have fabricated 5 µm wide by 1 mm long <i>C</i>-plane MgB₂ nanoribbons of 30 nm thickness that retain the bulk <i>T_c</i> of 39 K. Scanning electron microscopy shows that parasitic deposition on graphene is largely limited to photoresist contaminated areas or point and edge defects of the masking material that allow for Mg bonds to form. Furthermore, XPS analysis suggests the graphene is decoupled from the substrate during the MgB₂ deposition process, which allows for greater carrier mobility in the graphene. Lastly, the van der Waals bonding of graphene allows for easy physical exfoliation of the mask without damaging the MgB₂ device. Thus, this allows for isolation of superconducting devices on the semi-insulating SiC substrate if desired for the device application. Discussions in this talk will include synthesis considerations of developing these structures, such as chemical modification of the graphene during MgB₂ deposition, and the electrical properties of as-grown nanoribbons and other devices fabricated.