Combining Superconductivity, Wide Bandgaps and Magnetism in Epitaxial Cubic Nitride Heterostructures

Group III-Nitride materials have found applications in optoelectronics and photonic devices. Recent research has pursued the integration of this well-established material system with transition-metal nitrides to create complex heterostructures with additional magnetic or superconducting functionality. The thermodynamically stable wurtzite phase of group III-Nitrides is ubiquitous in the optoelectronics industry due to the large variation in direct bandgap which can be tuned from the infrared to the deep ultraviolet. The wurtzite phase of GaN has a direct bandgap of 3.4 eV, can be made both n-type or p-type, and imbued with room temperature ferromagnetism when doped with transition metals. The III-N family also has a metastable zinc blende allotrope which is much less explored; the bandgap of cubic-GaN is reduced to 3.2eV but remains direct. Unlike the wurtzite phase, cubic-GaN is centrosymmetric and therefore does not have issues with polarization, and the higher symmetry simplifies interfacing with the other cubic materials. The rocksalt structured transition metal nitrides are of interest for applications requiring high chemical and thermal stability, high hardness, superconductivity, or plasmonics. ZrN and NbN are well-known refractory superconductors with critical temperatures of 10K and 16K, respectively. Additionally, the similar lattice constant of cubic GaN (4.50Å) with ZrN (4.58Å) and NbN (4.44Å) results in an estimated lattice mismatch of the metal nitrides of only -1.4%, and +1.4%, respectively, suggesting the possibility of epitaxial and strain-tunable growth.<br/><br/>This work will discuss the molecular beam epitaxy synthesis of cubic-GaN and the additiona of magnetic transition metal elements. The primary focus will be on the epitaxial integration of cubic-GaN with known superconducting nitrides. The hexagonal-free nature of the GaN and epitaxial relationship with the transition metal nitrides are confirmed via <i>in-situ</i> reflection high energy electron diffraction, <i>ex-situ</i> X-ray diffraction, photoluminescence, and transition electron microscopy. Electrical transport and optical properties of transition metal nitrides deposited directly on 3C-SiC(001), cubic-GaN(001), and wurtzite GaN (0001) substrates are compared. The growth windows for GaN and some metal nitrides are close, which allows for deposition of epitaxial metal-dielectric heterostructures with sharp interface control. Epitaxial synthesis of a cubic wide-bandgap material with tunable magnetic functionality and superconducting metallic nitrides opens a new world of possibilities in band engineering, metamaterials, spintronics, and quantum science. This will create an avenue for new device architectures for hierarchical matter by combining materials with dissimilar properties with atomic layer precision.