Research Projects

Transition metal nitrides (TMNs) are an emerging class of materials and have attracted significant interest in recent years for their applications in plasmonics, thermoelectrics, optoelectronic, and refractory nano-electronic devices. Chromium nitride (CrN) is one of the most exciting TMNs due to its unusual electronic, magnetic, and structural phase transition at ~ 272 K, and has been researched extensively for fundamental physics studies and technological applications. At room-temperature, bulk CrN is semiconducting with rocksalt (Fm-3m) crystal structure, but at a lower temperature (TN ~ 272 K), it transforms into a metal with orthorhombic (Pnma) crystal symmetry. Concurrent with the structural and electronic transition, the magnetic phase of CrN also changes from paramagnetic at room temperature to anti-ferromagnetic at low temperature. Such simultaneous electrical, structural and magnetic transition is predicted to be driven by the magnetic stress due to spin ordering of Cr atoms around the Neel temperature (TN) but has not been experimentally verified. Though the phase transitions have been observed in bulk CrN before, CrN thin films do not guarantee such transition always. Previous research on thin film CrN concluded that the phase transition strongly depends on the stoichiometry of CrN films, and in most cases, the disappearance of transition was not well-understood. Recently, we have deposited stoichiometric nominally single-crystalline CrN thin films on (001) MgO substrates and demonstrated the phase transition in relaxed films at ~ 272 K reproducibly. We have also developed Cr2N/CrN metal/semiconductor lateral heterostructures that exhibit high thermoelectric power factors through stoichiometry control. However, the phase transition temperature in CrN must be increased to room temperature and above for practical device implementations. Moreover, experimental verification of magnetic stress as the origin behind the phase transition in CrN also must be demonstrated. In this work, we propose to utilize strain engineering in ultra-thin CrN films to achieve room temperature metal-insulator phase transition, and verify the underlying phase transition mechanism. We hypothesize that compressive epitaxial strain in CrN would increase its magnetic exchange interactions and lead to a substantial increase in its Neel temperature. As (001) CrN exhibits ~ 3% and ~ 5% higher lattice constant compared to (001) KTaO3 and (001) SrTiO3, respectively, we plan to achieve compressive strain by depositing ultra-thin CrN on SrTiO3 and KTaO3 substrates. In addition, we also plan to use GdScO3 and NdScO3 substrates to enforce compressive stress on CrN films. Successful demonstration of structural, magnetic, and electronic phase transition in CrN thin films at and above room temperature would enable CrN based sensor, switch, data storage, and memory device applications.