Center for Functional Nanomaterials Seminar

This work focuses on the fabrication and characterization of tantalum nitride-based superconducting devices, including a simple Superconducting Quantum Interference Device (SQUID) incorporating TaN/Al2O3/TaN junctions. A comprehensive microfabrication process was established using photolithography in an ISO Class 5 cleanroom environment, followed by structural, morphological, chemical, and low-temperature electrical characterization to investigate the properties of superconducting thin films and tunneling structures. TaN thin films were deposited on Si/SiO2 substrates by reactive DC magnetron sputtering under optimized growth conditions that avoided target poisoning. The structural, morphological, and chemical properties of the films were investigated using X-ray diffraction (XRD), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS), respectively. These complementary techniques enabled the investigation of the crystalline structure, surface morphology, and chemical composition of the films, including the identification of the superconducting δ-TaN phase. Superconducting device structures were subsequently fabricated through photolithographic patterning and thin-film processing. A chromium sacrificial layer deposited by electron-beam evaporation was used to transfer lithographic patterns onto the TaN films, enabling the fabrication of structures designed for cryogenic electrical measurements in the Van der Pauw configuration. In addition, a photoresist-based lift-off process was implemented to define Al2O3 insulating barriers deposited by Atomic Layer Deposition (ALD). The superconducting properties of the TaN films and TaN/Al2O3/TaN structures were investigated through low-temperature electrical transport measurements using a PPMS DynaCool system. Resistance-versus-temperature measurements revealed superconducting critical temperatures as high as 9.91 K, while the TaN/Al2O3/TaN structure exhibited a superconducting transition at 9.27 K. In another measurement configuration, a finite residual resistance associated with the insulating barrier was identified. Current-voltage, differential resistance, and differential conductance measurements performed over a temperature range of 2-12 K were used to investigate charge transport across the junctions and examine features consistent with superconducting tunneling and Cooper-pair transport. Additionally, TaN/native oxide/Ag tunnel junctions were fabricated and characterized. The naturally formed oxide layer on the TaN surface served as the insulating barrier between the superconducting TaN electrode and the metallic silver electrode. Low-temperature I-V measurements revealed a nonlinear electrical response consistent with tunneling transport across the oxide barrier, providing further insight into the electrical properties of TaN-based tunnel junctions. To evaluate the quality of electrical contacts to TaN, transmission line method (TLM) structures incorporating chromium and gold contacts were fabricated by photolithography. Room temperature I-V measurements performed using a probe station exhibited linear behavior for both Cr/TaN and Au/TaN contact systems, consistent with ohmic contacts. These results provided a basis for assessing metal/TaN interfaces and their suitability for superconducting device fabrication.