Harrison A. Walker

In heterostructures made from polar materials, e.g., AlN–GaN–AlN, the nonequivalence of the two interfaces is long recognized as a critical aspect of their electronic properties; in that, they host different 2D carrier gases. Interfaces play an important role in the vibrational properties of materials, where interface states enhance thermal conductivity and can generate unique infrared‐optical activity. The nonequivalence of the corresponding interface atomic vibrations, however, is not investigated so far due to a lack of experimental techniques with both high spatial and high spectral resolution. Herein, the nonequivalence of AlN–(Al_0.65Ga_0.35)N and (Al_0.65Ga_0.35)N–AlN interface vibrations is experimentally demonstrated using monochromated electron energy‐loss spectroscopy in the scanning transmission electron microscope (STEM‐EELS) and density‐functional‐theory (DFT) calculations are employed to gain insights in the physical origins of observations. It is demonstrated that STEM‐EELS possesses sensitivity to the displacement vector of the vibrational modes as well as the frequency, which is as critical to understanding vibrations as polarization in optical spectroscopies. The combination enables direct mapping of the nonequivalent interface phonons between materials with different phonon polarizations. The results demonstrate the capacity to carefully assess the vibrational properties of complex heterostructures where interface states dominate the functional properties.

Niobium nitride (NbN) is an important material for superconducting electronics because of its relatively high transition temperature in comparison to other conventional superconductors. Recent advances in the use of gas field focused ion beams for material modification motivate directly written NbN electronics. In this work, we study the electrical transport properties of ultra-thin film NbN microbridges irradiated with a focused helium ion beam. Twenty 4- $\mu$ m wide strips were structured into an NbN thin film and irradiated with a helium ion microscope with ion fluences ranging from $1 \times 10^{18}$ He $^+/$ cm $^{2}$ to $3 \times 10^{18}$ He $^+/$ cm $^{2}$ . We report the temperature and magnetic field dependence of the transport properties. At a higher dose of $1 \times 10^{20}$ He $^+$ , the irradiation reduces the critical temperature of a narrow region resulting in a planar superconductor-reduced $T_{c}$ superconductor-superconductor (SS'S) Josephson junctions. This establishes that NbN can be modified in this manner for nanoelectronics, opening up possibilities for superconducting logic circuits and other higher-speed and high-temperature applications.

In this report, the development of reactively sputtered ultra-thin niobium nitride (NbN) films and their fabrication into patterned structures using 193 nm optical lithography on 300 mm scale is presented. The target composition of NbN film with Cu encapsulation showed a critical temperature ( T_c ) of ∼9.5 K (at a thickness of ∼50 nm), with c ) as a function of film thickness. The variation of T_c of copper encapsulated NbN as a function of Nb to N ratio, and position on the wafer is reported, along with the variation of T_c and I_c as a function of linewidth. These results underscore the potential of ultra-thin NbN films deposited over a large area, for a broad range of applications in quantum computing, photon detection and superconducting circuits operating at GHz frequencies.