Scanning Tunneling Spectroscopy Studies of AlN Tunnel Barriers

Typical Josephson junctions (JJ's) utilize alumina (AlO$_{x})$ tunnel barriers due primarily to the reliable thermal oxidation procedure that yield high quality Nb-Al-AlO$_{x}$-Nb JJs in the low and moderate ($\le $ 10$^{4}$ A/cm$^{2})$ critical current density (J$_{c})$ regime[1]. However, AlN provides the possibility of forming ultra-thin barriers with fewer defects, and hence lower sub-gap leakage currents, and thus could improve device performance in the high J$_{c}$ regime [2-4]. We present results from an X-ray photoelectron spectroscopy (XPS) and scanning tunneling spectroscopy (STS) study of thin AlN layers on Nb formed by reactive radio frequency (rf) sputtering from an AlN target in a mixture of Ar and N gases. The XPS spectra indicates that O is generally incorporated into the nitride layer during growth in high and near-ultra-high vacuum, thus forming AlO$_{x}$N$_{y}$. The STS measurements reveal that these AlO$_{x}$N$_{y}$ layers exhibit an increase in bandgap with increased N content in the process gas. Decreased band-tails and improved surface stability suggest the barrier defect density can be modified through moderate post-growth annealing. We will provide suggestions for optimization of rf sputtered AlO$_{x}$N$_{y}$ layers for use in high J$_{c}$ Nb and NbN based JJs. [1] Miller, APL 63, 1423 (1993) [2] Wang, APL 64, 2034 (1994) [3] Kleinsasser, IEEE TAS 5, 2318 (1995) [4] Kaul, JMRS 20, 3047 (2005)