Reactive sputter deposition and annealing of nanometer scale NiO thin films for metal-insulator-metal tunnel junction diodes

Highlights: • MIM tunnel diode NiOx insulating layer. • NiOx films deposited by reactive sputtering. • NiO films inhomogeneous in the growth direction. • Ellipsometer measurement of vertical inhomogeneity. • Reduction of optical absorption of NiO films by annealing. - Abstract: The increased switching speed of metal-insulator-metal (MIM) tunnel diodes over existing diodes has the potential to open new applications including infrared energy harvesting and high frequency detectors. The work here analyzes the effect of the insulator layer (NiOx) properties on performance of such Ni-NiOx-Cr based junctions. This work focuses on these films because they are critical for device performance. Thickness must be measured and controlled in the nanometer range for the requisite junction tunneling to take place. At the same time the other fabrication challenges, while significant, have practically been met. The films were deposited by reactive magnetron sputtering of a Ni target in an atmosphere containing oxygen. Test films ranged from 10 to 30 nm in thicknesses and were smooth as determined by atomic force microscopy. Initial ellipsometry examination showed that the as-deposited films were inhomogeneous in the growth direction and exhibited high optical absorption across the visible wavelength range (midrange k = 0.8 and up). While this is not unexpected considering what is known about nucleation and initial growth, it is undesirable for the intended use. Consequently treatment of the films was carried out by annealing in an oxygen atmosphere at 400 °C for 3, 6, and 12 min. This resulted in both a significant decrease in the optical absorption and a dramatic improvement in film homogeneity. Examination using Secondary Ion Mass Spectroscopy did not show significant increases in film oxygen suggesting that atomic rearrangement rather than oxidation occurred during annealing. To confirm test structure fabrication, performance of MIM junctions with the annealed films was investigated using current-voltage measurements. The results were correlated with capacitance-voltage measurements.