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We present complete experimental determinations of the tunnel barrier parameters (two barrier heights, junction area, dielectric constant, and extrinsic series resistance) as a function of temperature for submicrometer Ni--NiO--Ni thin-film tunnel junctions, showing that when the temperature-invariant parameters are forced to be consistent, good-quality fits are obtained between I--V curves and the Simmons equation for this very-low-barrier system (measured [empty set] [approximately equal to] 0.20 eV). A splitting of [approximately equal to] 10 meV in the barrier heights due to the different processing histories of the upper and lower electrodes is clearly shown, with the upper interface having a lower barrier, consistent with the increased effect of the image potential at a sharper material interface. It is believed that this is the first barrier height measurement with sufficient resolution for this effect to be seen. A fabrication technique that produces high yields and consistent junction behavior is presented as well as the preliminary results of inelastic tunneling spectroscopy at 4 K that show a prominent peak at -59 meV, shifted slightly with respect to the expected transverse optic phonon excitation in bulk NiO but consistent with other surface-sensitive experiments. We discuss the implications of these results for the design of efficient detectors for terahertz and IR radiation. OCIS codes: 240.7040, 160.3900, 040.3060, 040.5160, 130.0250, 190.4360.