Conduction and Rectification in NbOx- and NiO-Based Metal-Insulator-Metal Diodes

Conduction and rectification in nanoantenna-coupled NbO_x- and NiO-based metal-insulator-metal (MIM) diodes ("nanorectennas") are studied by comparing new theoretical predictions with the measured response of nanorectenna arrays. A new quantum mechanical model is reported and agrees with measurements of current-voltage (I-V) curves, over 10 orders of magnitude in current density, from [NbO_x(native)-Nb₂O₅]- and NiO-based samples with oxide thicknesses in the range of 5-36 nm. The model, which introduces new physics and features, including temperature, electron effective mass, and image potential effects using the pseudobarrier technique, improves upon widely used earlier models, calculates the MIM diode's I-V curve, and predicts quantitatively the rectification responsivity of high frequency voltages generated in a coupled nanoantenna array by visible/near-infrared light. The model applies both at the higher frequencies, when high-energy photons are incident, and at lower frequencies, when the formula for classical rectification, involving derivatives of the I-V curve, may be used. The rectified low-frequency direct current is well-predicted in this work's model, but not by fitting the experimentally measured I-V curve with a polynomial or by using the older Simmons model (as shown herein). By fitting the measured I-V curves with our model, the barrier heights in Nb-(NbO_x(native)-Nb₂O₅)-Pt and Ni-NiO-Ti/Ag diodes are found to be 0.41/0.77 and 0.38/0.39 eV, respectively, similar to literature reports, but with effective mass much lower than the free space value. The NbO_x (native)-Nb₂O₅ dielectric properties improve, and the effective Pt-Nb₂O₅ barrier height increases as the oxide thickness increases. An observation of direct current of ∼4 nA for normally incident, focused 514 nm continuous wave laser beams are reported, similar in magnitude to recent reports. This measured direct current is compared to the prediction for rectified direct current, given by the rectification responsivity, calculated from the I-V curve times input power.