Current–Voltage Characterization of Transparent ITO/ZnO:B/ZnO:(Al + In)/Ag Schottky Diodes Prepared with Multilayer Films by Sol–Gel Deposition

We fabricated a ZnO-based Schottky diode via the deposition of a ZnO film co-doped with Al + In (4 at.%) on a boron-doped ZnO film (8 at.%). Each film was prepared by layering coatings (2, 3, 4, and 5 layers) by sol–gel deposition. The finished diode consists of the combination of seven layers (each layer with a thickness of around 90 nm). The total thickness of the diode is around 700 nm. The films were previously studied and structurally, optically and electrically characterized. Additionally, for comparative purposes, we fabricated and characterized un-doped ZnO films. The energy bandgap values of the un-doped films, mono-doped films, and co-doped films were 3.30 eV, 3.32 eV, and 3.34 eV, respectively. X-ray diffraction did not show traces of different phases from hexagonal Wurtzite-type ZnO. The electrical resistivity values obtained were 386, 4.44 × 10⁴, and 3.37 Ω-cm, respectively. The junction diodes were built by depositing layers of the high-resistivity material (ZnO:B) on ITO conductor substrates, followed by the deposition of layers of the low-resistivity material (ZnO:Al + In) on the same substrate. The I–V characteristics of these diodes were analyzed in terms of the number of the deposited layers (or the different thickness of the films). The results show a Schottky-type behavior in the dark and under light (spot lamp of 160 W), which is controlled by the thickness of the resistive layer. From the I–V curves, the characteristic parameters including barrier height, ideality factor, and series resistance were calculated. From the transconductance (g_m=dI/dV), it was possible to identify the presence of all the layer–layer interfaces. Depending on the thickness of the resistive ZnO:B film, we found a region of negative differential resistance and a region of visible light detection.