Trap characterization and conductance quantization in phosphorus-doped ZnO memory devices

This work addresses the trap characterization and conductance quantization of bipolar resistive switching devices using transparent phosphorus-doped ZnO (ZnO:P) films. The average transmittance in the visible light region and optical band-gap of the ZnO:P films are around 95.6% and 3.4 eV, respectively. Based on the analysis of current-voltage characteristics at high resistance state (HRS) in the structure of Ni/ZnO:P/Ni, the current conduction mechanism is dominated by hopping conduction. Accordingly, the trap spacing and trap energy level in ZnO:P films are extracted to be around 1.5 nm and 280-370 meV, respectively. The trap spacing remains about constant during set/reset cyclic switchings. Meanwhile, the trap energy level is decreased as the set/reset switching increases, which leads to the raised leakage current at HRS. In addition, the conductance values read at small voltages exhibit quantized effect during the set processes. The histogram of normalized conductance reveals that the conductance levels and peaks initially appear at 1 G0 and 2 G0, and shift to 5.5 G0, 7 G0, and 8 G0 after the set/reset switching cycle is more than about 100 times, where G0 (=77.5 μS) is the quantum of conductance.