Magnetization and photoluminescence tuned by resistive switching in copper doped zinc oxide structures

Abstract

The resistive switching phenomena have attracted wide attention for its applications in non-volatile memory devices in which the resistance of the material can be adjusted reversibly by applying an external electric bias. Room temperature ferromagnetism (RT FM) and materials with magnetization tuned by resistive switching are attractive and interesting physical phenomena that have impact in the fundamental developments of spintronic devices. In the aspect of UV emitting devices, the near band edge (NBE) emission tuned by resistive switching is also an attractive phenomenon for applications.

An Al/ZnO:Cu/n^+-ZnO:Ga/c-sapphire structure was fabricated using the pulsed laser deposition (PLD) method. The bipolar resistive switching property of the structure is confirmed by the hysteresis behaviour shown by the current-voltage (I-V) measurement.

Superconducting quantum interference device (SQUID) measurement shows that the sample is ferromagnetic at room temperature. The saturated magnetic moment of the sample is tunable with a reduction of about 30 % during the transition from the high resistance state (HRS) to the low resistance state (LRS). The saturated magnetic moment can be recovered when the sample is switched back into HRS. X-ray photoelectron spectroscopy (XPS) study shows that the occupancy ratio of the Cu^+:Cu^(2+) increases and the concentration of the oxygen vacancies (V_o) remains constant when the sample is switched from HRS to LRS. The reduction in magnetization has been explained by the shift in Fermi level, which decreases the effectiveness of the Cu^(2+)-V_o-Cu^+ magnetic alignments as described by the indirect double-exchange model.

The photoluminescence (PL) properties of the structure are found to be adjustable with the resistive switching process. During the transition from HRS to LRS, the PL signal intensities of the near band edge emissions (NBE) at 3.15 eV, 3.22 eV and 3.32 eV are observed to be reduced by about 30 % while the PL signal intensities of the defect emissions at about 2.36 eV are not changed. Capacitance-voltage (C-V) measurement shows similar hysteresis behaviour as the I-V data. This implies that a change in the interfacial charges distribution might occur which is not expected by the conventional conducting filament model for resistive switching. The physics leading to the resistive switching tuning of saturated magnetic moment and near band edge emission intensity is discussed.