Pulsed laser deposition of ZnO and VO2 films for memristor fabrication

Memristors are resistive switching memory devices which have attracted much attention over the last years for high-density memory applications because of their simple structure, small cell size, high speed, low power consumption, potential for 3-D stacking and excellent compatibility with the complementary metal-oxide-semiconductor (CMOS) technology [1]. Beside nonvolatile memory applications, memristors have been also proposed for other different applications including biosensors [2] and neuromorphic [3] circuits. The device structure is simply an oxide material sandwiched between two metal electrodes. The switching behavior is not only dependent on the oxide material but also on the choice of metal electrodes and their interfacial properties. For this reason switching characteristics of many metal oxide films (e.g. TiO2, NiO, TaO2, HfO2) and metal contacts have been studied [1]. ZnO has attracted much attention as oxide material for resistive switching application, due to its abundance in nature, which means low cost, and compatibility to CMOS technology [4] in terms of process integration and device scalability down to nanometric sizes. VO2 is also a promising candidate as switching element for data storage [5]. In this work we report on the fabrication and electrical characterization of microscale ZnO and VO2 memristors. ZnO-based memristors have active areas ranging between 2 7 2 mm2 and 300 7 300 mm2. VO2-based memristors have instead dimensions between 100 7 100 mm2 and 300 7 300 mm2. Both oxides were deposited by pulsed laser deposition (PLD) on FTO (FluorineTin- Oxide) glass substrates at the conditions reported in [6]. After the oxide deposition, top metal contact were defined by direct laserwriting microlithography and subsequent lift-off. All devices were electrically characterised at room temperature by performing two-probe I\u2013V measurements by means of a custom developed electronic circuit. For all devices the typical I\u2013V curve of hysteresis was achieved by sweeping the applied voltage in the range \u20133 to 3 V. An analysis of the high resistance state (HRS) and low resistance state (LRS) against the device size showed that for both material systems the LRS is independent on the device area, suggesting that the ON-state of the device is dominated by a local, filamentaryphenomenon. Moreover, smaller devices exhibit a HRS increasing with device size decreasing. Finally, ZnO devices have proven to be more suitable for memory application than VO2-based device, exhibiting a higher ROFF/RON ratio