Fabrication and characterization of microscale HfO2-based Memristors

Memristors are metal/insulator/metal devices whose resistance can be switched between two different states (i.e. the low resistive state LRS, and the high resistive state, HRS) by applying a proper voltage value over the two metal contacts [1], [2]. Their simple structure makes memristors prone to extreme down scaling and 3-D stacking potentiality, and excellent compatibility with the complementary metal-oxide-semiconductor (CMOS) technology. Moreover, because of their low power consumption and high speed, memristors are rightly considered the elemental bricks for a next generation of high-density nonvolatile memories. HfO2 has attracted much attention as an oxide material for memristor application, due to its natural compatibility to CMOS technology [2]. Pulsed laser deposition (PLD) is a low cost thin films growth technology, which has been widely utilised for depositing high quality HfO2 thin films [3]. It suits perfectly memristors’ fabrication needs, because it gives the possibility to finely modify oxides properties by changing growth parameters. In this work we report on the fabrication and electrical characterization of microscale Pt/HfO2/Cu memristors (see Fig. 1). To this aim, HfO2 thin films were grown by PLD on Si/SiO2/Ti/Pt substrates. Afterwards, 200 nm-thick Cu pads of 50 × 50 μm2 were defined by direct laser-writing microlithography and subsequent lift-off. All devices were electrically characterised at room temperature by performing two-probe I–V measurements by means of a Versastat 3 (Princeton Applied Research) connected to a Karl Suss probe station. The bias voltage was swept from – 0.5 to 0.5 V while simultaneously measuring the current. Fig. 2 shows the typical bistable I–V curve shown by the fabricated devices. Resistances at the LRS and HRS were measured at 0.42 V and resulted RON = 19 W and ROFF = 3921 W, respectively. Consequently, the ROFF/RON ratio was 195, which is suitable for memory applications. The endurance of these devices has not proved satisfactory yet, probably due to the absence of any current compliance protection in our measurement setup which led, after a small number of cycles, to devices failure due to oxide breakdown. We noticed also a larger variability of ROFF with the number of measured devices than the variability of RON. In conclusion, microscale HfO2-based memristors grown by PLD showed a remarkable ROFF/RON ratio but a poor endurance. This suggests that further work must address the lifetime of our devices, especially by investigating the HfO2 thin film quality and applying a proper current compliance.