Unipolar resistive switching in spin-coated cobalt ferrite thin films

Abstract

The resistive switching (RS) phenomenon refers to the reproducible switching between a high resistance state (HRS) and a low resistance state (LRS) observed in a metal/oxide/metal (MOM) configuration [1]. The RS effects in binary oxide materials such as NiO, TiO 2 , and TaOx has been studied intensively for their potential applications in nonvolatile resistive random access memory (RRAM) due to their simple structure, fast operating speed, low power consumption, and good compatibility with the complementary metal oxide semiconductor processing. More recently, ferrite materials have been discovered as potential RRAM materials showing excellent RS behaviors [2-4]. In this work we report the preparation of textured cobalt ferrite (CoFe 2 O 4 , CFO) thin films by sol-gel and spin-coating and the study on the RS properties of Pt/ CoFe 2 O4/Pt structures. The (111)-oriented polycrystalline CoFe 2 O 4 thin films were prepared on Pt/TiO 2 /SiO2/Si substrate by spin-coating a 0.2 M sol obtained by dissolving the precursors Co(NO 3 ) 2 6H 2 O and Fe(NO 3 ) 2 9H 2 O in 2-methoxyethanol. Scanning electron spectroscopy revealed that the average layer thickness of one coating is about 50 nm. The CFO thin films were exposed to different heat treatment process, i.e., without annealing, annealing at 500°C, and annealing at 700°C. The pristine I-V characterization showed that the resistivity of the CFO thin film decreased with the heat treatment temperature, and only CFO thin films annealed at 700°C showed repeatable RS. Furthermore, the thickness of the CFO thin film was adjusted by the number of coating varied from 1 to 4. Pt top electrodes of 100 nm thick and 90 × 90 μm2 area were then deposited using e-beam evaporation to fabricate the Pt/CFO/Pt structures. All Pt/1-layered CFO/Pt structures showed a quite low resistance of a few ohms and no switching was observed. Most of the MOM structures based on 2-layered CFO thin films showed increased resistance around 10 Ω in the initial states, and forming-free unipolar resistive switching (URS) was observed. When the layer number increased to more than 3, the initial states of the Pt/CFO/Pt structures were of high resistance higher than 50 kΩ, and stable and repeat-able URS with a HRS/LRS ratio about 1000 can be induced with a forming process (Fig. 1). Based on the thickness dependence of the URS behavior observed, we analyzed the RS behaviors of the textured CFO thin films in detail with respect to the forming voltage and the conducting mechanism. Our results revealed that Pt/CFO/Pt structures with 2 or 3 layers of CFO thin films show the most stable RS behavior with a much narrow distribution in the set voltage.