Revealing the nanostructure of filamentary HfO2-based memristors by scanning precession electron diffraction (SPED)

Abstract text (incl. figure legends and references)

Introduction: Resistive random-access memory (RRAM) devices are under intense research as they have low-power consumption, excellent scalability, enhanced storage density and, high speed operation [1]. The resistive switching phenomenon is thought to be governed by the formation and dissolution of a conductive nanofilament when a voltage is applied [2,3]. As previously investigated, the electrical behaviour of hafnia-based RRAM devices is correlated to the local crystallographic texture and microstructure [4].

Objectives: SPED was used to investigate two different Cu/HfO2/Pt RRAM devices. The layer structure is shown in fig1(a). The goal was to map the microstructure and establish a structure-property correlation.

Materials & methods: By using reactive molecular beam epitaxy (RMBE) 20 nm thick HfO2 thin films were grown on top of sputtered Pt bottom electrodes. The top Cu electrode was deposited with different ways by an (i) ion beam etching (IBE) procedure and by (ii) simple lift out (SLO). Electron transparent cross sectional TEM lamellas were fabricated using a JEOL JIB 4600F Focused Ion Beam. For SPED acquisition, a scanning nanoprobe (in 2D real space) was controlled with a NanoMEGAS P1000 scanning unit integrated into a JEOL-ARM200F and a dynamic Medipix3 detector was used for synchronized electron diffraction patterns (EDP) acquisition (in 2D reciprocal space).

Results: Experimental EDP was cross correlated and statistically matched with suitable simulated EDP with the software ASTAR to generate the orientation fig2(a,c) and phase fig2(b,d) maps. For both devices the orientation and grain size (5 -20 nm) of the dielectric is similar as they were grown under the same conditions. From the phase maps, the monoclinic phase of the HfO2 is revealed and matches to the XRD data. The results show that the Cu is homogeneously grown on the SLO devices and that it is not well distributed in the IBE devices. In the last ones, unexpected Pt traces were found which might have an influence on the electrical performance of the device. In fig1(b) the leakage currents of both devices are shown, in SLO the magnitude is lower than in IBE which could be due to the inhomogeneous Cu and the Pt traces on top of HfO2.

Conclusion: The generated maps confirm the monoclinic phase of the HfO2 dielectric layer and reveal a similar grain distribution in both SLO and IBE devices. Also, from the phase maps, the SLO devices show well distributed layers which might explain their electrical performance.

[1] Zahoor et al. Nanoscale Res Lett 15 (90) (2020)

[2] R Waser et al., Adv. Mater. 21 (25–26) (2009) 2632-2663

[3] M. Lanza et al., Appl. Phys. Lett. 100 (12) (2012) 123508.

[4] S. Petzold, A. Zintler et al., Adv. Electron. Mater. 5 (10) (2019) 1900484

The authors acknowledge funding from the ERC "Horizon 2020" Program under Grant No. 805359-FOXON and Grant No. 957521-STARE, from the DFG grant No. 384682067, from the BMBF under contract 16ESE0298 and 16MEE0154, from the DAAD, from the EPSCR and support by the framework of the the WAKeMeUP and StorAIge projects.

Fig1: (a) The thin film stacks of the two different studied devices: IBE and SLO. (b) Their corresponding leakage current measurements.

Fig2: SLO devices: (a) orientation map of Cu/HfO2/Pt device. Each colour reveals an orientation and (b) phase map of the three layers: Cu in red, m-HfO2 in blue and Pt in green. IBE devices: (c) orientation map and (d) phase map. Scale bar: 50 nm.