(Invited) Hard X-Ray Photoelectron Spectroscopic Study on High-k Dielectrics Based Resistive Random Access Memor
Authors: Nagata Takahiro , Yamashita Yoshiyuki , Yoshikawa Hideki , Chikyow Toyohiro 
Published: February 2020

Resistive random access memory (ReRAM) has been proposed as a new application for oxide materials and advanced to the commercial manufacturing stage. An oxide sandwiched between two metal electrodes shows reversible electric field–induced resistance switching behaviors. These are many resistivity changing mechanisms for the oxide based ReRAM structures such as the insulator–metal transition in perovskite oxides and conductive-bridging (CBRAM). In our research, we focus on the CBRAM with nano-electrolyte reaction, whose advantage is the low forming voltage. The CBRAM with nano-electrolyte reaction comprises the generation and rupture of a metal filament using a metal such as Ag and Cu acting as a fast mobile ion in oxides. Hafnium oxide (HfO2), which is used as a high-k gate insulator for advanced complementary metal-oxide-semiconductor (CMOS) technologies, has shown resistance switching phenomena and been increased interest in the use of HfO2 and related oxides as potential ReRAM materials.1) To put the oxide based ReRAM on practical applications, understanding on controls of metal/oxide interface is essentially important.
Here, we employed hard x-ray photoelectron spectroscopy (HX-PES) under bias operation. HX-PES is a powerful tool for investigating the electronic structure and chemical state of the surface/interface of stacking structures for nanoelectronics devices without any degradation because it has a longer photoelectron mean free path than conventional x-ray photoelectron spectroscopy using Al Kα radiation (hν= 1486.6 eV). The detection depth of HX-PES with an energy of 6 keV is approximately three times deeper than that of conventional XPS, so the photoelectron from a metal/oxide interface, which works as an electrical device, can be detected by HX-PES. With this method, bias-induced compositional changes around the metal/oxide interface during device operation have been directly observed. HX-PES was performed at the SPring-8 BL15XU undulator beamline. The incident X-ray energy and the total energy resolution were 5.95 keV and 240 meV, respectively.
We have demonstrated resistance switching using HfO2 film with a Cu top electrode for nonvolatile memory applications, and revealed the Cu diffusion into the HfO2 layer during the conductive filament formation process. Resistive switching was clearly observed in the Cu/HfO2/Pt structure by performing current-voltage measurements. The current step from a high resistive state to a low resistive state was of the order of 103-104, which provided a sufficient on/off ratio for use as a switching device. The filament formation process was investigated by employing HX-PES under bias operation. The application of a bias to the structure reduced the Cu2O state at the interface and the intensity ratio of Cu 2p3/2/Hf 3d5/2, providing evidence of Cu2O reduction and Cu diffusion into the HfO2 layer. These results also provide evidence that the resistance switching of the Cu/HfO2/Pt structure originates in a solid electrolyte reaction containing Cu ions. HX-PES also revealed the top or bottom electrode dependences of interface reduction or oxidization, and ion migration behaviors.2-5) In the presentation, we will show the details of the correlation between the switching mechanism and the interface reaction in the electrode/high-k dielectrics based ReRAM structure.6)

We are grateful to HiSOR, Hiroshima Univ. and JAEA/SPring-8 for the development of HX-PES at BL15XU of SPring-8. The HX-PES measurements were performed under the approval of the NIMS Beamline Station (Proposal Nos. 2009A 4600, 2010B 4600, 2011A 4611, 2011B 4613, and 2012A 4613).
References: (1) M. Haemori, T. Nagata, et. al. Appl. Phys. Express, 2, 061401 (2009), (2) T. Nagata, et. al. Appl. Phys. Lett. 97, 082902 (2010), (3) T. Nagata, et. al. Appl. Phys. Lett. 99, 223517 (2011), (4) T. Nagata et al. Journal of Materials Research, Vol. 27, 869 (2012), (5) T. Nagata, et. al. Jap. J. Appl. Phys. 54, 06FG01 (2015), (6) T. Nagata et al. ACS Combinatorial Science, Vol 15, 435 (2013).