(Invited) Photoelectron Spectroscopic Study on High-k Dielectrics Based Nanoionics-Type ReRAM Structure under Bias Operation

Resistive random access memory (ReRAM) has been proposed as a new application for oxide materials. An oxide sandwiched between two metal electrodes shows reversible electric field–induced resistance switching behaviors. Hafnium oxide (HfO₂), which is used as a high-k gate insulator, has also shown resistance switching phenomena and been increased interest in the use of HfO₂ and related oxides as potential ReRAM materials [1]. For the oxide based ReRAM with the conductive filament formation model, two mechanisms of resistance switching have been proposed. One is the nanoionics model, which comprises the generation and rupture of a metal filament using a metal such as Cu acting as a fast mobile ion in oxides. The other model is that of oxygen vacancy nucleation at the metal/oxide interface. 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 to examine the electronic structure of Pt or Cu/HfO₂ ReRAM structures in an operating device. 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.

The interface electronic states of 10-nm-thick Pt or Cu top electrodes/HfO₂/Pt structures were measured with HX-PES in the SPring-8 BL15XU undulator beamline. The incident X-ray energy was 5.95 keV. In the case of the Pt/HfO₂ interface, applying a forward bias increased the Pt–O bonding peak as shown in Fig 1, indicating evidence of Pt electrode oxidization and oxygen vacancy formation around the interface [2]. In contrast, the application of a bias to the Cu/HfO₂ interface reduced the copper oxide bonding state, providing evidence of oxygen reduction and Cu diffusion into the HfO₂ layer [3,4]. We achieved direct observation of oxygen migration at the metal/HfO₂ interface under device operation, which is the key to controlling the electrical properties of oxide based ReRAM. Based on these results, we demonstrated the control of the switching voltage and the initial conductive filament formation process of the Ta−Nb binary oxide ((Ta_xNb_1−x)₂O₅) ReRAM structure using a combinatorial method [5]. The relationship between the interface structure and the electrical properties also will be discussed in detail at the presentation.