Memristive Devices Formed By ALD Metal Oxide Growth on a Hafnium Layer – Study of the Interfacial HfO2 Formation

Abstract

Redox-based memristive devices (ReRAM) are considered for future energy-efficient non-volatile memory. Memristive cells enable high speed data access, low power consumption and high scalability, which is a key requirement for their use as artificial synapses in neuromorphic computing. Applications in networks of up to 106 devices require a scaling of the cell volume to about 103 nm3. For a typical valence-change-mechanism (VCM)-type switching cell with a metal oxide, like HfO2, sandwiched between two metal electrodes of different oxidation enthalpy, this means that the individual layer thickness is in the range of a few nanometers. Therefore, a control of the device properties demands for a control of interfacial reactions during device fabrication. Here, we are going to present a careful analysis of the oxidation of an Hf electrode layer during atomic layer deposition (ALD) of different metal oxides, these are Al2O3, TiO2, and HfO2. The in-situ encapsulation of a reactive metal electrode by means of an ALD metal oxide layer enables a better process control compared to stack structures formed by breaking the vacuum conditions.In this study, plasma and thermal ALD processes were used to grow ultrathin layers of Al2O3, TiO2, and HfO2 on top of Hf metal films, after transfer under UHV conditions. The growth of homogeneous metal oxide films of a few nanometer in thickness enabled a systematic analysis of the HfO2 formation at the Hf/metal oxide interface by means of angle dependent X-ray photoelectron spectroscopy (XPS). The results show that the properties of the interfacial HfO2 layer can be controlled by the metal oxide deposited on top as well as by the process chemistry and deposition temperature. In contrast to the TiO2 film, the Al2O3 layer clearly serves as oxidation barrier. In addition, the chemistry and structure of the HfO2 interfacial layer were analyzed from XPS and transmission electron microscopy analysis.Memristive device were fabricated from the Hf/HfO2/MOx stacks by depositing Pt top electrodes. The initial current of the devices scales with the pad size indicating the high quality of the insulating oxide layers. Consistent with VCM-type memristive devices the cells required an electroforming voltage to enable bipolar-type resistive switching. In this presentation, the trade-off between the two functionalities of oxidation protection versus enabling of a low electroforming voltage will be discussed.