Improved Resistive Switching Properties in HfO 2 -based ReRAMs by Hf/Au Doping
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Improved Resistive Switching Properties in HfO2-based ReRAMs by Hf/Au Doping Xiaoli He, Natalya A. Tokranova, Wei Wang and Robert E. Geer College of Nanoscale Science and Engineering, University at Albany, Albany, NY 12203 ABSTRACT Emerging NVM devices have been extensively studied as candidates to extend density scaling and power reduction beyond Si-based flash. Recently, resistive-random-access-memory (ReRAM) devices in the form of metal-insulator-metal (MIM) structures have attracted substantial attention due to their potential scalability, low power operation, and high speed. HfO2 is attractive compared to other transition metal oxides from the vantage point of CMOS process compatibility. Here, we investigate doped HfO2 with a Pt top electrode on an n+-Si substrate. By doping HfO2 with Hf or Au, improved resistive switching properties have been demonstrated in terms of enhanced cycling endurance and lower switching voltages for SET and RESET. The improvements were attributed to doping-induced oxygen vacancies. In addition, Cu-doped HfO2 devices have exhibited multilevel resistive switching. INTRODUCTION Due to the technical and physical limits in highly scaled Si-based flash nonvolatile memories (NVM), emerging NVM devices such as ferroelectric RAM (FeRAM) [1], magnetoresistive RAM (MRAM) [2], phase-change RAM (PCRAM) [3], and recently emerging resistive RAM (ReRAM) [4] have emerged as potential replacements. ReRAM is considered to have promising advantages because of its potential for low power consumption, fast switching and high integration density [4]. Many transition metal oxides including ZrO2 [5], HfO2 [6, 7], and TiO2 [8], have been found to exhibit resistive switching. Different models have been proposed to explain resistive switching, such as formation and rupture of metallic filaments [5, 6], trapcontrolled space limited conduction [9], and oxygen migration [8], the universal origins of the resistive switching properties are still a topic of debate. Regardless of the specific switching model, defects and impurities in the oxide layer are expected to play an important role. Previous studies have shown that introducing external defects or traps could improve performance [9]. In this paper, we use HfO2 as the insulator layer for ReRAM devices, due to its CMOS process compatibility, with Pt as the top electrode and n+-Si as the bottom electrode. By doping HfO2 with Hf/Au, improved resistive switching properties have been found in terms of enhanced cycle endurance, lower switching voltages for SET and RESET, and more uniform resistive switching. In addition, multilevel resistive switching of Cu-doped HfO2 devices was demonstrated. EXPERIMENT Resistive switching memory devices (~ 800 µm in diameter) were fabricated on an n+-Si wafer after a chemical cleaning in 5% HF and H2SO4:H2O2 (3:1) solutions. A 30 nm layer of HfO2 was deposited via reactive e-beam evaporation of Hf in an O2 flow. A mini e-beam evaporator system (SPECS EBE-1) with flux stabilization was used for the Hf evaporation. The formation of HfO
