Multilevel Resistive Switching in Hf-Based Rram

Abstract

Resistive random-access memory (RRAM) device has attracted wide attention for next-generation nonvolatile memory (NVM) application due to simple structure, high cycling endurance, good retention, low power consumption and fast switching time (1, 2). In recent years, RRAM devices are extensively researched for multi-level data storage (3, 4). RRAM devices can be driven to various resistance states by applying suitable compliance current values during the switching operation. In this work, multilevel resistive switching behaviors of RRAM devices with three different dielectric materials like HfO2, HfAlO2 and HfZrO2 are investigated with fixed top/bottom electrodes. To control the resistance state, a thin Al2O3 layer (1 nm) was inserted between the bottom electrode and switching dielectric (Ti/TiN/Al2O3/HfAlO2 or HfZrO2 or HfO2/Ti/TiN). The fabrication details of these devices are presented elasewhere (5). Fig. 1 shows DC endurance cycling on the LRS/HRS resistances of the three RRAM structures, (a) HfAlO2, (b) HfZrO2 and (c) HfO2 for different increasing compliance currents. The endurance cycling is performed with 20 cycles per compliance level. As can be seen from the figure, different low resistance states (LRS) and high resistance states (HRS) are well resolved after dc endurance cycling for HfAlO2 and HfZrO2 based RRAM devices. Devices with HfO2, on the other hand, does not show any multilevel LRS or HRS behavior for increase in compliance current. The reason is possibly due to the lack of oxygen vacancies available in the HfO2-RRAM structure. When HfZrO2 was subjected to a post deposition annealing (PDA) in nitrogen environment, the resistance distribution of LRS and HRS was improved, possibly due to improvement of dielectric quality while the oxygen vacancy concentration remained the same. To further improve the resistance window an additional 2 nm thick TiN layer was added before the Ti layer on top electrode of HfZrO2 based RRAM. The cycle to cycle distribution of HRS/LRS resistance of HfZrO2 based RRAM was significantly improved while the compliance current for switching was increased. In summary, process optimization and electrode selection with HfZrO2 as the switching layer can provide significant multilevel storage in RRAM devices.