Alexander Hardtdegen

Forming-free metal-oxide ReRAM by oxygen ion implantation process

We propose a new method for obtaining forming-free ReRAM devices by oxygen ion implantation (O<inf>2</inf> IIP) in the metal oxide film during the device fabrication process. By tuning the implantation dose, as-fabricated devices can be transformed into the ON state. Subsequent standard RESET and SET switching cycles reveal that the forming-free devices switch in a similar way to reference (formed) devices. The devices also show good R<inf>OFF</inf>/R<inf>ON</inf> ratio (>200), retention (10⁴ sec@125°C) and endurance reliability (10⁶ cycles), showing the absence of any device degradation caused by the O<inf>2</inf> IIP process. This method is applied on both (PVD) Ta<inf>2</inf>O<inf>5</inf> and (ALD) HfO<inf>2</inf> nanoscale ReRAM devices, demonstrating the versatile applications of the technique.

Internal Cell Resistance as the Origin of Abrupt Reset Behavior in HfO2-Based Devices Determined from Current Compliance Series

The resistive switching behavior in different HfO2/TiO2 nano crossbar structures of 100 x 100 nm2 size is analyzed by means of DC voltage sweeps. The devices fabricated from 3 nm thin ALD layers of HfO2 and TiO2 sandwiched between Pt and Hf or Ti electrodes show VCM-type bipolar resistive switching after electroforming. For increased compliance current (cc) during set from 50 μA to 800 μA, the set current runs into self- limitation while the reset behavior changes from gradual to abrupt. A model is defined with an internal resistance being in series with the local resistive switch. A recursive algorithm is applied to the cc series for calculation of the series resistor and evaluation of the intrinsic switching characteristic of HfO2-based cells. The intrinsic LRS turns out to be current compliance controlled and to follow the universal switching rule. Supported by compact modelling, we show that an abrupt reset behavior might arise even for materials with a gradual intrinsic reset characteristic in consequence of an internal series resistor.