Effect of Stoichiometry of TiN Electrode on the Switching Behavior of TiN/HfO x /TiN Structures for Resistive RAM
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Effect of Stoichiometry of TiN Electrode on the Switching Behavior of TiN/HfOx/TiN Structures for Resistive RAM Katrina A. Morgan1, Ruomeng Huang1, Stuart Pearce1, Le Zhong2, Liudi Jiang2 and C. H. de Groot1 1 Nano Research Group, Electronics and Computer Science, University of Southampton, Southampton, Hampshire, SO171BJ, UK. 2 Engineering Materials Research Group, University of Southampton, Southampton, Hampshire, SO171BJ, UK. ABSTRACT Two types of TiN/HfOx/TiN devices have been fabricated where the top 200nm TiN electrode has been deposited by two different sputtering methods; reactive, using a titanium target in a nitrogen environment, and non-reactive, using a titanium nitride target. Characterization of the materials shows that the reactive TiN is single-phase stoichiometric TiN with a sheet resistance of 7Ω/square. The non-reactive TiN has a sheet resistance of 300Ω/square and was found to contain significant amounts of oxygen. The resistive switching behavior differs for both devices. The reactive stoichiometric TiN device results in bipolar switching with a Roff/Ron ratio of 50. The non-reactive TiN results in unipolar switching with a Roff/Ron ratio of more than 103, however this device shows poor reproducibility. These results show that an oxygen rich layer between the top electrode and insulator affects the Roff value. It supports the theory of oxygen vacancies leading to the formation of conductive filaments. INTRODUCTION Resistive random access memory (RRAM) is a promising candidate for the next generation of non-volatile memory due to its simple structure which offers low power, high-density memory [1].The theory of the switching mechanism and the filament formation are still not fully understood [2], although for transitional metal oxides, oxygen vacancies are believed to be involved in a redox-based reaction [3]. Switching properties of RRAM devices have been measured in order to investigate the role of the electrode stoichiometry. The importance of the electrode material type was shown in [4, 5] and interfacial layers between the electrodes and oxides have been shown to directly affect the switching properties [6-8]. Little has been reported, however, on the stoichiometry of the electrode. In this work, different deposition techniques have been used to vary the stoichiometry of the top electrode and its effects on the switching properties have been investigated. Two sputtering techniques were used to deposit TiN, reactive and non-reactive sputtering. For non-reactive TiN, argon ions collide with a TiN target. TiN is ejected from this target in the opposite direction resulting in deposition of TiN on the substrate. For reactive TiN, argon ions collide with a Ti target. Titanium atoms travel towards the substrate resulting in a Ti layer. An additional plasma source is then used to create nitrogen plasma, reacting with the thin layer of Ti, forming TiN. X-ray diffraction (XRD), energy-dispersive Xray (EDX), X-ray photoelectron spectroscopy (XPS) and sheet resistance measurements were used to character
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