Fabrication and modelling of ZnO based resistive switching devices for non-volatile memory applications

Abstract

Non-volatile memories (NVMs) are widely used for storage in many electronic devices in the form of memory cards, flash drives etc. for their small sizes, low power consumption and portability. Semiconductor technology based on Moore’s law is already on the verge of its physical limits. To gratify the needs of rapidly advancing and condensing computational technology, a number of emerging NVM technologies such as magnetic-RAM (MRAM), ferroelectric RAM (FRAM), phase change RAM (PCRAM) have been proposed and are under investigation. Among other NVM technologies, resistive RAM (RRAM) or memristor has shown a great potential. Memristive device gains memory property from dual resistive state of the device, in which the device can be switched from one state to another by an applied potential across it.Disclosed herein is a method of fabricating a non-volatile resistive switching memory element and memristordevice with optimized conditions of dual ion beam sputtering (DIBS) system. A two-terminal metal-insulator-metal (MIM) resistive memory cell showing memristive behavior has been fabricated with ZnO as a switching layer sandwiched between two aluminum (Al) electrodes.A number of fabrication techniques such as atomic layer deposition, chemical vapor deposition, pulsed laser deposition, radio-frequency (RF) sputtering, molecular beam epitaxy (MBE) have been deployed to realize resistive memory devices. As compared to other physical vapor deposition (PVD) techniques, dual ion beam sputtering (DIBS) system has been widely recognized to produce high-quality compound semiconductor films with reasonably better compositional stoichiometry, uniformity, and adhesion to the substrate surface. The system is equipped with one RF ion deposition source and one direct current coupled (DC) assist ion source. The primary ion source precisely focuses a neutralized ion beam onto a target. Secondary DC assist ion source enhances the density of the films coupled with superior growth uniformity and film adhesion to the substrate by assisting in reduction of columnar growth of thin films. Besides these, other salient features of DIBS system are high-quality growth with reduced surface roughness on a larger substrate area, in-situ substrate pre-cleaning before carrying out growth process. To the best of authors’ knowledge, the fabrication of resistive memory device by DIBS system has not been reported till date. Therefore, the study of the switching characteristics of ZnO and corresponding relation withelectrical, optical, elemental and structural properties is quite significant, when we have sought to realize ZnO based resistive memory device. While fabricating, the growth temperature, the oxygen partial pressure [O2/(O2+Ar)%] and the thickness of device have been optimized sequentially keeping other parameters constant. In an implementation, the method in the beginning investigates the effect of growth temperature on the electrical properties of the switching material, ZnO films, keeping gas partial pressure and film thickness unaltered. In the next stage, oxygen partial pressure is optimized keeping the growth temperature fixed at the optimum value obtained from the previous stage and constant ZnO film thickness. In the subsequent stage, film thickness is optimized keeping growth temperature and oxygen partial pressure fixed at their respective optimized valuesWe report various aspects of Dual Ion Beam Sputtering (DIBS) deployed fabrication of ZnO based Resistive Switching (RS) using reactive (Al) and inert electrodes (Pt and Au). Devices have been investigated to understand the role of interfaces in RS and analyze corresponding performance parameters. Schottky, Schottky-like, and Ohmic interfaces have been identified in terms of usage of different electrode combinations. Besides, interface-limited conduction (ILC) which is affected by nature of electrodes, there is considerable effect of bulk parameters in terms of bulk-limited conduction (BLC), which can be controlled during deposition of switching layer by DIBS. RS is affected by many anomalies such as the deterioration of device in environmental conditions, or for change in concentration/configuration of oxygen vacancies (VO)/non-lattice oxygen ions (IO) with bias and with number of read/write cycles. Device performance is found to be affected by the configuration of electrodes as well, i.e., top or bottom electrodes. Bipolar RS (BRS) and Unipolar RS (URS) have been evaluated for different combinations ofelectrodes. The formation/dissolution of AlOx layer at metal/semiconductor interface (Al/ZnO) to impart bipolar switching to ZnO based RS device have also been investigated for one or both Al electrodes Compliance current (Icc), i.e. maximum current through device and stop voltage (Vstop), i.e. maximum stop voltage upon device, have been used as bias characteristics to evaluate the effect upon conduction behavior. Effect of change in Icc for constant Vstop and vice-versa upon high resistance state (HRS) and low resistance state (LRS) has been evaluated in terms of change in bulk defects, i.e. oxygen vacancy and interface inhomogenity, i.e. AlOx formation/dissolution. The device shows unipolar resistive switching (URS) at lower Icc (<=0.1 mA) and transits irreversibly into bipolar resistive switching (BRS) at higher Icc (>=1 mA). Thedominant conduction behavior of device is found to be homogenous resistive switching (HoRS). The conduction in HRS and LRS is found to be space charge limited current conduction. Conductive AFM results of thin film switching layer support the HoRS mechanism and also illustratethe role of interfacial oxide in attributing BRS behavior in Al/ZnO/Al resistive switch. The effects of bulk defects (oxygen vacancies, non-lattice oxygen ions, trap charges) as Bulk-Limited Conduction (BLC) and interface anomalies (disorder-induced interface states, Schottky barrier formation/dissolution) as Electrode-Limited Conduction (ELC) for a resistive switch or memristor have been exhaustively studied. Distribution of bulk defects with applied bias which governs switching and the forming-free behavior of device has been illustrated. Its role in formation/dissolution of interfacial oxide is found to affect Schottky barrier considerably. The present work significantly contributes towards a further understanding and modeling of the conduction mechanisms for a wide range of resistive switches.