Over the past decades, the demand for semiconductor memory devices has been steadily increasing, and is currently experiencing an unprecedented boost due to the development and expansion of artificial intelligence. Among emerging high-density non-volatile memories, resistive random-access memory (RRAM) is one of the best recourses for all kind of applications, such as neuromorphic computing or hardware security [1]. Although many materials have been evaluated for RRAM development, some of them with excellent results, HfO2 is one of the established materials in CMOS domain due to its compatibility with standard materials and processes [2].The main goal of this work is to study the switching capability and stability of HfO2-based RRAMs, as well as to explore their ability in the field of analogue applications, by analyzing the evolution of the resistance states that allow multilevel control. Indeed, analogue operation is a key point for achieving electronic neural synapses in neuromorphic systems, with synaptic weight information encoded in the different resistance states. This research has been carried out over a wide temperature range, between 40 and 340 K, as we are interested in testing the extent to which performance is maintained or modified, with a view to designing neuromorphic circuits that are also suitable in the low-temperature realm. We aim to prove that these simple, fast, high integration density structures can also be used in circuits designed for specific applications, such as aerospace systems.The RRAM devices studied in this work are TiN/Ti/8 nm-HfO2/TiN metal-insulator-metal (MIM) capacitors. Dielectric layers were atomic layer deposited (ALD). It has been demonstrated that the Ti coat in the top electrode acts as a scavenger that absorbs oxygen atoms from the HfO2 layer, and facilitates the creation of conductive filaments of oxygen vacancies [3]. In fact, the oxygen reservoir capability of Ti is well known, as it is able to attract and release oxygen atoms from or to the HfO2 layer during the RRAM operation [4]. The clustering of vacancies extends through the entire thickness of the oxide and, after an electroformig step, it joins the upper and lower electrodes and the device reaches the low resistance state (LRS). By applying adequate electrical signals, the filaments can be partially dissolved, which brings the device into the high-resistance state (HRS), with lower current values. The set process brings the device to the LRS state, while the reset one brings it to the HRS. The dependence of electrical conductivity on external applied electrical excitation allows triggering the device between the both states in a non-volatile manner [5].The experimental equipment used consisted of a Keithley 4200-SCS semiconductor parameter analyzer and a Lake Shore cryogenic probe station. Fig.1 shows current-voltage cycles measured at different temperatures; the averages values at each temperature, both in logarithmic and linear scale, are also shown. The functional window increases as temperature decreases.The evolutions of set and reset voltage values with temperature are depicted in Fig.2, whereas the current values (measured at 0.1 V) corresponding to the LRS and HRS can be seen in Fig.3. LRS resistance decreases as temperature increases, in agreement with semiconductor behaviour, probably due to a hopping conduction mechanism. Both set and reset voltages decrease as temperature increases; the reset process is smoother at high temperatures. The reduction in reset voltage variability as temperature increases is very notable.Finally, Fig. 4 shows a picture of the transient behaviour; in the right panel of the same figure, the amplitudes of the current transients in the reset state have been included in the external loop.To sum up, the resistive switching phenomena is studied in a wide temperature range. The LRS shows semiconducting behavior with temperature, most likely related to a hopping conduction mechanism. Switching voltages decrease as temperature increases, with a notable reduction in reset voltage variability. An excellent control of intermediate resistance state is shown through current transients at several voltages in the reset process.REFERENCES[1] M. Asif et al., Materials Today Electronics 1, 100004 (2022).[2] S. Slesazeck et al., Nanotechnology 30, 352003 (2019).[3] Z. Fang et al., IEEE Electron Device Letters 35, 9, 912-914 (2014).[4] H. Y. Lee et al., IEEE Electron Device Letters 31, 1, 44-46 (2010).[5] D. J. Wouters et al., Proceedings of the IEEE 103, 8, 1274-1288 (2015). Figure 1
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Sep 27, 2024
Kristian Stølevik Olsen + 3
Open Access
