Impact of the switching mode on the read noise of ReRAM devices

Valence change mechanism (VCM)-based memristive devices are interesting candidates for computing in memory and neuromorphic applications. For these devices read noise is a characteristic which is influenced by a variety of factors like the switching mode, namely the area-dependent and the filamentary mode. In this paper we use TiOx-based devices as an example system exhibiting both modes. This allows to only investigate the effect of the modes while excluding other influences. We find that the read noise in the area-dependent mode is lower than for the filamentary mode and that abrupt current jumps are primarily seen for the filamentary mode. This has to be taken into account when choosing the right operation mode for a specific application.


INTRODUCTION
One of the main challenges of computing nowadays is the von Neumann bottleneck.This means that the run time of tasks is limited by the speed of the data exchange between the central processing unit and memory.This problem could potentially be overcome by computing in memory using memristive devices [4,8].
Various applications require specific characteristics which can be provided by redox-based resistive random-access memory (ReRAM) devices based on the valence change mechanism (VCM).These are metal/metal oxide/metal structures with asymmetric metal layers which are chosen such that one Schottky interface exists.The metal Permission to make digital or hard copies of part or all of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page.Copyrights for third-party components of this work must be honored.For all other uses, contact the owner/author(s).NANOARCH '23, December 18-20, 2023, Dresden, Germany © 2023 Copyright held by the owner/author(s).ACM ISBN 979-8-4007-0325-6/23/12. https://doi.org/10.1145/3611315.3633248layer with this interface is called the active electrode (AE).The resistive change is primarily realized by varying the concentration of oxygen vacancies close to the AE.For VCM devices the areadependent and the filamentary switching mode are distinguished.The area-dependent mode is mainly based on an interfacial exchange of oxygen ions close to the AE.In the filamentary mode the oxygen vacancies are rather relocated within the oxide layer [10].
In this study, we focus on the read noise as this is a central parameter for matching application and operation mode.In some computational tasks like the vector matrix multiplication (VMM) the read noise should be minimized [5] whereas for the training of deep neural networks overfitting might be prevented by a certain level of stochasticity [6].In previous studies the read noise of different device stacks was compared according to the conduction mechanism [7] or the switching mode [2].In this study we compare the read noise of the two switching modes in the same stack.We investigate TiO  -based cross point VCM devices with a device area of 500 x 500 nm 2 .We use devices consisting of a 25 nm Pt/ 1 nm Al 2 O 3 / 7 nm TiO  / 5 nm Cr/ 15 nm Pt stack [1].These devices can permanently be switched from the area-dependent to the filamentary mode by applying a sufficiently high voltage (forming).The area-dependent mode of the device under test shows gradual switching, a low power consumption and a lower cycle-to-cycle variability, whereas the filamentary switching mode is reported

CDF of SNRavg
Figure 2: (a): Three traces of both switching modes.In the filamentary traces shown in red some abrupt jumps are seen, which are absent in the area-dependent traces (blue lines).Worst-case (b) and average (c) signal to noise ratio of both modes depending on the median of the respective trace.The SNR distribution is shifted a little bit towards lower values by forming a filament.Still the median SNR for the filamentary mode is significantly higher than for the switching layers with deep defect states [7]. to provide faster and abrupt switching as well as a higher cycleto-cycle variability [1].In general, gradual switching can still be achieved in filamentary devices by applying well-chosen switching and fabrication parameters [3].The conductivity of the areadependent mode scales with the device area, whereas this is not the case for the filamentary mode.The IV -characteristics of the area-dependent and filamentary switching modes of the studied TiO  -based cells can be seen in Fig. 1 (a) and (b), respectively.

RESULTS
We performed read variability measurements on devices in the high resistive state (HRS) and evaluated the data of both modes according to our previous work [7].The parameters of the measurements are listed in Table 1.Three exemplary traces for each mode are shown in Fig. 2 (a).We see some abrupt jumps in the filamentary traces whereas fewer jumps with lower amplitude are observed for the area-dependent mode.
From the traces we calculated the worst-case and average signal to noise ratio (SNR) according to Schnieders et al. [7].The SNR data of both modes are shown in Fig. 2 (b) and (c).We see that the median current of the filamentary mode is about one order of magnitude higher compared to the area-dependent mode.Furthermore, the mean SNR of the filamentary mode is slightly lower than for the area-dependent traces, as current jumps of high relative amplitude are rather seen for the filamentary mode.
By forming the devices under test, the amplitude of the read noise can be increased and by this tuned according to the application.In our previous study [7] we compared the read noise of switching oxides with shallow (type 1) and deep (type 2) defect states.Here, we showed that the read noise of the type 1 material TiO  is increased by forming.Still the frequency and amplitude of current jumps is lower compared to the filamentary switching type 2 materials.

CONCLUSION
The read noise is a characteristic of VCM-devices which has to be taken into consideration when matching application and device.This paper investigated the read noise of the area-dependent and filamentary switching mode of 500 x 500 nm 2 TiO  -based devices.The worst and average case SNRs are higher for area-dependent devices.Hence, the low read variability along with the gradual switching of the area-dependent mode is favorable for high precision computation in memory tasks like the VMM.Other applications like random number generators [9] or the training of artificial neural networks can better be implemented with the filamentary mode.Thus, considering both switching modes yields higher flexibility.

Figure 1 :
Figure 1: Switching curves of the area-dependent (a) and filamentary (b) mode.The area-dependent switching mode shows a gradual resistance change for SET and RESET operation.The filamentary mode shows an abrupt SET and a gradual RESET event.The arrows indicate the switching direction.