Influence of Graphene Interlayers on Electrode-Electrolyte Interfaces in Resistive Random Accesses Memory Cells

Mater. Res. Soc. Symp. Proc. Vol. 1729 © 2015 Materials Research SocietyDOI: 10.1557/opl.2015.

Influence of Graphene Interlayers on Electrode-Electrolyte Interfaces in Resistive Random

Accesses Memory Cells

Michael Lübben1, Panagiotis Karakolis2, Anja Wedig1, Vassilios Ioannou2, Pascal Normand2,

Panagiotis Dimitrakis2, Ilia Valov1 1Peter Gruenberg Institut, Forschungszentrum Jülich GmbH, Jülich, Germany 2 Institute of Nanoscience and Nanotechnology, NCSR Demkritos, Athens, Greece

ABSTRACT The behavior of the redox-based resistive switching memories is influenced by chemical

interactions between the electrode and the solid electrolyte, as well as by local environment. The

existence of different chemical potential gradients is resulting in nanobattery effect lowering the

stability of the devices. In order to minimize these effects we introduce a graphene layer at the

active electrode – solid electrolyte interface. We observe that graphene is acting as an effective

diffusion barrier in the SiO2-based electrochemical metallization cells and acts catalytically on

the electrochemical processes prior to resistive switching.

INTRODUCTION

Redox-based resistive switching memory (ReRAM) cells are the most emerging memory

devices for technology nodes <16 nm according to the latest edition of the international

technology roadmap for semiconductors (ITRS) [1]. Mainly studied are ReRAMs showing either

the vacancy change (VCM) or electrochemical metallization mechanism (ECM) [2]. It is well

known that the resistance switching in these devices is governed by the electrochemical

interaction between the terminal electrodes and the solid-state electrolyte material [3]. The

chemical instability and the presence of (electro)-chemical potential gradients at the

electrode/electrolyte interface are the main issues for ReRAM cells that affect both performance

and reliability [4,5]. We aim to control the electrochemical interaction at the interfaces by

inserting an intermediate layer between the electrode and electrolyte as shown for other systems

in order to serve as diffusion barrier and prevent intermixing and chemical interactions [6] as

shown for e.g. Ag/GeSx based devices [7]. Graphene was tested as such intermediate layer as the

thinnest electronically conductive film with sufficient electronic conductivity in plane and

mostly insulating out of plane, possessing good mechanical properties and the ability to

homogeneously cover the active electrode. Graphene monolayers (GML) have unique properties

in terms of conductivity, mechanical strength, diffusion barrier, transparency, heat conductivity

etc. [8] and thus graphene has been used as electrode material in many applications [9].

Recently, graphene (G) has been used as an intermediate layer between the electrode and

the metal oxide material in two-terminal ReRAM devices. It has been demonstrated that in VCM

ReRAM [10] device the GML offers higher low-resistance-state (LRS) and better high-

resistance-state (HRS) uniformity, which are both important for ReRAM characteristics. The

built-in series resistance due to the GML substantially raises the LRS resistance, effectively

reduces current overshoot, and significantly lowers the programming power. The GML may

serve as an oxygen capture reservoir in addition to being a passive barrier. Finally, it was found

[11] that by increasing the number of graphene layers from one to three, the series resistance

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between the active electrode and the metal oxide increases, which allows the easy integration of

a selector device to realize one-selector one-resistor (1S1R) structures.

In this work, results on the performance of graphene-modified G-ReRAM cells

Ag/G/SiO2/Pt and Cu/G/SiO2/Pt ECM cells where GML is deposited at the active

electrode/metal oxide interface are shown. The GML transfer process was characterized and

graphene layers were tested as an effective diffusion barrier by secondary ion mass spectrometry.

The G-ReRAM cells were characterized by cyclic voltammetry, prior to the resistive switching

event, subsequently being compared to the reference cells without GML.

EXPERIMENT

Graphene Monolayer (GML) transfer process

Several techniques have been developed in order to transfer GML on various substrates.

The technique used is similar to the technique of etch-back of a copper foil as described in

reference [12]. The GML on copper foil used in this work was commercially available. In this

technique, the Cu/GML/PMMA foil is put in a copper etching solution (see Fig. 1(a)). When the

copper is etched off, the GML/PMMA film is floating on the liquid etchant surface (see Fig.

1(b)). Next, we put the target substrate in the solution and the film is attached on substrate’s

surface by Van der Waals forces (see Fig. 1(c)).

(a) (b) (c) (d)

Figure 1 The three steps of the etch-back process for GML transfer on a substrate: (a) the

GML on Cu is covered by PMMA and is put in a copper etching solution, (b) the Cu is etched

and the GML/PMMA film is floating, (c) the floating film is transferred on the substrate and

(d) transfer is completed.

Iron (III) chloride (FeCl3) was used as an etchant of the copper foil in the process. It must be

noted that in order to: a) eliminate any contamination of the graphene/PMMA film and the

substrate with iron (Fe) or chloride (Cl) residues and b) avoid any interaction of the substrate

with chloride ions, a two-step cleaning procedure was performed. Ammonium persulfate (APS)

was tested as well. The complete process is summarized below:

Step A: Cu Etch

1. PMMA / Graphene sample on Cu foil (~25 μm) was cut in size of a small sample

2. 25 ml of FeCl3 solution (17.4 g FeCl3 / 100 ml DI water) for Cu etching

3. Sufficient time to etch Cu (~50 min) Step B: Sample cleaning – Removal of any metallic residues

1. 10 ml of DI water was rinsed on the sample to clean Cu etchant. Wait for 5 minutes (repeat 6-

8 times)

2. HCL: DI solution (1:2) was rinsed to remove metal particles

3. Rinse with DI water

4. Repeat steps 2 & 3 (at least twice)

5. Rinse with DI water (repeat at least 8 times)

6. Transfer PMMA / Graphene on the substrate

7. Samples left to dry overnight

Step C: PMMA removal

1. Samples cleaned overnight in acetone and IPA respectively

Fabrication of ReRAM cells

ReRAM cells were prepared on oxidized silicon substrates covered with 5 nm TiO2 as adhesion

layer and 30 nm Pt as bottom electrode. Both layers were in situ deposited with DC sputtering.

For the devices with tantalum and its corresponding oxide RF sputtering was used. Here, 15 nm

metal was deposited. One sample was covered with graphene as described above. Another

tantalum sample was left in ambient air, to have similar storage conditions. Afterwards, 15 nm of

tantalum pentoxide was deposited, prior to a photolithography step for the 30 nm thick platinum

top electrodes. The SiO2 based samples were prepared by coverage of the platinized substrates

with 30 nm silicon dioxide films, which were deposited via electron beam evaporation at 0.01

nm/s. On the oxide, graphene was transferred before the formation via photolithography and

deposition (metal lift-off) of 30 nm Cu electrodes and subsequently capped by a 30 nm Pt thin

layer using the same e-beam evaporation system.

Figures 2 and 3 are typical images of samples with GML transferred on SiO2 (left figure)

and Ta2O5 (right figure). Images were taken just after the transfer and the subsequent drying

overnight (Step B-7), i.e. PMMA film is not removed in order to have a clear picture of the

sample’s area covered by GML.

Figure 2 Graphene on SiO2substrate. Figure 3 Graphene on Ta2O5 substrate.

RESULTS

Graphene Monolayer (GML) characterization by Raman Spectroscopy

Following the transfer procedure described in previous section, GMLs were transferred

on 280 nm thick SiO2 layers, thermally grown on Si wafers. In order to characterize the quality

of the transferred GMLs, Raman microscopy was used extensively. Measurements were

performed at room temperature with a Renishaw spectrometer with a laser at 514.5 nm. The

resulted spectra for one of the samples are presented in figure 4. The D band (indication of

presene of defects) is minimized while the G and 2D bands are very sharp. The sharpness of the

GML

2D bands indicates that a GML monolayer is measured which is expected and verifies the

uniform transfer of our graphene layer. Also, as calculated, the ratio of 2D - D peak intensity is

very high, which also indicates that the transfer process results in high quality graphene

monolayer.

(a)

(b)

Figure 4 (a) Optical microscope image for sample shown at figure 2 (named P2). Cross-marks

denote the points where the Raman spectra shown in (b) were measured.

SIMS results:

The effectiveness of the graphene as a diffusion barrier has been studied by secondary ion

mass spectrometry (SIMS). Samples with and without graphene were annealed at 200 °C for 2

h. The depth profiles presented in figure 5 show that for the samples with graphene on the Ag

electrode there is no significant intermixing, whereas for the reference sample without

graphene silver atoms diffused into the SiO2. This reveals the capability of graphene to

function as a diffusion barrier in ReRAM devices.

Electrical characterization

Cyclic voltammetry was performed on stacks consisting of 30 nm Pt/30 nm SiO2/graphene/30

nm Cu. The measurements were performed at different sweep rates and compared to results

from previous measurements without graphene between the SiO2 electrolyte and the copper

electrode. The obtained cyclic voltamograms are shown in figure 6. The intermediate graphene

layer has a significant influence on the redox-behavior of the cells prior to the forming or

switching event. Obviously the oxidation peaks shift to smaller voltage values while the

current increases. In comparison to the standard cell in Fig. 6 the graphene seems to have

catalytic impact on the oxidation of the active copper electrode while the reduction is slightly

depressed. This leads to an ON-switching after few cycles. Without graphene the switching

processes are shifted to higher voltages.

(a) (b)

Figure 5 SIMS profiles. (a) Ag/SiO2 stack without graphene (b) Ag/SiO2 stack with

graphene at the interface.

(a) (b)

(c)

(d)

Figure 6 (a)-(b) Cyclic voltammetry with switching events for the Cu/G/SiO2/Pt samples at of 129 mV/s

and 4.1mV/s sweep rates respectively. (c) Cyclic voltammetry on the reference ReRAM cell without

graphene as intermediate layer [13]. (d) Schematic stack of the tested devices.

CONCLUSIONS

In this paper we have shown that graphene acts as a diffusion barrier for Ag and stops the

intermixing of the active electrode and the solid electrolyte. It is also increasing the catalytic

activity of the active electrodes as shown in cyclic voltammetry experiments. Memory cells with

graphene layer are switching at lower voltages compared to cells without graphene. We also

discuss the technological specifics and challenges in preparing and characterizing these type

devices.

ACKNOWLEDGMENTS

This research has been financially supported by the Greek-German bilateral joint research project

“G-ReRAM” supervised by the GSRT project Nr. GER_2316 and BMBF project Nr. 03X0140.

Dr A. Kontos from INN-NCSR “Demokritos” is highly acknowledged for the measurements of

Raman spectra.

REFERENCES

1. The International Technology Roadmap for Semiconductors-ITRS 2011 Edition. (2011).

2. R. Waser and M. Aono, “Nanoionics-based resistive switching memories”, Nature Materials,

6, 833 (2007)

3. I. Valov, “Redox-Based Resistive Switching Memories (ReRAMs):Electrochemical Systems

at the Atomic Scale”, ChemElectroChem, 1, 26–36 (2014)

[4] I. Valov, E. Linn, S. Tappertzhofen, S. Schmelzer, J. van den Hurk, F. Lentz & R. Waser,

“Nanobatteries in redox-based resistive switches require extension of memristor theory”, Nat.

Commun. 4, 1771 (2013)

[5] S. Tappertzhofen, E. Linn, U. Bottger, R. Waser, I. Valov, “Nanobattery Effect in RRAMs -

Implications on Device Stability and Endurance”, IEEE Electr. Dev. Lett. 35, 208 (2014)

[6] J. van den Hurk, A.C. Dippel, D.Y. Cho, J. Straquadine, U. Breuer, P. Walter, R. Waser, I.

Valov, “Physical origins and suppression of Ag dissolution in GeSx-based ECM cells”, Phys

Chem Chem Phys, 16, 18217 (2014)

[7] D.-Y. Cho et al. “Direct Observation of Charge Transfer in Solid Electrolyte for

Electrochemical Metallization Memory”, Adv. Mater. 24, 4552 (2012)

8. R. S. Ruoff, C. W. Bielawski, and D. R. Dreyer, “From Conception to Realization: An

Historial Account of Graphene and Some Perspectives for Its Future”, Angew. Chem. Int. Ed.

(2010), 49, 9336 – 9345

9. G. N. Dash, S. R. Pattanaik, S. Behera, “Graphene for Electron Devices: The Panorama of a

Decade”, J. Electron Device Society 2, 77-104 (2014)

10. Hong-Yu Chen, He Tian, Bin Gao, Shimeng Yu, Jiale Liang, Jinfeng Kang, Yuegang Zhang,

Tian-Ling Ren, H.-S. Philip Wong, “Electrode/Oxide Interface Engineering by Inserting Single-

Layer Graphene:Application for HfOx–Based Resistive Random Access Memory”, IEDM 2012

Techn. Digest, 489-492

11. Yuchao Yang , Jihang Lee , Seunghyun Lee , Che-Hung Liu , Zhaohui Zhong , and Wei Lu,

“Oxide Resistive Memory with Functionalized Graphene as Built-in Selector Element”, Adv.

Mater., 26, 3693–3699 (2014)

12. J. W. Suk, A. Kitt, C.W. Magnuson, Y. Hao, S. Ahmed, J. An, et al., “Transfer of CVD-

Grown Monolayer Graphene onto Arbitrary Substrates” ACS Nano,5, 6916–6924 (2011)

13. S. Tappertzhofen, S. Menzel, I. Valov, R. Waser, “Redox Processes in Silicon Dioxide Thin

Films using Copper Microelectrodes”, Appl. Phys. Lett., 99, 203103 (2011)