Abstract— We report an innovative method of fabricating atomic switching junctions using an LB film of cadmium stearate in the gap between an inert electrode and a chalcogenide solid electrolyte. The thickness of the monolayer is about 2.8nm and it helps ensure a constant gap size maintaining identical characteristics in a large number of switching junctions. We used a new energetic plasma process to create the chalcogenide solid electrolyte using a sulfur containing plasma for converting thin films of copper into a mix of covelite phase CuS and chalcocite phase Cu2S.

I. INTRODUCTION The atomic switch is based on the growth and shrinkage of nanoscale metal protrusions from solid electrolytes such as chalcogenides across a nanometer sized gap.1 Terabe et al. first demonstrated the atomic switch by using a 1nm silver thin film sandwiched between an Ag2S bottom electrode and a platinum top electrode.1 The switch was initially “on” and when a bias was applied, the 1nm of silver was incorporated into the Ag2S layer after ionizing. This created the 1nm gap in which further switching was caused by the bridging of Ag nanofilaments. The main difficulty with this approach is the accuracy and reproducibility of the 1nm thin film of silver. Most metal deposition begins with the nucleation of islands or clusters, which can be larger than 1nm, before coalescing into a thin film. What is left behind is a very bumpy uneven surface that may or may not be the desired 1nm in thickness. In particular, the grains in Ag thin film deposited with conventional techniques are very large and form discrete islands when the film is less than 10nm in thickness.2 The grains and spikes on the surfaces of a thicker metal film are much larger than the size of the molecules and this was found to contribute to device shorting in molecular scale electronic devices.

We describe our solution to accurately and precisely control the nanometer sized gap using a monolayer of an organic film deposited by the LB technique. The structure, shown in Figure 1, combines both the molecular switch and

1Integrated NanoDevices and Systems Research, Department of Electrical and Computer Engineering, University of California Davis, Davis, CA 95616.

2Quantum Science Research, Hewlett Packard Laboratories, MS 1123, 1501 Page Mill Rd, Palo Alto, CA, 94304-1100.

*Contacting Author. M. Saif Islam is with the Electrical and Computer Engineering, University of California Davis, Davis, CA 95616. (phone: +1-530-754-6732; fax: +1-530-752-8428; e-mail: saif@ece.ucdavis.edu).

the atomic switch where the gap size can therefore be controlled by the size of the molecule. Our approach was to

intentionally form nanoscale filaments through an electrochemical reaction of a solid electrolyte across the insulating organic monolayer.

In addition to using an LB film to provide the spacing, we also tried a new way of producing the chalcogenide. In our study, we compared both a H2S vapor and we introduce a new sulfidization process using a sulfur containing plasma. Unlike the Ag2S based devices of Terabe et al1, we used CuSx to construct the atomic switching junctions in this work. Since the plasma process is a very energetic process, the reaction or conversion time is significantly reduced.

II. EXPERIMENTAL PROCEDURE Our devices consisted of 1-10μm × 1-10μm cross-bar

junctions with top electrode containing the solid electrolyte separated by a LB film of cadmium stearate which is stearic acid, with CdCl2 providing the cadmium salts. It is an 18 carbon chain that should be an electrically insulating film while also providing a gap length of 2.8nm. The structure started with the lithographically defined deposition of 100nm Pt on a silicon substrate coated with 100nm of SiO2 followed by a 100W, 100mTorr O2 plasma cleaning for 5min. A monolayer of cadmium stearate (~2.8nm) was then deposited using the LB technique followed by an e-beam

Nanoscale Switching Junctions Based on an Organic Monolayer of Molecules and Solid Electrolytes

Chad Johns1, Doug A. A. Ohlberg2, Shih-Yuan Wang2, R. Stanley Williams2, and M. Saif Islam1

Pt electrode

Molecular monolayer

CuxSCu

Pt electrode

CuxS

Pt electrode

Molecular monolayer

CuxSCu

Pt electrode

CuxS

Figure 1: An organic monolayer is used to separate the top and bottom electrodes while also providing a precise gap size for the bridging of a metal filament from the solid electrolyte.

1-4244-0608-0/07/$20.00 © 2007 IEEE. 1306

Proceedings of the 7th IEEEInternational Conference on Nanotechnology

August 2 - 5, 2007, Hong Kong

deposited 4nm Cu blanket deposition. The LB depositions were done over unbuffered water which had a pH of ~ 5.8 at 21 °C. Cadmium chloride was added to a concentration of 1 mM. Sulfidization involved exposing the sample to a 50W, 101mTorr H2S plasma for various times ranging from 1min, 5min, and 10min. Rutherford backscattering (RBS) analysis of the copper films exposed to the H2S plasma or H2S gas revealed a combined stoichiometry consisting of both the covelite (CuS) and chalcocite phase (Cu2S). The top electrode consisted of a 5nm Cu deposited through a shadow mask and followed by 10nm Pt.

We prepared another sample with a bottom electrode of 3nm Ti and 10nm Ag followed by a blanket layer of 10-12nm ZnS. Initially a monolayer of distearoylphosphatidylcholine (DSPC), which is also two C18 alkane chains (~2.8nm) held together by a positive polar head, was deposited using the LB technique. But it was found not make a good LB film with the majority of the devices shorted through pin holes in the monolayer from metal penetration during top electrode deposition. The yield significantly increased when stearic acid was mixed with DSPC in a 1:1 ratio to generate a high quality film. The top metal electrode consisted of e-beam evaporated 5nm Ti and 10nm Pt.

III. RESULTS AND DISCUSSION As seen in Figure 2a, switching at around 7V-8V was

observed for the first sample with a plasma treatment of only one minute. The switching ratio, defined as the ratio of on to “off” currents at the switching edge, was observed to be above 103. As the device was cycled, the transition from “on” to “off” occurred quickly giving a sharp switching edge. In contrast, the transition from “off” to “on” degraded as the device was cycled. This section where the current is not constant is due to the volatile dynamic formation of the filament in which ions are coming out of solution and bridging the gap. Typically the breaking of the filament causes a nice sharp switching edge whereas the formation of the filament as shown in the Figure 2a-c cases a fluctuation in the current. In fact, many devices showed one-sided switching with very good transitions from “on” to “off”.

As the plasma treatment time was increased to 5min, the switching voltage decreased to around 1V (Figure 2b) but it became more volatile. This was likely due to more of the copper layer being converted to CuxS. Additionally, the longer plasma treatment or ion bombardment could have increased the surface mobility of the now looser copper ions thus allowing them to bridge the gap at a lower voltage. The longest plasma treatment of 10min had a very low yield, 1 of 20 devices tested. Therefore it was believed to be too long of a treatment, possibly removing the solid electrolyte layer.

Further research is needed to find the optimal plasma treatment time but it seems to occur somewhere near 5 minutes.

There was a mixture of devices displaying initial open circuits (“off” state) before switching and initial conductance (on state) before switching. Ideally, if the monolayer was pinhole free, it should be insulating and the device should be initially an open circuit or in the “off” state. So, some areas contained pinholes in the monolayer of cadmium stearte causing metal filaments to penetrate during deposition of the top electrode.

We also tested the device structure that was fabricated using a 1:1 mixed monolayer of

Figure 2: (a) The copper thin film was sulfidized by exposing to an H2S plasma for 1min. Switching voltage was observed to be around 7-8V. The transition from off to on degraded as the device was cycled. (b) The plasma treatment was increased to 5min which lowered the switching voltage to around 1V. (c) Sample was exposed to H2S vapor for 13.5 hours and switching was observed around 5V. In contrast to the first sample, the formation of the filament became more stable as the device was cycled. (d) Structure of device consisting of a bottom electrode of platinum followed by a monolayer of cadmium stearate. The blanket layer of copper was sulfidized by either the plasma treatment or vapor.

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distearoylphosphatidylcholine and stearic acid to separate the bottom electrode of titanium, silver, and ZnS from the top electrode of titanium and platinum. Silver is known to diffuse rapidly into ZnS by an interstitial mechanism and its fast diffusion in the ZnS layer possibly contributed to lowering the switching voltage and fast and stable formation of switching filaments.3 A device structure without the Ag layer was found to switch at higher than 10 times the voltage for “on” and “off” states. The majority of the devices tested were initially open and the yield was very high 80% tested showing some switching. Titanium has been shown to be reactive with the functional groups of organic monolayers, thus preventing penetration.4 This is believed to be the reason for the high observation of devices found in the “off”

state rather than a completely pinhole free monolayer. Figure 3 shows a representative I-V plot with the structure.

As can be seen, the device is very symmetric with about the same switching voltage of 1.88V, except opposite in polarity. For the transition from conduction (“on”) to non-conduction (“off”) the average voltage at which this occurred was 1.86V with a standard deviation of 0.36V. The switching of device from “off” to “on” required an average voltage of -1.89V with a standard deviation of 0.32V.

One of our switching junctions was cycled over 50 times while still showing switching characteristics. About 29 cycles switched very sharply with both transitions. Further cycles showed one sided switching, mainly the “on” to “off” edge. If only one side or the transition from “on” to “off” was counted, the number of good switching cycles would greatly increase. It seems the formation of the filament is very dynamic and therefore doesn’t give a nice transition edge whereas the breaking of the filament does.

IV. CONCLUSION A new technique was demonstrated using an LB film of

cadmium stearate to create a 2.8nm gap between an inert electrode and a chalcogenide solid electrolyte. With the length of each monolayer about 2.8nm, it has the distinct benefit of maintaining precise control over the gap size. A fast energetic plasma process was presented using a sulfur containing plasma to convert thin films of copper into a combination of covelite (CuS) and chalcocite phase (Cu2S). A ZnS based crossbar atomic switching junction was also demonstrated for the first time.

ACKNOWLEDGMENT This work at University of California-Davis was partially

supported by NSF CAREER grant # 0547679, a UC Davis research grant and a CITRIS grant sponsored by the Hewlett-Packard Laboratory.

REFERENCES [1] Terabe, K.; Hasegawa, T.; Nakayama, T.; Aono, M.

“Quantized conductance atomic switch”, Nature 2005, 433, (7021), 47-50.

[2] Islam, M. S.; Jung, G. Y.; Ha, T.; Stewart, D. R.; Chen, Y.; Wang, S. Y.; Williams, R. S. “Ultra-smooth platinum surfaces for nanoscale devices fabricated using chemical mechanical polishing”, Applied Physics A -Materials Science & Processing 2005, 80, (6), 1385-1389.

[3] Cusdin, A.; Anderson, J. C. “Conductivity switching in ZnS single-crystal platelets”, Journal of Physics D: Applied Physics 1970, 3, (11), 1776-1781.

[4] Jung, D. R.; Czanderna, A. W.; Herdt, G. C., “Interactions and penetration at metal/self-assembled organic monolayer interfaces”, Journal of Vacuum Science & Technology a-Vacuum Surfaces and Films 1996, 14, (3), 1779-1787.

Figure 3: Two representative curves with the switching direction are shown. The voltage was varied on top electrode which consisted of titanium and platinum. The titanium reacts with the functional group of the monolayer thus preventing penetration.

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