TiOx Memristors with Variable Turn-On Voltage using Field-Effect for Non-Volatile Memory

Pradeep Pai, Faisal K. Chowdhury, Tien-Vinh Dang-Tran and Massood Tabib-Azar Electrical and Computer Engineering

University of Utah Salt Lake City, USA

azar.m@utah.edu

Abstract— This work demonstrates the first use of field-effect based memristors that exhibit variable turn-on voltage. Turn-on voltage as low as 4V is achieved in this work and the switching characteristics is reproducible. The novelty of this work compared to earlier work based on metal-insulator-metal (MIM) structure is in introducing an auxiliary electrode to the MIM structure that produces variation in the turn-on voltage of the device through field effect. A gate bias of ±2V varies the drain-source turn-on voltage by ±2V around the turn-on voltage at zero gate bias. The simplicity in the device structure can allow easy integration to form an array of these devices to constitute a memory module. The leakage current in the off state is less than 10pA, which would enable low power operation and long hours of data storage.

I. INTRODUCTION Technology and consumer needs have created a need for

high package density memory devices. The current CMOS technology allows high package density but also increases the power consumption. In the past, non-CMOS based non-volatile memory devices have been realized with the help of solid electrolytes (SE) [1], MEMS switches [2,3], MIM devices [4] etc. The SE and MIM devices have very low switching voltages less than 5V, compared to the MEMS switches that require 10s of volts. Also, the absence of any moving parts in SE and MIM devices make them mechanically robust and limit their speed only by the ionic mobility. Various insulators like amorphous silicon [5], TiO2 [6], CuOx [7] and methylsilesquioxane [8] etc. have been used in the MIM configuration to realize memristors. Out of these, TiO2 is very popular for its CMOS compatibility and ease of characterization. Extensive work has been done by various research groups to understand the switching mechanism in the TiO2 based memristors [9-12].

This work uses oxidized Ti as the insulating layer. Due to the coarse nature of the oxidation process used here, the oxidized Ti is assumed to have a non-stoichiometric structure and is considered to be TiOx. The objective of this work is to extend the dynamic switching range of a memristor to suit different applications. This functionality is realized by introducing an auxiliary electrode to the MIM stack that

controls the switching threshold of the insulator through field effect.

II. THEORY OF OPERATION The memory associated with a memristor is in the state of

its resistance. A memristor can have a high resistance state (HRS) or a low resistance state (LRS) that defines the two bits of memory. The resistance states can be reversibly switched from one to the other by applying suitable voltage. TiO2 exhibits a variety of resistive switching (RS) mechanisms. The RS mechanism can be unipolar or bipolar depending on how the material is conditioned. The electrical conduction mechanism through TiO2 is from the drifting of oxygen vacancies which act as defects. An initially non-conducting TiO2 is made conductive by continuously sweeping voltage across it to form conductive filaments through it. This process is called electroforming. The filaments are formed by accumulation and alignment of locally available oxygen vacancies.

Depending on the current compliance set during the electroforming stage, the material could be unipolar RS (URS) or bipolar RS (BRS). In case of URS, the switching between high and low states can be performed solely using either positive or negative voltage. In contrast, BRS requires one voltage polarity to switch from a HRS to LRS, and a reversed polarity to switch it back from LRS to HRS. This work focuses on the BRS. There are different theories that explain the principle behind BRS through formation and breaking of conductive filaments, or trapping and de-trapping of defects at interfaces, or sweeping of defects from one electrode to the other. Details of different BRS switching mechanisms are provided in [12]. However, the basic underlying principle behind all these theories is the redox reaction of oxygen vacancies. Finding the mechanism behind BRS is out of scope of this work. This work merely utilizes the BRS technique to perform memristive switching and demonstrates the effect of introducing an auxiliary electrode that varies the RS voltage.

Use of Pt/TiO2/Pt to perform memristive switching is very common. Pt is used as the conductive electrodes due to its high oxygen ion mobility, which readily allows trapping and releasing of interfacial oxygen ions to atmosphere. This makes Pt highly resistant to oxidation. TiO2 can be deposited by

This work was partly supported by DARPA MPD program.

978-1-4673-4642-9/13/$31.00 ©2013 IEEE

various techniques. In this work, TiOx was obtained by the oxidation of sputtered Ti.

The operating principle of the variable switching voltage memristor is demonstrated schematically in fig 1. For simplicity, the memristive switching is assumed to be due to the sweeping of oxygen vacancies from one electrode to the other. The oxygen vacancies are drawn towards the anode. In the presence of a third electrode, the e-field in the TiOx layer is a vector sum of the e-field due to VDS and VGS. So, the VGS can aid or inhibit the effect of VDS on the oxygen vacancies. In the actual device, the gate electrode is not symmetric with respect to the drain and source electrodes as shown in fig 2. Thus, the effect of VGS can be expected to be stronger than the case with electrodes arranged as in fig 1.

Fig 1. Schematic representation of the effect of gate field-effect on the motion of oxygen vacancies that changes the threshold e-field for turn on.

III. FABRICATION A 4" p-type Si wafer is thermally oxidized to form 100nm

insulating oxide. 5nm/50nm/10nm of Ti/Pt/Ti are sputtered on top of this and patterned to form the source/drain electrode as shown in fig 2a. 5nm/50nm of Ti/Pt is again sputtered on and patterned to form the drain/source electrode, fig 2b. The drain and source electrodes are re-sputtered with 100nm Pt and patterned in the electrical connection pad regions to make them mechanically robust. The wafer is then placed in a furnace at 500°C for 10 minutes to oxidize the sandwiched Ti to form TiOx, fig 2c. 50nm HfO2 is deposited by atomic layer deposition and patterned to form the gate oxide, fig 2d. 100nm Pt is sputter deposited and patterned to form the gate electrode, fig 2e.

Fig 3 shows images of the fabricated device. The source-drain electrodes are 3µm wide and 10µm long. The overlap between the source-drain electrodes is less than 1µm and this defines the width of the active region. The length of the active

region is roughly 15nm which is defined by the thickness of the oxidized Ti layer.

Fig 2. Schematic representation of the fabrication process. Pt/Ti layers are sputter deposited and patterned by lift-off. TiOx is formed by oxidizing sandwiched Ti layer at 500°C for 10 minutes. HfO2 is used to insulate the gate electrode from source-drain electrodes.

Fig 3. Optical microscope image of the tunable memristor with the SEM image shown in the inset. The source-drain overlap defines the active region of the device and is less than 1µm wide.

IV. RESULTS AND DISCUSSION The devices are tested using the FET characterization

program in the Agilent 4156 C Semiconductor Parameter Analyzer. Initially the devices exhibit a random switching behavior with different turn-on voltages. The voltage between drain-source is swept from 0 to a certain VDS and back to 0. VDS is swept both in positive and negative directions. The devices undergo the electroforming phase with repeated switching cycles until they develop a stable switching behavior. This typically takes at least 30-40 repeated switching with a current compliance of 10nA. At smaller current compliance (sub 100nA), the device shows a

10um

DS

G

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(c)Platinum HfO2 Ti TiOx

Silicon Thermal SiO2

Source Drain

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VS = 0V

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VS = 0V

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VD = +V

Motion of oxygen vacancies

Motion of oxygen vacancies

Platinum HfO2 TiOx

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reversible switching behavior by turning-OFF while the voltage is swept from VDS to 0. Higher current compliance is needed to latch the device to the ON state.

A. Ohmic switching Ohmic switching occurrs at current compliance of 1µA

and greater. The switching is considered ohmic because the device exhibits a linear I-V after switching from HRS to LRS. The switching voltage varies between 3 and 10 volts for different devices due to the slight difference in their source-drain overlap. Ohmic switching reported in earlier work occurrs for higher current compliances of the order of a few hundred micro-amps up to a few milli-amps [12]. The reason for low current switching in this work is due to the extremely small volume (1µm×1µm×15nm) of the active region.

The current is limited to 1µA to prevent device damage from dielectric breakdown of TiOx film. The switching data is plotted in fig 4. The voltage sweep sequence is numbered for clarity. The device is initially in the HRS (OFF state), indicated by (1). At a certain drain-source threshold voltage, the device switches to the LRS (ON state), indicated by (2). The device retains the ON state when the voltage is decreased to 0, indicated by (3-4-5). The device remains in the ON state for any subsequent voltage sweeps (6-7-8-9). Switching from ON state to OFF state is performed by sweeping the voltage in the reverse direction. Similar switching is observed for negative VDS sweep as shown in the third quadrant of fig 4. The device retains the OFF and ON state successfully for voltages less than the turn-ON voltage. This voltage range is sufficient to determine the state of the memristor, which constitutes the read operation in a non-volatile memory element. It must be noted that the gate electrode is connected to ground during these measurements.

Fig 4. Resistance switching characterization of the memristor. The device resistance is high in the OFF state indicated by the nearly zero current lines up to the turn-on voltage. The turn-ON voltage corresponds to the threshold e-field required to form the conductive filaments in the TiOx film. The device is turned OFF by applying a reverse voltage. The device response to positive and negative voltage is almost symmetric.

B. Gate control through field effect The effect of gate bias is tested by sweeping VDS and

recording the ID, while changing VGS in steps. The thick HfO2 gate oxide layer prevents the gate from starting its own memristive switching with respect to the source-drain electrodes. Since the measurement focuses only on the effect of VGS on the turn-on voltage VDS(ON), it is not necessary to latch the device to the ON state. Hence, the current compliance is set to a low value of 10nA. The measurements are repeated 4 times for every value of VGS. The effect of gate bias on the turn-on voltage is evident from the plot in fig 5.

It can also be noted that the effect of gate bias is not symmetric. The turn-on voltage increases with gate bias for positive VDS, whereas it decreases (absolute value) with gate bias for negative VDS. The cause for this behavior is understood from the physical arrangement of S-D-G electrodes in fig 2. For positive values of VDS and VGS, the effective e-field in the TiOx film increases since they are in the same direction. This should lower the turn-ON voltage and vice-versa for opposing VGS and VDS. However, the measurement data in fig 5 is exactly the opposite. From this, it can be deduced that the source and drain electrodes are interchanged for these sets of measurements. Also, the turn-on voltages are different for positive and negative VDS since the gate electrode is not equidistant from source and drain electrodes. The reason for the hysteresis in the switching behavior is not clearly understood. Further experiments are necessary to understand the origin of hysteresis, which would also explain the difference in hysteresis between measurements.

Fig 5. Characterization of the gate field-effect on the memristor switching behavior. The turn-on voltage of the memristor increases when VDS and VGS appear in opposing directions in the TiOx film and reduces when they are in the same direction.

To observe the effect of gate voltage at a finer scale, the device is tested in the VGS-ID mode at a constant VDS. Fig 6 plots the measurement data obtained for VDS=5V (fig 6a) and VDS=6V (fig 6b). In both the plots, the effect of VGS is clearly seen from the drop in ID at higher VGS. The trend is consistent with theory at both values of VDS where the threshold voltage

-1.20E-08

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increases with VGS. This corroborates with the data obtained in fig 5.

Fig 6. Gate effect characterization curves showing the relation between VGS and ID at a constant VDS. The drop in current is indicative of the switching threshold voltage VDS(ON). As expected, a higher gate voltage shows a greater threshold voltage in compliance to the measurements in fig 5.

V. CONCLUSION This work successfully demonstrates the working of a

tunable switching voltage memristor. Different switching characteristics are observed based on the current compliance, which corroborates with earlier work. The use of small volume of TiOx for active region allows memristive switching at low compliances. But this also makes the device vulnerable to dielectric breakdown at higher currents. The device can be equipped to handle larger currents by using thicker and wider TiOx film. Further tests are needed to characterize the switching speed of the device.

ACKNOWLEDGMENT The devices were fabricated at Utah Nanofab. The authors

thank Nanofab staff for their technical help.

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[7] L. L. Wei et al., “Gradual electroforming and memristive switching in Pt/CuOx/Si/Pt systems”, Nanotechnology, vol. 24, 2013, p. 325202.

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[9] M. H. Lee et al., “Study on the Electrical Conduction Mechanism of Bipolar Resistive Switching TiO2 Thin Films using Impedance Spectroscopy”, Appl. Phys. Lett., vol. 96, 2010, p. 152909.

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[11] D. S. Jeong, H. Schroeder and R. Waser, “Mechanism of Bipolar Switching in a Pt/TiO2/Pt Resistive Switchign Cell”, Phys. Rev. B, vol. 79, nr. 19, 2009, p. 195317.

[12] K. M. Kim et al., “Electrically Configurable Electroforming and Bipolar Resistive Switchign in Pt/TiO2/Pt Structures”, Nanotechnology, vol. 21, 2010, p. 305203.

(b)

-1.20E-08

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VGS (V)

I D(A

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VDS = 6VVDS = -6V

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4.00E-09

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VGS (V)

I D(A

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VDS = 5VVDS = -5V