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Cite this: Nanoscale, 2014, 6, 2161
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Evolution of the shape of the conducting channel in complementary resistive switching transition metal oxides Kyung Jean Yoon, Seul Ji Song, Jun Yeong Seok, Jung Ho Yoon, Tae Hyung Park, Dae Eun Kwon and Cheol Seong Hwang* Ultimate control of the defect distribution and local conduction path in a bipolar resistive switching (BRS) Pt/ TiO2/Pt sample, which was in a unipolar reset state, is provided by means of voltage pulsing and the resulting time-transient current analysis. The limited amount of oxygen vacancies in this system allowed reversibly switching-diode-like current–voltage curves, which was also confirmed in another Magne´li-phasecontaining Pt/WO3/Pt sample. Such careful control of the defect distribution allowed the achievement of a complementary resistive switching (CRS) curve even from a single switching layer. The unlimited
Received 12th October 2013 Accepted 24th November 2013
vacancy source in the Pt/TiO2/TiO2x/Pt sample did not allow the switching-diode type and the CRS behavior. The data retention of the on-state in the BRS was critically dependent on the shape of the
DOI: 10.1039/c3nr05426h
rejuvenated conduction channel. The required time to lead to the rejuvenation of the conducting
www.rsc.org/nanoscale
channel was 70–100 ns when the threshold voltage for the BRS set of 1 V was applied.
I.
Introduction
Resistance-switching random access memory (RRAM) is one of the leading contenders for the next-generation non-volatile memory. Due to its exceptional device performance and superb compatibility with current silicon technology, it has recently attracted widespread interest. TiO2 is one of the most representative transition metal oxides to show such a phenomenon, and thus, has been intensively studied mainly in association with its defect structures.1–6 The unipolar resistive switching (URS) phenomenon in this material, wherein set [high resistance state (HRS) / low resistance state (LRS)] and reset (LRS / HRS) switching occur under the same bias polarity, is attributed to the formation and rupture of the local conducting laments (CFs), which are made of the Magn´ eli phases (TinO2n1, where n ¼ typically 4 or 5), within the lm. When a Pt/ TiO2/Pt resistive switching (RS) sample is in the URS reset state, the electrical states of the Magn´ eli CF ruptured region could undergo various bipolar resistive switching (BRS) phenomena, wherein set switching and reset switching occur in the opposite bias polarity, depending on the history of the bias voltage application (polarity) and the height of the bias voltage. The BRS behavior varied from the electronic switching type,7,8 where the bias-polarity-dependent electron trapping and detrapping due to the asymmetric potential barrier could explain such behavior well, to the ionic switching type, where the shi in the Department of Materials Science and Engineering and Inter-university Semiconductor Research Center, Seoul National University, Seoul 151-744, Korea. E-mail: cheolsh@ snu.ac.kr
This journal is © The Royal Society of Chemistry 2014
oxygen vacancies between the Pt/TiO2 interface and the TiO2/ Magn´ eli phase is responsible for the switching-diode-like RS behaviour.9,10 All these interesting RS behaviours could be ascribed to the migration of a limited amount of oxygen vacancies within the local region of the Magn´ eli-CF ruptured region. Complementary resistive switching (CRS) is an intriguing RS element that contains the switching and selection devices in one structure, and, thus, is desirable for the cross-bar array (CBA).11–15 In the original suggestion of CRS by Linn et al., two anti-serial electrochemical metallization cells comprised one memory element, wherein either of the two anti-serial cells is always in the HRS, thereby suppressing the sneak current in the CBA, while the CRS cell can be written and read by the application of an appropriate voltage pulse to it. Recently, Balatti et al. reported that a similar CRS operation can be achieved even from a single RS layer of HfO2 (Pt/HfO2/Pt structure) by noting that the directionality of the conical conducting path in the HfO2 can be adjusted upward or downward using the appropriately controlled voltage pulses.16,17 In this case, one of the key prerequisites was to limit the available density of the oxygen vacancies within the CF region; otherwise, the always-off state of the HfO2 CRS cannot be attained. This triggered an idea that a similar CRS-type operation might be possible in the URS-reset TiO2 RS memory cell. Furthermore, even more careful control of the voltage application may provide direct evidence of the transition from one resistance state to another via the intermediate state, which may pave the way for even analogue type operation. Therefore, this study provides further insights into the URS-reset TiO2 RS system, based on the time-transient
Nanoscale, 2014, 6, 2161–2169 | 2161
View Article Online
Nanoscale
behavior of each resistive switching mode and the evaluation of the properties depending on the atomically controlled shape of the conducting laments.
Published on 27 November 2013. Downloaded by North Dakota State University on 23/10/2014 02:33:51.
II.
Experimental section
A Pt/50 nm thick TiO2/Pt sample was used as the main sample in this study. The details of the sample fabrication process are reported elsewhere.7–10 Briey, the TiO2 thin lm was grown on a 100 nm thick Pt bottom electrode (BE) that had been sputtered onto a SiO2/Si substrate via plasma-enhanced atomic layer deposition (PEALD) at a wafer temperature of 250 C using Titetra-isopropoxide and plasma-activated O2 as the Ti-precursor and the oxygen source, respectively. Pt/TiO2/TiO2x/Pt and Pt/ WO3/Pt samples were also fabricated as the comparison samples. Pt/TiO2/TiO2x/Pt was fabricated by depositing a 15 nm thick TiO2 via the identical PEALD on a 100 nm thick Ti lm grown via DC magnetron sputtering on Pt/SiO2/Si. During the ALD of TiO2, the Ti underlayer was oxidized to TiO2x, which works as the unlimited source or sink of the oxygen vacancies during the BRS. A WO3 lm was deposited via reactive RF-magnetron sputtering using a W-target under a 30% O2–70% Ar atmosphere with a working pressure of 15 mtorr at room temperature (50 W RF power was applied to the 3-inch-diameter target), and was subsequently annealed in an O2 atmosphere at 400 C for 30 min to obtain the desired RS phenomenon. The substrate for the WO3 lm was also Pt/SiO2/Si. Circular Pt top electrodes (TEs) with a diameter of 300 mm were commonly fabricated using an electron beam evaporation method through a metal shadow mask. No post-annealing was performed aer the TE fabrication. The electrical test was performed in either the pulse-type voltage application [with the HP 81110A pulse generator (PG) as the voltage-programmed current source and the Tektronix 684C oscilloscope (OSC) as the current monitor] or the voltage sweep mode using a semiconductor parameter analyser (HP 4145B). The electrical test sequences for the RS performance evaluation are described in the following Results section. All the bias voltage was applied to the TE while the BE was grounded.
III.
Results and discussion
The ionic BRS begins from the URS-reset state of the Pt/TiO2/Pt memory. First, the sample was electroformed using a compliance current (Icomp) of 20 mA, and then URS-reset in the voltage sweep mode. Subsequent application of an appropriate electric eld can drive the oxygen ions in the Magn´ eli CF-ruptured region of the sample that induced the occurrence of the ionic BRS. To accomplish the ionic BRS and to avoid rejuvenation of the Magn´ eli phase CF during the BRS set process, the ICC should be settled at much smaller values than that of the URS (typically
Published on 27 November 2013. Downloaded by North Dakota State University on 23/10/2014 02:33:51.
PAPER
Cite this: Nanoscale, 2014, 6, 2161
View Journal | View Issue
Evolution of the shape of the conducting channel in complementary resistive switching transition metal oxides Kyung Jean Yoon, Seul Ji Song, Jun Yeong Seok, Jung Ho Yoon, Tae Hyung Park, Dae Eun Kwon and Cheol Seong Hwang* Ultimate control of the defect distribution and local conduction path in a bipolar resistive switching (BRS) Pt/ TiO2/Pt sample, which was in a unipolar reset state, is provided by means of voltage pulsing and the resulting time-transient current analysis. The limited amount of oxygen vacancies in this system allowed reversibly switching-diode-like current–voltage curves, which was also confirmed in another Magne´li-phasecontaining Pt/WO3/Pt sample. Such careful control of the defect distribution allowed the achievement of a complementary resistive switching (CRS) curve even from a single switching layer. The unlimited
Received 12th October 2013 Accepted 24th November 2013
vacancy source in the Pt/TiO2/TiO2x/Pt sample did not allow the switching-diode type and the CRS behavior. The data retention of the on-state in the BRS was critically dependent on the shape of the
DOI: 10.1039/c3nr05426h
rejuvenated conduction channel. The required time to lead to the rejuvenation of the conducting
www.rsc.org/nanoscale
channel was 70–100 ns when the threshold voltage for the BRS set of 1 V was applied.
I.
Introduction
Resistance-switching random access memory (RRAM) is one of the leading contenders for the next-generation non-volatile memory. Due to its exceptional device performance and superb compatibility with current silicon technology, it has recently attracted widespread interest. TiO2 is one of the most representative transition metal oxides to show such a phenomenon, and thus, has been intensively studied mainly in association with its defect structures.1–6 The unipolar resistive switching (URS) phenomenon in this material, wherein set [high resistance state (HRS) / low resistance state (LRS)] and reset (LRS / HRS) switching occur under the same bias polarity, is attributed to the formation and rupture of the local conducting laments (CFs), which are made of the Magn´ eli phases (TinO2n1, where n ¼ typically 4 or 5), within the lm. When a Pt/ TiO2/Pt resistive switching (RS) sample is in the URS reset state, the electrical states of the Magn´ eli CF ruptured region could undergo various bipolar resistive switching (BRS) phenomena, wherein set switching and reset switching occur in the opposite bias polarity, depending on the history of the bias voltage application (polarity) and the height of the bias voltage. The BRS behavior varied from the electronic switching type,7,8 where the bias-polarity-dependent electron trapping and detrapping due to the asymmetric potential barrier could explain such behavior well, to the ionic switching type, where the shi in the Department of Materials Science and Engineering and Inter-university Semiconductor Research Center, Seoul National University, Seoul 151-744, Korea. E-mail: cheolsh@ snu.ac.kr
This journal is © The Royal Society of Chemistry 2014
oxygen vacancies between the Pt/TiO2 interface and the TiO2/ Magn´ eli phase is responsible for the switching-diode-like RS behaviour.9,10 All these interesting RS behaviours could be ascribed to the migration of a limited amount of oxygen vacancies within the local region of the Magn´ eli-CF ruptured region. Complementary resistive switching (CRS) is an intriguing RS element that contains the switching and selection devices in one structure, and, thus, is desirable for the cross-bar array (CBA).11–15 In the original suggestion of CRS by Linn et al., two anti-serial electrochemical metallization cells comprised one memory element, wherein either of the two anti-serial cells is always in the HRS, thereby suppressing the sneak current in the CBA, while the CRS cell can be written and read by the application of an appropriate voltage pulse to it. Recently, Balatti et al. reported that a similar CRS operation can be achieved even from a single RS layer of HfO2 (Pt/HfO2/Pt structure) by noting that the directionality of the conical conducting path in the HfO2 can be adjusted upward or downward using the appropriately controlled voltage pulses.16,17 In this case, one of the key prerequisites was to limit the available density of the oxygen vacancies within the CF region; otherwise, the always-off state of the HfO2 CRS cannot be attained. This triggered an idea that a similar CRS-type operation might be possible in the URS-reset TiO2 RS memory cell. Furthermore, even more careful control of the voltage application may provide direct evidence of the transition from one resistance state to another via the intermediate state, which may pave the way for even analogue type operation. Therefore, this study provides further insights into the URS-reset TiO2 RS system, based on the time-transient
Nanoscale, 2014, 6, 2161–2169 | 2161
View Article Online
Nanoscale
behavior of each resistive switching mode and the evaluation of the properties depending on the atomically controlled shape of the conducting laments.
Published on 27 November 2013. Downloaded by North Dakota State University on 23/10/2014 02:33:51.
II.
Experimental section
A Pt/50 nm thick TiO2/Pt sample was used as the main sample in this study. The details of the sample fabrication process are reported elsewhere.7–10 Briey, the TiO2 thin lm was grown on a 100 nm thick Pt bottom electrode (BE) that had been sputtered onto a SiO2/Si substrate via plasma-enhanced atomic layer deposition (PEALD) at a wafer temperature of 250 C using Titetra-isopropoxide and plasma-activated O2 as the Ti-precursor and the oxygen source, respectively. Pt/TiO2/TiO2x/Pt and Pt/ WO3/Pt samples were also fabricated as the comparison samples. Pt/TiO2/TiO2x/Pt was fabricated by depositing a 15 nm thick TiO2 via the identical PEALD on a 100 nm thick Ti lm grown via DC magnetron sputtering on Pt/SiO2/Si. During the ALD of TiO2, the Ti underlayer was oxidized to TiO2x, which works as the unlimited source or sink of the oxygen vacancies during the BRS. A WO3 lm was deposited via reactive RF-magnetron sputtering using a W-target under a 30% O2–70% Ar atmosphere with a working pressure of 15 mtorr at room temperature (50 W RF power was applied to the 3-inch-diameter target), and was subsequently annealed in an O2 atmosphere at 400 C for 30 min to obtain the desired RS phenomenon. The substrate for the WO3 lm was also Pt/SiO2/Si. Circular Pt top electrodes (TEs) with a diameter of 300 mm were commonly fabricated using an electron beam evaporation method through a metal shadow mask. No post-annealing was performed aer the TE fabrication. The electrical test was performed in either the pulse-type voltage application [with the HP 81110A pulse generator (PG) as the voltage-programmed current source and the Tektronix 684C oscilloscope (OSC) as the current monitor] or the voltage sweep mode using a semiconductor parameter analyser (HP 4145B). The electrical test sequences for the RS performance evaluation are described in the following Results section. All the bias voltage was applied to the TE while the BE was grounded.
III.
Results and discussion
The ionic BRS begins from the URS-reset state of the Pt/TiO2/Pt memory. First, the sample was electroformed using a compliance current (Icomp) of 20 mA, and then URS-reset in the voltage sweep mode. Subsequent application of an appropriate electric eld can drive the oxygen ions in the Magn´ eli CF-ruptured region of the sample that induced the occurrence of the ionic BRS. To accomplish the ionic BRS and to avoid rejuvenation of the Magn´ eli phase CF during the BRS set process, the ICC should be settled at much smaller values than that of the URS (typically
