Article pubs.acs.org/JPCC
Conductive Atomic Force Microscopy Investigation of Switching Thresholds in Titanium Dioxide Thin Films M. Trapatseli, D. Carta, A. Regoutz, A. Khiat, A. Serb, I. Gupta, and T. Prodromakis* Nanoelectronics and Nanotechnology Research Group, Southampton Nanofabrication Centre, Electronics and Computer Science, Faculty of Physical Sciences and Engineering, University of Southampton, Southampton, United Kingdom S Supporting Information *
ABSTRACT: Titanium dioxide thin films have attracted increasing attention due to their potential in next-generation memory devices. Of particular interest are applications in resistive random access memory (RRAM) devices, where such thin films are used as active layers in metal−insulator−metal (MIM) configurations. When these devices receive a bias above a certain threshold voltage, they exhibit resistive switching (RS), that is, the resistance of the oxide thin film can be tuned between a high resistive state (HRS) and a low resistive state (LRS). In the context of this work, we have used conductive atomic force microscopy (C-AFM) to identify the resistive switching thresholds of titanium dioxide thin films deposited on Si/SiO2/Ti/Pt stacks to be used in memory devices. By performing a set of reading/writing voltage scans over pristine areas of the thin films, we have identified the critical thresholds, which define a reversible operation (soft-breakdown, SB) via localized changes in electrical resistance across the film and an irreversible operation (hard-breakdown, HB) that includes both changes in local electrical resistance and thin film topography. We have also assessed the transition from SB to HB when thin films are stimulated repeatedly with potentials below the identified onsets of HB, validating a history dependent behavior. This study is therefore aimed at presenting new insights in RRAM device programmability, reliability, and eventually failure mechanisms.
1. INTRODUCTION In the past decade, titanium dioxide thin films have been extensively studied for their application in nanoelectronics,1−3 energy,4−6 photocatalysis,7 and storage.8−10 So far, TiO2 thin films were considerably employed in resistive random access memory (RRAM) cells, serving as an active layer sandwiched between two metal electrodes in a meta-insulator−metal (MIM) stack.8 Due to their simple structure, RRAM cells can be easily incorporated into single devices or crossbar arrays of low power consumption, high density integration and high speed operation.10 RRAM cells can support abrupt alterations between two or more resistive states that occur after applying appropriate biasing schemes. In most cases, two distinct resistive states can be distinguished, commonly referred to as high resistive state (HRS) or OFF state and low resistive state (LRS) or ON state.9,11 Several transition metal oxides exhibit resistive switching (RS) behavior such as TaOx,12 HfOx,13 NiOx,14 WOx,15 CuO,16 and SrTiOx.17 However, TiO2 is one of the most celebrated ones due to its wide band gap, high thermal stability, and high dielectric constant.8 The RS mechanism is still not well understood and it remains a matter of debate. The mechanisms that have been proposed so far to explain the RS behavior can be mainly classified in two categories: localized interface-type18 and homogeneous filament-type.19,20 The filament-type mechanism is based on the formation and © 2015 American Chemical Society
migration of defects (such as oxygen vacancies) in the metal oxide, driven by the applied voltage.21,20 The motion of defects within the film results to formation of reduced oxide species in form of highly conductive filaments (CF) perpendicular to the electrodes.22 The reversible formation and rupture of CF cause the oxide film to toggle between LRS and HRS.23 In the interface-type mechanism, the barrier height at the interface between the titanium dioxide and the electrodes can be modified by a voltage bias without accompanying structural changes of the oxide, leading to different resistance states.18 A typical current−voltage (I−V) characteristic recorded after voltage sweeps of Pt/TiO2/Pt devices is shown in Figure 1a. At a certain voltage, corresponding to the RESET, the resistance of the film increases abruptly bringing the film to HRS. By reversing the voltage, the device can be brought back to LRS via a SET process. When a soft breakdown (SB) occurs, an abrupt increase of current is observed. However, no major physical deformation of the top electrode is observed, as revealed by the atomic force microscopy (AFM) image in the inset of Figure 1a. Devices operating in SB maintain their last assumed state through subsequent low-voltage (below switchReceived: February 18, 2015 Revised: April 30, 2015 Published: May 5, 2015 11958
DOI: 10.1021/acs.jpcc.5b01672 J. Phys. Chem. C 2015, 119, 11958−11964
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Figure 1. Representative I−V characteristic of a device (a) after SB and (b) after HB. Insets: AFM topographies of the devices top electrodes. (c) Schematic of the resistive switching dynamics in RRAM cells.
surface, thus adding a resistive component in the tip−sample system.22 In C-AFM the tip is in physical contact with the oxide layer, resulting in a setup which is closer to the MIM configuration. However, even C-AFM measurements can be affected by additional resistive components such as adsorbates or impurities on both tip and sample surfaces. The main advantage of C-AFM over STM is the fact that C-AFM measurements can be performed under different atmospheres and in ambient conditions. So far, C-AFM characterization of TiO2 thin films designed for RRAM applications has been mainly used for the evaluation of conductive filaments,33,34 the identification of switched areas35 and for the investigation of the RS mechanism at the very early stage of electroforming.36 Two main methodologies have been reported for studying TiO2 thin films using C-AFM. The first consists of switching the RRAM cell, delaminating the top electrode using techniques such as focused ion beam (FIB),16 wet etching,37,33 mechanical cleaving,34,35,38 AFM etching,30 and finally, scanning the bare oxide film using a C-AFM tip. However, by using this approach the oxide film can be affected during the electrode removal process. The second method consists of switching in situ the bare film by using the C-AFM tip as a nanosized top electrode.29,39−41 In the present work, we have investigated the switching dynamics of a 10 nm thick titanium dioxide thin film deposited by reactive sputtering on a Si/SiO2/ Ti/Pt support following the second approach, that is, by switching the films in situ using a C-AFM tip as a top electrode. Most C-AFM studies on TiO2 refer to thicknesses in the range of 25−60 nm.34,36,37 However, downscaling the physical thickness of the film is crucial for achieving low voltage switching, which is extremely important for low-power applications.33 The switching threshold which causes SB (hereafter called VSB), was identified as the minimum voltage that induces changes in lateral current distribution but not in topography and the switching threshold which causes HB (hereafter called VHB), was identified as the minimum voltage that induces changes both in lateral current distribution and topography. The transition to HB region by performing recurrent scans with voltages lower than the VHB has been also assessed, validating a history dependent behavior. Two
ing threshold) sweeps. When applying very high voltage or when performing a large number of switching cycles the device enters the hard breakdown (HB) region and finally fails. In HB, the device may still be able to switch electrically, but this is accompanied by the formation and proliferation of protrusions on the top electrode, as shown in Figure 1b. The protrusions accumulate irreversibly until the device fails completely, that is, becomes unable to switch electrically. It has been suggested that these irreversible topographic deformations are caused by the development of oxygen gas at the interface along with increase of temperature due to Joule heating causing deformation or rupture of the electrode.20,24 These dramatic events can cause permanent failure of devices. In Figure 1c, a schematic diagram of the RS dynamics in RRAM cells based on the I−V characteristics and AFM topographies shown in Figure 1a,b is presented. Three major areas can be identified: a “neutral” zone (shown in gray) corresponding to application of very low voltages that are not sufficient to alter the device’s resistance, a “reversible switching” zone (shown in green) corresponding to a higher voltage range where the device operates reliably and an “irreversible switching” zone at higher voltages where switching causes irreversible changes to device’s architecture and leads to permanent failure. Therefore, the evaluation of SB and HB threshold voltages in thin films is important in order to improve device reliability. The active layer can be engineered to lower the SB voltage threshold (key for low-power operation) and to increase the HB voltage threshold (key for enhanced endurance and reliability).25−27 However, the identification of SB and HB voltage thresholds is not trivial due to the fact that these events take place at the nanoscale. Therefore, the use of characterization tools with high spatial lateral resolution is essential. Conductive atomic force microscopy (C-AFM) is an ideal technique for this purpose as it offers simultaneous in situ recording of spatial topography and current distributions with a nanometer scale lateral resolution roughly equal to the end radius of the tip.22,28−30 Scanning tunneling microscopy (STM) has also been used for nanoscale electrical characterization of thin films as it has advantage over other methods due to the atomic scale resolution.31,32 In STM the tip is in close proximity from the 11959
DOI: 10.1021/acs.jpcc.5b01672 J. Phys. Chem. C 2015, 119, 11958−11964
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The Journal of Physical Chemistry C titanium dioxide films with varying oxygen content were investigated, a near-stoichiometric TiO2−x and a mixed valence TiOx. The effect of oxygen content on switching thresholds has been evaluated. These results provide original insights for engineering the critical thresholds VSB and VHB to result in operation with improved cycling endurance.
2. EXPERIMENTAL METHODS 2.1. TiO2−x and TiOx Thin Films Preparation. A conductive substrate stack was fabricated as follows: a Si wafer was thermally oxidized to yield a 200 nm thick, insulating SiO2 layer. Thereafter, an adhesion layer of 5 nm Ti and a bottom electrode of 200 nm Pt were deposited by magnetron reactive sputtering. This substrate was patterned and the 10 nm thick active layers were deposited on top. The sputtering settings for the near-stoichiometric TiO2−x were 8 sccm O2 and 35 sccm Ar at the cathode with a power of 2 kW and 15 sccm O2 and a power of 2 kW in the additional plasma source. In addition, the process was lambda controlled at a set point of 365 mV. For the mixed valence TiOx thin film the cathode was set to 2 kW with 10 sccm O2 and 35 sccm Ar. The additional plasma source was not used. 2.2. X-ray Photoelectron Spectroscopy (XPS) Characterization. XPS was used to evaluate the stoichiometry of TiO2−x and TiOx films. A Thermo Scientific Theta Probe Angle-Resolved X-ray Photoelectron Spectrometer (ARXPS) system with a monochromated Al Kα X-ray source (hν = 1486.6 eV) operating at a base pressure of 2 × 10−9 mbar was used to record all spectra. Data were collected over an area of approximately 400 × 400 μm2 and with a pass energy of 200 and 50 eV for survey and core level spectra, respectively. The X-ray source was operated at 6.7 mA emission current and 15 kV anode bias and photoelectrons are collected over a cone of ±30° with the lens mounted at 40° in respect to the sample surface. A 180° double focusing hemispherical analyzer with two-dimensional PARXPS detector are part of this system. All spectra were corrected for any charge shifts by aligning them to the C 1s core level at 285.0 eV and all data were analyzed using the Avantage software package. 2.3. C-AFM. C-AFM measurements were performed using a Bruker MultiMode Nanoscope V system equipped with a current sensing C-AFM module at room temperature and atmospheric pressure. Conductive Pt/Ir coated Si tips with a cantilever spring constant of 0.2 N/m and nominal radius of 12 nm were used (Bruker, SCM-PIC) with the current sensitivity being set at 1 nA/V. Before performing the C-AFM measurements on the film, the conductivity of the tip was confirmed by scanning a conductive reference sample. In order to minimize the tip wear, a minimum set point of 0.1 V was applied. The DC voltage bias was applied between the bottom electrode and the tip with the latter connected to virtual ground. Therefore, all voltages mentioned in the text will be referred to as the polarity of the bottom electrode.
Figure 2. XPS spectra of TiO2−x and TiOx thin films: (a) survey; (b) Ti 2p core level, including higher binding energy satellites S3/2 and S 1/2; (c) detailed view of the lower binding energy peaks corresponding to Ti3+ and Ti0 populations.
2b, the main core lines 2p3/2 and 2p1/2 can be observed along with the charge transfer satellites S3/2 and S1/2 at higher binding energies.42 The stoichiometry of the TiO2−x was determined to be TiO1.94. It has to be noted that, while the 2p3/2 peak of the TiO2−x film is consistent with Ti in the oxidation state 4+, the TiOx film exhibits two additional peaks on the lower binding energy side of the 2p3/2 peak, which are identified as (I) Ti0 and (II) Ti3+ (Figure 2c). Their energies coincide with average reference energies calculated from the NIST X-ray Photoelectron Spectroscopy Database,43 which are 453.9 eV for Ti0 and 457.2 eV for Ti3+. 3.2. C-AFM Identification of Soft and Hard Breakdown Voltage Thresholds. In Figure 3, the I−V characteristics obtained from an arbitrary point of the surface of the pristine TiO2−x (blue curve) and TiOx (black curve) thin films are shown.
3. RESULTS AND DISCUSSION 3.1. XPS Characterization. The oxidation states of Ti in TiO2−x and TiOx thin films were evaluated by performing XPS analysis (Figure 2). As shown in the XPS survey spectra (Figure 2a), both films exhibit the expected core levels in photoemission, Ti 2s, 2p, 3s, 3p, and O 1s and 2s, as well as a C 1s peak. Detailed views of the Ti 2p core level spectra are shown in Figure 2b,c. In Figure
Figure 3. I−V characteristics of TiO2−x (black line) and TiOx (blue line) thin films obtained via C-AFM. Arrows indicate sweeping directions. Top left inset figure: schematic illustration of the C-AFM experimental setup. 11960
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Figure 4. Evaluation of VSB for TiO2−x and TiOx films. TiO2−x: (a) Spatial topography and (b) current distribution after reading at 5 V. (e) Spatial topography and (f) current distribution after writing inside the dashed white square with 6.5 V. TiOx: (c) Spatial topography and (d) current distribution after reading at 1.5 V. (g) Spatial topography and (h) current distribution after writing inside the dashed white square with 1.7 V.
temporarily since the oxide is finally resting in HRS, manifesting that the states are volatile. In order to identify the VSB and VHB thresholds, we have performed reading/writing voltage sweeps over an extended area of the TiO2−x and TiOx thin films. In particular, we used the following procedure:
The I−V characteristics were recorded using the C-AFM tip as nanoelectrode, as shown in the top left inset of Figure 3. A 12.5 nA compliance current was employed for preventing damage to the tip. Starting from 0 V, the voltage was swept toward positive voltages up to 8 V, returned to 0 V, swept toward negative voltages up to −8 V and then brought back to 0 V for a complete switching cycle. Both I−V characteristics follow the typical memristor signature, that is, a pinched hysteresis loop.44 For each bias polarity there is a characteristic voltage threshold above which the thin film is switched from HRS to LRS. In particular, voltage potentials above ∼5.5 V and ∼6 V induced an increase of conductivity for both thin films, setting them to LRS (bottom right inset of Figure 3). By sweeping the voltage below −6 V and below −5 V for TiO2−x and TiOx, respectively, an abrupt increase in current is observed, setting again the local state of the film to LRS. It is interesting to note that the acquired I−V characteristics are asymmetric, in agreement with experimental data of fully functional micrometer sized MIM devices comprising such films. However, the voltages required to SET the film using the nanometer-scale AFM tip as top electrode are much higher than those required when using standard micrometer sized electrodes in a conventional probe station (Figure 1a).45 This could be due to the presence of adsorbates or impurities on tip or sample surface caused by the atmospheric environment. It has been acknowledged that atmospheric environment causes complications on C-AFM measurements.28 In particular, Choi et al.33 have observed differences in the total value of current measured in vacuum and in air. Therefore, effects due to the presence of unwanted adsorbates such a moisture cannot be excluded in the present work. The I−V characteristics reported in Figure 3 show that positive potentials induce higher current in the TiOx film compared to the TiO2−x. This can be explained by the higher conductivity of the mixed valence TiOx thin film due to the presence of reduced Ti species. It has to be noted that both CAFM I−V characteristics are type II,46,47 where the applied potential induces an increase of conductivity but only
(a) An area of 5 × 5 μm2 was scanned with an appropriate reading voltage. Topography and current images were simultaneously acquired. (b) An inner area of 2 × 2 μm2 was then scanned with an appropriate writing voltage. (c) The scan area was restored to the 5 × 5 μm2 and any modification of topography and current were evaluated using the same reading voltage as in (a). Reading voltages of 5.0 and 1.5 V were used for the TiO2−x and TiOx thin films, respectively. In order to identify the first threshold VSB, steps a−c were repeated in pristine areas until a change in current (and no change in topography) was detected between two consecutive reading maps. Correlation between topography and current maps for the identification of VSB in TiO2−x and TiOx thin films are reported in Figure 4. The spatial topographic and current distributions of 5 × 5 μm2 areas of the TiO2−x thin film after reading with 5.0 V are reported in Figure 4a and b, respectively. Spatial topographic and current distributions of the same areas after writing a 2 × 2 μm2 inner area with 6.5 V (indicated by the white dashed squares) are shown in Figure 4e and f, respectively. Likewise, Figure 4c,d shows the spatial topographic and current distributions of 5 × 5 μm2 areas of the TiOx thin film after reading at 1.5 V, and Figure 4g,h shows the spatial topographic and current distributions of 5 × 5 μm2 after writing the inner 2 × 2 μm2 areas with 1.7 V. In both TiO2−x and TiOx films, the 2 × 2 μm2 areas have been switched to HRS after the writing operation at 6.5 and 1.7 V, respectively with a decrease in current without change in topography. Therefore, 6.5 and 1.7 V can be identified as VSB in TiO2−x and TiOx thin films, respectively (Table 1). In order to justify the results in detail, it 11961
DOI: 10.1021/acs.jpcc.5b01672 J. Phys. Chem. C 2015, 119, 11958−11964
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the VHB threshold after repetitive writing operations over the same scan area (VHBR). The initial topography and current maps of the TiO2−x and TiOx film recorded after reading at 5.0 and 1.5 V are shown in Figure 6a,b and 6c,d, respectively. The initial reading scans reveal uniform spatial topographic and current distributions for both films. However, changes in topography and current were observed in the inner writing area of 2 × 2 μm2 after scanning the TiO2−x thin film with 8 repetitive scans with 6.8 V (Figure 6e,f). Similar behavior was observed for the TiOx thin film after eight repetitive scans with 2.0 V (Figure 6g,h). Therefore, 6.8 and 2.0 V were identified as the VHBR voltages for the TiO2−x and TiOx films, respectively, being the lowest voltage values that cause current and topographical changes when scanning repetitively the same area. A summary of all identified threshold voltages is reported in Table 1. As expected, VHBR is slightly lower than VHB for both films (6.8 V vs 7.0 V for TiO2−x and 2.0 V vs 2.2 V for TiOx). It has to be noted that all initial reading current maps (Figures 4b,d, 5b,d, and 6b,d) reveal that both films are initially in the ON state, with uniform current distributions. The final reading current maps (Figures 4f,h, 5f,h, and 6f,h) show that the film has been switched to OFF state. Similar behavior has been observed by Muenstermann et al.38 and was explained with a homogeneous RS mechanism. In particular, since the tip radius is very small, the electric filed is concentrated in the region very close to the tip. When the tip is negatively biased (positive charge on the bottom electrode/ sample) it induces the formation of an oxygen vacancy depleted region with poor conductivity that causes the sample to switch to HRS. We believe that these results show new insights for understanding the switching dynamics of titanium dioxide thin films in devices, in particular, for optimizing the device’s operation and for avoiding failure.
Table 1. Reading Voltages (VR), Soft Breakdown Voltage Thresholds (VSB), Hard Breakdown Voltage Thresholds (VHB), and Hard Breakdown Voltage Thresholds after Repetitive Scanning (VHBR) for TiO2−x and TiOx Thin Films TiO2−x TiOx
VR (V)
VSB (V)
VHB (V)
VHBR (V)
5.0 1.5
6.5 1.7
7.0 2.2
6.8 2.0
In order to identify VHB, we have used the same protocol described above. However, in this case, steps a−c were repeated until changes in both current and topography were detected between two consecutive reading maps. The initial topography and current maps of the TiO2−x and TiOx film recorded after reading at 5.0 and 1.5 V are shown in Figure 5a,b and c,d, respectively. The initial reading scans are uniform in terms of spatial topographic and current distributions. However, a change in topography and current was observed in the inner writing area of 2 × 2 μm2 after scanning the TiO2−x film with 7.0 V (Figure 5e,f) and the TiOx film with 2.2 V (Figure 5g,h). In order to make clearer the change in topography of Figure 5e, the 2D topographical map is reported in the Supporting Information (Figure S1). Therefore, 7.0 and 2.2 V were identified as the VHB for TiO2−x and TiOx, respectively. These threshold voltages were consistent for different areas of the film and they were always identified with an error of ±0.1 V. As observed in the evaluation of VSB (Figure 4), in both films, the 2 × 2 μm2 areas have been switched to HRS after the writing operation. 3.3. C-AFM Identification of History Dependence Voltage Threshold. VSB and VHB were identified by scanning pristine areas of the films with increasing voltages until a change in current/topography was observed. However, one of the main problems of RRAM is that the HB is often observed after repetitive switching cycles. Therefore, we have also evaluated
4. CONCLUSIONS The resistive switching dynamics of a near-stoichiometric TiO2−x and a mixed valence TiOx 10 nm thin film were
Figure 5. Evaluation of VHB for TiO2−x and TiOx films. TiO2−x: (a) Spatial topography and (b) current distribution after reading at 5 V. (e) Spatial topography and (f) current distribution after writing inside the dashed white square with 7.0 V. TiOx: (c) Spatial topography and (d) current distribution after reading at 1.5 V. (g) Spatial topography and (h) current distribution after writing inside the dashed white square with 2.2 V. 11962
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Figure 6. Evaluation of VHBR voltage after eight repetitive scans for TiO2−x and TiOx films. TiO2−x: (a) Spatial topography and (b) current distribution after reading at 5 V. (e) Spatial topography and (f) current distribution after writing inside the dashed white square with 6.8 V. TiOx: (c) Spatial topography and (d) current distribution after reading at 1.5 V. (g) Spatial topography and (h) current distribution after writing inside the dashed white square with 2.0 V.
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investigated using C-AFM. I−V characteristics recorded using the C-AFM tip as nanoelectrode reveal asymmetric bipolar switching behavior for both films. Switching voltage thresholds at which SB (changes in current distribution but not in topography, VSB) and HB (changes in current distribution and topography, VHB) occur were identified by performing sets of reading/writing C-AFM scans on pristine areas of both films. The writing operations switch all films to HRS suggesting a homogeneous resistive switching mechanism. For the TiO2−x thin film, VSB was found to be 6.5 V and VHB 7.0 V, while for the TiOx, VSB was found to be 1.7 V and VHB 2.2 V. In agreement with solid-state equivalent devices, HB can be also achieved after recurrent scanning of a pristine area with the same voltage (VHBR). VHBR was found to be 6.8 V for TiO2−x and 2.0 V for TiOx. We believe that the identification of SB and HB thresholds is highly beneficial for engineering the switching dynamics of RRAM devices in a tailored fashion.
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ASSOCIATED CONTENT
S Supporting Information *
2D topographical map of the TiO2−x thin film after soft breakdown (corresponding to Figure 5e in the manuscript). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b01672.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel.: +44 (0)23 8059 8803. Fax: +44 (0)23 8059 3029. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors wish to acknowledge the financial support of EPSRC EP/K017829/1 and EU-FP7 RAMP. 11963
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DOI: 10.1021/acs.jpcc.5b01672 J. Phys. Chem. C 2015, 119, 11958−11964
