Ti-Doped GaOx Resistive Switching Memory with Self-Rectifying

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Ti-doped GaOx Resistive Switching Memory with Self-rectifying Behavior by using NbOx/Pt Bilayers Ju Hyun Park, Dong Su Jeon, and Tae Geun Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10266 • Publication Date (Web): 15 Nov 2017 Downloaded from http://pubs.acs.org on November 16, 2017

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ACS Applied Materials & Interfaces

Ti-doped GaOx Resistive Switching Memory with Self-rectifying Behavior by using NbOx/Pt Bilayers Ju Hyun Park, Dong Su Jeon and Tae Geun Kim* School of Electrical Engineering, Korea University, Anam-dong, Seongbuk-gu, Seoul 136-701, Republic of Korea Keywords: self-rectifying, resistive switching, selectivity, Schottky emission, crossbar array.

ABSTRACT

Crossbar arrays (CBA) with resistive random access memory (ReRAM) constitute an established architecture for high-density memory. However, sneak paths via unselected cells increase the total power consumption of these devices, and limit the array size. To eliminate such sneak path problems, we propose a Ti/GaOx/NbOx/Pt structure with self-rectifying resistiveswitching (RS) behavior. In this structure, to reduce the operating voltage, we used a Ti/GaOx stack to increase the number of trap sites in the RS GaOx layer, through interfacial reactions between the Ti and GaOx layers. This increase enables easier carrier transport with reduced electric fields. We then adopted a NbOx/Pt stack to add rectifying behavior to the RS GaOx layer. This behavior is a result of the large Schottky barrier height between the NbOx and Pt layers. Finally, both the Ti/GaOx and NbOx/Pt stacks were combined to realize a self-rectifying ReRAM device, which exhibited excellent performance. Characteristics of the device include a low

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operating voltage range (-2.8–2.5 V), high on/off ratios (~20), high selectivity (~104), high operating speeds (200~500 ns), a very low forming voltage (~3 V), stable operation, and excellent uniformity for high-density CBA-based ReRAM applications.


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1. INTRODUCTION A considerable amount of attention has been paid to resistive random access memory (ReRAM) because of the advantages of these devices, such as their simple structure, nonvolatile characteristic, high-speed operation, and suitability for fabrication using complementary metaloxide semiconductor (CMOS) processes 1. Of the materials used in ReRAM, metal-oxide or nitride materials such as Ta2O5, HfO2, NiO, ZrO2, ZnO, GaOx, SiNx, and AlN have been actively investigated for use as resistive switching (RS) layers

2-9

. These RS layers are sandwiched

between two metal electrodes, to form ReRAM devices in a small area, which makes it possible to realize a crossbar array (CBA) architecture with the smallest memory cell size. The CBA is an ideal architecture for high-density memory functions, for either planar or three-dimensional (3D) structures. However, when ReRAM devices are used in the CBA architecture, sneak-path currents flow through the unselected neighboring cells in the CBA, significantly reducing the read margin between the set and reset states, leading to a read disturb error and a reduction of device density 10. To avoid these problems inherent to the CBA, various switching elements, such as transistors (in the so-called 1T1R configuration), diodes (1D1R), and selectors (1S1R), have been connected in series with memory elements

11-13

. However, neither of the 1T1R and 1D1R

structures is suitable for high-density memories, and both require high-temperature device fabrication processes. In addition, the requirement for an additional selective layer with a memory element in 1S1R structures is unsuitable for vertical 3D integration. For example, because the selector requires an additional area, it is difficult to implement a 4F2 density when the feature size is scaled to below the thickness of the 1S1R stack

14

. Although complementary

resistive switching (CRS) structures have been introduced as another solution to the problems


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associated with CBA structures, they require a complicated and destructive read operation 15. In recent years, self-rectifying ReRAM devices without any additional switching elements have been investigated. Some groups have used thin-film bilayer structures consisting of RS layers, and rectifying layers, created by optimizing the Schottky barrier height (SBH). Yoon et al. reported stable self-rectifying characteristics using Ta2O5/HfO2 bilayers inserted between TiN and Pt electrodes

16

. Jeon et al. reported selector-free RS memory behavior, with a high

resistance ratio and nonlinearity, using a Schottky barrier between a BiFeO3 nano-island and Nb:STO layer

17

. However, these self-rectifying devices have high operating voltages, limiting

the possible size of arrays for high-density CBA architectures, due to voltage drops in the electrode lines. In this paper, we propose a Ti/GaOx/NbOx/Pt structure for low-voltage, self-rectifying ReRAM operation. We first fabricated a Pt/NbOx junction with a rectifying property, resulting from a large SBH between the Pt and NbOx layer, without using a forming process. Note that NbOx materials are widely used as selectors for threshold-switching behavior, due to a thermally induced insulator-to-metal transition

18

. However, although these NbOx-based selectors exhibit

bidirectional-rectifying characteristics, they require a forming process and accurate stoichiometry control. In addition, because with these selectors, the off-current is very high and the selectivity is low, possible implementation in next-generation memory modules that require high density and low power characteristics is limited. The Pt/NbOx layer was then stacked in series with an RS GaOx layer, to realize a self-rectifying ReRAM device. GaOx-based ReRAM devices, operating with space charge limited current or Poole-Frenkel emission mechanisms, with an energy bandgap of 4.8 eV, have low operating voltages and good reliability, because of their use of carrier traps, such as oxygen vacancies 19-23. Therefore, we used GaOx as a RS layer, and Ti as


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a highly reactive electrode, to increase the number of trap sites, through interfacial reactions between the GaOx and Ti layers. Because of these reactions, carrier transport can occur in electric fields of low magnitudes. By optimizing the structure and operating characteristics, we obtained a high on/off ratio, high selectivity, a low forming voltage, high operating speeds, and uniform device performance from the Ti/GaOx/NbOx/Pt structure.

2. EXPERIMENTAL SECTION Device Fabrication. To fabricate the Ti/GaOx/NbOx/Pt device, a 30-µm-wide pattern corresponding to the geometry of the bottom electrode, was first prepared on a SiO2/Si substrate, using a photolithographic process. Subsequently, a 100-nm-thick Pt layer was deposited on the substrate, using a reactive radio-frequency (RF) sputtering method, at room temperature. To define the active region, a square 100 µm × 100 µm pattern was prepared using a photolithographic process. A 30-nm-thick NbOx film was then deposited in Ar/O2 gas environment, with a flow rate of 20/10 sccm, at a base pressure of ~2 × 10−6 Torr, and a working pressure of ~10 mTorr. After that, we deposited a 10-nm-thick GaOx layer on the NbOx/Pt film using an RF sputtering method. Here, the RS GaOx layer was grown in Ar/O2 gas environment, to make it more resistive, with a flow rate of 20/15 sccm using a Ga2O3 target. Finally, we deposited a 100-nm-thick Ti layer for the top electrode, and the photoresist used for both top and bottom electrodes was lifted-off in the same process. The current-voltage (I-V) switching behavior was measured using a Keithley 4200 semiconductor parameter analyzer. Furthermore, we calculated the maximum array size, for quantitative analysis of the anti-crosstalk characteristic.


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AES and FE-TEM Analyses and sample preparation. A Ti/GaOx/NbOx/Pt structure was prepared for Auger electron spectroscopy (AES) analysis, performed with the PHI 700 tool. The atomic concentration of the five elements (i.e., O, Ti, Ga, Nb, Pt) was measured using the AES depth profile, collected with a sputtering rate of 60 Å/min on SiO2 film. A sample for transmission electron microscopy (TEM) was prepared by milling and thinning the Ti/GaOx/NbOx/Pt structure, using a focused ion beam (FIB, QUAMTA 200 3D). Cs-corrected scanning TEM (STEM) and energy dispersive X-ray (EDX) analyses were performed using a field emission TEM (FE-TEM, XFEG Titan), with the Cs-corrector at an accelerating voltage of 300 kV.

XPS Analyses and Sample Preparation. A Ti/GaOx/Pt structure was prepared as a sample for X-ray photoelectron spectroscopy (XPS) analysis. The XPS analyses were performed using a 24.5 W monochromatic Al Kα radiation source (PHI 5000 VersaProbe, ULVAC-PHI). The binding energy ranges of the narrow-scan spectra for the O 1s and Ga 2p core levels were between 524 and 540 eV, and 1118 and 1160 eV, respectively, with a resolution of 0.125 eV/step. The scan area was 100 µm × 100 µm.

3. RESULTS AND DISCUSSION Figures 1a and 1b show the schematic drawing and the scanning electron microscope (SEM) image of the proposed Ti/GaOx/NbOx/Pt structure, respectively. The size of the metal pad, and line width of the top and bottom electrodes, is 150 µm × 150 µm and 30 µm, respectively. Thus,


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by considering the area overlapped by two 30-µm-wide electrode lines, the actual area of the RS layer is 900 µm2. To confirm if the cell structure was deposited as designed, each layer was analyzed using AES. Figure 2a shows the atomic concentration of the five elements (i.e., O2, Ti, Ga, Nb, Pt) constituting the ReRAM cell, measured from an AES depth profile, collected with a sputtering rate of 60 Å/min on SiO2 film. In this figure, each layer is clearly identified, and the ratio of Nb to O atoms in the NbOx layer is estimated to be approximately 1:4. However, we observed that the atomic ratios were non-uniform at the interface between the Ti and NbOx layer, and Ti atoms existed in the GaOx layer. The ratio of Ga to O atoms in the GaOx layer without Ti is also confirmed to be approximately 1:3.5 (or 2:7) from the O/Ga ratio of the O and Ga contents in Figure 2a. Because Ti is a highly reactive metal

24

, the reaction of Ti and GaOx atoms at the

interface was expected. Figure 2b shows a cross-sectional TEM image of the Ti/GaOx/NbOx/Pt structure (top panel). The bottom panel of Figure 2b shows an EDX mapping image of the five elements (i.e., O, Ti, Ga, Nb, Pt), which clearly displays the distribution of elements inside the structure. Interestingly, a Ti-doped GaOx (GaOx:Ti) layer, formed at the interface between the Ti and GaOx layer, was observed using the EDX mapping image. To understand what happened at the Ti/GaOx interface, we analyzed the region of the Ti and GaOx layers, indicated by dotted lines in Figure 2a, using an XPS measurement. For this measurement, we prepared a Ti/GaOx/Pt/SiO2/Si structure using the fabrication process detailed in the experimental section. The energy of the Ar+ ion beam was set to 3 keV during the sputter etching process. Figures 2c and 2d show the typical chemical binding states of the O 1s and Ga 2p core levels, respectively, as functions of the etching depth, where low and high etch levels represent top regions (in the vicinity of Ti) and bottom regions (in the vicinity of Pt) of the Ti/GaOx/Pt/SiO2/Si structure,


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respectively. In Figure 2c, the binding energy of the O 1s level was shifted toward a higher binding energy, as the sputter etching level increased (form low to high etch levels). This result indicates that various oxygen combinations exist at the interface, as well as in the GaOx layers. In particular, a non-lattice O-Ti combination is dominant near the interface, compared to within the GaOx layer. For reference, the O 1s peaks in the GaOx film can be deconvoluted into two components, corresponding to lattice oxygen (O-Ga, 530.9 eV), and non-lattice oxygen (oxygen vacancies, 532.1 eV)

25

. In contrast, in the TiOx film, the lattice oxygen (O-Ti) and non-lattice

oxygen peaks were located at 530 eV and 531.7 eV, respectively 26. The Ga 2p levels were also shifted toward a higher binding energy, as shown in Figure 2d. The Ga 2p peaks were located at 1117 and 1143.7 eV, near the interface, which correspond to Ga-Ti peaks

27

. In addition, the

1118.5 peak was dominant for the higher etching level, which means Ga-O combinations became dominant 28. Based on these results, we conclude that for Ti/GaOx bilayers, the bond between Ga and O atoms can be broken easily, owing to the high reactivity of Ti, leading to the formation of Ga-Ti and O-Ti combinations at the interface. Using these XPS results, we found that the number of trap sites acting as conducting paths, such as oxygen deficient states or oxygen vacancies, increase at the interface of the Ti/GaOx structure. Hence, carrier transport occurs easily, with reduced electric fields. To optimize the rectifying characteristics of the NbOx layer, we fabricated a Ti/NbOx/Pt structure, with a large SBH at the interface, and measured I-V switching curves. Conventionally, NbOx materials exhibit threshold-switching behavior after a forming process, due to a thermally induced insulator-to-metal transition 18. However, in this study, the Schottky effect was created by joining low work function Ti (~4.3 eV) and high work function Pt (~5.45 eV), with an NbOx film, without a forming process. First, to determine the forming voltage at different currents,


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positive and negative bias voltages were applied to the Pt electrode while the Ti electrode was grounded, with a current compliance (CC) of 5 mA. The initial forming process occurred at approximately 1.7 V and -3.5 V, respectively, as shown in Figure 3a. After the forming process, threshold-switching behavior was observed in the I-V curve of the Ti/NbOx/Pt structure, as shown in Figure 3b. The current was increased by sweeping the voltages from 0 to 2 V. The “on” state was reached at 1.25 V, and the device was switched to the “off” state by reducing the positive voltage to 1.1 V. Similar behavior was observed for negative bias voltages. However, when the positive and negative bias voltages were swept with a CC of 1 mA, a Schottky-diodelike characteristic occurred without the forming process. The I-V characteristic was measured by injecting electrons from the Ti electrode to the NbOx film (positive bias), and from the Pt electrode to the NbOx film (negative bias), as shown in the insets of Figure 3c. These behaviors are attributed to the difference in the SBH at each interface where electrons are injected. The work function of Ti (~4.3 eV) is closer to the electron affinity of the NbOx film (4.2 eV), in contrast to the work function of Pt (~5.45 eV) 29. Therefore, electron injection at the Ti interface is enhanced, compared to injection at the Pt interface. To understand the mechanism controlling the rectifying phenomenon, we observed the reverse current transport behavior of the Ti/NbOx/Pt structure. We plotted the current as a function of the square root of voltage, as shown in Figure 4a. The current was proportional to the square root of voltage, indicating that reverse current transport is dominated by Schottky emission. The Schottky emission current density can be expressed as follows 30: ( /( ))

= ∗ exp

,

(1)


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where A* is the effective Richardson constant, ΦB is the barrier height, T is the temperature, d is the barrier thickness, q is the charge of an electron, k is Boltzmann’s constant, εr is the dynamic dielectric constant, and J is the current density. Figure 4b shows a conventional Richardson plot [ln (J/T2) vs. 1/kT] of the Ti/NbOx/Pt structure in the reverse current region, measured between 25 and 125 °C. The SBHs for the given bias voltages (-0.15 – -0.6 V) were calculated using the slope of the fitted linear graph. Therefore, as shown in Figure 4c, the effective SBH at zero bias was calculated as ~1.003 eV, using the Richardson plot. It is worth noting that the rectifying characteristic was achieved in the Ti/NbOx/Pt structure using only the difference in the work functions, without the forming process. Next, NbOx/Pt stacks were combined with RS Ti/GaOx layers, to realize a self-rectifying ReRAM device in the form of a Ti/GaOx/NbOx/Pt structure. The Pt electrode at the bottom of the device was biased, whereas the Ti electrode at the top was grounded. The CC was set to 1 mA, to prevent complete breakdown of the films, particularly the NbOx layer. As shown in Figure 5a, during the first positive voltage sweep, the current in the high resistance state (HRS) was lower than during the next positive voltage sweep. This discrepancy indicates that the density of trap sites in the Ti/GaOx layer is low in the pristine state, but increased by the positive voltage bias, causing the resistance to change from the HRS to the low resistance state (LRS) (curve 1). Following this process, also called the forming process, a negative voltage bias is applied from 0 to -2 V, which reset the device to the HRS (curve 2). This phenomenon corresponds to the recombination of the oxygen ions and oxygen vacancies by the electric field, due to the high SBH in the NbOx/Pt interface. Subsequently, the HRS was switched again, by sweeping the voltage from 0 to 3 V, with a CC of 1 mA (curve 3). The device showed reproducible I–V curves


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following testing with identical set and reset processes for over 100 DC sweep cycles. The inset in Figure 5a shows an I–V curve scaled linearly. Compared to the RS characteristics of the Ti/GaOx/Pt structure (without NbOx layer)19, current levels in the negative bias region are significantly lowered because of higher Schottky barrier height at the NbOx/Pt interface (e.g., rectifying effect) for the proposed Ti/GaOx/NbOx/Pt structure, while operating voltages in the positive bias region are slightly increased to ~1 V because of additional voltages applied to the NbOx film. More detailed conceptual drawings relating to the mechanism for the operation of the proposed device, are presented in Figure S-1 in the Supporting Information section. To determine the read voltage, the on/off ratio was calculated depending on the positive voltage of a random device, as shown in Figure 5b. With this device, a maximum on/off ratio of 70 was recorded, at 0.7 V. Therefore, during the endurance test, the current in the HRS and LRS were read at 0.7 V, as shown in Figure 5c. The device exhibited stable RS characteristics, maintaining the initial on/off ratio (~20) for more than 100 cycles. In addition, the pulse switching properties of the device were analyzed in AC pulse mode, to define the program and erase speeds. In the program test, the device was completely switched from the HRS to the LRS, at 2.5 V/200 ns. Conversely, the device state was changed from the LRS to the HRS, at -2.8 V/500 ns (see Figure S-2). Finally, to estimate the maximum array size possible with the proposed device, we calculated a read margin using anti-crosstalk characteristics. We assumed a worst-case scenario for reading the crossbar, and chose a one bit-line pull up scheme. In the CBA, the worst case for reading the HRS occurs when the total resistance of the unselected cells is minimized, which means that all unselected cells are set to the LRS. When all unselected cells are set to the HRS, the total resistance of the unselected cells is maximized, leading to the worst case for the LRS read of the


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selected cell 31. The one bit-line pull-up scheme can be derived by connecting the word line and bit line for the selected cell, while the other bit lines are floating

32

. The N × N CBA can be

simplified to a 2 × 2 array, with an equivalent circuit, as shown in Figure 6a. When a read voltage is applied to the selected bit line, current flows through the unselected cells (the so-called sneak path current), as well as the selected cell. As shown in Figure 6a, the LRS current of the unselected cell (R2) is suppressed in the negative voltage region, due to the self-rectifying behavior of our device. However, this behavior is dominant only when the value of N is small, because the effect of R1 and R3 is negligibly low. On the other hand, if the value of N were larger, the effect of R1 and R3 would no longer be negligible, due to the low value of R2. Therefore, we calculated the maximum size of the CBA by considering these effects. A detailed calculation method is given in Figure S-3 of the Supporting Information section. Using the parameters extracted from the I-V curve of our self-rectifying device and the equivalent circuit, the maximum value of N, with an at least 10 % read margin, was calculated as 2×103 at a read voltage of 0.7 V, which is suitable for high-density CBAs without a selector element.

4. CONCLUSIONS

In summary, we demonstrated a self-rectifying RS memory characteristic in a Ti/GaOx/NbOx/Pt structure. Here, the Ti/GaOx layer was chosen to improve the RS performance, by increasing the number of trap sites in the RS GaOx layer, through interfacial reactions. The NbOx/Pt junction layer was chosen as a selector, to utilize the difference in the SBHs of the NbOx and Pt layers. From XPS analyses, we found that Ti reacted with GaOx to form either Ti-O or Ti-Ga combinations at the Ti/GaOx interface. This implies that the increase in the number of trap sites at the GaOx surface, due to the Ti-doping effect, can reduce the operating voltage. Self-rectifying


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characteristics were implemented in the proposed cell structure without a forming process, using the difference between the work functions of the Pt (~5.45 eV) and NbOx layer. The effective SBH in this cell was calculated to be 1.003 eV. Consequently, the proposed ReRAM cell exhibited stable RS characteristics for more than 100 DC sweep cycles, a sufficiently large on/off ratio (~20), high operating speeds (200~500 ns), and high selectivity (~104). In addition, the anti-crosstalk behavior of the cell can suppress the sneak path current, allowing integration of this structure in a CBA. We believe that study of the proposed ReRAM cell structure offers important information for the creation of high-density CBA architectures.

SUPPORTING INFORMATION Conceptual drawings associated with the mechanism for the operation of the proposed device (Figure S-1), programing and erasing characteristics measured under AC pulse mode (Figure S2), and the calculation method for establishing the maximum size of an array (Figure S-3), are included in the Supporting Information.

AUTHOR INFORMATION Corresponding Author *Corresponding author; E-mail: [email protected], Phone: +82-2-3290-3255, Fax: +82-2-9245119 ACKNOWLEDGMENTS This work was supported by a National Research Foundation of Korea (NRF) grant, funded by the

Korean

government

(Ministry

of

Science,

ICT

&

Future

Planning,

No.


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2016R1A3B1908249). The authors are also grateful for support from the Samsung semiconductor research center at Korea University.

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Figure Captions Figure 1. (a) Schematic drawing, and (b) SEM images of the proposed Ti/GaOx (10 nm)/NbOx (30 nm)/Pt structure.

Figure 2. (a) Atomic concentration of the five elements (i.e., O2, Ti, Ga, Nb, Pt) in the Ti/GaOx/NbOx/Pt structure measured from AES depth profiles. (b) A cross-sectional TEM image of the device (top panel). The bottom panel shows the EDX mapping image of the five elements (i.e., O2, Ti, Ga, Nb, Pt). XPS depth profiles measured for the (c) O 1s, and (d) Ga 2p core levels of the Ti/GaOx/Pt structure.

Figure 3. (a) Current-voltage (I-V) characteristics of the Ti/NbOx/Pt structure under positive and negative voltage biases, with a current compliance (Icc) of 5 mA, for estimating the forming current (If) and voltage (Vf). (b) I-V curve measured for a selector after positive forming process. When a positive bias is applied, the current increases to Icc at the threshold voltage (Vth). The current decreases when the applied bias is reduced to the hold voltage (Vhold). (c) Schottkydiode-like characteristic observed without a forming process, and schematic band diagrams for negative and positive biases (inset).

Figure 4. (a) Schottky emission fit of ln (J/T2) vs V1/2. (b) Temperature-dependent Schottky emission fit of the Ti/NbOx/Pt structure at reverse bias, measured between 25 and 125 °C. (c) Schottky barrier heights extracted from Figure 4b, for various voltages.


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Figure 5. (a) Self-rectifying characteristic of the Ti/GaOx/NbOx/Pt structure for voltages between -2 and 3 V. (b) On/off ratio as a function of voltage, which is extracted from a random I-V curve from Figure 5a. (c) Endurance test performed with 100 DC sweep cycles, at a read voltage of 0.7 V.

Figure 6. (a) Equivalent circuit of an N × N crossbar array (CBA) for the one bit-line pull-up read scheme. (b) Dependence of the normalized read margin, ∆Vout/Vpu, on the word line number (N), at a read voltage of 0.7 V.


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Figure 1


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Figure 2


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Figure 3


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Figure 4


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Figure 5


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Figure 6


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Graphic for manuscript


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Ti-Doped GaOx Resistive Switching Memory with Self-Rectifying

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