Thermionic Emission Based Resistive Memory with Ultrathin

Article Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Thermionic Emission Based Resistive Memory with Ultrathin Ferroelectric BiFe1−xCrxO3 Films Deposited by Mineralizer-Free Microwave-Assisted Hydrothermal Synthesis Gitanjali Kolhatkar,† Fabian Ambriz-Vargas,† Bernhard Huber,†,‡ Reji Thomas,§ and Andreas Ruediger*,† Institut National de la recherche scientifique, centre Énergie, Matériaux, Télécommunications, 1650 Boulevard Lionel-Boulet, Varennes, Québec, J3X 1S2, Canada ‡ Munich University of Applied Sciences, Department of Applied Sciences and Mechatronics, Lothstrasse 34, 80335 Munich, Germany § Lovely Professional University, Jalandar - Delhi G.T. Road, Phagwara, Punjab 14411, India †

S Supporting Information *

ABSTRACT: We develop resistive switching memory devices employing ultrathin (∼2.5 nm) BiFe1−xCrxO3 ferroelectric films, deposited by microwave-assisted hydrothermal (MWHT) synthesis, as the potential barrier. BiFeO3 is a multiferroic material highly suitable for nonvolatile semiconductor memories due to its polar and magnetic ordering at room temperature. Chromium is incorporated to enhance the material’s multiferroic properties. As compared to more commonly used physical and chemical vapor deposition tools, MWHT is an extremely cost-efficient tool that provides high reproducibility and stoichiometry control. Using no mineralizer and thereby reducing leakage currents and structural disorder, the ferroelectric phase is reached in our films after 2 cycles of microwave irradiation, and atomic step-terraces can be seen on the surface, attesting to a highly ordered film growth. Using platinum (Pt) top electrodes and conductive substrates (Nb:SrTiO3 and Pt) as bottom electrodes, memory devices are fabricated. Resistive switching is confirmed through electrical characterizations, and resistance ratios of ∼20 and ∼105 are obtained for the SrTiO3/BiFe1−xCrxO3/Pt and Pt/ BiFe1−xCrxO3/Pt designs, respectively. By correlating the band alignment of both designs to their current−voltage behavior, we demonstrate that thermionic emission is the dominant charge transport mechanism. Under quasi-static conditions, the SrTiO3/ BiFe1−xCrxO3/Pt devices are switched over 35 cycles on 10 different electrodes.

1. INTRODUCTION Due to their simple structure (1 transistor-1 capacitor) and their fast write and read access (10 ns/10 ns), dynamic random access memories (DRAM) have been a candidate of choice in the semiconductor memory market for many decades.1 However, they present important drawbacks such as scalability limit and a volatile nature.2 Resistive random access memories (Re-RAM) offer a promising solution to overcome these limitations. They combine good scalability with low energy consumption (nonvolatile nature), low switching energies, high operation speed (write/read time, 5 ns/5 ns), high endurance (1010 cycles), and a simple structure (1 transistor-1 resistor, 1T1R).1 These devices exploit the principle of resistive switching, a reversible phenomenon where the resistance of a material is varied under an electric field.3 Re-RAM structures consist of a thin insulating layer sandwiched between two electrodes. They are characterized by two resistance states, a high resistance state (HRS), and a low resistance state (LRS). Reversible switching between the HRS and the LRS is achieved by bipolar voltages to the device.4−6 Among Re-RAMs, ferroelectric tunnel junctions (FTJ) are among the most promising contenders to overcome the limitations of DRAM.7,8 FTJs consist of a few © XXXX American Chemical Society

nanometer thick layer of ferroelectric material, or barrier between two metal electrodes.9−11 Due to the low thickness of the barrier, the electrons have a finite probability of coherent quantum-mechanical tunneling. In such ultrathin films, the electrons might overcome the potential barrier through three different mechanisms: direct tunneling, Fowler-Nordheim tunneling, and thermionic emission.12 While direct tunneling has been suggested for use in FTJs, resistive memory devices based on thermionic emission have not yet been systematically investigated. Thermionic charge transport is considered in barrier layers too thick for coherent tunneling. Bismuth ferrite (BiFeO3) has attracted broad attention due to its multiferroic nature, i.e., the simultaneous presence of ferroelectric and magnetic ordering13,14 at room temperature. FTJs based on BiFeO3 have already been reported, attesting to this material’s potential for such applications.15 In addition, it presents photovoltaic properties16 while belonging to the magnetoelectric class.17 Due to all of these properties, BiFeO3 Received: December 14, 2017 Revised: February 12, 2018 Published: February 21, 2018 A

DOI: 10.1021/acs.cgd.7b01745 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

were hydrothermally etched, following a method previously described.36 After the etching, a flat substrate surface (rms of ∼0.15 nm) showing atomic terraces-steps with an average height of 0.22 nm was obtained (see the Supporting Information, section 1). Thin-Film Deposition. For the deposition of BiFe1−xCrxO3 (equimolar, 0.002 M), the precursors used were Sigma-Aldrich 99.999% bismuth(III) oxide (Bi2O3), 99.5% iron(III) hydroxide oxide (FeO(OH)), and 99.9% chromium(III) oxide (Cr2O3). The deposition time was varied by changing the number of cycles, where 1 cycle corresponds to 10 min at 120 W of nominal microwave output power. Knowing that the hydrothermal reactor’s pressure limit is 83 bar and using the vapor pressure diagram, the deposition temperature was estimated to ∼280 °C. After each cycle, the hydrothermal reactor was left to cool down inside the microwave oven for 2 h to avoid overpressure. Nanoscale Characterization. Atomic force microscopy (AFM), piezoresponse force microscopy (PFM), and conductive atomic force microscopy (C-AFM) measurements were performed with an AISTNT SmartSPM system. The topography was mapped through AFM measurements in tapping mode using a 0.5 Hz scan rate and a Nanosensor AFM probe with a 10 nm silicon (Si) tip. PFM measurements were effectuated to study the ferroelectric behavior of the ultrathin films: the conductive substrate acted as the bottom electrode while an ∼30 nm radius Pt-Ir coated Si tip, through which an AC voltage was applied, was placed in contact with the films to act as a mobile top electrode. Local hysteresis curves were acquired through a quasi-static DC voltage sweep performed at a given position on the sample. PFM maps were obtained by poling different square shaped regions with an electrical bias potential, with a negative voltage first (green squares), and then with a positive voltage (blue squares) (Figure 1).

can be used for a wide range of applications, such as nonvolatile memories18 as well as for energy harvesting devices. The bulk crystal structure of BiFeO3 is the rhombohedrally distorted perovskite. It belongs to the R3c symmetry group and displays spontaneous polarization along one of the [111] pseudo-cubic axes.19 Its lattice parameters are a = b = c = 5.63 Å and α = β = γ = 59.4°, and it has high Néel and Curie temperatures, of ∼640 K and ∼1100 K, respectively.20 Its antiferromagnetism arises from the G-type ordering, where the Fe3+ are surrounded by six nearest neighbors of opposite spin,21−23 while its ferroelectricity originates from the dangling bonds that result from the Bi3+ lone pairs stereochemically active.24 To improve the material’s ferroelectric properties through a decrease in the leakage current, other atoms such as lanthanum (La), manganese (Mn) can be added to the crystal.25−28 Cr3+ ions can also be used to enhance the material’s magnetic properties through the Fe3+and Cr3+ interactions.29 So far, BiFeO3 has been synthesized by different techniques, such as radio frequency (RF)-magnetron sputtering,30 molecular beam epitaxy (MBE),31 chemical solution deposition,32 metalorganic chemical vapor deposition (MOCVD),33 pulsed laser deposition (PLD),34 hydrothermal synthesis,35 and microwave-assisted hydrothermal synthesis.36 Microwave-assisted hydrothermal synthesis provides many advantages over the others. Similarly to the conventional hydrothermal process, it is inexpensive, does not necessitate any sophisticated equipment, and operates at low temperature while providing good stoichiometry control.37 This technique accelerates the crystallization kinetics, and the reactions occur much faster than in conventional hydrothermal synthesis, which in turn reduces the energy consumption.38 To increase the growth speed and facilitate the nucleation of BiFeO3, mineralizers, such as potassium hydroxide (KOH), are often used.39−41 This mineralizer assists in the dissolution of the precursors and the crystallization of the film. However, the K+ introduced by the use of KOH creates an aliovalent substitution and acts as a dopant, thereby generating a substantial electrical leakage and a structural disorder that increases with the K+ content.42 In addition, a secondary product that is also ferroelectric at room temperature, potassium nitrate (KNO3), can be formed due to the reaction of KOH with the nitrite present in the precursors.43 Here, we report on a mineralizer-free microwave-assisted hydrothermal synthesis of BiFe1−xCrxO3. The only atom present during the reaction and crystal growth not to be part of the crystal structure is hydrogen. We optimize the deposition parameters by studying the effects of deposition time on the ferroelectric properties of the films. The resistive switching effect in our films is demonstrated through electrical measurements. Two different designs are compared; the first is a SrTiO3/BiFe1−xCrxO3/Pt structure, while the second is a Pt/BiFe1−xCrxO3/Pt configuration. The electronic band alignment is reconstructed for both designs.

2. EXPERIMENTAL SECTION Substrate Surface Preparation. The films were synthesized using a microwave-assisted hydrothermal process on conductive polycrystalline Si/SiO2/Al2O3/Pt substrates and on (111) oriented Nb (0.5 wt %) doped SrTiO3 single crystal substrates. Details regarding the experimental setup can be found elsewhere.44 Prior to deposition, the Si/SiO2/Al2O3/Pt substrates were ultrasonically rinsed in acetone, isopropanol, methanol, and deionized water before being annealed at 650 °C for 30 min under an oxygen flow (80 sccm) in a programmable furnace (MTI Corporation) to stabilize the Pt microstructure. The (111) oriented Nb-doped SrTiO3 substrates

Figure 1. (a) 1 μm × 1 μm topography maps, (b) 3 μm × 3 μm PFM phase images, and (c) local phase-hysteresis loops of BiFe1−xCrxO3 samples synthesized on Si/SiO2/Al2O3/Pt substrates for 1 and 2 cycles of microwave-irradiation, respectively. B



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Chemical Characterization. Confocal Raman spectroscopy was employed to characterize the chemical composition of the films. The Raman spectra were acquired using a Horiba system (473 nm solid state blue Cobolt 04-01 laser, Synapse Back-Illuminated Deep Depletion 1024 × 256 CCD from Horiba Scientific). The films’ composition was analyzed by X-ray photoelectron spectroscopy (XPS) using a VG Escalab 220i XL (Al Kα source, 15 kV, 20 mA, normal emission geometry, 20 eV pass energy). The adventitious hydrocarbon C 1s peak (285.00 ± 0.05 eV) was used to reference all the reported binding energies, and the Au 4f line (84.00 ± 0.05 eV) to calibrate the spectrometer energy scale.

3. RESULTS AND DISCUSSION Optimization of the Synthesis Parameters. To optimize the deposition parameters, the films were synthesized on conductive polycrystalline Si/SiO2/Al2O3/Pt substrates for different deposition times. The topography of the films studied by AFM is presented in Figure 1a. For both samples, the surface roughness is relatively low, with a value of 1.7−2.0 nm, reflecting the surface roughness of the platinized silicon substrate. The PFM maps (Figure 1b) reveal ferroelectricity already after 1 cycle of microwave irradiation (10 min at 120 W). The preferential spontaneous polarization faces downward, and the region poled with a positive voltage (blue square) is brighter while the negatively poled region (green square) reveals a light shadow. After 2 cycles of microwave radiation, the signal-to-noise ratio of the piezoelectric signal improves. The local hysteresis measurements (Figure 1c) attest to the ferroelectric nature of the films after 1 and 2 cycles of deposition. The coercive voltages are close to +4 V and −4 V after 1 and 2 cycles. This qualitative analysis reveals that the coercive field does not vary with the deposition time. We conclude that the reaction is stable after 2 cycles, i.e., 20 min of effective growth duration. Ferroelectric Analysis of the Films. The polycrystalline Si/SiO 2/Al2 O3 /Pt substrates are not well adapted for BiFe1−xCrxO3 thin-film synthesis. The large grains present on their surface greatly deteriorate the quality of the films, which could explain the weak ferroelectric effect observed. To improve the quality of the thin films, the deposition was repeated in the optimized conditions on conductive (111) oriented Nb-doped SrTiO3 substrates. The (111) substrate orientation was chosen to enhance ferroelectric switching in the film.45 The surface roughness of the sample thereby obtained is very smooth, with an rms of ∼0.2 nm on each terrace (Figure 2a). The atomic step-terraces of the substrates are still visible, with an average height of ∼0.22 nm, as expected for a (111) oriented Nb:SrTiO3 substrate (d111 = a√3 = 0.225 nm).19 This figure demonstrates a Frank−van der Merwe or layer-by-layer growth which occurs when the smallest nucleus grows in 2D, resulting in the formation of uniform planar sheets.46 The PFM phase image, presented in Figure 2b, reveals that the polarization homogeneously shifts by 180° between the two regions written with opposite voltages, and states of downward (yellow-brown regions, blue square) and upward (black regions, green square) polarizations can be observed. The pristine state indicates a downward preferential polarization. While the positive voltage was applied on a square shaped region, the positively poled area has rounded corners, as a result of a minimization of domain wall energy under creep conditions.47 The local hysteresis curve (Figure 2c) further confirms the ferroelectric nature of the thin film. Coercive voltages of +4 V and −6 V are extracted, indicating that the coercive field is slightly higher than the one

Figure 2. Typical (a) 1 μm × 1 μm AFM image of the BiFe1−xCrxO3 ultrathin film, still exhibiting the terrace features of the SrTiO3:Nb substrate with an rms of ∼0.2 nm per terrace, including in the inset a profile showing the atomic step-terraces, (b) 3 μm × 3 μm PFM phase-map, (c) local phase-hysteresis loop, and (d) 3 μm × 3 μm amplitude map obtained from a BiFe1−xCrxO3 sample grown on a (111) oriented Nb-doped SrTiO3 substrate for 2 cycles of microwave irradiation.

obtained on the films deposited on Si/SiO2/Al2O3/Pt substrates. In the corresponding amplitude map (Figure 2d), the darker regions, corresponding to the minima in the amplitude, indicate the domain walls with an apparent width of ∼150 nm. In the negatively polarized region, the amplitude is larger than in the positively polarized area, similarly to the region in the virgin state, due to the downward polarization. In conclusion, the quality of the thin film (surface roughness and ferroelectricity) was greatly improved using an atomically flat (111) oriented Nb-doped SrTiO3 substrate, and a flat ferroelectric BiFe1−xCrxO3 ultrathin film was synthesized. Moreover, in the absence of a mineralizer, leakage currents and structural disorder are minimized, and the ferroelectricity measured here originates from the BiFe1−xCrxO3 film only and not from any potassium-related secondary product. The thickness of the film was determined by a transmission electron microscope (TEM) to be ∼2.5 nm (see the Supporting Information, section 2). The effect of hydrogen-related defects appears to be small, certainly hydrogen-related leakage currents do not interfere with polarization reversal. Composition of the Films. To study the composition of the ferroelectric BiFe1−xCrxO3 ultrathin film, Raman measurements were conducted on this sample. Due to the low thickness of the film, the Raman signal from the substrate strongly overshadowed the signal of the film, and no information regarding the BiFe1−xCrxO3 layers could be obtained by this method. However, simultaneously with the ultrathin film, particles of BiFe1−xCrxO3 are formed during the homogeneous nucleation in the liquid medium in microwave-assisted hydrothermal synthesis. Those particles, which sediment at the bottom of the hydrothermal reactor, undergo the same reaction as the films and thus can give information regarding the film composition. We performed Raman measurements on the powders formed during the synthesis of BiFe1−xCrxO3 ultrathin films on (111) oriented Nb-doped SrTiO3 substrates. Several peaks attributed to the BiFe1−xCrxO3 phase can be seen on the spectrum, presented in Figure 3. The peaks observed at 139, 210, and 435 cm−1 are attributed to the A1(LO) modes of C



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of those devices, the electronic band structure is established and the band alignment of both structures was determined using XPS prior to the current−voltage (I−V) analysis, using a method reported in literature.52 The valence band offset (VBO) at the Nb-SrTiO3/ BiFe1−xCrxO3 interface is given by VBO1 = (E Bi4f7/2 − VBM)BFCrO − (ESr3d5/2 − VBM)STO − (E Bi4f7/2 − ESr3d5/2)BFCrO/STO

(1)

Similarly, the VBO at the BiFe1−xCrxO3/Pt interface is given by VBO2 = (E Bi4f7/2 − E Pt4f7/2)BFCrO/Pt + (E Pt4f7/2 − E F)Pt − (E Bi4f7/2 − VBM)BFCrO

(2)

The VBM values were determined using the Kraut method.53 As depicted in Figure 4, the energy difference (ESr3d5/2 − VBM)STO = 130.82 ± 0.05 eV and (EPt4f7/2 − EF)Pt = 70.05 ± 0.05 eV were measured on clean Nb-doped (111) SrTiO3 and polycrystalline Si/SiO2/Al2O3/Pt substrates, respectively. In the same way, the line separations (EBi4f7/2 − VBM)BFCrO = 157.89 ± 0.05 eV, (EBi4f7/2 − ESr3d5/2)BFCrO/STO = 26.10 ± 0.05 eV, and (EBi4f7/2 − EPt4f7/2)BFCrO/Pt = 87.60 ± 0.05 eV were determined from the BiFe1−xCrxO3 films deposited on Nb-doped (111) SrTiO3 and polycrystalline Si/SiO2/Al2O3/Pt substrates, respectively. Inserting these values in eq 1 gives VBO1 = 0.97 ± 0.05 eV, and in eq 2, VBO2 = 0.21 ± 0.05 eV. Furthermore, the electric potential step height (φ) is given by

Figure 3. Typical Raman spectrum of the BiFe1−xCrxO3 powder after 2 cycles of microwave irradiation.

the Bi−O bond, while the peaks at 263, 305, 345, and 540 cm−1 correspond to the E(TO) modes of the Fe−O bonds.48 The remaining peak, located at ∼825 cm−1, is related to the Cr−O bond.49,50 No other phase related to Fe2O3, Bi2O3, or any other Bi compound was observed. Furthermore, the structural properties of the films were investigated by X-ray diffraction (see the Supporting Information, section 3), and the BiFe1−xCrxO3 (222) peak was observed, suggesting at least partial crystallinity in our films.51 BiFe1−xCrxO3 Resistive Switching. Two different series of devices were fabricated using BiFe1−xCrxO3 deposited under the optimized conditions described above (2 cycles, 0.002 M). The bottom electrode is different between both series. The first design is the Nb-SrTiO3/BiFe1−xCrxO3/Pt device, where a Nbdoped (111) SrTiO3 substrate was used as the bottom electrode (BE), while, in the second design, the Pt/ BiFe1−xCrxO3/Pt device, a polycrystalline Si/SiO2/Al2O3/Pt substrate was employed. In both cases, Pt was used as the top electrodes (TE). To understand the charge transport behavior

φ = Eg − VBO

(3)

where Eg is the band gap. Due to the low Cr content in the films, Eg was taken to be equal to the band gap of BiFeO3 (2.8 eV54). The potential step barriers were determined to be 1.83 ± 0.07 eV at the Nb:SrTiO3/BiFe1−xCrxO3 interface, and 2.59 ± 0.07 eV at the BiFe1−xCrxO3/Pt interface. The calculated values correspond to the potential barriers for a film with spontaneous polarization, in this case, downward as shown by the PFM

Figure 4. XPS spectra acquired on (a) a Pt reference substrate, (b) BiFe1−xCrxO3/Pt, (c) BiFe1−xCrxO3 reference film, (d) a Nb-SrTiO3 reference substrate, and (e) BiFe1−xCrxO3/SrTiO3. D



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and the conductive substrate, acting as a bottom electrode, while the generated current was recorded. A typical I−V curve is illustrated in Figure 6b. A hysteretic behavior is observed, as indicated by the two different resistance states. The initial state is the HRS (“OFF” state), linked to the upward polarization. When the negative threshold voltage is reached, the resistance switches to the LRS state (“ON’” state), linked to the downward polarization. For a downward polarization, toward the Nb-doped SrTiO3 bottom electrode, the negative screening charges will build up in the electrode at the Nb-doped SrTiO3/ BiFe1−xCrxO3, bending down the conduction band in the process. As a result, the potential barrier height will decrease and current will flow. This corresponds to the LRS (“ON” state). Inversely, in the case of an upward polarization (HRS, “OFF” state), toward the Pt TE, the conduction band will bend upward due to a positive screening charges buildup at the Nbdoped SrTiO3/BiFe1−xCrxO3 interface, thereby increasing the potential barrier height and preventing the current to flow (see the Supporting Information, section 5).11,12 The I−V curve can therefore be separated into four different segments, as illustrated in Figure 6b. As mentioned previously, in ultrathin ferroelectric (FE) films, current can flow across the potential barrier through different transport mechanisms.12 To identify the dominant mechanism for the “OFF” and “ON” states, the curves were analyzed as depicted in Figure 6c,d, respectively. Figure 6c, corresponding to segment 3 of the I−V curve, shows that, in the “ON” state, I ∝ e √V. Such a relation indicates that the dominant mechanism is thermionic

results. The reconstructed electronic band line-ups are illustrated in Figure 5.

Figure 5. Schematic diagrams of the electronic band line-up for (a) the Nb:SrTiO3/BiFe1−xCrxO3/Pt, and (b) the Pt/BiFe1−xCrxO3/Pt designs.

To study the resistive switching in our ferroelectric material, we performed C-AFM measurements on the BiFe1−xCrxO3 ultrathin films deposited on (111) Nb-doped SrTiO3 in the configuration depicted in Figure 6a. ∼45 nm thick Pt electrodes with a diameter of ∼300 μm were deposited on the sample’s surface by radio frequency magnetron sputtering using a shadow mask (see the Supporting Information, section 4). A conductive Pt-Ir coated Si tip was placed in contact with one of these Pt electrodes, and a voltage was applied between the tip

Figure 6. (a) Schematic of the C-AFM configuration used for I−V characterization, and (b) typical I−V curve acquired by this method on BiFe1−xCrxO3 ultrathin films deposited on (111) Nb-doped SrTiO3, showing the “OFF” (segments 1 and 4) and “ON” (segments 2 and 3) states. The current plot of segments 3, describing thermionic emission, and 4, showing a diode behavior, are shown in (c) and (d), respectively. E



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Figure 7. (a) Typical I−V curve measured on a Pt/BiFe1−xCrxO3/Pt device showing the “ON” (segments 1 and 4) and “OFF” (segments 2 and 3) states, and (b) current plot of segment 4 indicating thermionic emission.

parasitic, yet different, interfacial layers at both film−electrode interfaces. This resulted in an electrostatic potential barrier profile with a trapezoidal shape. The symmetry breakdown can thus be attributed to different interface states,57 as depicted in Figure 8, where the downward and upward polarizations lead to “ON” and “OFF” states, respectively.58

emission.55 In this case, the charge carriers overcome the potential barrier through thermal activation, allowing the current to flow through the barrier according to ⎡ q ⎛ ⎜⎜φ − J = A*T 2 exp⎢ − ⎢⎣ kBT ⎝ B

qV /d 4πε0εr

⎞⎤ ⎟⎟⎥ ⎠⎥⎦

(4)

where A* is the effective Richardson’s constant, T is the absolute temperature, kB is the Boltzmann constant, q is the electronic charge, φB is the potential barrier height, V is the voltage, d is the BiFe1−xCrxO3 layer thickness, ε0 is the permittivity in vacuum, and εr is the optical dielectric constant.55,56 Figure 6d, corresponding to segment 4 of the I−V curve, reveals an exponential relation between the current and the voltage in the “OFF” state, indicative of a Schottky diode, where a negligible current flows through the barrier until the coercive voltage is reached, at ∼ −4 V and ∼ +5 V. The resistance values for the “ON” and “OFF” states were determined at a read voltage of ∼0.3 V and a resistance change of 1 order of magnitude was obtained, with an ROFF/RON ratio of ∼20. The voltage values indicated here correspond to the voltage applied on the tip. A typical I−V curve obtained by C-AFM on a Pt/ BiFe1−xCrxO3/Pt device is illustrated in Figure 7a. Similarly to the device fabricated with the Nb-doped SrTiO3 BE, the “ON” and “OFF” states can be observed. Plotting the current in the ON state using a logarithmic scale as a function of √V (Figure 7b) reveals that the electrons travel across the potential barrier by thermionic emission, as was previously shown for the Nb:SrTiO3/BiFe1−xCrxO3/Pt device. In the same way as for the SrTiO3/BiFe1−xCrxO3/Pt design, the change in resistivity is caused by a change in the polarization direction. However, in the Pt/BiFe1−xCrxO3/Pt design, at thermodynamic equilibrium, the same efficient screening occurs at both interfaces, resulting in a rectangular barrier. In such a configuration, no current can flow through the barrier. Yet, Garcia et al. state that the asymmetry necessary for the current flow does not originate solely from different electrodes, but also from other parameters such as different interface terminations, an ultrathin dielectric layer at one of the interfaces, or pinned interface dipoles.12 In our case, the ferroelectric films were first synthesized with the microwave-assisted hydrothermal method. Then, the top Pt electrodes were deposited by RF magnetron sputtering. As a result, the vacuum was not maintained throughout the entire device fabrication process, which led to the formation of

Figure 8. Band diagram schematic of the Pt/BiFe1−xCrxO (BFCO)/Pt device in (a) the low resistance state (ON state) and (b) the high resistance state (OFF state).

This confirms that the conduction process does not vary when changing the electrodes. However, an ROFF/RON ratio of ∼1 × 105 was obtained here, much higher than the value measured on the Nb:SrTiO3/BiFe1−xCrxO3/Pt device. The difference in TER ratio between the two sets of devices is due to what has recently been referred to as the “giant electrode effect”.59 This effect was attributed to the microscopic interfacial effect,10 where the displacement of Fe3+ ions upon polarization switching modifies the structure at the electrode/ ferroelectric interface, which in turn modifies the barrier profile. As a result, the change in potential barrier height (Δφ) between the ON and OFF states varies with the material the electrode is made of, resulting in the different ROFF/RON ratios. Such a value (∼1 × 105) is comparable to filamentary switching in Re-RAM cells.60 This high resistance ratio is beneficial for memory applications since the two states can be easily distinguished by read-out electronics. Such a value is appropriate for 1 transistor1 resistance memory devices. The Nb:SrTiO3/BiFe1−xCrxO3/Pt device switched over 35 cycles under quasi-static conditions on a set of 10 different electrodes (see the Supporting Information, section 6). Pulsed F


Crystal Growth & Design measurements to establish fatigue and retention behavior are underway.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b01745. The Nb:SrTiO3(111) substrate surface preparation, TEM measurements giving the film thickness, X-ray diffraction characterization of the films, the deposition and structural properties of the Pt top electrodes, a band diagram schematic of the “ON” and “OFF” states of the Nb:SrTiO3/BiFe1−xCrxO3/Pt memory devices, and an endurance and reproducibly study of the Nb:SrTiO3/ BiFe1−xCrxO3/Pt memory devices (PDF)

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ACKNOWLEDGMENTS

G.K. is thankful for an FRQNT Postdoctoral scholarship and F.A.-V. for an individual FRQNT MELS PBEEE 1M scholarship and for the financial support of CONACyT (National Council of Science and Technology-Mexico). B.H. gratefully acknowledges financial support through the Bayerische Forschungsallianz. A.R. acknowledges the generous support through an NSERC discovery (RGPIN-2014-05024) and an NSERC strategic (STPGP 506953-17) grant.

4. CONCLUSION In summary, we present thermionic emission based resistive memory devices fabricated using microwave-assisted hydrothermal synthesis to deposit ultrathin ferroelectric BiFe1−xCrxO3 films, acting as the potential barrier. We present a recipe to synthesize ultrathin ferroelectric BiFe1−xCrxO3 films by microwave-assisted hydrothermal synthesis without mineralizer. For an optimized deposition time (20 min, 2 cycles), we obtained ferroelectric BiFe1−xCrxO3 films with a very low thickness of ∼2.5 nm on (111) oriented Nb-doped SrTiO3. Atomic step-terraces were observed on the surface of the ultrathin film by AFM, with a step height of ∼0.22 nm, indicating a highly ordered growth of the films. The composition of the sample was characterized by Raman spectroscopy, confirming the presence of BiFe1−xCrxO3. The absence of mineralizer reduces the leakage currents and structural disorder in the films, and ensures that the ferroelectric effect observed is indeed due to BiFe1−xCrxO3 and not to, e.g., KNO3. After depositing Pt top electrodes on the ultrathin ferroelectric BiFe1−xCrxO3 films, a C-AFM analysis was conducted on both Nb:SrTiO3/BiFe1−xCrxO3/Pt and Pt/ BiFe1−xCrxO3/Pt structures. In both cases, the I−V curves evidence a resistive switching effect, where thermionic emission is the dominant conduction mechanism in the LRS. The electronic band line-ups were reconstructed, and potential barrier values of φ1 = 1.83 eV and φ2 = 2.59 eV, and φ1 = φ2 = 2.59 eV were determined for the Nb:SrTiO3/BiFe1−xCrxO3/Pt and Pt/BiFe1−xCrxO3/Pt designs, respectively. Resistance ratios of ∼20 for the Nb:SrTiO3/BiFe1−xCrxO3/Pt device and ∼105 for the Pt/BiFe1−xCrxO3/Pt device were obtained, indicating a stronger effect in the design using similar electrodes. This study attests to the potential using thermionic emission for resistive memories, and of microwave-assisted hydrothermal synthesis to fabricate such devices.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Gitanjali Kolhatkar: 0000-0003-0848-4751 Fabian Ambriz-Vargas: 0000-0002-9069-9710 Notes

The authors declare no competing financial interest. G



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Thermionic Emission Based Resistive Memory with Ultrathin

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