Ferroelectric Tunnel Junction for Dense Cross-Point

Transcription

1 Supporting Information Ferroelectric Tunnel Junction for Dense Cross-Point Arrays Hong-Sub Lee, Wooje Han, Hee-Yoon Chung, Marcelo Rozenberg,, Kangsik Kim, Zonghoon Lee, Geun Young Yeom, and Hyung-Ho Park*, *corresponding author: Hyung-Ho Park, Department of Materials Science and Engineering, Yonsei University, Seodaemun-Ku, Seoul , Korea Laboratoire de Physique des Solides, CNRS-UMR 8502 Université Paris-Sud, Orsay 91405, France IFIBA-Conicet and Departamento de Física, FCEN, Universidad de Buenos Aires, Ciudad Universitaria Pabellón I, (1428) Buenos Aires, Argentina School of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan , Korea Department of Advanced Materials Science and Engineering and SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon, Kyunggi-do , Republic of Korea S-1

2 Table of contents A. Carrier transport mechanisms in metal/insulator/metal structure B. Perovskite manganite family Pr 0.98 Ca 0.02 MnO 3 and Ca 0.98 Pr 0.02 MnO 3 C. GI-WAXD of BaTiO 3 /Pt (111) reference and the X-ray photoelectron spectroscopy D. Piezoresponse force microscopy E. Simulation method for the number of N of cross point array A. Carrier transport mechanisms in metal/insulator/metal structure In general, the carrier transport mechanisms in metal/insulator/metal structure as Figure 1c considered by direct tunneling (DT), Fowler Nordheim tunneling (FNT), thermionic emission (TI) and the J DT, J FNT, and J TI can be written as shown below in Equations (S1), (S2), and (S3) respectively. 1 j DT = C exp[α{(φ B,2 ev 3 2 ) 2 (φb,1 + ev 3 2 ) 2 }] α 2 [ φ B,2 ev 2 φ B,1+ ev 2 ] 2 sinh [ 3eV α { φ 4 B,2 ev φ 2 B,1 + ev }] (S1), 2 where C = 4em e,ox 9π 2 ħ 3, α = 4d 2m e,ox 3ħ(φ B,1 +ev φ B,2 ), and m e,ox is the effective tunneling electron mass. d, φ B,1, and φ B,2 are the thickness of the potential barrier, the potential barrier height between insulator and electrodes 1 and 2, respectively. J DT passes through the rectangular barrier as shown in Figure 1c. 2 j FNT = e3 m e E 2 exp [ 8π 2m 3 e,ox φ 2 B ] (S2), 8πhm e,ox φ B 3he E S-2

3 basically, J FNT is the physical phenomenon to J DT that is also the function of the height and width of the potential barrier as shown in Equation (S2). However, J FNT only appears when the rectangular and/or trapezoidal potential barrier are tiled to a triangle shape by the applied electric field and J FNT exponentially increases with decreasing potential barrier width. Therefore, in the rectangular and/or trapezoidal potential barrier, the carrier transport mechanism transfers from J DT at a low voltage region to J FNT at a high voltage region. 3 j Schottky = A T 2 exp [ 1 (φ k B T B e3 E )] 4πε 0 ε ifl (S3), A** is the effective Richardson s constant, ε o is the permittivity of the vacuum, and ε ifl is the permittivity of the insulator. J TI indicates passing electrons over the potential barrier and the barrier height is reduced by image force lowering, called the Schottky effect. In Equation (S3), the current density can be described for sufficiently high voltages (approx. V >100 mv at room temperature, i.e., approximately 3k B T/e). 4 When we set the parameters as d=3.2 nm, φ B,1 =φ B,2 =1 ev, m e,ox = m e, ε ifl =10, A**=10 6 Am -2 K -2, and T=300 K, the current-voltage curve of J DT, J FNT, and J TI was indicated as Figure 1c. B. Perovskite manganite family Pr 0.98 Ca 0.02 MnO 3 and Ca 0.98 Pr 0.02 MnO 3 Figure S1a and b shows a schematic band diagram of the Mn 3d band of PMO and CMO, respectively. In the perovskite manganite family RE 3+ 1-xAE 2+ xmno 3 (RE: rare earth AE: alkaline earth), RE and AE outer electrons were transferred to the oxygen atom to complete the O 2p shell. 5-7 S-3

4 Figure S1. Schematic diagrams of Mn-O octahedron and Mn 3d band of (a) Pr 0.98 Ca 0.02 MnO 3 and (b) Ca 0.98 Pr 0.02 MnO 3, electrodes 1 and 2 with perovskite manganite family. Therefore, excess holes/electrons were primarily located in Mn 3d by stoichiometry x, which thus controls the Mn 3d filling. In the basic structure of the compound series Pr 1- xca x MnO 3, the Mn 3d e g band was singly occupied, with one electron per site, for x = 0, and it was the Mott insulator by on site coulomb repulsion U with Jahn Teller distortion as shown in Figure S1a. The completely depleted e g band as x=1 was the band insulator as shown in Figure S1b. This is consistent with the fact that both ends were insulators (Mott or band insulators, respectively for x=0 or 1), while the intermediate compounds were semiconductors. 5-9 Therefore, The PMO and CMO of this study were a hole-doped Mott insulator and an electron-doped band insulator. S-4

5 Current (A) Au/Ca doped PMO/Pt Au/Pr doped CMO/Pt Voltage (V) Figure S2. I-V characteristics of Au/ Pr 0.98 Ca 0.02 MnO 3 and Ca 0.98 Pr 0.02 MnO 3 / Pt structure. Figure S2 shows I-V characteristics of reference PMO and CMO thin film (thickness: 5 nm) using Au top and Pt bottom electrodes. The dot size of Au top electrode was 50 μm diameter. As shown in the Figure S2, the PMO and CMO films indicated ohmic behavior because they have smaller or almost same work function when compared with Au and Pt electrodes as shown in Figure 3 of the manuscript. As described in the Figure S1, PMO shows higher resistance than CMO though Ca doped PMO and Pr doped CMO have the same doping concentration (refer to Figure S4). C. GI-WAXD of BaTiO 3 /Pt (111) reference and the X-ray photoelectron spectroscopy S-5

6 Figure S3. GI-WAXD of BaTiO 3 /Pt (111) reference. The 2D intensity distribution (left), the horizontal (or out-of-plane) component of the scattering vector q y is plotted along the x-axis, and the vertical (or in-plane) component of the scattering vector q z is plotted along the y-axis. The observed diffraction peaks are addressed according to Miller indices (hkl). And the 1D diffraction intensity curve according to xy cut (red line). The incident angle was 0.06 o. Therefore BTO (111) plane which was formed to parallel with Pt (111) was not observed at vertical cut. And as the red line in the Figure S3, BTO (111) and Pt (111) planes were observed in same direction with different d-spacing. As observed dead layer in Figure 2f and g of the manuscript, BTO film on Pt (111) also had the dead layer as shown in Figure S3 (right). As shown in the 1D diffraction intensity curve of Figure S3, the intensity was observed between BTO (111) and Pt (111) peaks. S-6

7 Figure S4. The X-ray photoelectron spectroscopy for the dopant concentration using reference films as Pr 0.98 Ca 0.02 MnO 3 and Ca 0.98 Pr 0.02 MnO 3. The atomic percent of Pr and Ca dopant were confirmed to 0.5~0.6 at% as their stoichiometry (Pr 0.98 Ca 0.02 MnO 3 /Pt and Ca 0.98 Pr 0.02 MnO 3 ) Pt reference films which were measured by high energy resolution experiment settings on an XPS equipped (Thermo Scientific K-alpha) with a monochromatic Al K-alpha X-ray source. S-7

8 D. Piezoresponse force microscopy Figure S5. (a) Piezoresponse force microscopy (PFM) phase image of the surface region (5 5 μm 2 ) of BTO/CMO/Pt reference sample after downward and upward polarizations written with ±5 V. Hysteretic behavior of the PFM (b) amplitude and (c) phase signals. Ferroelectric properties of the BTO/CMO/Pt reference film was investigated at room temperature using the PFM. The local out-of-plane piezoresponse was measured on the bare surface of the reference film. Applying a constant voltage ±5 V to PFM tip and scanning the reference film surface, we poled different regions as shown in Figure S5a. Figure S5b and c showed a clear hysteresis behavior as both amplitude and phase (~180 difference) signals, respectively. E. Simulation method for the number of N of cross point array S-8

9 As shown in Figure 1a (a cross-point array circuit), in the worst condition (the selected HRS cell surrounded by unselected LRS cells), the relationship between the number of word lines N and readout margin (RM) can be simulated by the Kirchhoff equation S4, RM = V V pu (N) = R pu ([R read ] [ 2R read 2 ])+R N 1 pu R pu ([R read ] [ 2R read 2 ])+R N 1 pu (S4) where R pu is the resistance of the pull-up resistor that can be optimized from (R read *R read ) 1/2. 10,11 When we read the resistance of selected HRS cells (selected R HRS ), the (2R read /2)/(N-1) should be greater than the selected R HRS. If the (2R read /2)/(N-1) is less than the selected R HRS, the sensing margin is reduced as shown in Equation (S4). Therefore, V read is defined from the consideration of the RS ratio@v read with R HRS (V read )/R LRS (V read /2) to maximize the number of N while V write is fixed by the RS voltage (a greater R write /2 reduces the voltage drop in the write operation). This study defined the V read to -1.5 V, in which the voltage showed a maximized N. References (1) Pantel, D.; Alexe, M. Electroresistance Effects in Ferroelectric Tunnel Barriers. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, (2) Gruverman, A. Tunneling Electroresistance Effect in Ferroelectric Tunnel Junctions at the Nanoscale. Nano Lett. 2009, 9, (3) Fowler, R. H.; Nordheim, L. Electron Emission in Intense Electric Fields. Proc. R. Soc. London, Ser. A 1928, 119, S-9

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