Giant tunability of the two-dimensional electron gas at the interface of -Al 2 O 3 /SrTiO 3

Transcription

1 Supporting Information Giant tunability of the two-dimensional electron gas at the interface of -Al 2 O 3 /SrTiO 3 Wei Niu,, Yu Zhang, Yulin Gan, Dennis V. Christensen, Merlin V. Soosten, Eduardo J. Garcia-Suarez, Anders Riisager, Xuefeng Wang,* Yongbing Xu, Rong Zhang, Nini Pryds and Yunzhong Chen* Department of Energy Conversion and Storage, Technical University of Denmark, Risø Campus, 4000 Roskilde, Denmark National Laboratory of Solid State Microstructures, School of Electronic Science and Engineering, Nanjing University, Nanjing, China Center for Catalysis and Sustainable Chemistry, Department of Chemistry, Technical University of Denmark, 2800 Lyngby, Denmark To whom correspondence should be addressed: yunc@dtu.dk; xfwang@nju.edu.cn 1

2 Sample Growth and Device Fabrication The GAO thin films were grown by pulsed laser deposition (PLD) using a KrF laser ( =248nm) with a repetition rate of 1 Hz. Prior to the growth of GAO, an amorphous LMO (a-lmo) layer was deposited on TiO2-terminated STO (001) substrates at room temperature. 1, 2 Then using the optical lithography to pattern the a-lmo/sto structures, subsequently, the exposed a-lmo layers were removed by selective wet chemical etching. 2 With a lift-off procedure, the STO substrate is patterned into a Hall bar geometry after removing the residual photoresist. The patterned STO substrates were transferred to the PLD chamber again for the growth of GAO at high temperatures. The growth of GAO was performed at 650 o C with an oxygen background pressure of mbar. At the end of deposition, the film was cooled down at the same growth pressure without post-annealing. For the patterned sample, the film thickness was controlled by the growth rate, which was determined with the unpatterned samples by in-situ reflection high-energy electron diffraction (RHEED) oscillations. 2, 3 After the growth, a platinum plate was pasted on the LMO region as the side gate electrode (Supporting Information Figure S1). Finally, a drop of ionic liquid gel was placed on the surface of the patterned 2DEG channel to form the EDLT. Ionic liquid Gel Fabrication The ionic liquid gel was choose due to the high capacitance and easy controllability. 4 The electrolyte was formed by the gelation of a triblock copolymer poly(styrene-blockmethylmethacry-late-block-styrene) (PS-PMMA-PS) in an ionic liquid 1-ethyl-3- methylimidazolium-bis(trifluoromethanesulfonyl) amide (EMI-TFSI). A drop of ionic liquid gel was pasted over the device, covering both the 2DEG channel and the Pt coplanar side gate. 2

3 Transport Properties The transport properties of GAO/STO interfaces were measured in a CRYOGENIC cryogenfree measurement system, with the temperature ranging from 300K down to 2 K and magnetic field up to 16 T. Electrodes were bonded with aluminum wires by ultrasonic bonding equipment. During all the transport measurements, the applied currents were within 10 A. Special care was taken to avoid heating effect. The magnetic fields were applied perpendicular to the interface. For the IL-assisted gating measurements, ionic liquid is frozen below 180 K totally, so that the leak current, IL, is negligible compared with source current, IS, (Supporting Information Figure S2) which ensures the accuracy of the transport measurements. Before the measurements, the gate voltage was applied and kept 20 min for the redistribution of ions. During the whole cooling down process, the gate voltage kept a constant Vg. When changing the Vg, the EDLT device was heated up to 230 K to melt the ionic gel and lose the polarization and a new Vg was reset for another run of transport measurement. It should be noted that the EDLTs returned to the initial states after removing the gate voltage, which rules out the electrochemical reaction between the ionic liquid and channel and the possibilities of the electric-field-induced oxygen vacancy migration. 5 The negative gate voltages are confined within -1.5 V because the resistance become too insulating under higher negative voltage leading to a bad signal-to-noise ratio. 3

4 Pt Ionic Liquid Figure S1. An optical micrograph of the GAO/STO EDLT. The scale bar is 0.5 mm. 4

5 I S Current (A) 10-8 I L T (K) V g = 3V Figure S2. Source-drain current (IS) and leak current (IL) as a function of temperature at Vg= 3V. When the temperature is lower than 200 K, the leak current is smaller than 1 na, much lower than the source-drain current. 5

6 R (W) V +2V T (K) Figure S3. Temperature-dependent resistance at low temperature at Vg=2 V, 3 V, respectively and fitting results by Kondo effect. The resistance minimum can be described by a simple Kondo model 5, 6 : R(T) R0 at b R K (T/ T K) (S1) where R0 is the residual resistance, qt 2 and pt 5 are the contributions from electron-electron and electron-phonon interactions, respectively. Last term comes from Kondo effect, which we use an empirical from for the universal function, T K 2 R K T/T K = R K (T = 0)( T T )s K (S2) T K = T K /(2 1 s 1) 1 2 Thus the final reduced formula is: b 1 R(T) R 0 at RK,0( ) 1 s 2 1 (2 1)(T/ T ) the parameter s is fixed at for S=1/2 system. 5, 6 K s (S3) (S4) Table S1 Fitting results deduced by the Kondo model at Vg= 2V and 3V, respectively. R0 (W) a b RK (W) TK ( ) 2 V V

7 R xx (W/ ) R xy (W) V +3V Rxy Rxx fitting fitting (a) -300 (b) B (T) B (T) Figure S4. Two-band fitting of Rxx and Rxy. (a, b) Rxx and Rxy as a function of magnetic field at 2 K, where the applied gate voltage is 3V. In the two-band model, the Hall resistance and the longitudinal resistance can be expressed as 7, 8 : R H = R + R 0 R 1 + (B/B W ) 2 ; ρ xx = ρ + ρ 0 ρ 1 + (B/B W ) 2 (S4) where B= 0H, stands for the magnetic field, and the values at zero and infinite fields can be simplified as: R 0 = n 1μ n 2 μ 2 (n 1 μ 1 + n 2 μ 2 ) 2 ; R 1 = (n 1 +n 2 ) ρ 0 = 1 n 1 μ 1 + n 2 μ 2 ; ρ = n 1μ 2 + n 2 μ 1 (n 1 +n 2 ) 2 μ 1 μ 2 (S5) (S6) and the characteristic magnetic field scale: B W = n 1μ 1 + n 2 μ 2 (n 1 + n 2 )μ 1 μ 2 (S7) The analyzed data based on two-band model show a clear evidence that the nonlinear behavior is due to the filling of another sub-bands, dyz/dxz. Therefore, when the carrier density beyond nl, multi-bands appear across the Fermi level. 7

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