Dynamics of an Electrically Driven Phase Transition in Ca₂RuO₄ Thin Films: Nonequilibrium High‐Speed Resistive Switching in the Absence of an Abrupt Thermal Transition

Abstract

In Mott‐type resistive switching phenomena, which are based on the metal–insulator transition in strongly correlated materials, the presence of an abrupt temperature‐driven transition in the material is considered essential for achieving high‐speed and large‐resistance‐ratio switching. However, this means that the freedom of material/device design in applications is significantly reduced for this type of switching by the strict requirement of transition abruptness. Here, high‐speed, abrupt resistive switching with a switching time of 140 ns is demonstrated in epitaxial films of Ca2RuO4/LaAlO3 (001), which is a material with a nonthermal metal–insulator transition driven by current, despite the complete absence of an abrupt thermal transition in the resistivity–temperature characteristics. Highly smooth negative‐differential‐resistance behavior, very high cycling stability, and an endurance over 106 cycles are also demonstrated in the current–voltage and current–time characteristics, which confirm the nonstochastic nature of the abrupt switching. These results suggest that strict control of the resistivity–temperature characteristics is not necessarily required in a material with a nonthermal‐type metal–insulator transition to obtain high‐speed resistive switching because of the independence of the dynamics from those of the thermal transition, and this phenomenon potentially has important advantages in resistive switching applications.

Significant effects of epitaxial strain on the nonlinear transport properties in Ca2RuO4 thin films with the current-driven transition

“…While the current-induced metal-insulator transition of Ca 2 RuO 4 is generally hardly observed in epitaxial thin films grown by vacuum deposition methods such as pulsed laser deposition, [26][27][28][29][30] we have recently shown that the currentinduced transition becomes observable in nonvacuum-grown epitaxial films with increased resistivity of Ca 2 RuO 4 , which were grown by solid phase epitaxy at atmospheric pressure. 12,21) The Ca 2 RuO 4 thin films examined in this study were thus grown by the nonvacuum solid-phase epitaxy technique on various substrate materials, and the substrate effects on the crystal structures and nonlinear transport properties were investigated. In this study, 100 nm thick Ca 2 RuO 4 thin films were grown on single-crystal substrates of YAlO 3 (001), LaAlO 3 (001), LaSrGaO 4 (001), NdGaO 3 (110), (La,Sr)(Al,Ta)O 3 (LSAT) (100), and NdCaAlO 4 (100).…”

“…We first fabricated amorphous Ca 2 RuO x precursor thin films on the above six kinds of substrates by pulsed laser deposition, and then the solid-phase epitaxial growth of the Ca 2 RuO 4 thin films was conducted from the precursor films at a growth temperature of 1300 °C in an Ar (99%) + O 2 (1%) atmosphere with a pressure of 1.0 atm, as described in our previous papers. 12,21) Prior to precursor deposition, the YAlO 3 (001) and LaAlO 3 (001) substrates were etched with concentrated HCl solutions, and the other substrates were preannealed at 1300 °C (the same conditions as for the film growth) to form atomically flat step-and-terrace structures on their surfaces. [31][32][33] The crystal structures and surface morphologies of the Ca 2 RuO 4 thin films were analyzed by X-ray diffraction (XRD) and atomic force microscopy using a SmartLab (Rigaku), a D8 Discover (Bruker AXS Inc.) and a NanoCute (Hitachi High-Tech Co.).…”

“…The detailed configurations of the in-plane electrodes were described in our previous papers. 12,21) The current-voltage measurements of the Ca 2 RuO 4 thin films were conducted at stage temperatures of 8-590 K in a cryogenic probe station system (Nagase, GRAIL-20-305-6-LV) and a high-temperature probe station system using a semiconductor parameter analyzer (Keysight 4156C).…”

“…Materials with metal-insulator phase transitions have been considered particularly promising for device applications because of the highly reliable high-operation-speed conductivity changes in the materials driven by the transitions. 1,2) By applying electric fields or injecting currents, a variety of nonlinear transport properties have been demonstrated in the materials with metal-insulator transitions, [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] and important application functions have been derived from the transport nonlinearity. In principle, materials with metal-insulator phase transitions show sharp resistive switching 4,5,8,10,12,14,17,18,[20][21][22][23] and negative differential resistance (NDR) [3][4][5][6][7][8][9][10][11][13][14][15][16]…”

“…24) The layered orthorhombic perovskite Ca 2 RuO 4 is known as a representative platform for this class of metal-insulator transitions. 5,6,9,10,[12][13][14][15]21,25) In the currentvoltage characteristics of Ca 2 RuO 4 , clear nonlinear transport phenomena (both switching and NDR) have been observed based on the current-driven transition, 5,6,9,10,[12][13][14][15]21) and because of the nonthermal origin of the transition, the driving mechanisms have been indicated to be not predominated by thermal runaway due to Joule heating but by current injection itself, 9,14,21) unlike those in materials such as VO 2 . In the nonlinear transport phenomena, additionally, previous studies have demonstrated that the unit cell dimensions of Ca 2 RuO 4 progressively and continuously change with current injection (and with progression of the current-induced transition), originating from the strong electron-lattice coupling in the material.…”