Reversible insulator-metal transition of LaAlO₃/SrTiO₃ interface for nonvolatile memory

Abstract

We report a new type of memory device based on insulating LaAlO₃/SrTiO₃ (LAO/STO) hetero-interface. The microstructures of the LAO/STO interface are characterized by Cs-corrected scanning transmission electron microscopy, which reveals the element intermixing at the interface. The inhomogeneous element distribution may result in carrier localization, which is responsible for the insulating state. The insulating state of such interface can be converted to metallic state by light illumination and the metallic state maintains after light off due to giant persistent photoconductivity (PPC) effect. The on/off ratio between the PPC and the initial dark conductance is as large as 10⁵. The metallic state also can be converted back to insulating state by applying gate voltage. Reversible and reproducible resistive switching makes LAO/STO interface promising as a nonvolatile memory. Our results deepen the understanding of PPC phenomenon in LAO/STO, and pave the way for the development of all-oxide electronics integrating information storage devices.

Figure 1. Chemical mapping with atomic resolution of LaAlO₃/SrTiO₃ interface.

(a) High resolution STEM image of LAO/STO interface. The LAO and STO regions are clearly distinguished from each other in those regions far from interface (>~u.c.). (b) High-magnification STEM image of LAO/STO interface, showing the detailed microstructures close to the interface. (c) to (g) are atomically resolved chemical mapping of Al, La, Ti, Sr, and O respectively, obtained in the same area as (b). Scale bars, 1 nm.

Figure 2. Temperature dependent resistance of LAO/STO interface.

The samples show an insulating behavior in the dark, while changes to a metallic state under illumination and even light off 10 minutes later. Inset shows the schematic of device structure for photoconductivity measurement.

Figure 3. Photoelectric response of LAO/STO.

(a), (b) Temporal evolution of photocurrent excited by 325 nm laser at 300 K and 10 K, respectively. (c), (d) Photocurrent response with excitation by 514 nm laser at 300 K and 10 K, respectively. Insets are photoresponse of bare STO for comparison. The 325 nm and 514 nm laser powers are 17 μW and 3.7 mW, respectively. The illumination spot size is ~ 10 μm in diameter. Voltage bias is 0.2 V for all LAO/STO measurements, while 1 V and 5 V for bare STO with illumination of 325 nm and 514 nm lasers, respectively.

Figure 4. Decay behavior of resistance variation after light off.

(a) Normalized resistance variation as a function of time after switching off the illumination of 325 nm laser at different temperatures. The decay curves are well fitted by Kohlrausch stretched exponential expression △R ~ exp[-(t/τ)^β]. Here τ and β are decay time constant and exponent. (b) ln τ vs the reciprocal of temperature (1/T).

Figure 5. Memory effect of insulating LAO/STO interface.

(a) Revisable memory operation cycles. The 514 nm laser illumination was used for Write operation to produce the On state. The back gate voltage was used for Erase operation to return to the Off state. The source-drain bias voltage of 0.2 V was applied to perform the Read operation. (b) Voltage pulses applied to the back gate. Duration is ~ 2 seconds. The experiments were performed at 300 K.

Figure 6. Schematic energy band diagram showing mechanism of memory effect.

(a) Electrons are trapped in the potential well at the interface after light illumination. (b) The potential barrier is lowered by gate electric field, leading to photocarriers recombination.