Giant Electroresistive Ferroelectric Diode on 2DEG

Abstract

Manipulation of electrons in a solid through transmitting, storing, and switching is the fundamental basis for the microelectronic devices. Recently, the electroresistance effect in the ferroelectric capacitors has provided a novel way to modulate the electron transport by polarization reversal. Here, we demonstrate a giant electroresistive ferroelectric diode integrating a ferroelectric capacitor into two-dimensional electron gas (2DEG) at oxide interface. As a model system, we fabricate an epitaxial Au/Pb(Zr(0.2)Ti(0.8))O3/LaAlO3/SrTiO3 heterostructure, where 2DEG is formed at LaAlO3/SrTiO3 interface. This device functions as a two-terminal, non-volatile memory of 1 diode-1 resistor with a large I+/I- ratio (>10(8) at ± 6 V) and I(on)/I(off) ratio (>10(7)). This is attributed to not only Schottky barrier modulation at metal/ferroelectric interface by polarization reversal but also the field-effect metal-insulator transition of 2DEG. Moreover, using this heterostructure, we can demonstrate a memristive behavior for an artificial synapse memory, where the resistance can be continuously tuned by partial polarization switching, and the electrons are only unidirectionally transmitted. Beyond non-volatile memory and logic devices, our results will provide new opportunities to emerging electronic devices such as multifunctional nanoelectronics and neuromorphic electronics.

Figure 1. Schematic diagram of an electroresistive ferroelectric diode on 2DEG.

Ferroelectric capacitor working as an electroresistor and 2DEG working as a diode are combined in Au/PZT/LAO/STO heterostructure.

Figure 2. Structural characterization of the PZT/LAO/STO heterostructure.

a. Surface morphology of the PZT (50 nm)/LAO (4 nm)/STO obtained by AFM. b. Cross-sectional high-resolution TEM image with [100] zone axis. The scale bar is 2 nm. For better visibility, the TEM image is artificially colored. c. XRD out-of-plane θ-2θ scan for PZT/LAO/STO heterostructure. d. ϕ-scan of the 101 PZT and 101 STO diffraction peaks.

Figure 3. Electrical characterization of Au/PZT/LAO/STO heterostructure.

a. Current versus voltage measurement between 50μm diameter Au top electrode and 2DEG bottom electrode. The arrows and numbers represent the measurement sequence. The possible photocurrent effect in the LAO/STO interface was excluded by limiting light exposure to sample during electrical measurement. b. The current measurement under +1 V_read (red square) and −1 V_read (blue circle) as a function of the applied voltage between −6 V and +6 V. The insets show the voltage profile of the applied pulses. c. Repeatable switching characteristic. The bottom graph shows the applied voltage pulses on top electrode, and the top graph exhibits the concomitant current changes under +1 V_read. d. The non-volatile switching characteristic. The red and blue squares represent the on and off state, respectively. The insets show out-of-plane PFM phase images (top: phase image with the unit of degree, bottom: amplitude image with an arbitrary unit) after writing with −10 V_dc (dark) and +10V_dc (bright). Over 24 hours, the ferroelectric domains are stable, which is consistent with the stable on/off resistance states. The scale bar in the PFM image is 1 μm. e. Reproducible switching characteristic. On/off current levels are measured at 25 different cells.

Figure 4. Memristive functions simulated by Au/PZT/LAO/STO heterostructure.

a. Analogue-type resistance change. The bottom graph shows the profile of writing voltage pulses on top electrode, and the top graph exhibits the concomitant current changes under +1 V_read. b. Unidirectional signal transmission. The bottom graph shows the applied voltage pulses on top electrode, and the top graph exhibits the concomitant current changes under +1 V_read (red square) and –1 V_read (green circle).