Illumination Effect on Bipolar Switching Properties of Gd:SiO2 RRAM Devices Using Transparent Indium Tin Oxide Electrode
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To discuss the optoelectronic effect on resistive random access memory (RRAM) devices, the bipolar switching properties and electron-hole pair generation behavior in the transparent indium tin oxide (ITO) electrode of Gd:SiO2 thin films under the ultraviolet (λ = 400 nm) and red-light (λ = 770 nm) illumination for high resistance state (HRS)/low resistance state (LRS) was observed and investigated. In dark environment, the Gd:SiO2 RRAM devices exhibited the ohmic conduction mechanism for LRS, exhibited the Schottky emission conduction and Poole-Frankel conduction mechanism for HRS. For light illumination effect, the operation current of the Gd:SiO2 RRAM devices for HRS/LRS was slightly increased. Finally, the electron-hole pair transport mechanism, switching conduction diagram, and energy band of the RRAM devices will be clearly demonstrated and explained.
KeywordsNonvolatile memory Illumination effect Gadolinium Silicon oxide RRAM
Magnetic random access memory (MRAM), ferroelectric random access memory (FeRAM), and phrase change memory (PCM) devices are indispensable to various nonvolatile electronic applications in portable electron devices [1, 2, 3, 4]. Because of the excellent compatibility integrated circuit (IC) processes, long retention cycles, low operation voltage, and low electric consumption, the various resistive random access memory (RRAM) devices are investigated and discussed in recent memory device search [5, 6, 7, 8, 9, 10]. Among these RRAM device applications, the different metal element-doped silicon dioxide thin films prepared by various physical vapor disposition methods are widely considered and fabricated [1, 2, 3, 4, 5, 6, 7, 8, 9, 10].
According to previous studies, the bipolar resistance switching and initial metallic filament forming properties of the various structure RRAM devices using indium tin oxide (ITO) electrode for the high resistance state (HRS) and low resistance state (LRS) are investigated for experimental details [5, 6, 7, 8, 9, 10, 11, 12]. Besides, the illumination effect induced the electron-hole pair generation in switching operation current of the RRAM devices for the transparent ITO electrode is not widely discussed.
In this study, the ITO/Gd:SiO2/TiN structure of the RRAM devices was prepared by gadolinium-doped SiO2 layer between of titanium nitride (TiN) and ITO electrode. In addition, the bipolar switching resistive properties of Gd:SiO2 RRAM devices for HRS/LRS affected by the ultraviolet (λ = 400 nm) and red-light (λ = 770 nm) illumination effect were also discussed later.
Results and Discussion
In Fig. 1(a), the typical I-V switching curves of the Gd:SiO2 thin film RRAM device was exhibited the bipolar switching behavior properties. After the initial electrical forming process in Fig. 1(b), the LRS/HRS states of the Gd:SiO2 RRAM device was reached and observed. To define reset process, the operation switching current of the devices was gradually decreased from LRS to HRS by sweeping the positive bias over the reset voltage. To avoid the failure and broken situation of RRAM devices, the compliance current was limited to 1 μA. For inverted bipolar switching resistive behaviors, the transmission electron in metallic filament path early captured by the lots of oxygen vacancy in ITO top electrode of Gd:SiO2 RRAM devices was proved and investigated in Fig. 1(a) .
To describe the physical mechanism for optoelectronic effect on ITO electrode of the RRAM devices, the electron-hole pair carrier generated in conduction mechanism and electron transport path diagram was explained in Figs. 2 and 3. In Fig. 2(a), the RRAM device for HRS was transferred from the Schottky emission mechanism to Poole-Frankel mechanism in illumination effect environment [9, 10, 11]. In Fig. 2(b, c), the electrons of initial metallic filament path in the Gd:SiO2 thin film RRAM devices jumped from the defect activation energy, induced the leakage current, and exhibited the Poole-Frankel mechanism in illumination environment.
In Fig. 3(a), the RRAM device for LRS was transferred from the Schottky emission mechanism to ohmic conduction mechanism in illumination environment. In Fig. 3(b), the RRAM devices exhibited the Schottky emission conduction for high applied voltage. The barrier height of oval-shaped depletion region in ITO thin films was formed by the oxygen-rich atoms surrounding tip metallic filament. In Fig. 3(c), the ohmic conduction mechanism was caused by lots of intrinsic carrier generation of electron transport behavior in metallic filament of Gd:SiO2 thin films.
In HRS, the oxygen ions return the TiN electrode and recombined the metallic filament tip in Gd:SiO2 thin films for high positive applied voltage in Fig. 5a. In Fig. 5b, the transmission electron of ITO electrode overcome the barrier height in Gd:SiO2 thin film region which was also found for the Schottky conduction mechanism. For continuing positive applied voltage, the electron was departed from the trap and exhibited the Poole-Frankel conduction mechanism in Fig. 5c.
For the ultraviolet (λ = 400 nm) and red-light (λ = 770 nm) illumination environment, the bipolar switching properties and conduction mechanism of Gd:SiO2 RRAM devices using transparent ITO electrode for HRS/LRS states were measured and investigated. Besides, the switching operation current for LRS/HRS was slightly increased by ultraviolet and red-light illumination effect. For the Schottky emission mechanism transferred to the Poole-Frankel mechanism in illumination environment for HRS, the leakage current of RRAM devices was caused by electron jump from the defect activation energy. For illumination environment effect in LRS, the Schottky emission mechanism transferred to ohmic conduction of the RRAM devices induced by lots of electron-hole pair generation was proved.
This work was performed at the National Science Council Core Facilities Laboratory for Nano-Science and Nano-Technology in the Kaohsiung-Pingtung area and was supported by the National Science Council of the Republic of China under Contract MOST. 104-2633-E-272-001 -MY2.
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