† Corresponding author. E-mail:
Project supported by the National Key Research and Development Program of China (Grant No. 2017yfb0405600), the National Natural Science Foundation of China (Grant Nos. 61404091, 61274113, 61505144, 51502203, and 51502204), and the Natural Science Foundation of Tianjin City (Grant Nos. 17JCYBJC16100 and 17JCZDJC31700).
The impacts of HfOx inserting layer thickness on the electrical properties of the ZnO-based transparent resistance random access memory (TRRAM) device were investigated in this paper. The bipolar resistive switching behavior of a single ZnO film and bilayer HfOx/ZnO films as active layers for TRRAM devices was demonstrated. It was revealed that the bilayer TRRAM device with a 10-nm HfOx inserted layer had a more stable resistive switching behavior than other devices including the single layer device, as well as being forming free, and the transmittance was more than 80% in the visible region. For the HfOx/ZnO devices, the current conduction behavior was dominated by the space-charge-limited current mechanism in the low resistive state (LRS) and Schottky emission in the high resistive state (HRS), while the mechanism for single layer devices was controlled by ohmic conduction in the LRS and Poole–Frenkel emission in the HRS.
Resistive random access memory (RRAM) is considered as the best potential candidate for the next generation of non-volatile memory because of its simple structure, good scalability, fast write/read speed, and low power consumption.[1,2] With the emergence of transparent electronics, transparent RRAM (TRRAM) has many prospective applications in invisible electronics based on integration of wide-bandgap semiconductors. It has attracted many researchers to develop see-through memory units instead of traditional silicon-based data storage devices. Recently, many different materials, such as ZnO, TiO2, SiOx, HfO2, IGZO, AlN, and egg albumen, have been discovered as effective active layers for TRRAM.[3–9] Transition metal oxides have great advantages not only in compatibility with CMOS processes, but also in their thermal and chemical stability, as well as high transparency in the visible region. ZnO has been considered as an excellent active layer for RRAM due to good performance of resistive switching properties such as the coexistence of unipolar and bipolar switching behaviors, transparency, and flexible application. However, variation in critical switching parameters and power consumption remain challenges in single layer ZnO films for potential TRRAM applications.[3,10,11] Multilayering is one effective way to improve the resistive switching behavior.[12–17] In this paper, the single layer TRRAM device (SL-TRRAM) with ITO/ZnO/ITO structure and effects of inserting layer thickness of HfOx film on resistance switching properties of bilayer TRRAM device (BL-TRRAM) were investigated. In addition to being forming-free and having a lower reset current, the BL-TRRAM devices with 10-nm HfOx thickness had better uniformity compared to SL-TRRAM. Moreover, the average transmittance of the devices was above 80% in the visible range. The switching mechanism of the SL-TRRAM device in the low resistive state (LRS) and high resistive state (HRS) was consistent with ohmic conduction and Poole–Frenkel emission, while the BL-TRRAM was controlled by the space-charge-limited current (SCLC) and Schottky emission mechanism.
A 50-nm ZnO thin film was deposited by radio-frequency (RF) magnetron sputtering using a ZnO ceramic target on a commercial ITO-coated glass substrate in argon-oxygen (32:8) at 200 °C. For the bilayer TRRAM, HfOx films with different thicknesses (5 nm, 10 nm, and 15 nm) were sputtered on ZnO film in an ambient environment of Ar (36 sccm) and O2 (6 sccm). 150-nm ITO was deposited as the top electrode (TE) in an Ar environment with a metal mask 300 μm in diameter. Optical transmittance of the device was measured using a Lambda 750 UV/VIS spectrophotometer. The crystal structure and surface morphology of the ZnO film and HfOx film were analyzed by x-ray diffraction (XRD) and atomic force microscopy (AFM), respectively. The resistive switching characteristics of TRRAM devices were measured using an Agilent B1500 semiconductor parameter analyzer. The bias was applied on the TE while the bottom electrode (BE) was grounded.
As shown in Fig.
There were major obstacles in using the ZnO single layer for practical applications, including non-uniformity in the distribution of operation voltage and high operation current. Therefore, bilayer HfOx–ZnO TRRAM devices with three different thicknesses of inserting layer HfOx film were fabricated to improve the resistive switching behavior. The reset current decreased as the inserting layer thickness increased from 5 nm to 10 nm, while there was no significant change from 10 nm to 15 nm, as shown in Fig.
Because of its relatively outstanding performance, the device with 10-nm thick HfOx inserting film was further studied. As shown in Fig.
Next, the surface morphology of the ZnO and HfOx films and crystal structure of the HfOx film deposited on ITO/glass substrate were analyzed using AFM and XRD, respectively, the results of which are shown in Fig.
To clarify the conduction mechanisms of the devices, I–V curves for both LRS and HRS states were fitted to various electrical conduction mechanisms with double logarithmic curves. For the SL-TRRAM, as shown in Figs.
Due to the high Gibbs free energy of HfOx thin films, the oxygen ions in ZnO migrate into HfOx, making HfOx less oxygen-deficient but more insulative. Moreover, because of the broken chemical bond of ZnO, free oxygen ions could be generated with the increased electric-field, and then migrate into HfOx in the HRS, leading to enhanced conductivity of ZnO and insulation of HfOx. Hence, electron transportation from the bottom electrode to the top electrode must overcome the barrier at the HfOx/ZnO interface, as shown in Fig.
In summary, the performance of SL-TRRAM and BL-TRRM devices were demonstrated in this paper. Both of the devices had a high transmittance above 80% in the visible region. The BL-TRRAM device with 10-nm-HfOx thickness had more stable and uniform resistive switching behaviors than SL-TRRAM and was forming-free, while SL-TRRAM required a forming process. The conduction mechanisms of LRS and HRS are consistent with Ohmic conduction and Poole–Frenkel emission for SL-TRRAM, and SCLC and Schottky emission for BL-TRRAM, respectively. The results of this study indicate that the BL-TRRAM device has great potential for use in transparent memory devices.
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