Optical, Structural, and Resistive Switching Characteristics of Atomic-Layer-Deposited ZnO Films with Their Thickness Variation
Authors: Lee Taehoon , Jung Yongchan , Seong Sejong , Kim Seon Yong , Park In-Sung , Ahn Jin-Ho 
Published: February 2020

Resistive random access memory (RRAM) which has simple structure of metal-insulator-metal (MIM) resistor has been focused as a promising device to be applied for memory device due to its non-volatile characteristics and low power consumption. Especially, ZnO films have emerged for applications to transparent or semi-transparent devices in the range of visible light, owing to their wide band gap (3.3 eV) and high transmittance. The resistor with ZnO insulator shows the low-operation reset or set voltage below ±2 V. However, the crystallinity, conduction path, refractive index, energy band gap, and oxygen vacancy concentration of ZnO films vary with the film thickness. That is, as the optical, structural, and electrical properties of ZnO film depend on the thickness of the deposited film, the resistive switching characteristics of resistor made with ZnO such as operation voltage and on/off ratio also the function of its thickness.
In this study, we fabricated the ZnO insulators at lower temperature with atomic layer deposition (ALD) method and investigated their optical, structural, and electrical properties. The MIM resistor devices were also made with ZnO insulator with different thickness and their resistive switching characteristics were evaluated. The switching parameters were analyzed with the relation of material properties.
The MIM resistors with different thickness of ZnO films were fabricated on p-type Si substrates. First, Ru films of 100 nm were deposited by DC sputtering on the Si substrates at room temperature. Afterward, 20, 40, 60, 80, and 100 nm-thick ZnO insulators were deposited on Ru films by ALD technique at 150 °C with the use of diethylzinc and water. Top 100 nm-thick Al electrodes were formed on the insulator by DC sputtering and the resistor cells with 100 µm × 100 µm were patterned by lift-off lithography.
The thickness and band gap of each ZnO film were confirmed by spectroscopic ellipsometry. Band gap of ZnO film was increased from 3.0 eV to 3.15 eV with the increase of thickness from 20 nm to 60 nm. The crystallinity were also increased with the higher thickness of ZnO insulators, determined by XRD. The switching characteristics of Al/ZnO/Ru MIM resistors were obtained from the current-voltage (J-V) curves measured by a semiconductor parameter analyzer. Furthermore, as the ZnO film thicken, the switching parameters also changed to the higher values, that is, the higher reset and set voltages, and even higher on/off current ratio.
From the above results, it is concluded that the optical band gap of ZnO and resistive switching factors of the resistor with ZnO insulator are related to the thickness of a ZnO film. However, as the on/off current ratio and operation voltages are trade-off, good signal identification also causes the huge power consumption. Therefore, the optimized thickness of ZnO insulator is needed to operate good performance resistive switching memory.

Figure 1