**Chao-Feng Liu , Xin-Gui Tang \* , Lun-Quan Wang, Hui Tang , Yan-Ping Jiang, Qiu-Xiang Liu, Wen-Hua Li and Zhen-Hua Tang**

School of Physics & Optoelectric Engineering, Guangdong University of Technology, Guangzhou Higher Education Mega Center, Guangzhou 510006, China

**\*** Correspondence: xgtang@gdut.edu.cn; Tel.: +86-20-3932-2265

Received: 15 July 2019; Accepted: 2 August 2019; Published: 4 August 2019

**Abstract:** The resistive switching (RS) characteristics of flexible films deposited on mica substrates have rarely been reported upon, especially flexible HfO2 films. A novel flexible Au/HfO2/Pt/mica resistive random access memory device was prepared by a sol-gel process, and a Au/HfO2/Pt/Ti/SiO2/Si (100) device was also prepared for comparison. The HfO2 thin films were grown into the monoclinic phase by the proper annealing process at 700 ◦C, demonstrated by grazing-incidence X-ray diffraction patterns. The ratio of high/low resistance (off/on) reached 1000 and 50 for the two devices, respectively, being relatively stable for the former but not for the latter. The great difference in ratios for the two devices may have been caused by different concentrations of the oxygen defect obtained by the X-ray photoelectron spectroscopy spectra indicating composition and chemical state of the HfO2 thin films. The conduction mechanism was dominated by Ohm's law in the low resistance state, while in high resistance state, Ohmic conduction, space charge limited conduction (SCLC), and trap-filled SCLC conducted together.

**Keywords:** resistance switching; high/low resistance; oxygen defect; conduction mechanism
