The effect of top contact on ZnO write-once–read-many-times memory

1 Introduction Zinc oxide (ZnO) is a II–VI wide-band-gap compound semiconductor, which has promising applications in light-emitting diodes [1], field-effect transistors [2], laser diodes [3], solar cells [4], sensors [5], etc. due to its environmental friendliness, abundant availability in nature, highly evolved growth technologies, compatibility with metal–oxide–semiconductor technology, and suit-ability for fabrication of small-size devices [6, 7]. Recently, ZnO materials have been utilized to fabricate resistive random access memory (RRAM) devices [8–12]. It was found that different metal contacts have significant effect on the performance of RRAM [13]. At the same time, write-once– read-many-times memory (WORM) devices, in which the data storage is permanent, as required for archival storage of video images and for non-editable databases, have attracted a great deal of interest [14, 15]. Previously, our group fabricated a WORM device with ZnO film [16]. The OFF/ON state resistance ratio for the ZnO WORM device is over 10 4 and can be well sustained for more than 100 years and perfectly endure reading cycles of 10 8. The conducting filaments consisting of oxygen vacancies are responsible for the switching mechanism. In this Letter, the