Impacts of HfO2/ZnO Stack-Structured Charge-Trap Layers Controlled by Atomic Layer Deposition on Nonvolatile Memory Characteristics of In-Ga-Zn-O Channel Charge-Trap Memory Thin-Film Transistors

Abstract

We fabricated the charge-trap memory thin film transistors (CTM-TFTs) using InGaZnO (IGZO) active channel and HfO2/ZnO stack-structured charge-trap layer (CTL). To investigate the effects of the number and thickness of HfO2 layers inserted between the ZnO within the stack structured CTLs on the device characteristics, 2-nm-thick HfO2 thin films were inlaid once, twice, and four times, and 4-nm-thick HfO2 layers were introduced twice between the ZnO layers. The CTM-TFTs using the stack structured CTLs with 4-nm-thick HfO2 layers showed good memory characteristics, including large memory window (MW) of 25 V and rapid program/erase (P/E) speed of 500 $\mu \text{s}$ because of high total trap density of HfO2 with a sufficient thickness to provide charge-trap centers. On the contrary, relatively narrow MW of 16 V and slower P/E speed of 100 ms were obtained for memory device using the stacked CTL with four HfO2 layers of 2 nm. The HfO2 layer with a thickness as thin as 2 nm was supposed to act as just dielectric films deactivating the trapping or migration of electron charges due to too thin film thickness. The gate-stack structures confirmed from STEM images suggested that the modulations in memory device characteristics with different CTL structures resulted from the variations in designs of stack structured CTLs when the interface qualities within the gate-stacks were well prepared. Moreover, the detailed fabrication conditions were found to be important control parameters to reproducibly obtain reliable memory device characteristics.