Investigation of the effects of interface carrier concentration on ZnO thin film transistors fabricated by atomic layer deposition

Abstract

Thin films of ZnO were deposited by atomic layer deposition (ALD) at different process temperatures and the resulting chemical and electrical characteristics were investigated. As the process temperature increased, the ZnO film exhibited an increase in carrier concentration from 1.3 × 1015 to 2.1 × 1019 cm−3, and a decrease in resistivity from 6.7 × 103 to 8.2 × 10−3 Ω cm. We utilized this temperature dependence of the electrical properties and fabricated thin film transistors (TFTs) at different temperatures with both single and double channel layers. In the ZnO-TFT with a single channel layer, the overall device performance of the ZnO-TFT, such as the Ion/Ioff ratio and subthreshold swing (SS), was degraded as the entire channel resistance decreased. In contrast, the ZnO-TFT with a double channel layer could control the turn-on voltage and threshold voltage by suppressing the increase in the off-current. The Ion/Ioff ratios were 8.6 × 105, 2.2 × 106 and 4.7 × 105 and the subthreshold swings exhibited 0.60 V/decade, 0.71 V/decade and 0.68 V/decade for the TFT with interface channel layers deposited at 120 °C, 140 °C and 160 °C, respectively. The saturation mobility slightly increased from 1.267 to 1.912 cm V−1s−1 as the process temperature of the interface channel layer increased.