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 x 10(15) to 2.1 x 10(19) cm(-3), and a decrease in resistivity from 6.7 x 10(3) to 8.2 x 10(-3) Omega 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 I-on/I-off 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 x 10(5), 2.2 x 10(6) and 4.7 x 10(5) 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 degrees C, 140 degrees C and 160 degrees C, respectively. The saturation mobility slightly increased from 1.267 to 1.912 cm V-1 s(-1) as the process temperature of the interface channel layer increased.