High-performance ZnO thin film transistors with sputtering SiO_2/Ta_2O_5/SiO_2 multilayer gate dielectric

摘要

A high-performance ZnO thin film transistor ZnO-TFT with SiO_2/Ta_2O_5/SiO_2 (STS) multilayer gate insulator is fabricated by sputtering at room temperature. Compared to ZnO-TFTs with sputtering SiO_2 gate insulator, its electrical characteristics are significantly improved, such as the field effect mobility enhanced from 11.2 to 52.4 cm~2/V s, threshold voltage decreased from 4.2 to 2 V, and sub-threshold swing improved from 0.61 to 0.28 V/dec. The improvements are attributed to the high gate capacitance (from 50 to 150 nF/cm~2) as well as nice surface morphology by using dielectric with high k Ta_2O_5 sandwiched by SiO_2 layers. The capacitance-voltage characteristic of a metal-insulator-semiconductor capacitor with the structure of Indium Tin Oxide/ STS/ZnO/Al was investigated and the trap charges at the interface or bulk is evaluated to be 2.24 × 10~(12) cm~(-2). From the slope of C~(-2) versus gate voltage, the doping density N_D of ZnO is estimated to be 1.49 × 10~(16) cm~(-3).