High-performance polycrystalline SiGe thin-film transistors using Al₂O₃ gate insulators

The use of aluminum oxide as the gate insulator for low temperature (600 °C) polycrystalline SiGe thin-film transistors (TFT's) has been studied. The aluminum oxide was sputtered from a pure aluminum target using a reactive N₂O plasma. The composition of the deposited aluminum oxide was found to be almost stoichiometric (i.e., Al₂O₃), with a very small fraction of nitrogen incorporation. Even without any hydrogen passivation, good TFT performance was measured on devices with 50-nm-thick Al₂O₃ gate dielectric layers, Typically, a field effect mobility of 47 cm²/Vs, a threshold voltage of 3 V, a subthresbold slope of 0.44 V/decade, and an on/off ratio above 3 × 10⁵ at a drain voltage of 0.1 V can be obtained. These results indicate that the direct interface between the Al₂O₃ and the SiGe channel layer is sufficiently passivated to make Al₂O₃ a better alternative to grown or deposited SiO₂ for SiGe field effect devices.