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

The use of aluminum oxide as the gate insulator for low temperature (600/spl deg/C) polycrystalline SiGe thin-film transistors (TFTs) has been studied. The aluminum oxide was sputtered from a pure aluminum target using a reactive N/sub 2/O plasma. The composition of the deposited aluminum oxide was found to be almost stoichiometric (i.e., Al/sub 2/O/sub 3/), with a very small fraction of nitrogen incorporation. Even without any hydrogen passivation, good TFT performance was measured an devices with 50-nm-thick Al/sub 2/O/sub 3/ gate dielectric layers. Typically, a field effect mobility of 47 cm/sup 2//Vs, a threshold voltage of 3 V, a subthreshold slope of 0.44 V/decade, and an on/off ratio above 3/spl times/10/sup 5/ at a drain voltage of 0.1 V can be obtained. These results indicate that the direct interface between the Al/sub 2/O/sub 3/ and the SiGe channel layer is sufficiently passivated to make Al/sub 2/O/sub 3/ a better alternative to grown or deposited SiO/sub 2/ for SiGe field effect devices.