Nucleation Control for High Quality Polycrystalline GexSi1-x Thin Film Transistors

The initial growth of polycrystalline Si (poly-Si) and poly-GexSi1-x on thin thermal oxide has been studied using a Rapid Thermal MultiProcessing (RTMP) reactor and in situ reflectometry. The deposition temperatures were in the range of 500-625°C and the total pressure between 0.1-1.2 mbar. Pure SiH4 and GeH4 were used as source gases with a constant flow ratio of 30:5 sccm. Based on the results of the nucleation experiments, Thin Film Transistors have been made in poly-Si and poly-GexSi1-x. Electrical characterization shows that the addition of Ge to the active thin film reduces the transconductance in the transistor, while the Low Temperature Low Pressure Chemical Vapor Deposition (LPCVD) oxide exhibits a lower breakdown voltage than the thermal oxide. Also, a low ION/IOFF current ratio is found due to the high leakage current of most TFTs. Still, some good TFTs have been made providing evidence that the fabrication of good quality poly-GexSi1-x Thin Film Transistors is possible.