EFFECTS OF GE ON MATERIAL AND ELECTRICAL-PROPERTIES OF POLYCRYSTALLINE SI1-XGEX FOR THIN-FILM TRANSISTORS

Si1-xGex films deposited by very low pressure chemical vapor deposition (VLPCVD), with and without plasma enhancement, and by low-pressure CVD (LPCVD) were studied to explore the effects of Ge on various properties of poly-Si1-xGex films. As the Ge content increases, growth rates increase for thermal and plasma-deposited polycrystalline and amorphous films. The transition temperature for poly-to-amorphous deposition is lower, and the crystallization of amorphous films is faster. Enlarged grain sizes are achieved with Ge in films grown thermally, with plasma enhancement, and by solid-phase crystallization, with grain sizes as large as 1.3 mu m in films of 1000 Angstrom thickness. Increased n-type and p-type Hall mobilities and decreased p-type resistivities are achieved as functions of Ge content in poly-Si1-xGex films with x less than or equal to 0.20. The Hall mobility in an n-type poly-Si0.80Ge0.20 film is 50 cm(2)/V-s, the highest value reported to date for poly-Si1-xGex materials. Poly-Si1-xGex thin-film transistors (TFTs) have been fabricated and evaluated, leading to a proposal of a poly-Si-capped Si1-xGex TFT structure.