DEFECTS IN AMORPHOUS AND SOLID-PHASE EPITAXIAL SILICON

The microstructural morphology of amorphous Si (a-Si) layers deposited in ultrahigh vacuum, as well as crystalline Si grown by solid phase e pitaxy (SPE), was studied as a function of Al doping and vapour beam i ncidence angle. The microstructure of the films was investigated using cross-section transmission electron microscopy. All a-Si layers have a columnar structure, with an average column width of less-than-or-equ al-to 5 nm. The direction of the columns abruptly changes with the cha nge of deposition direction and shows local column tilts and void form ation at substrate surface irregularities. These built-in defects in t he a-Si films also influence the defect structure in epitaxial Si film s grown by SPE. Voids are initially aligned along the column direction s and extra voids form owing to irregularities of the columnar structu re. Doping of amorphous Si with Al to 10(18)-10(20) cm-3 does not leav e detectable effects in the amorphous structure itself, but will incre ase the void density of the re-grown SPE Si layers. Furthermore, segre gation of Al resulting in metallic inclusions in the amorphous crystal line interface causes metal induced crystallization of Si at temperatu res far below the normal SPE regrowth temperature, thus preventing the formation of single crystalline silicon in a single-step process.