Influence of nitrogen trifluoride and nitrogen plasma treatment on the formation of hillocks during aluminum induced crystallization of a-Si:H

Aluminum induced crystallization of hydrogenated amorphous silicon (a-Si:H) was studied. Al was evaporated on the a-Si:H films deposited in an ultrahigh vacuum plasma enhanced chemical vapor deposition system. These films were annealed at 300 °C for 30 min. X-ray diffraction revealed full crystallization of the a-Si:H film after annealing. The surface morphology (especially, the hillock formation) was studied using scanning electron microscopy on two types of substrates: silicon and borosilicate glass. Annealed films yield a very rough morphology due to a very large density of hillocks on the surface. The use of nitrogen and fluorine (from N₂ and NF₃ plasmas, respectively) as stuffing atoms to prevent the formation of hillocks at the surface is reported. This study shows that the surface of the samples treated with nitrogen has the same hillocks of the untreated samples for both silicon and glass substrate. Whereas fluorine treated samples show a smooth surface and the hillocks disappeared in comparison with untreated samples on glass substrates. For samples made on silicon substrates, the size of hillocks was reduced considerably relative to the untreated samples. All these results are presented and discussed on the basis of various parameters. A model is proposed for the low temperature aluminum assisted crystallization of a-Si:H.