Mechanism of Germanium-Induced Perimeter Crystallization of Amorphous Silicon

Abstract

We report a study aimed at highlighting the mechanism of a new amorphous silicon crystallization phenomenon that originates from the perimeter of a germanium layer during low-temperature annealing ͑500°C͒. Results are reported on doped and undoped amorphous silicon films, with thicknesses in the range 40-200 nm, annealed at a temperature of 500 or 550°C. A comparison is made of crystallization arising from Ge and SiGe layers and the role of damage from a high-dose fluorine implant is investigated. Plan-view scanning electron microscope images show that perimeter crystallization is only present in amorphous silicon films with thicknesses ഛ100 nm, and that the crystallization width increases with decreasing film thickness and increasing doping level. Cross-sectional scanning electron microscope images show that the perimeter crystallization originates from grains at the bottom of the amorphous silicon film. The perimeter crystallization phenomenon disappears when the amorphous silicon is implanted with fluorine and when an Si 80% Ge 20% layer is employed instead of germanium. The perimeter crystallization is due to the formation of large grains as a result of an increased growth rate of pre-existing grains and this is attributed to the strain generated by the thermal expansion of the germanium layer during anneal. The crystallization of amorphous silicon ͑␣-Si͒ has been intensively investigated for application in active-matrix flat-panel displays and 3D electronics.1-10 Several techniques have been studied for the fabrication of high-quality polycrystalline silicon ͑poly-Si͒ with large crystallite size, low grain-boundary defect density, and high carrier velocity. These include excimer laser crystallization ͑ELC͒, 1-3 solid-phase crystallization ͑SPC͒, 4,5 metal-induced crystallization ͑MIC͒, 6,7 and metal-induced lateral crystallization ͑MILC͒. [8][9][10] This latter approach has the advantage of giving good control over the location of the grain boundaries but the disadvantage of introducing metal contamination into the ␣-Si, with deleterious effects on transistor performance. 11Germanium has also been proposed as a seeding agent for the lateral crystallization of ␣-Si 12,13 because of its better compatibility with scaled complementary metal oxide semiconductor ͑CMOS͒ technology. The lower melting point of germanium than silicon was reported to reduce the incubation time for the crystallization of the underlying amorphous silicon film by the formation of a SiGe layer at the Ge-Si interface.12,13 Improved values of mobility were obtained for thin-film transistors fabricated in ␣-Si layers crystallized using a germanium seed. 12,13 In a previous article, we reported a new low-temperature ͑500°C͒ ␣-Si crystallization phenomenon that occurs at the perimeter of a Ge layer grown on ␣-Si through a window in a SiO 2 layer.14 This phenomenon was termed perimeter crystallization and the results suggested the presence of an additional crystallization mechanism in the early stages of the crystallization process. Perimeter c...