Polycrystalline Si thin films and devices: I. Seed selection through ion channeling II. Thin-film transistors

Part I of this thesis investigates the ."seed selection through ion channeling" process, employed to modify the grain size and crystallographic texture of polycrystalline Si thin films deposited on SiO 2. In each of the three experiments, polycrystalline Si films prepared by the low-pressure chemical-vapor deposition technique were used as the starting material. Si self-implantation, at a fixed angle and energy (normal incidence and 200 keV, or 600 from normal incidence and 350 keV) but with variable doses (1-20x 1014 cn 2 ), was then used to selectively amorphize the material. Ideally, the implantation step should amorphize a major fraction of the film and preserve only those crystallites that are properly oriented for ion channeling. A low-temperature furnace anneal, at 6000C for 48 h in an N2 ambient, was then carried out to crystallize the film. Ideally, the crystallization should proceed due entirely to the growth of the surviving crystallites, and should exclude any effect due to spontaneous nucleation. The overall results obtained demonstrate that, by optimizing the implant dose, one can produce strong fiber-textured polycrystalline Si films on Si0 2. The production of restricted-fiber-textured films, on the other hand, will have to await the availability of more sophisticated implantation and annealing equipment. Part II of this thesis investigates the metal-oxide-semiconductor thin-film transistors fabricated on the modified polycrystalline Si films. For transistors fabricated with a maximum processing temperature of 600*C, with deposited gate oxide, it was shown that the grain-size enlargement (achieved through amorphization-crystallization) can lead to significant improvements in the device characteristics. For transistors fabricated with a maximum processing temperature of 800*C, with thermally grown gate oxide, it was shown that both the grain size and grain orientation can be important parameters in determining the device performance; for grain sizes of the order of 1-2 yim, a stronger {110} texture can, in fact, lead to better devices than a larger average grain size. These results demonstrate how the surface effects, in addition to the grain-boundary effects, can limit the performance of polycrystalline Si thin-film transistors. Thesis Supervisor: Rafael Reif Associate Professor Electrical Engineering and Computer Science