Aluminum interacts with amorphous silicon (a-Si:H) at an annealing temperature beyond 170 degrees C. However, by increasing the phosphorus concentration in the a-Si:H alloys, the Si recrystallization rate at the Al/a-Si:H interface decreases appreciably even when annealed at 250 degrees C. As the phosphorus concentration is increased to a very high level, the recrystallization process will eventually be stopped. Applying this concept, the surface of the phosphorus doped a-Si:H is pretreated by PH3 plasma before the deposition of the aluminum, the annealing results show that this pretreatment step can perfectly protect the upper surface of a-Si:H even when annealed at 300 degrees C. This technology can be applied to the a-Si:H thin film transistor with the Al metal contact and the a-SiNx:H passivation layer and can be deposited at the higher temperature to achieve better quality.