Stress and dopant activation in solid phase crystalized Si films

摘要

Defect creation mechanisms during solid phase crystallization (SPC) of Si thin films were investigated with PECVD amorphous precursor samples produced with various deposition temperatures and thicknesses. These precursor films were implanted withdopant and then crystallized to obtain both SPC and dopant activation. The doping efficiency was found to decrease with the tensile stress level as measured by Raman shift. The stress shows a decrease as the precursor deposition temperature and thicknessare lowered. Furthermore, a lower level of stress is induced by rapid thermal annealing when the annealing temperature is high enough to soften the glass substrate on which the films were deposited. We show that by control of stress during the SPC step,intragrain defect density can be lowered and electronic quality of the resulting polycrystalline Si films can be improved. Based on these observations, we propose the following tentative model to explain the defect creation: during SPC, tensile stressevolution is considered to result from the volumetric contraction of Si film when it transforms from the amorphous to crystalline phase. This contraction is retarded by the substrate, which imposes a tensile stress on the film. A high level of stressleads to formation of structural defects inside the grains of the resulting polycrystalline material. These defects trap carriers or complex with the dopant reducing doping efficiency.