Resolution and contrast in near-field photocurrent imaging of defects on semiconductors

To understand the factors that determine the resolution and contrast in near-field photocurrent (NPC) imaging of electrically active defects on semiconductor surfaces, a two-dimensional carrier diffusion model with a localized generation term is employed. Numerically simulated images of photo-generated carriers are compared with experimental NPC images acquired using a near-field scanning optical microscope (NSOM). Good agreement in image lateral resolution, contrast, and feature sharpness is obtained. It is found that the NPC lateral resolution of dislocation defects is limited not by the minority carrier diffusion length in the defect-free region as previously believed, but rather by either the defect size or the NSOM tip aperture size, whichever is larger. The NPC contrast increases with larger defect size, smaller tip aperture size, longer diffusion length in the defect-free region, and shorter diffusion length inside the defect. By comparing the theoretical results to experimental NPC data, an estima...