Scallop-shaped silicon nanowire structures with improved optical absorption compared to planar designs are studied to enhance NIR sensitivity and suppress crosstalk simultaneously. The influence of substrate thickness, guard-ring doping and DTI structures on lateral carrier diffusion is systematically investigated using TCAD simulations to assess the severity of crosstalk. The model is first validated against published data for silicon planar PDs. The results show that deep and highly doped guard-rings of the same type as the substrate effectively block lateral electron flow without compromising the external quantum efficiency in the case of thin substrates (10-25 mm). In contrast, for thick substrates (150-300 mm), guard-rings of the opposite doping type to the substrate exhibit superior crosstalk suppression by electrically collecting laterally diffusing carriers. DTI structures with comparable depths effectively suppress crosstalk only in thin substrates, while their effectiveness becomes limited for thick substrates due to the insufficient blockage of deep diffusion paths.