Spatial uniformity of responsivity for silicon, gallium nitride, germanium, and indium gallium arsenide photodiodes

For almost a decade, the National Institute of Standards and Technology (NI ST) has supplied to its customers calibrated photodiode standards and speci al tests of photodetectors for absolute spectral responsivity from 200 nm t o 1800 nm. During this time spatial responsivity measurements have been mad e on several dozen Hamamatsu silicon S1337-1010BQ photodiodes. We have foun d that the spatial responsivity changes with wavelength, sometimes signific antly as the wavelength approaches the bandgap. The most significant change s appear to be caused by defects in the photodiode material and are not app arent over most of the wavelength region where the photodiode operates. The change in spatial uniformity with wavelength can significantly contribute to measurement uncertainties. These measurements have been repeated and the spatial responsivities have remained constant over several years. Measurem ents have also been made on other types of Si, GaN, Ge, and InGaAs photodio des. The measurement equipment, method, and results are presented.