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 (NIST) has supplied to its customers calibrated photodiode standards and special tests of photodetectors for absolute spectral responsivity from 200 nm to 1800 nm. During this time spatial responsivity measurements have been made on several dozen Hamamatsu silicon S1337-1010BQ photodiodes. We have found that the spatial responsivity changes with wavelength, sometimes significantly as the wavelength approaches the bandgap. The most significant changes appear to be caused by defects in the photodiode material and are not apparent 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. Measurements have also been made on other types of Si, GaN, Ge, and InGaAs photodiodes. The measurement equipment, method, and results are presented.