UC Davis

A novel method was used to epitaxially grow P-i-N germanium photodiode structures for next-generation imaging at the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory (LLNL). Semiconductor device simulations in Silvaco Atlas were used to find the minimum impurity concentration within the intrinsic region. Following this the photodiode structure was optimized to minimize the depletion bias without sacrificing temporal performance. The epitaxial wafers meanwhile were used to fabricate photodiodes of various intrinsic thicknesses to test the mass-attenuation coefficients of epitaxial germanium to ensure the external quantum efficiency of the fabricated devices matches those predicted by the massattenuation coefficients of germanium as measured by the National Institute of Standards and Technology (NIST) organization. Tests were performed at the Manson laboratory at LLNL from 1keV to 8keV energy x-rays, and at the Advanced Light Source (ALS) at the Lawrence Berkeley National Laboratory (LBNL) from 6keV to 28keV energy x-rays. Following this, temporal response tests were performed by evacuating 60ps x-rays from germanium photodiodes at the ALS. Quantum efficiency was found to match those predicted by NIST. Evacuation times were found to be under two nanoseconds of both fabricated germanium photodiodes and reference silicon photodiodes, predicting excellent evacuation time.