  The eXciton Franz-Keldysh (XFK) effect is observed in GaN p-n junction diodes
via the spectral variation of photocurrent responsivity data that redshift and
broaden with increasing reverse bias. Photocurrent spectra are quantitatively
fit over a broad photon energy range to an XFK model using only a single fit
parameter that determines the lineshape, the local bias ($V_{l}$), uniquely
determining the local electric field maximum and depletion widths. As expected,
the spectrally determined values of $V_{l}$ vary linearly with the applied bias
($V$) and reveal a large reduction in the local electric field due to
electrostatic non-uniformity. The built-in bias ($V_{bi}$) is estimated by
extrapolating $V_{l}$ at $V=0$, which compared with independent C-V
measurements indicates an overall $\pm$0.31 V accuracy of $V_{l}$. This
demonstrates sub-bandgap photocurrent spectroscopy as a local probe of electric
field in wide bandgap diodes that can be used to map out regions of device
breakdown (hot spots) for improving electrostatic design of high voltage
devices.
