  Owing to its wide (3.4 eV) and direct-tunable band gap, gallium nitride (GaN)
is an excellent material platform for UV photodetectors. GaN is also stable in
radiation-rich and high-temperature environments, which makes photodetectors
fabricated using this material useful for in-situ flame detection and
combustion monitoring. In this paper, we use a GaN photodetector to measure
ultraviolet (UV) emissions from a hybrid rocket motor igniter plume. The
normalized photocurrent-to-dark current ratio (NPDR) is a performance metric
which simultaneously captures the two desired characteristics of high
responsivity and low dark current. The NPDR of our device is record-high with a
value of 6 x 10$^{14}$ W$^{-1}$ and the UV-to-visible rejection ratio is 4 x
10$^6$. The photodetector shows operation at high temperatures (up to
250{\deg}C), with the NPDR still remaining above 10$^9$ W$^{-1}$ and the peak
wavelength shifting from 362 nm to 375 nm. The photodetector was placed at
three radial distances (3", 5.5", and 7") from the base of the igniter plume
and the oxidizer-to-fuel ratio (O2/CH4) was varied. The data demonstrates a
clear trend of increasing current (and thus intensity of plume emission) with
increasing fuel concentration and decreasing separation between the
photodetector and the plume. By treating the plume as a black body, and
calculating a radiative configuration factor corresponding to the geometry of
the plume and the detector, we calculated average plume temperatures at each of
the three oxidizer-to-fuel ratios. The estimated plume temperatures were
between 850 and 950 K for all three combustion conditions. The temperature is
roughly invariant for a fixed fuel concentration for the three tested
distances. These data demonstrate the functionality of GaN as a material
platform for use in harsh environment flame monitoring.
