Gallium Nitride Photodetector Measurements of UV Emission from a Gaseous CH4/O2 Hybrid Rocket Igniter Plume

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.