High responsivity, low dark current ultraviolet photodetector based on AlGaN/GaN interdigitated transducer

An ultraviolet (UV) photodetector employing the two-dimensional electron gas (2DEG) formed at the AlGaN/GaN interface as an interdigitated transducer (IDT) is characterized under optical stimulus. The 2DEG-IDT photodetector exhibits a record high normalized photocurrent-to-dark current ratio (NPDR, $6\times10^{14}$). In addition, we observe a high responsivity ($7,800$ A/W) and ultraviolet-visible rejection-ratio ($10^{6}$), among the highest reported values for any GaN photodetector architecture. We propose a gain mechanism to explain the high responsivity of this device architecture, which corresponds to an internal gain of $26,000$. We argue that the valence band offset in the AlGaN/GaN heterostructure is essential in achieving this high responsivity, allowing for large gains without necessitating the presence of trap states, in contrast to common metal-semiconductor-metal (MSM) photodetector architectures. Our proposed gain mechanism is consistent with measurements of the scaling of gain with device channel width and incident power. In addition to high performance, this photodetector architecture has a simple two-step fabrication flow that is monolithically compatible with AlGaN/GaN high electron mobility transistor (HEMT) processing. This unique combination of low dark current, high responsivity and compatibility with HEMT processing is attractive for a variety of UV sensing applications

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.Comment: Accepted to IEEE Aerospace Conference 201