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 to make UV photo detectors. GaN is also stable in radiation-rich and high-temperature environments, which makes photo detectors fabricated using this material useful for in-situ flame detection and combustion monitoring. In this paper, we use a GaN photo detector to measure ultraviolet (UV) emissions from a hybrid rocket motor igniter plume. The GaN photo detector, built at the Stanford Nanofabrication Facility, has 5 μm wide regions of AlGaN/GaN two-dimensional electron gas (2DEG)electrodes spaced by intrinsic GaN channels. In most applications, the ideal photodetector would exhibit a high responsivity to maximize the signal, in addition to a low dark current to minimize quiescent power. A performance metric which simultaneously captures these two values is the normalized photocurrent-to-dark current ratio (NPDR), defined as the ratio of responsivity to dark current, with units of W^-1. The NPDR of our device is record-high with a value of 6 × 10¹⁴ W^-1 and the UV-to-visible rejection ratio is 4 × 10⁶. The high rejection ratio is essential as it eliminates cross-sensitivity of the detector to visible light. The spectral response can be modeled as a rectangular window with a peak responsivity of 7,800 AW^-1 at 362 nm and a bandwidth of 16 nm. The photo detector shows operation at high temperatures (up to 250°C). The NPDR still remains above 10⁹ W^-1 at the higher temperatures, and the peak wavelength shifts from 362 nm to 375 nm at 250°C. The photo detector 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 to alter the size and strength of the plume. The current measured from the device was proportional to the intensity of the emission from the plume. The data demonstrates a clear trend of increasing current with increasing fuel concentration. Further, the current decreases with larger separation between the photo detector and the plume. A calibration curve constructed from the responsivity measurements taken over four orders of magnitude was used to convert the current into incident optical power. 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.

1. Introduction

Ultraviolet (UV) photo detectors have many diverse uses, such as chemical analysis for environmental applications, communication between satellites, UV astronomy, flame detection for fire alarms, and combustion monitoring [1], [2]. The parameters that determine photo detector performance include signal-to-noise ratio, response time, dark current, and responsivity [2], [3]. Responsivity is the ratio of the photocurrent to the incident optical power. The responsivity divided by the dark current is known as the normalized photocurrent-to-dark current ratio (NPDR), which incorporates the parameters that govern a large signal amplitude and low quiescent power in a single performance metric [4]–[6]. Another important metric is the UV-to-visible rejection ratio, which describes the cross-sensitivity of the detector to visible light.