Abstract-We report solar-blind Al Ga 1 N-based heterojunction p-i-n photodiodes with low dark current and high detectivity. After the p+ GaN cap layer was recess etched, measured dark current was below 3 fA for reverse bias values up to 6 V. The device responsivity increased with reverse bias and reached 0.11 A/W at 261 nm under 10-V reverse bias. The detectors exhibited a cutoff around 283 nm, and a visible rejection of four orders of magnitude at zero bias. Low dark current values led to a high differential resistance of 9.52 10 15 . The thermally limited detectivity of the devices was calculated as 4.9 10 14 cm Hz 1 2 W 1 .Index Terms-AlGaN, dark current, detectivity, heterostructure, high-performance, p-i-n photodiode.
SOLAR-BLIND detectors with long-wavelength cutoff around 280 nm have important applications including missile plume sensing, flame detection, chemical-biological agent sensing, and covert space-to-space communications [1]. With the advent in material growth of high-quality Al Ga N ternary alloys, AlGaN-based solar-blind photodetectors emerged as a potential alternative for the photomultiplier tube (PMT) and silicon-based solar-blind detector technology. They have the advantage of intrinsic solar-blindness, and therefore, do not need complex and costly filters. In addition, AlGaN-based solar-blind detectors can operate under harsh conditions due to their wide bandgap and robust material properties [2]. Several research groups have demonstrated high-performance solar-blind photodetectors using Al Ga N material systemDetectivity is an important detector performance parameter which gives the signal-to-noise performance of the device. For low noise detection, detectivity should be as high as possible. The typical detectivity of a cooled PMT is about cm Hz W around 300 nm [13]. A comparable detectivity performance ( cm Hz W at 275 nm) was reported recently with a solar-blind AlGaN-based back-illuminated p-i-n photodiode [14]. In this letter, we report the design, fabrication, and characterization of solar-blind AlGaN p-i-n photodiodes with record dark current and detectivity performance. The measured dark current was below 3 fA at 6-V re- verse bias and a PMT-exceeding solar-blind detectivity of cm Hz W at 267 nm was achieved. The p-i-n photodiode wafer was grown by metal-organic chemical vapor deposition on sapphire substrate. The detector structure was designed for front (p-side) illumination. The active absorption region of the photodiode was formed with a 100-nm-thick unintentionally doped Al Ga N layer which was sandwiched between a 250-nm-thick n+ GaN layer and a 10-nm-thick p-type-doped Al Ga N layer. To improve the p-ohmic contact quality, a 30-nm-thick p-type GaN cap layer was grown on top of p-Al Ga N layer. A five-step microwave compatible semiconductor fabrication process was utilized to complete the device fabrication [15]. In the first two steps, ohmic contacts were formed. A 250-nm deep dry-etch for n+ ohmic contact was done via CCl F -based reactive ion etching (RIE). This was follow...