LowDark Current Mesa-Type AlGaN Flame Detectors

This study characterizes and reports on the fabrication process of AlGaN flame photodetectors with an Al0.1Ga0.9N/GaN superlattice structure. The AlGaN flame photodetectors exhibited a low dark current (∼ 1.17×10−10 A at bias of−5 V) and large rejection ratio of photocurrent (∼ 2.14×10−5 A at bias of−5 V) to dark current, which is greater than five orders of magnitude. Responsivity at 350 nm at a bias of −5 V was 0.194 A/W. Quantum efficiency, η, was 0.687 at a reverse bias of 5 V.


INTRODUCTION
Recently, the detector for blue/ultraviolet (UV) wavelengths has been studied widely.An effective UV detector should operate even in strong visible background light.Correspondingly, detector sensitivity in a UV region should be larger than that in the visible region.Ultraviolet detectors are practical at flame sensing, missile plume detection, and spaceto-space communications [1,2].Since flame luminescence in the UV spectrum is weak, high responsivity and low current noise are necessary to prevent misdetection.Also a large spectral selectivity is important to efficiently reject solar light [2].
The effectiveness of applying III-V nitride detectors for UV wavelengths has been demonstrated [2][3][4][5][6][7][8][9][10].By adjusting the aluminum fraction of Al x Ga 1−x N-based photodiodes, the band gap energy varies at 3.4-6.2eV and shifts the cutoff wavelength from 365 nm (x = 0) to 200 nm (x = 1) [1].However, the ratio of the UV light photocurrent to the visible light photocurrent is roughly 3-4 orders of magnitude or lower.Yeh et al. reported that AlGaN/GaN strained-layer superlattice (SLS) structure can increase acceptor ionization efficiency and hole concentration in the GaN p-i-n photodiode [10].This study describes the fabrication of and characterizes mesa-type Al 0.1 Ga 0.9 N flame photodetectors with an Al 0.1 Ga 0.9 N/GaN superlattice structure that has a low dark current and a visible-to-UV light rejection ratio of 6 orders of magnitude.
The surface of the p-type GaN layer was then partially etched using photolithography and inductively coupled plasma-reactive ion etching (ICP-RIE) technology until the n-type GaN layer was exposed, indicating that the mesa structure was formed.The SiN x layer was then evaporated as an insulation layer.An open was formed by photolithography to expose the surface of the p-GaN layer.The Ni/Au (50 Å/80 Å) transparent contact was evaporated onto the surface of the p-GaN using an electron-beam evaporator, and thermally annealed in ambient pure oxygen at 550 • C for 10 minutes to form the p-metal.Finally, the Ti/Al/Ti/Au (15 nm/50 nm/100 nm/1000 nm) contacts were formed simultaneously on the exposed n-type GaN layer as n-metal and a bonding pad, and on the Ni/Au transparent contact as a bonding pad. Figure 2  The current-voltage (I-V) characteristics of the AlGaN photodiode were measured using an HP 4155B semiconductor parameter analyzer.Responsivities were determined by a spectrum meter (Hitachi U-3010).All measurements were made at room temperature.

RESULTS AND DISCUSSION
Figure 3 shows plots of the I-V characteristics of photodiodes measured in the dark (dark current) and under illumination (photocurrent) at reverse biases from 0 V to 20 V. The photocurrent was approximately 2.14 × 10 −5 A and the dark current was approximately 1.17×10 −10 A at a bias of 5 V. Therefore, a large photocurrent-to-dark-current contrast ratio exceeded 5 orders of magnitude.The orders of magnitude were markedly higher than other structures in other studies due to the addition an unintentionally doped SLS structure between the p-GaN layer and Al 0.1 Ga 0.9 N absorption layer in this study.The AlGaN/GaN superlattice structure could change the orientation of threading dislocations, so that it resulted in a low dark current.Also, the superlattice structure would cause the incline of the band gap in high electrical field, enhanced the impact of the hole to grow many electron-hole pairs and increased the photocurrent.
Figure 4 presents a plot of responsivity as a function of wavelength for an AlGaN flame photodetector.High responsivity is evident at wavelengths of 360-320 nm at reverse biases of 3 V and 5 V.The responsivity at 350 nm at a where I ph is the photocurrent, P inc is the incident power, and η, q, c, h, and λ are quantum efficiency, electron charge, velocity of light, Planck constant, and light wavelength, respectively.Using (1), quantum efficiency, η, was 0.687 at a reverse bias of 5 V. On the other hand, the dashed line in Figure 4 is flame spectrum near the UV range of 200-420 nm.The portion of relative high intensity was at 030-400 nm, which matches the high responsivity region of the AlGaN photode- tector.Therefore, the AlGaN photodetector can be used for flame detection under strong visible background light.

CONCLUSIONS
In summary, AlGaN p-i-n photodiodes grown by MOCVD technology are characterized.The dark current and photocurrent of AlGaN p-i-n photodetectors were 1.17×10 −10 A and 2.14 × 10 −5 A at a bias of −5 V, and the photocurrent rejection ratio was 6 orders of magnitude.Responsivity and quantum efficiency, η, at 350 nm at a bias of −5 V were 0.194 A/W and 0.687, respectively.The portion of relatively high intensity in the flame spectrum was at 300-400 nm, which matches the high-responsivity region of the AlGaN photodetector.Therefore, the AlGaN photodetector can be used for flame detection under a strong visible background light.

Figure 3 :Figure 4 :
Figure 3: Dark and illuminated (λ = 350 nm) I-V characteristics of AlGaN flame photodetectors at reverse biased from 0 V to 20 V.