High Temperature GaN and AlGaN Photovoltaic Detectors for UV Sensing Applications
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ABSTRACT AlGaN photodiode detectors are grown on (0001) sapphire by RF atomic nitrogen plasma molecular beam epitaxy. Both individual detectors and 1 x 10 element arrays are fabricated. The individual detectors have active areas of 0.5 mm2 , 1.0 mm 2, and 2.0 mm2 . Individual elements in the lx10 detector arrays range in size from 250x250 Pm to 450x450 ýtm. The detectors are fabricated using a chlorine-based reactive ion etch (RIE) and refractory metal ohmic contacts. At room temperature, GaNp-i-n photovoltaic detectors show peak responsivity at 360 nm as high as 0.198 A/W, corresponding to an internal quantum efficiency of 85%. These detectors also exhibit five orders of magnitude of rejection for radiation longer than 500 nm. The electrical and spectral characteristics of these detectors are examined at elevated temperatures. The short wavelength UV responsivity remains fairly constant at elevated temperatures, while the peak responsivity actually increases with increasing temperature. The smooth surface morphology of heavily doped p-type material grown by MBE makes possible diode structures with a p-type bottom layer. The effect of the spectrally broader p-type material in the photodiode responsivity will be discussed.
INTRODUCTION GaN and its ternary alloys with aluminum, AlGaN, are extremely robust and chemically inert materials. The wide bandgap of this material system lends itself to the fabrication of efficient ultraviolet (UV) photodetectors which are capable of sensitive UV detection even in a high background of visible and infrared radiation1- 2. These qualities make the AIGaN material system ideally suited for sensing applications in high temperature, hostile environments such as encountered in combustion processes. The bandgap of AlGaN can be adjusted between 365 nm (3.4 eV) and 200 nm (6.2 eV). This allows photodetectors to be fabricated which have peak sensitivities tuned for UV emission bands from important combustion species such as hydroxyl radical (OH). The applicability of UV sensors for combustion monitoring is illustrated in Figure 1. This figure shows the optical emission spectrum from an acetylene/oxygen flame under different gas/oxygen ratios. In clean burning, oxygen rich flames, an intense UV band at
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Mat. Res. Soc. Symp. Proc. Vol. 482 01998 Materials Research Society
310-320 nm is observed from electronically excited OH radicals. As the gas/oxygen ratio is increased, the UV emission intensity goes down, and the emission intensity in the visible, due to hydrocarbon radical, goes up. Superimposed on these emission spectra is the spectral responsivity curve of a 10% Al AIGaN photovoltaic detector. This material has a maximum responsivity near the peak of the OH band emission and has very little responsivity in the visible and infrared. An AlGaN sensor could be used to monitor the OH radical UV component of flames. Because of its sensitivity to just the UV emission, the AlGaN flame sensor could be used to provide -fastON/OFF detection for flames even in very hot background environm
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