First demonstration and performance of AlGaN based focal plane array for deep-UV imaging

We present several prototypes to extend the range of AlGaN focal plane arrays from near UV to deep UV range (200 nm - 4 nm). Arrays include 320x256 pixels with a pitch of 30 μm and are based on Schottky photodiodes. AlGaN is grown on a silicon substrate. After a flip-chip hybridization, silicon substrate is thinned and removed by dry etching. The tricky point is to maintain the membrane integrity. By using a honeycomb structure in the Si substrate, after hybridization, we were able to keep the membrane plane and rigid, avoid the crack expansion, and thus maintain the membrane integrity. The structure includes an Al .35 Ga .65 N active layer grown on a thick Al .55 Ga .45 N window layer, with a graded AlGaN layer in between. The high quality materials are grown by MBE. The Al .55 Ga .45 N window layer is also thinned by dry etching down to the gradual layer and desertion layer where a higher internal electric field takes place. The results show that the dry etching process doesn't affect the readout circuit properties. The dark current is negligible and non uniformity in etching slightly contributes into a constant offset. The measured noise factor, a bit more than 100 electrons rms, is due to reset noise in the integration capacitance and in other parasitic capacitances. With a peak response at 300 nm of 35%, the responsivity is 1% at 266 nm and in the deep UV range. The spectral responsivity measured on a synchrotron line at a wavelength of 2nm reaches more than 200% due to multiple photoexcitation.