AlGaN heterostructure solar-blind avalanche photodiodes (APDs) were fabricated on a double-polished AlN/sapphire template based on a separate absorption and multiplication (SAM) back-illuminated configuration. By employing AlGaN heterostructures with different Al compositions across the entire device, the SAM APD achieved an avalanche gain of over 1×105 at an operated reverse bias of 92 V and a low dark current of 0.5 nA at the onset point of breakdown. These excellent performances were attributed to the acceleration of holes by the polarization electric field with the same direction as the reverse bias and higher impact ionization coefficient of the low-Al-content Al0.2Ga0.8N in the multiplication region. However, the Al0.2Ga0.8N layer produced a photocurrent response in the out of the solar-blind band. To retain the solar-blind detecting characteristic, a periodic Si3N4/SiO2 photonic crystal was deposited on the back of the AlN/sapphire template as an optical filter. This significantly improved the solar-blind characteristic of the device.

To improve the solar-blind/visible-blind photocurrent response rejection ratio of solar-blind photodetectors, we designed and fabricated a high-performance SiO 2 /SiN x 1-D photonic crystal (PC) ultraviolet (UV) filter on (0 0 0 1) double-polished sapphire substrate. When depositing SiN x , we found that employing NH 3 as nitrogen precursor instead of N 2 can simultaneously improve the peak reflectivity of filter stopband in the designed visible-blind region and the transmissivity in the solar-blind region. Research shows that it is associated with the H atom concentration in SiN x and the generation of Si-O-N transition layer at SiO 2 /SiN x interface. Finally, we obtained a high-performance SiO 2 /SiN x PC UV filter with a stopband reflectivity over 90% from 285 to 345 nm and a transmissivity over 80% in the solar-blind region. The UV filter is also demonstrated to have a good effect in improving the solar-blind/visible-blind photocurrent response rejection ratio of a back-illuminated AlGaN photodetector by depositing it to the back of the detector.