SWIR photodetector development at Fraunhofer IAF

Abstract

For surveillance and reconnaissance applications in the short-wave infrared (SWIR) spectral range, the imaging systems have to cope with usually very low photon flux densities. Thus, dark-current and noise characteristics of the focal plane array (FPA) are demanding. On the other hand, the challenge of detecting extremely low photocurrents can be mitigated by utilizing an internal gain as provided by avalanche photodiodes (APDs). Fraunhofer IAF has recently started the development of InGaAs-based SWIR detectors. We report on the current development status covering design considerations, epitaxy, process technology and electro-optical characterization. Detector structures based on both, classical InGaAs PIN homojunction diodes as well as InGaAs/InAlAs APDs in separated-absorption-grading-charge-and-multiplication layer heterostructures, have been grown by molecular beam epitaxy on InP. Diodes structures were fabricated with a dry-etch mesa process and a subsequent dielectric passivation of the mesa sidewalls. High-resolution FPAs with 640 x 512 pixels and a 15 μm pixel pitch based on PIN diodes have been assembled to a SWIR camera system in cooperation with AIM Infrarot-Module GmbH. Design variations, in particular for the APDs, were assisted by band-edge-profile simulations. APD test structures as well as fan-out hybrids have been characterized, revealing gain values larger than 300 at room temperature.