RESIST - Readout-ic for Extended Shortwave-infrared Imaging Staircase-apd Technology

Silicon APDs have been commercialized for visible and near infrared (NIR) applications. Fiber optical communication has also led to commercialization of single element short wave infrared (SWIR) APDs that can sense the 1.55mm laser wavelength commonly used in these applications. More recently, SWIR APD based focal plane arrays (FPA) are being commercialized for scanning and flash LiDAR (LADAR) receivers for autonomous driving. The 1.55mm band is relatively eye safe compared to the NIR band which has led to increased interest in these receivers. InGaAs, germanium and some other III-V alloy based APDs are becoming commercially available for imaging up to 1.6mm. DoD has been pursuing longer cut-off (2mm) device development as the lasers in this band (i) provide improved eye safety to soldiers in the field, (ii) are not detectable by commercial SWIR sensors, (iii) offer better solar background rejection, (iv) improved atmospheric penetration, and (v) other phenological advantages. Because of the above-mentioned range of applications and advantages over existing systems, DoD has a clear established need for 2mm cut-off APDs. However, the developments of such a device face the following problems: Due to the longer cut-off, the band-gap of these devices is narrow, leading to increased dark current. To overcome the dark current the APDs must be cooled making the systems more expensive, bulky, and power hungry. The APD gain mechanism has an increased statistical noise related to the gain mechanism. This noise is characterized by an excess noise factor. If this factor is high, the APDs cannot be operated at high gains as the excess noise limits the signal to noise ratio (SNR). HgCdTe (MCT) APDs have been demonstrated to have very low excess noise factor and a tunable bandgap to achieve extended SWIR cut-off but at cryogenic temperatures and at high cost. To overcome all of the above limitations, our team has developed a novel 2-micron cutoff linear mode APD with custom DARPA ROIC to handle multiple-return events in close succession, as well as providing broader band passive (or active) imaging of the scene. Even though this promising APD device has been demonstrated by our team, it now needs to be transitioned to a camera demonstration. As detailed in the proposal our team has all the key expertise required to demonstrate the military relevance of 2-micron linear mode APD by improving the TRL and MRL of the technology. Specifically, the team is working on transitioning the technology from university research to a fieldable camera. The proposed work will fabricate the ROIC, fabricate extended SWIR APDs, build sensor chip assemblies and demonstrate performance in a lab camera. This project significantly enhances the state of the art in low light extended SWIR sensing by taking the technology to an imaging demo.