Large Format LADAR Receiver Arrays Based on Near Infrared HgCdTe Avalanche Photodiodes

Most seeker systems usually operate at long distances. The return laser signal from the target is not only attenuated by absorption, reflection and scattering by air-borne gas, dust and liquid particles, but also by the emissivity and reflectivity variations of the target surface. High bandwidth detectors with internal gain are required. Avalanche photodetectors (APDs) are best suited for this purpose due to their high gain-bandwidth characteristics. Fortunately, the MCT material can be bandgap engineered to achieve a large asymmetry in the hole to electron ionization coefficient. This in turn leads to high gain-bandwidth MCT APDs with minimal excess noise. Using advanced growth technique like molecular beam epitaxy (MBE), it is possible to tailor heterostructure MCT materials that would exclusively use electron ionization or hole ionization. Recent developments in monitoring techniques like in-situ spectroscopic ellipsometry can lead to high quality MCT heterostructures with optimized interfaces. This, in combination with better nucleation methods and other geometric design considerations, make it now possible to grow very low-defect materials. Consequently, it is now possible to produce shot-noise limited MCT APDs that operate over a wide infrared wavelength range in single element and array formats. We propose to use our extensive experience in HgCdTe growth by molecular beam epitaxy and device processing to fabricate large format near infrared (1064nm) HgCdTe avalanche photodiodes on silicon substrates. They will be incorporated into LADAR receiver modules and integrated with MCT-based MWIR/LWIR thermal imagers.