Superlattice Materials for Very-Long Wavelength Infrared Detectors (VLWIR)

The fabrication of large format, highly sensitive focal plane arrays (FPAs) sensing at very long wavelengths in the infrared region (VLWIR), i.e. beyond 15um, is highly desirable for the Air Force's space-based applications. Currently, only limited typesof arrays such as 256 X 256 Si:As are available in this spectral region. However, these extrinsic detectors suffer from several fundamental limitations, such as low quantum efficiencies, relatively low detection sensitivities and low operatingtemperatures. Intrinsic detectors made from HgTe/CdTe superlattices (SLs) have several advantages over them, including lower noise, higher operating temperatures and higher quantum efficiencies. The cut-off wavelength of HgTe/CdTe SLs can easily be tunedto the VLWIR by adjusting the layer thickness of the HgTe layers. This results in greater cut-off uniformity than that achievable by controlling the alloy composition of bulk HgCdTe. Another important advantage of HgTe/CdTe SLs compared to bulk HgCdTeincludes the ability to achieve p-type doping without high temperature annealing. Furthermore, these SLs can be grown on Si substrates, which would pave the way to megapixel hybrid or possibly monolithic VLWIR FPAs. Through the design of HgTe/CdTe SLelectronic band structures and device properties, and in-situ control of layer thickness, we proposed the growth of high quality SLs for the fabrication of high performance infrared detectors with cut-off wavelengths between 20-25 microns and operatingtemperatures above 40 K.