This Small Business Innovation Research Phase I project examines a new material, indium-thallium-phosphide (InTlP), which has been suggested as an alternative to mercury-cadmium-telluride (HgCdTe) for long wavelength infrared (8 to 12 micrometers) detectors for infrared focal plane arrays applications. Theoretical calculations suggest that addition of Tl to InP can reduce the bandgap of InP from 1.35 eV (0.9 micrometers) to less than 0 to 0.1 eV, i.e., around 8 to 12 micrometers. Though these theoretical predictions have yet to be verified (since InTlP has not been made by any growth technique), preliminary results for InTlSb have demonstrated that InTlSb films can be grown by metalorganic chemical vapor deposition (MOCVD) and that addition of Tl to InSb can reduce the bandgap to 0.15 eV (8 micrometers). InTIP is expected to have properties superior to InTlSb, including complete solid solubility over the entire alloy phase diagram, a near lattice-match to InP, and the potential for integration of detector arrays and InP read-out circuitry. Further, InTlP offers many potential advantages over HgCdTe and III-V quantum well infrared photo-detectors (QWIPs); it has higher mechanical strength, lower substrate cost, and better compositional uniformity than HgCdTe. In comparison to QWlPs, it offers normal incidence detection and higher quantum efficiency. The objective of Phase I is to demonstrate InTlP photoconductive detectors operating at two different wavelengths beyond 2 micrometers(less than .5 eV). High performance detectors based on Tl-based alloys will provide a low-cost replacement for HgCdTe infrared focal plane arrays. In addition to yielding a new class of more economical IR detectors, this material system can be useful for lasers operating in the mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) regions, and for industrial and biomedical thermography.
