A comparative study and performance characteristics of ion-implanted and heterojunction short-wave infrared HgCdTe focal-plane arrays

Abstract

Short-wave infrared (SWIR) HgCdTe focal-plane arrays (FPAs) with a cutoff wavelength of 2.5 µm have been produced using both planar ion-implanted and heterojunction-mesa device structures. The two-dimensional FPAs are comprised of a 320 × 256 format with 30-µm pixel pitch and are cooled by a multistage thermo-electric (TE) cooler. Measured RoA values of the two types of device structures show similar results below about 130 K because of the performance-limiting effect of the surface passivation of the heterojunction. However, a substantial difference is seen above 130 K and up to 300 K between the two structures types, with the heterojunction-mesa p-on-n device having an order of magnitude higher RoA value than the planar ion-implanted n-on-p configuration. The difference in the RoA values is reflected in the FPA images of the two different device types, where at 200 K, both FPAs display a clear picture with the n-on-p implanted device having a somewhat lesser resolution. However, no image can be seen from the planar-implanted FPA at 300 K, whereas the heterojunction-mesa FPA still exhibits a notable image at this temperature. These differences are examined and are attributed largely to higher diffusion and generation-recombination (g-r) currents that are thought to be prevalent in the ion-implanted n-on-p device structure. Yet, baking studies carried out show the ion-implanted diodes to be slightly more robust, as experiments reveal that they tend to survive a 120°C heat treatment longer than the mesa devices, which tend to degrade after a certain period of time. The nature of n-type donors in ion-implanted diodes is discussed, and a new theory based on Te antisites is proposed to explain recent experimental findings.