Numerical modeling and optimization of HgCdTe infrared photodetectors for thermal imaging

Koçer, Hasan
This thesis presents a detailed investigation of the performance limiting factors of long wavelength infrared (LWIR) and very long wavelength infrared (VLWIR) p on n HgCdTe detectors through numerical simulations at 77 K incorporating all considerable generation-recombination mechanisms including trap assisted tunneling (TAT), Shockley-Read-Hall (SRH), Auger and radiative processes. Numerical simulations under dark and illuminated conditions were performed with different absorber layer thicknesses, material compositions (cut-off wavelengths), trap density, and trap energy level. The results identify the relative strength of the dark current generation mechanisms by numerically extracting the contribution of each G-R mechanism on the detector characteristics with various cut off wavelengths and practically achievable material parameters. While the provided information can be used as a guide for optimizing the device processing conditions and detector structure, it also enlights the importance of various intrinsic mechanisms on the detector sensitivity. The results show that the dominant sensitivity degrading trap level depends on the detector cut-off wavelength being about 0.7Eg for LWIR HgCdTe sensors (cut-off wavelength=10 µm) instead of 0.5Eg which is generally believed to be the most efficient R-G level. TAT related 1/f noise dominates the sensor noise even under small reverse bias voltages at a trap density as low as 1E14 cm-3 for sensors with cut-off wavelength >11 µm. Considering the fact that trap densities below this level are rarely reported for HgCdTe material, exceptionally trap-free material is required to achieve desirable imaging performance with these sensors. Simulation results show that Auger mechanism has twofold effect on the sensitivity of the sensor by increasing the dark current and decreasing the photo current of the detector. As to our knowledge, this work is one of the most comprehensive simulation based investigations of the HgCdTe detector performance providing important results that can be used as a guide for optimization of the detector performance in order to meet the demanding requirements of the third generation thermal imagers.
Citation Formats
H. Koçer, “Numerical modeling and optimization of HgCdTe infrared photodetectors for thermal imaging,” Ph.D. - Doctoral Program, Middle East Technical University, 2011.