Development of high performance long wavelength infrared HgCdTe focal plane arrays

Barutçu, Berna
This thesis study reports the characterization of long wavelength infrared (LWIR) HgCdTe photovoltaic detectors with the p on n structure and 9.5 μm cut-off wavelength grown by molecular beam epitaxy on CdZnTe substrates and fabricated with different passivation processes. The characterization study was conducted at both pixel and focal plane array (FPA) levels. While the detectors exhibit diffusion dominated dark current at temperatures at and above 130 K under reverse bias voltages typically used in imaging applications, the major component of the dark current is trap assisted tunneling (TAT) at low temperatures (80 K). We carried out a detailed characterization study on the detector pixels with 30 μm pitch in order to identify the properties of the traps establishing the generation-recombination (G-R) and the TAT currents. Dark current analysis study yielded a hole trap in the n-side at an energy of 0.36Eg (Eg is the bandgap of the absorber material) measured from the valence band edge with a density in the order of 1014 cm-3. While this trap has been reported in the literature, we explored the further properties of this trap including the capture cross section characteristics exhibiting capture barrier as well as the effect of it on the signal to noise ratio of the detector through the G-R and TAT components of the dark current contributing to the 1/f noise of the detectors. vi Shunt (ohmic) leakage current was observed in the detectors fabricated with different passivation processes including CdTe and CdTe/ZnS subjected to different annealing conditions. The absence of this leakage current in a detector fabricated with the same material but with properly applied and processed CdTe passivation suggests that it originates from the surface. We have also observed that the shunt current component introduces significant 1/f noise with a noise coefficient, α, in the order of that of the G-R current (10-3). The TAT current is observed to introduce 1/f noise to exhibit noise current at 1 Hz expressed as 0.8x10-6(ITAT)0.57. The detectors exhibit a peak specific detectivity of ~1x1011 cmHz1/2/W at 78 K with f/2 optics corresponding to temporal noise equivalent temperature difference of ~20 mK with an integration time as low as 400 μs. We also present the characteristics of a mid-format (320x256) focal plane array (FPA) fabricated with the same material in order to assess the FPA level performance of the detectors.
Citation Formats
B. Barutçu, “Development of high performance long wavelength infrared HgCdTe focal plane arrays,” Thesis (M.S.) -- Graduate School of Natural and Applied Sciences. Electrical and Electronics Engineering., Middle East Technical University, 2019.