The effects of dislocations on very-long-wavelength infrared (VLWIR) HgCdTe photodiodes (cutoff wavelength >14 lm at 40 K) have been determined experimentally and analyzed. The photodiodes are in the back-illuminated configuration, fabricated from HgCdTe p-on-n double-layer heterostructure (DLHJ) films grown at BAE Systems by liquid phase epitaxy (LPE) onto lattice-matched (111) CdZnTe substrates. Arrays were hybridized to silicon ROICs to form focal plane arrays (FPAs). After characterization for dark current and response, the arrays were dehybridized and stripped of their metals and passivation layers. Dislocations were revealed using a Hä hnert and Schenk (H&S) etch. Pixel traceability was maintained throughout the analysis, permitting one-to-one correlation between photodiode performance and dislocation density measured within that photodiode. We found that response and dark current were correlated to etch pit density (EPD), which we assumed to be equal to dislocation density. Our results support earlier dislocation studies on larger-bandgap HgCdTe, which showed response was only weakly impacted by EPD, while dark current was strongly affected by EPD. Measured EPD values ranged from low 10 5 to low 10 7 cm -2 . Potential causes for this range in EPD are discussed.
Atmospheric remote-sensing have been one of the primary drivers toward longer wavelength infrared sensors beyond the 8 to 12 um atmospheric window typically used for terrestrial imaging systems. This paper presents the recent performance improvement attained with very long wavelength infrared (VLWIR) focal plane arrays, by the stringent control of the small bandgap HgCdTe material quality. Array operability is further enhanced by design using a 2:1 super-pixel detector format scheme with programmable bad element de-select and our new detector input offset optimization circuitry within each unit cell. Focal plane arrays with peak quantum efficiencies in excess of 80 percent, and cutoff wavelengths out to 15 µm have NEI operabilities around 95 percent for mid 10 14 ph/s-cm 2 fluxes operating near 50 K. Average NEI of 3.5 x 10 10 ph/s-cm 2 was demonstrated for a 14 µm cutoff wavelength focal plane array, consisting of over 55,000 elements, operating with an effective sample time of 87.5 ms. IntroductionRecent strides in very long wavelength infrared (VLWIR) HgCdTe focal planes have reinvigorated their use in VLWIR atmospheric remote sensing applications. These VLWIR focal plane arrays, comprised of HgCdTe photovoltaic diodes, have demonstrated unprecedented operability for large area arrays with detector cutoffs beyond 14 µm. Significant improvement in array yield have occurred over the last 10 years due to all aspects of the array fabrication, including larger and more uniform substrates with tighter process controls, and better understanding of defects which have led to successful mitigation strategies [1-3]. The readouts for these arrays have also improved and several features have been added specifically to address yield issues to achieve producible high operability performance above 95 percent [4]. The two main features are SuperPixels, where each pixel is sub-divided into several sub-elements that can be individually deselected if found defective, and through input bias optimization on a pixel by pixel basis to remove CMOS threshold variations across the chip. This latter feature allows the reverse bias of each detector to be optimized. The data also clearly shows the operability benefit of using smaller pixel higher density FPAs to cover the same focal plane area, demonstrating that the majority of the defects are randomly distributed defects that are much smaller than a pixel.In this paper we discuss the VLWIR detector technology at BAE Systems in section 2, followed by the excellent photodiode array performance presented in section 3. The operability performance of two staring VLWIR FPA systems using these detector arrays are discussed in section 4. The first FPA is a 256 x 256 array on 40 µm pitch specifically designed for low background applications with a frame rate of 100 Hz and a charge capacity of 11 million electrons. Each pixel is divided into a 2 x 2 array of sub-elements for SuperPixel de-selection. The baseline operation of this device uses the best 3 of every 4 sub-elements per pixel for optimal...
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