Small-Pitch HgCdTe Photodetectors

If we can make wavelength-sized detectors, we approach the limit at which smaller detectors have no further advantage for imaging focal plane arrays with practical (f/1-2) optics. Of course, this must be accomplished without compromising performance—a challenge for 5-lm devices for which the perimeter, the currents of which depend on passivation quality, is very large compared with the area of the device. This paper describes the development of small LWIR HgCdTe detectors and compares dark current performance with that of larger basic devices, as described by ‘‘Rule 07’’, a well-known rule of thumb which gives the HgCdTe dark-current density characteristics of the best reported diodes as a function of device cut-off wavelength and operating temperature. Low cross-talk requires a fully-depleted absorber layer sufficiently thick to provide adequate quantum efficiency (QE). Preliminary results show dark-current densities are more than a factor of ten below the Rule 07 trend line. With these dark-current densities, the measured 40% non-antireflection-coated QE in the 8–10 lm region is more than adequate to achieve background-limited performance with the margin under tactical backgrounds for the fast (f/1), diffraction-limited optics required for the small pixels.