Fabrication and characteristics of nonequilibrium VLWIR detectors with HgTe/CdTe superlattice active regions

P + -v-N + structures with HgTe/CdTe superlattice absorber regions were grown by MBE to give cut-off wavelengths in the very long wavelength infrared (14 μm and longer) at temperatures below 80 K. The superlattice period of one sample was 98.3a according to its x-ray diffraction profile, very close the intended 98.5a for a 48.5a HgTe/50.a Hg 0.05 Cd 0.95 Te superlattice. The cut-off wavelengths of another sample were approximately 14 and 18 μm at 77 and 40K, respectively, as determined by optical absorption and spectral response measurements. A first batch of devices annealed at 150°C or 180°C for 1 hr after the deposition of 50a thick gold films showed relatively low R 0 A values (approximately 0.3 ohm-cm 2 ). This was interpreted to be due to the formation of a junction near the boundary of the superlattice and a high carrier concentration region of HgCdTe alloy. A second batch of devices was annealed at 120°C for 1 hr or 5 min. to decrease the gold diffusion depth. The electrical properties showed higher R 0 A values (approximately 8 ohm-cm 2 ). The detectivities of the second batch devices at 77K were in the range of 10 8 to 10 10 cmHz½/W and showed frequency dependence because the noise had frequency dependence. We observe very low knee frequencies (below 10Hz) in their noise spectra.