## 4 Selected Device Results
We believe the primary reason for this is that for these samples, the \(n\)-type absorber thickness is probably longer than the hole diffusion length, and therefore, the \(n\)-type absorber does not contribute to the QE effectively, as photo-generated carriers recombine before they are collected. This is supported by the observation from Figures 5b and 6b that the QE for both devices decreases noticeably with decreasing temperature. If the hole diffusion length is longer than the \(n\)-type absorber over the 50-80 K temperature range where spectral QE measurements were obtained, we would expect the QE to stay relatively constant. For the pn-CBIRD sample Sb3587, in the \(p\)-type absorber section, the electron diffusion length is longer than the absorber thickness, so the photo-generated carriers are collected efficiently. Therefore, for Sb3587 the 1.7 \(\upmu\)m thick \(p\)-type absorber section actually contributes proportionally more to the net QE than its 3 \(\upmu\)m thick \(n\)-type absorber section. A possible secondary reason for Sb3583 having relatively worse QE performance than Sb3587 is that due to sample-to-sample variations, the hole diffusion length in Sb3587 could be somewhat longer than that in Sb3583. This would permit the \(n\)-type section of Sb3587 to contribute to the QE a little more effectively. In any case, the data suggest that the QE (and also the dark current) performance of both of these devices can be improved by decreasing the thickness of the \(n\)-type absorber. Figure 5: (a) Dark current density as a function of applied bias, measured at several temperatures, and (b) backside illuminated spectral QE, for an n-CBIRD detector fabricated from sample Sb3583. Figure 6: (a) Dark current density as a function of applied bias, measured at several temperatures, and (b) backside illuminated spectral QE, for a pn-CBIRD detector fabricated from sample Sb3587. The dark current characteristics of the pn-CBIRD Sb3587 is not as good as that for the n-CBIRD Sb3583. The presence of the homojunction between the \(p\)-type and \(n\)-type absorbers, as well as the surface \(p\)-\(n\) junctions on the \(p\)-type absorber sidewalls, makes it more challenging to achieve good dark current characteristics, especially in (V)LWIR devices where the absorber band gap is small. This suggests we might expect pn-CBIRD devices to have better dark current characteristics if the absorber band gap is larger (shorter cutoff wavelength), and therefore less susceptible to tunneling. Figure 7 shows the device results for a pn-CBIRD with shorter cutoff wavelength. Sb3589 has a cutoff wavelength of 10 \(\upmu\)m at 80 K, and its dark current characteristics appear to be much better behaved than those of the first pn-CBIRD sample Sb3587 (11.75 \(\upmu\)m cutoff at 70 K). At 79 K and 70 K, with \(-\)0.1 V bias, the dark current densities of Sb3589 are 4.9 \(\times\) 10\({}^{-5}\) and 3.7 \(\times\) 10\({}^{-6}\) A/cm\({}^{2}\), which are respectively 8.6 and 12.8 times higher than Rule 07. The QE at 7.5 \(\upmu\)m is 33% at 79 K, without AR coating. In general, the pn-CIBRD can achieve significantly higher QE than the n-CBIRD, but at the expense of having worse dark current characteristics. Evidently, for pn-CBIRDs with shorter cutoff wavelengths, the tunneling dark current is reduced, and the dark current characteristics tend to be better. Figure 8 shows the spectral-specific detectivity (\(D^{\star}\)) calculated from the spectral response for the three samples. The \(D^{\star}\) calculation includes the dark current shot noise, Johnson noise, and shot noise from the photocurrent in the spectral range shown in Figure 8 for 300 K background with f/2 condition. Figure 8: Spectral-specific detectivity (\(D^{\star}\)) at several temperature for non-anti-reflection coated detectors fabricated from Sb3583, Sb3587, and Sb3589. Figure 7: (a) Dark current density as a function of applied bias, measured at several temperatures, and (b) backside illuminated spectral QE, for a pn-CBIRD detector fabricated from sample Sb3589.