## 4 Selected Device Results  
We had previously presented (V)LWIR n-CBIRD, pn-CBIRD, and p-CBIRD device results in [47; 48]. In this section, we examine device results from three additional LWIR InAs/InAsSb T2 SLS CBIRD samples. The samples were grown on three-inch-diameter Te-doped GaSb (100) substrates in a Veeco Applied-Epi Gen III molecular beam epitaxy chamber equipped with valved cracking sources for the group V Sb2 and As2 fluxes. Square mesa photodiodes with Ti/Pt/Au contacts were fabricated using standard optical lithography for responsivity and dark current measurements. The devices were not passivated or treated with antireflection (AR) coating. Sb3583 is an n-CBIRD sample nominally consisting of a 0.36 \(\upmu\)m thick \(n\)-type (1 \(\times\) 10\({}^{17}\) cm\({}^{-3}\)) T2 SLS bottom contact; a 0.11 \(\upmu\)m thick, not intentionally doped (n.i.d.) \(n\)-type T2 SLS window layer; an n.i.d. \(n\)-type (estimated to be 1 \(\times\) 10\({}^{15}\) cm\({}^{-3}\)) T2 SLS absorber; a 0.25 \(\upmu\)m thick \(p\)-type (4 \(\times\) 10\({}^{16}\) cm\({}^{-3}\)) AlGaAsSb electron unipolar barrier; and a 0.15 \(\upmu\)m thick \(p\)-type (1 \(\times\) 10\({}^{18}\) cm\({}^{-3}\)) T2 SLS top contact. The absorber consists nominally of a 3 \(\upmu\)m thick graded region followed by a 0.5 \(\upmu\)m thick non-graded region (both are \(n\)-type). Sb3587 and Sb3589 are pn-CBIRD samples with the same basic structure, except that the absorber consists of a 3 \(\upmu\)m thick graded n.i.d. \(n\)-type region followed by a 1.7 \(\upmu\)m thick non-graded \(p\)-type (3 \(\times\) 10\({}^{15}\) cm\({}^{-3}\)) region. Figure 5 shows (a) the measured dark \(J\)-\(V\) characteristics at temperatures ranging from 30 K to 79 K, and (b) the spectral QE derived from back-side illuminated spectral responsivity measured at 50, 60, and 70 K under \(-\)0.1 V bias, for the n-CBIRD sample Sb3583. In general, the dark current characteristics of the Sb3583 n-CBIRD is fairly well behaved at below 0.3 V reverse bias. At 70 K and \(-\)0.1 V bias, the Sb3583 cutoff wavelength is 11.27 \(\upmu\)m, and the dark current density is 1.24 \(\times\) 10\({}^{-5}\) A/cm\({}^{2}\), which is a factor of 2.4 higher than that given by the Rule 07 [50]. At 60 K and \(-\)0.1 V bias, the dark current density is 1 \(\times\) 10\({}^{-6}\) A/cm\({}^{2}\), which is a factor of 6.8 higher than that given by the Rule 07. The QE for the Sb3583 is modest; without AR coating, the QE at 8.6 \(\upmu\)m is 15% at 70 K. In the backside-illuminated configuration, the light incidents from the GaSb substrate side. Without AR coating, approximately one-third of the light is reflected by GaSb; therefore, the maximum possible (external) QE is \(\sim\)67%. Alternatively, we may estimate the internal QE by multiplying the QE shown in Figure 5b by 1.5. Figure 6 shows the corresponding results for the pn-CBIRD sample Sb3587, which has a slightly longer cutoff wavelength of 11.75 \(\upmu\)m at 70 K. The dark current characteristics of Sb3587 seem to show significant tunneling contribution. At 70 K and 60 K, the dark current densities under \(-\)0.1 V bias are 3.9 \(\times\) 10\({}^{-4}\) and 2.1 \(\times\) 10\({}^{-4}\) A/cm\({}^{2}\), which are respectively 32 and 520 times higher than Rule 07. The 70 K QE for the Sb3587 at 8.6 \(\upmu\)m without AR coating is 32%, which is noticeably higher than that of Sb3583. We recall that the Sb3583 absorber consists of a 3 \(\upmu\)m thick graded \(n\)-type section and a 0.5 \(\upmu\)m thick non-graded \(n\)-type section, and that the Sb3587 absorber consists of a 3 \(\upmu\)m thick graded \(n\)-type section and a 1.7 \(\upmu\)m thick \(p\)-type section. The total absorber thicknesses for Sb3587 and Sb3583 are respectively 4.7 and 3.5 \(\upmu\)m, with a thickness ratio of \(\sim\)1.34. Normally, in devices where all the photo-generated carriers are effectively collected, we would not expect the QE ratio between the two samples to exceed the thickness ratio. Yet, the QE for Sb3587 is more than twice that of Sb3583.