## 3 Results
### Quantum Efficiency Measurements  
Figure 5a,b displays the QE as a function of temperature for two operating bias voltages Yb equal to \(-\)0.2 V and \(-\)0.4 V, respectively. The detector cut-off wavelength is 5 \(\upmu\)m at 150 K, and a QE value higher than 50% is extracted (without anti-reflection coating (ARC)), which is a remarkable value considering the absorption coefficient and the low hole mobility in the T2 SL structure [31; 32]. Nevertheless, it may be observed that the QE values are strongly dependent on the applied voltage in the considered temperature range (77-220 K).  
Figure 4: TRPL signal of the Ga-free XBn InAs/InAs\({}_{0.63}\)Sb\({}_{0.37}\) T2 SL structure at 80 K, 150 K and 250 K. Inset shows the minority carrier lifetime values extracted from measurements.  
For \(\mathrm{Vb}=-0.2\mathrm{V}\), we can observe (Figure 5a) that the 50% QE value is not reached for temperatures below 200 K. This may be due to the presence of an unwanted valence band offset (VBO) between the AL and BL which impedes the transport of hole minority carriers. This is not the case for \(\mathrm{Vb}=-0.4\mathrm{V}\) (Figure 5b), where this barrier seems to be overcome, resulting in higher QE values at low temperatures before saturation at 200 K.  
Figure 6 displays the QE values measured at \(\lambda=3.4\)\(\mathrm{\SIUnitSymbolMicro m}\) for different applied biases in the temperature range (77-220 K). The QE increases from 32% at T = 77 K and \(\mathrm{Vb}=-0.4\) V to reach 54% at T = 200-220 K and \(\mathrm{Vb}=-0.3\) V. This evolution with temperature is due to an increase in the hole minority carrier vertical diffusion length (along the growth axis), since absorption and lifetime measurements (Figures 2 and 4, respectively) showed no significant  
Figure 5: Front side illuminated spectral quantum efficiency taken at different temperatures (from 77 K to 220 K) and under two biases, \(\mathrm{Vb}=-0.2\) V (**a**) and \(\mathrm{Vb}=-0.4\) V (**b**).  
evolution in this temperature range. Moreover, a slope of the QE can be observed, mainly at low temperature. This slope is a signature of a non-optimized device in the considered range of temperature. Indeed, if the AL thickness is longer than the vertical diffusion length of the minority carriers, some of the photo-generated carriers recombine before reaching the CL. Thus, an increase in the depletion width with the bias is necessary to promote the full collection of minority carriers. However, the QE begins to saturate at high temperature (Figure 6). Indeed, taking into account minority carrier lifetime and mobility values in the Ga-free T2 SL structure [32], the vertical diffusion length increases with the temperature, allowing better collection of photo-generated carriers. At 150 K, a slight QE slope remains, while at 220 K, the QE completely saturates for biases lower than the operating bias (Vb\(=-0.4\) V). A QE value higher than 55% highlights that the transport of hole minority carriers is optimized at this temperature, but not completely at the 150 K targeted temperature operation. Indeed, at this temperature, the hole vertical diffusion length was estimated at around 3 \(\upmu\)m [32], in agreement with the chosen AL thickness.