## 4 Discussion  
As mentioned, the Sb distribution in InAsSb or even in the InAs layer can affect the strain, which could result in the observed energy bandgap changes versus \(T_{growth}\). In order to present the Sb distribution in the InAsSb layer in the Set I samples 4, 5, and 6, [PERSON]'s model was incorporated [23]. In this model, the Sb background (\(x_{b}\)) should be determined, and the \(R\) parameter corresponds to the rate of the As-Sb exchange in InAsSb. Knowing the T2 SL period (\(P\) = 9.4 nm) and intended Sb profile (\(x_{Sb}\) = 0.39), one can simulate the Sb distribution profile (z-direction) according to the following relations:  
* \(x_{sb}=x_{0}(1-R^{z})+x_{b}\) for \(0<z<N\);
* \(x_{sb}=x_{0}\big{(}1-R^{N}\big{)}R^{z-N}+x_{b}\) for \(N<z<P\). For the Set I samples 4, 5, and 6, we made the following assumptions related to [PERSON]'s model characteristic parameters versus growth temperature presented in Table 2.  
The SIMS measurements show the Sb background composition at the level of \(\sim\)5% [9]. The 3% Sb background, \(R\) = 0.7 (for the comparable growth temperature, \(T_{growth}\) = 420 \({}^{\circ}\)C) contributing to the similar Sb distribution in T2 SLs InAs/InAsSb were reported by [PERSON] et al. [22]. The InAsSb thickness (\(N\)) for which maximum Sb composition occurred was assumed at the level of 2 nm, assuming nominal InAsSb thickness 2.49 nm, similarly to the Sb composition distribution presented by [PERSON] et al. [22]. When we increased the \(T_{growth}\), the \(R\) parameter increased, which could result in higher Sb background composition (that is marked by arrows). Figure 6 presents the Sb composition distribution within the InAsSb layer (2.49 nm). We assumed the following \(R\) parameters for the Set I samples 4, 5, and 6: \(R\) = 0.9/0.7/0.4 and \(R\) = 0.8/0.7/0.6; these values correspond to the \(T_{growth}\) to present the 1D in-plane strain \(\Big{(}\varepsilon_{II}=\big{(}a_{f}-a_{GaSb}\big{)}/a_{GaSb}\big{)}\) distribution within InAsSb (out-of-plane strain could be calculated by the equation \(\varepsilon_{\perp}=-2C_{12}/C_{11}\) for (001) GaAs substrates). The InAsSb layer was in compression in relation to the GaSb buffer layer for in-plane and in tensile for out-of-plane direction. It must be stressed that an average (for samples 4, 5, 6) \(\Delta a\)/\(a\) GaAs-0\({}^{\text{th}}\)/GaSb was assumed as 7.271%. As presented in Figure 6a-d, a decrease in the exchange rate \(R\) parameter lowered Sb composition distribution and strain within the InAsSb layer in relation to GaSb. As mentioned above, the segregation efficiency \(R\) increases with growth temperature, as expected, the results of which are presented in Figure 6[27]. \begin{table}
\begin{tabular}{c c c c} \hline Parameter & 450 \({}^{\circ}\)C & 425 \({}^{\circ}\)C & 400 \({}^{\circ}\)C \\ \hline Sb background (\(x_{b}\)) & \(\uparrow\)5\% & 5\% (SIMS) [9] & \(\downarrow\)5\% \\ \hline \(R\) & \(\uparrow\)0.9 & 0.7[22] & \(\downarrow\) (0.4) \\ \hline Nominal Sb composition (\(x_{0}\)) & & 0.39 & \\ \hline Nominal InAsSb thickness [nm] & & 2.49 & \\ \hline InAsSb thickness (\(N\))-maximum \(x_{Sb}\) [nm] & & 2 & \\ \hline Period (\(P\)) [nm] & & 9.4 & \\ \hline \end{tabular}
\end{table} 
Table 2: [PERSON]’s Sb distribution profile for T2 SLs InAs/InAsSb for Set I samples 4, 5, and 6. The measured 100% cut-off wavelengths (\(\lambda_{100\%cut-off}\)) for Set I samples 4, 5, and 6 read \(\sim\)9.4 \(\upmu\)m (450 \({}^{\circ}\)C), \(\sim\)11.5 \(\upmu\)m (425 \({}^{\circ}\)C), and \(\sim\)12.8 \(\upmu\)m (400 \({}^{\circ}\)C) (Figure 7). Figure 6: The Sb distribution according to [PERSON]’s model and 1D in-plane strain (%) distribution in InAsSb in relation to the GaSb for assumed \(R\) exchange ranges: (**a,b**) \(R\) = 0.9/0.7/0.4 and (**c,d**), \(R\) = 0.8/0.7/0.6, corresponding to (**b**) \(T_{growth}\). Figure 7: Spectral response characteristic at 210 K for T2 SLs from Set I (samples 4, 5, and 6).