## 4 Results and Discussion
### Raman Scattering  
In Figure 2, we have reported our RT Raman scattering spectroscopy results recorded on GS-MBE-grown InP\({}_{1-x}\)Sb\({}_{x}\)/n-InAs samples (#T1, T2, T4, T5, see Table 1) by using a Reinshaw InVia spectrometer in the backscattering geometry in the 100-450 cm\({}^{-1}\) frequency range. The polarization-dependent Raman measurements (Section 4.1.1) of these samples are presented in Figure 3.  
Obviously, the selected frequency region (100-450 cm\({}^{-1}\)) in Figure 2 has covered the longitudinal optical \(\omega_{\rm LO}\) (transverse optical \(\omega_{\rm TO}\)) modes of InSb \(\sim\)193 cm\({}^{-1}\) (\(\sim\) 185 cm\({}^{-1}\)) and InP phonons \(\sim\)354 cm\({}^{-1}\) (\(\sim\)304 cm\({}^{-1}\)), respectively. For zb materials, while the \(\omega_{\rm TO}\) modes are usually forbidden in the backscattering configuration of the (001) surface, the phonons can be observed as a weak feature either due to slight misalignment of the sample and/or alloy disorder caused by the breakdown of wavevector \(\widetilde{\mathbf{q}}\) conservation rule [45]. The perusal of Figure 2 has clearly indicated InP-like \(\omega_{\rm TO}\) phonon as well as the defect-activated longitudinal acoustic (DALA) mode. In samples #T1, T2, while the DALA mode has occurred around \(\sim\)147.2 cm\({}^{-1}\), it shifted to a lower value of \(\sim\)143.4 cm\({}^{-1}\) for the #T4, T5 samples. An A\({}_{\rm 1g}\) phonon mode of antimony also appeared between \(\sim\)160 and 165 cm\({}^{-1}\), as its intensity is much weaker compared to the DALA peaks. Two additional alloy-disordered related weak features are perceived in our Raman measurements near \(\sim\)105 cm\({}^{-1}\) and \(\sim\)129 cm\({}^{-1}\). Besides observing the InP-like \(\omega_{\rm LO}\) modes in #T1, T2, the Raman study has also detected InP-like \(\omega_{\rm TO}\) mode near \(\sim\)298 cm\({}^{-1}\) which moved to a lower value at \(\sim\)295 cm\({}^{-1}\) in #T4, T5 samples (Figure 2). In addition to the InP-like optical phononfrequencies (\(\omega_{\rm LO}\), \(\omega_{\rm TO}\)), we have also identified an additional disorder-activated optical (DAO) band appearing as a shoulder of the \(\omega_{\rm TO}\) mode between \(\sim\)308 cm\({}^{-1}\) and 311 cm\({}^{-1}\).  
Figure 2: Results of our Raman scattering measurements in the backscattering geometry for four different GS-MBE-grown samples (see Table 1). Clearly, the Raman spectra have revealed only one set of composition-dependent InP-like \((\omega_{\rm LO}^{\rm InP},\omega_{\rm TO}^{\rm InP})\) modes. The InSb-like \(\left(\omega_{\rm LO}^{\rm InSb},\omega_{\rm TO}^{\rm InSb}\right)\) doublet has not been resolved. Disorder-activated longitudinal acoustic (DALA) and optical (DAO) phonons are also observed (see text).  
Unlike observing the InP-like optical phonon doublets, we could only identify a single broad InSb-like band (Figure 2) near \(\sim\)195 cm\({}^{-1}\) in GS-MBE-grown samples. Obviously, the InSb-like band in InP\({}_{1-x}\)Sb\({}_{x}\) is not resolved, as its large (\(\sim\)10 cm\({}^{-1}\)) full width at half maximum (FWHM) has a value larger than the separation of (\(\omega_{\rm{LO}}-\omega_{\rm{TO}}\)) \(\sim\)8 cm\({}^{-1}\) in InSb, at x = 1 [49]. We strongly feel that the low-intensity InSb-like \(\omega_{\rm{TO}}\) band as well as the gap mode of In\(P\):Sb near x \(\to 0\) are either buried in the vicinity of disordered optical bands and/or falling in the region where the density of phonon states is high [24; 25; 26; 27]. In Section 4.1.2, we will address this issue theoretically by calculating the gap mode of Sb in In\(P\) and LVM of P in In\(Sb\) using a realistic lattice dynamical model in the ATM-GF framework [59].