Supplementary material: Comment on Infrared dielectric function of GaAs1-xPx semiconductor alloys near the reststrahlen bands [Appl. Phys. Lett. 123, 172102 (2023)]

In the main text, we outline an updated version of the PM for GaAs(1-x)P(x) that explains, beyond existing CM and PM approaches, its phonon mode behavior, as apparent in the novel IR-ellipsometry Im{εr (ω,x)} data of Zollner et al. (Fig. S1, symbols). Sec. I reports on the main features of the cluster model (CM) and of the percolation model (PM). A common terminology is introduced to facilitate the discussion. Both the TO-frequency and the TO-intensity aspects are covered. In Sec. II, an updated version of the PM for GaAs(1-x)P(x) is presented (Fig. S2), supported by existing (Fig. S3) and novel (Fig. S4) ab initio phonon calculations at minimum (x~0,1 - TO-frequency aspect) and maximum (x~0.5 - TO-intensity aspect) alloy disorder. This updated PM scheme for GaAs(1-x)P(x) is directly confronted with Zollner's Im{εr (ω,x)} experimental data (Fig. S1, curves). An overview of the so far tested CM and PM approaches on GaAs(1-x)P(x), including the current PM one, helps to evaluate their relative bias and merits. The overview focuses on the sensitive Ga-P phonon signal at the critical GaAs0.725P0.275 composition (Fig. S5), used as a case study. Last, for the sake of completeness, we show how the latter signal is impacted within the PM by deviating from the ideal random As↔P substitution (Fig. S6).