Assessment of optical phonons in BeTe, Be${}_x$Zn${}_{1-x}$Te, p-BeTe epilayers and BeTe/ZnTe/GaAs (001) superlattices

Comprehensive simulations for the polarization-dependent far-infrared (FIR) reflectance and transmission spectra are reported to assess the longitudinal-optical (ω${}_{LO}$) and transverse-optical (ω${}_{TO}$) phonons in binary BeTe (ZnTe)/GaAs (001), ternary alloy Be${}_x$Zn${}_{1-x}$Te/GaAs (001) epilayers and BeTe/ZnTe/GaAs (001) superlattices. The Kramers-Krönig analyses are performed to achieve the optical constants of bulk materials for constructing their frequency dependent dielectric functions. Both s-[T${}_s$(ω)/R${}_s$(ω)] and p-[T${}_p$(ω)/R${}_p$(ω)] polarized spectral calculations are attained at an oblique incidence [i.e., Berreman effect (BE)] using a three-phase model in a multilayer optics approach. For perfect thin epilayers, the BE method has offered an unambiguous appraisal of ω${}_{LO}$ and ω${}_{TO}$ phonons in the p-polarized [T${}_p$(ω)/R${}_p$(ω)] FIR spectra; while in the p-doped BeTe epilayer, it justified the LO-plasmon (ω${}_{LOPL}^{+}$) coupled modes to be longitudinal in character. Careful assessment of the ω${}_{LOPL}^{+}$ by FIR spectroscopy offers an alternative, elegant and effective method, complementary to Raman scattering spectroscopy for estimating the free-carrier charge density η in technologically important doped semiconductor epilayers.