Infrared dielectric function and phonon modes of highly disordered (Al_xGa_{1 - x})_0.52In_0.48P

Dielectric function spectra and phonon modes of highly disordered (Al_xGa_{1 - x})_0.48In_0.52P, i.e., solid solutions with nearly randomly distributed cations, lattice matched to GaAs, are studied for A1 compositions x = 0, 0.33, 0.48, 0.7, 0.82, and 1 using far-infrared ellipsometry and Raman scattering. An anharmonic oscillator model approach is employed for line-shape analysis of the (Al_xGa_{1 - x})_0.48In_0.52P dielectric function. A complex phonon mode behavior for the random-alloy solid solutions is found: (i) two (one weak GaP-like and one strong InP-like or one weak InP-like and one strong A1P-like with TO-LO splitting) bands are present in Ga_0.52In_0.48P and A1_0.52In_0.48P, respectively, (ii) three (one weak GaP-like, one weak A1P-like, and one strong InP-like) bands dominate the quaternary compounds for x < 0.5, (iii) the GaP-like band is absent for x > 0.5, and (iv) three additional modes (AM's) with low polarity occur with small composition dependencies at AM₁ ∼ 313 cm^-1, AM₂ ∼ 351 cm^-1, and AM₃ ∼ 390-405 cm^-1, respectively. Results from polarized Raman measurements agree excellently with the mode scheme developed from the ellipsometry study. Modes AM₁ and AM₂ coincide with CuPt-type superlattice-ordering-induced lattice modes, predicted recently for Ga_0.52In_0.48P from first-principles calculations IV. Ozoliņš and A. Zunger, Phys. Rev. B 57, R9404 (1998)] and may be used to identify small degrees of ordering in A1GaInP by far-infrared ellipsometry.