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Determination of the complex dielectric function and the critical-point energies of (Al x Ga 1Ϫx) 0.51 In 0.49 P, over the full range of composition x and for photon energies E from 0.75 to 5 eV is reported from variable angle of incidence spectroscopic ellipsometry. Native-oxide effects on the (Al x Ga 1Ϫx) 0.51 In 0.49 P optical functions are removed numerically. The highly disordered state of the metalorganic vapor-phase epitaxy grown samples is analyzed by transmission electron microscopy. Optical anisotropy investigations revealed that the order-induced optical birefringence is negligible throughout. The augmentation of A. D. Rakić and M. L. Majewski ͓J. Appl. Phys. 80, 5909 ͑1996͔͒ to Adachi's critical-point model, i.e., consideration of Gaussian-like broadening function instead of Lorentzian broadening, is used for calculation of the isotropic (Al x Ga 1Ϫx) 0.51 In 0.49 P dielectric function. The optical functions spectra consistently match the experimental data, whereas previously reported model dielectric functions fail to reproduce the correct absorption behavior of the quaternary, especially near the fundamental band-to-band transition. The results are compared to those presented previously, and influence of spontaneous chemical ordering is discussed.

We study the optical properties of tensile strained GaAs1−yNy (0%⩽y⩽3.7%) single layers for photon energies from 0.75 to 4.5 eV and for wave numbers from 100 to 600 cm−1 using spectroscopic ellipsometry. The intentionally undoped GaAsN layers were grown pseudomorphically on top of undoped GaAs buffer layers deposited on Te-doped (001) GaAs substrates by metalorganic vapor phase epitaxy. We provide parametric model functions for the dielectric function spectra of GaAsN for both spectral ranges studied here. The model functions for photon energies from 0.75 to 4.5 eV excellently match dielectric function data obtained from a numerical wavelength-by-wavelength inversion of the experimental data (point-by-point fit). Critical-point analysis of the point-by-point fitted dielectric function is performed in the spectral regions of the fundamental band gap and the critical-point transitions E1 and E1+Δ1. The band-gap energy is redshifted whereas the E1 and E1+Δ1 transition energies are blueshifted with increasing y. For y⩽1.65% the observed blueshift of the E1 energy is well explained by the sum of the effects of biaxial (001) strain and alloying. The GaAsN layers show a two-mode phonon behavior in the spectral range from 100 to 600 cm−1. We detect the transverse GaAs- and GaN-sublattice phonon modes at wave numbers of about 267 and 470 cm−1, respectively. The oscillator strength of the GaN-sublattice resonance increases linearly due to alloying and tensile strain. We compare our results from the single layers with those obtained previously from tensile and compressively strained GaAsN/GaAs and GaAsN/InAs/GaAs superlattices.

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Dielectric function spectra and phonon modes of highly disordered (Al x Ga 1Ϫx ) 0.48 In 0.52 P, i.e., solid solutions with nearly randomly distributed cations, lattice matched to GaAs, are studied for Al compositions x ϭ0, 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 x Ga 1Ϫx ) 0.48 In 0.52 P dielectric function. A complex phonon mode behavior for the random-alloy solid solutions is found: ͑i͒ two ͑one weak GaP-like and one strong InP-like or one weak InP-like and one strong AlP-like with TO-LO splitting͒ bands are present in Ga 0.52 In 0.48 P and Al 0.52 In 0.48 P, respectively, ͑ii͒ three ͑one weak GaP-like, one weak AlP-like, and one strong InP-like͒ bands dominate the quaternary compounds for xϽ0.5, ͑iii͒ the GaP-like band is absent for xϾ0.5, and ͑iv͒ three additional modes ͑AM's͒ with low polarity occur with small composition dependencies at AM 1 ϳ313 cm Ϫ1 , AM 2 ϳ351 cm Ϫ1 , and AM 3 ϳ390-405 cm Ϫ1 , respectively. Results from polarized Raman measurements agree excellently with the mode scheme developed from the ellipsometry study. Modes AM 1 and AM 2 coincide with CuPt-type superlattice-ordering-induced lattice modes, predicted recently for Ga 0.52 In 0.48 P from first-principles calculations ͓V. Ozoliņš and A. Zunger, Phys. Rev. B 57, R9404 ͑1998͔͒ and may be used to identify small degrees of ordering in AlGaInP by far-infrared ellipsometry.