Optical functions, phonon properties, and composition of InGaAsN single layers derived from far- and near-infrared spectroscopic ellipsometry

We study the optical properties of compressively strained In_xGa_1-xAs_1-yN_y (x<0.13, y<0.03) single layers for photon energies from 0.75 to 1.3 eV (near infrared), and for wave numbers from 100 to 600 cm^-1 (far infrared) using spectroscopic ellipsometry. The intentionally undoped InGaAsN 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 InGaAsN for both spectral ranges studied here. The InGaAsN 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 polar strength f of the GaN sublattice resonance changes with nitrogen composition y and with the biaxial strain ε_xx resulting from the lattice mismatch between InGaAsN and GaAs. This effect is used to derive the nitrogen and indium content of the InGaAsN layers combining the observed f dependence with results from high-resolution double-crystal x-ray diffractometry and using Vegard's law for the lattice constants and the elastic coefficients C₁₁ and C₁₂. The calculated nitrogen concentrations reflect growth properties such as increasing N incorporation in InGaAsN with decreasing growth temperature, with increasing concentration of nitrogen in the gas phase, and with decreasing indium concentration in InGaAsN.