Far-infrared dielectric functions and phonon spectra of alloys determined by spectroscopic ellipsometry

We have used spectroscopic ellipsometry to determine the complex dielectric function of a series of ternary BexZn1−xTe thin films grown by molecular beam epitaxy. The II–VI semiconductor alloys were grown on InP substrates that had an InGaAs buffer layer. After the growth, X-ray diffraction experiments were performed in order to determine the alloy concentration. A standard inversion technique was used to obtain the dielectric functions from the measured ellipsometric spectra, obtained between 2000 nm (5000 cm−1) and 40,000 nm (250 cm−1). By modelling the dielectric function as a collection of oscillators, representing longitudinal and transverse optical phonons of the BexZn1−xTe lattice, we were able to recover the phonon spectra for this alloy system. It is argued that the additional phonon modes that are obtained from ellipsometry are best understood from the recently-proposed percolation model.