Observation of a linear dependence of the frequency splitting between GaAs and AlAs optical surface phonon as a function of Al concentration in AlxGa1-xAs

In this paper, we report on the high-resolution electron-energy-loss spectroscopy study of high-quality AlxGa1−xAs ternary alloys in the whole range of Al concentration, from x=0 to 1. The ternary alloys have been grown by molecular-beam epitaxy and analyzed in the same ultrahigh-vacuum (UHV) environment. For intermediate Al concentrations we observe two long-wavelength surface phonons which can be associated with the vibrations of Ga or Al atoms against As atoms, respectively. Precise values of the frequencies of these two modes versus the Al concentration are deduced from a semitheoretical fit of the spectra and compared with the theoretical model based on the coherent-potential approximation. The surface-phonon frequency splitting provides an alternative way of determining the aluminum concentration in such ternary alloys. The intensities of both losses are considered in the framework of the dielectric theory and some conclusions are drawn for the oscillator strengths of the two surface modes.