The infrared lattice-vibration spectra of ${\mathrm{Ga}}_{1-x}{\mathrm{Al}}_x\mathrm{As}$ mixed crystals have been studied by measuring the room-temperature reflectivity at near-normal incidence in the 200-600-${\mathrm{cm}}^{-1\mathrm{}}$ frequency range. Two distinct reststrahlen bands with frequencies near those of pure GaAs and pure AlAs were observed over the entire composition range investigated. Each band shifts to lower frequencies and decreases in amplitude as the concentration of the corresponding compound in the alloy is decreased. An analysis of the data using the Kramers-Kronig technique and classical dispersion theory gives the transverse and longitudinal mode frequencies. The changes of these mode frequencies with composition are interpreted on the basis of the random-element isodisplacement model.

• Received 22 September 1969

DOI:https://doi.org/10.1103/PhysRevB.1.1576