The far-infrared (20-410 ${\mathrm{cm}}^{-1\mathrm{}}$) reflectivity of the quaternary alloy ${\mathrm{In}}_{1-x}{\mathrm{Ga}}_x{\mathrm{As}}_y{\mathrm{P}}_{1-y}$ epitaxially grown and lattice-matched to an InP substrate has been measured for $y=0.22\mathrm{} \mathrm{to} 0.66$. The lattice vibrational frequencies obtained from the data are in substantial agreement with previous Raman results but one additional mode (223 to 254 ${\mathrm{cm}}^{-1\mathrm{}}$) is observed. All the observed lattice frequencies can be assigned on the basis of impurity modes in conjunction with InAs-, GaAs-, and InP-like vibrations.

• Received 26 December 1979

DOI:https://doi.org/10.1103/PhysRevB.21.5656