We have calculated the frequencies of zone-center optical phonons in ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As as well as ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As by using a modified random-element-isodisplacement model and obtained good agreement with experimental values. The optical phonons in ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As exhibit two-mode behavior as in the case of ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As. The force constants in terms of the alloy compositions were discussed.

• Received 20 July 1994

DOI:https://doi.org/10.1103/PhysRevB.50.18644