Crystalline structure of mixed ${\mathrm{Ga}}_{1-x}{\mathrm{Al}}_x\mathrm{As}$ and $\mathrm{Ga}{\mathrm{P}}_{1-x}{\mathrm{As}}_x$ crystals

Two-phonon processes have been used to study the crystalline structure of two mixed-crystal systems. The two-phonon spectra of ${\mathrm{Ga}}_{1-x}{\mathrm{Al}}_x\mathrm{As}$ show coupled optical zone-edge phonons, which are GaAs-like, AlAs-like, and also combinations of GaAs-like and AlAs-like zone-edge phonons. The latter combination is not observed in $\mathrm{Ga}{\mathrm{P}}_{1-x}{\mathrm{As}}_x$, the other mixed-crystal system studied here, where the linewidth of the GaP-like Raman mode increases and the Raman intensity rapidly decreases with increasing As content. We explain these results qualitatively by assuming a random distribution of Ga and Al atoms in ${\mathrm{Ga}}_{1-x}{\mathrm{Al}}_x\mathrm{As}$ crystals, and a distortion of the lattice structure as well as a departure from a random distribution of atoms in $\mathrm{Ga}{\mathrm{P}}_{1-x}{\mathrm{As}}_x$.