Raman investigation of anharmonicity and disorder-induced effects inGa1−xAlxAsepitaxial layers

Abstract

Raman scattering measurements between 20 and 450 K are reported in ${\mathrm{Ga}}_{1-x}{\mathrm{Al}}_x\mathrm{As}$ single crystals for the range $0\le x\le 1$. Peak-frequency and linewidth variations versus temperature and composition of the first-order modes are studied in the light of disorder and anharmonicity. The asymmetry of the LO mode is tentatively explained with the help of an elementary model, chiefly based on the knowledge of the general form of the dispersion curves in the zinc-blende structure. A detailed analysis of the anharmonic effects is carried out in the case of GaAs and the calculated cubic contribution is found to be dominant with respect to the quartic one. It is shown that the anharmonicity is not affected by the substitutional disorder in ${\mathrm{Ga}}_{1-x}{\mathrm{Al}}_x\mathrm{As}$. A resonant Raman study of an aluminum-rich composition $x=0.75\mathrm{}$) allows an accurate observation of disorder-induced modes corresponding to the Brillouin-zone edges $X$ and $L$ of the acoustic branches. Disorder-activated longitudinal- and transverse-acoustical structures exhibit the same resonance behavior as the first-order lines and are chiefly ${\Gamma }_1$ polarized.