Effect of pressure on optical phonon modes and transverse effective charges in $\mathrm{GaN}$ and $\mathrm{AlN}$

The pressure behavior of phonon modes of the hexagonal and cubic modifications of GaN and hexagonal AlN was investigated experimentally. The mode pressure coefficients were determined from Raman measurements at hydrostatic pressures up to 6 GPa. The low-frequency $E_2$ phonon in GaN exhibits a weak softening which is qualitatively similar to that of zone-boundary transverse acoustic modes of zinc-blende III-V semiconductors. In AlN the $E_2\mathrm{}\left(\mathrm{low}\right)\mathrm{}$ phonon frequency is essentially constant under pressure. For both materials an increase of the LO-TO splitting is observed, which results from the interplay between the pressure dependence of the high-frequency dielectric constant and Born’s transverse dynamical effective charge. The latter turns out to be nearly constant under pressure, a behavior deviating from that of other III-V semiconductors. The experimental findings are compared to results of ab initio calculations.