Band gap changes of GaN shocked to 13 GPa

The band gap of GaN under uniaxial-strain compression was determined using time-resolved optical transmission measurements in shock-wave experiments. Shock waves were generated by impacting the GaN samples with c-cut sapphire impactors mounted on projectiles fired by a gas gun. Impact velocities were varied to provide longitudinal stresses ranging from 4.5 to 13 GPa. An abrupt increase of the band gap is observed upon shock-wave compression, followed by a slower increase. By measuring the absorption threshold before and during shock compression of the GaN layer, the band-gap shift for a particular longitudinal stress was obtained. A linear fit to the data yields a band-gap shift of 0.02 eV/GPa. Comparison with ab initio calculations show that this slope lies between the calculated bounds for isotropic and uniaxial compression. Potential reasons for the differences are indicated.