Effects of internal electric field and carrier density on transient absorption spectra in a thin GaN epilayer

Abstract

Carrier dynamics in a thin GaN epilayer with a thickness of $0.15 \ensuremath{\mu}\mathrm{m}$ almost coherently grown on ${\mathrm{Al}}_{0.2}{\mathrm{Ga}}_{0.8}\mathrm{N}$ was investigated using transient absorption spectroscopy above and below the band gap. The absorption peak of excitons was observed at 3.521 eV without a pump beam. Just after the photoexcitation above the band gap, the bleaching occurred and the absorption peak of excitons disappeared due to high-density photogenerated carriers. At about 30 ps, the absorption peak was restored at 3.527 eV, as the carriers were decreased owing to radiative and/or nonradiative recombination. After that, the absorption spectra were larger than that without a pump beam. This is because excitonic absorption suppressed by both Franz-Keldysh and Stark effects is restored by photoinduced carriers as a result of the screening of the internal electric field.