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

Carrier dynamics in a thin GaN epilayer with a thickness of 0.15 mum almost coherently grown on Al0.2Ga0.8N 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.