Ultrafast Carrier Dynamics in Nitride-based wide band gap semiconductors: Hexagonal Boron Nitride and Aluminum Nitride

We used time-resolved pump-probe spectroscopy with sub-picosecond resolution to observe free carrier dynamics in hexagonal boron nitride (h-BN) and aluminum nitride (AlN). The samples were excited by two-photon excitation with a pump pulse at 267 nm and probed by an infrared beam spectrally tuned to the edge of the Reststrahlen band (e.g. 1650 cm^-1for h-BN). We observed strong Reststrahlen band softening with the injection of carriers due to the (LO)- phonon free-carrier plasma coupling effect. Lifetimes were observed from a few picoseconds to nanoseconds in h-BN and AlN.