Ultrafast electron dynamics in n-doped GaN was investigated using multiple-wavelength pump-probe techniques. A fast electron cooling with a time constant of 500 fs was observed, indicating the electron as the dominant carrier type in cooling processes. Electrons in band-tail states were found to relax at the same rate as conduction electrons, indicating fast (<500 fs) carrier capture into shallow band-tail states and fast scattering between shallow band-tail electrons and conduction band electrons. Our results agree well with the band-tailing model of Chakraborty and Biswas. Impurity screening potential was thus obtained. With a variation of pump photon energy, conduction band intervalley scattering of GaN was also studied. With a proper selection of pump wavelength, the electron cooling behavior was found to be delayed by intervalley returned electrons with a time constant on the order of 1 ps. By examining the fraction of the delayed cooling component, our data suggested an intervalley scattering threshold energy of 1.34 eV, which is the separation energy between the bottom of the U valley and Γ valley conduction band minimum in wurzite GaN.

• Received 6 January 1999

DOI:https://doi.org/10.1103/PhysRevB.59.13535