STUDY OF GAN FILMS BY PULSED-LASER PHOTOELECTRON-SPECTROSCOPY

We have studied surface defect states and conduction electrons in mole cular-beam epitaxy (MBE)-grown and metalorganic vapor phase epitaxy (M OVPE)-grown GaN films using the third and fourth harmonics of a Ti:sap phire pulsed laser to excite photoelectrons by one- and two-photon tra nsitions. By introducing a delay between the excitation and probe puls es in two-step excitations it is possible to measure the intermediate state lifetimes. Spectra of occupied surface defect states in the gap of n-type GaN are reported. Under intense photon pumping, conduction e lectron spectra are observed. The relaxation time of hot carriers in t he conduction band is less than 200 fs. (C) 1998 Elsevier Science B.V. All rights reserved.