GROWTH MECHANISMS IN EXCIMER-LASER PHOTOLYTIC DEPOSITION OF GALLIUM NITRIDE AT 500-DEGREES-C

The mechanisms of controlling laser-induced chemical vapour deposition of GaN at substrate temperatures between 350 and 650 degrees C have b een investigated. Ultraviolet (193 nm) photolytic decomposition of tri methylgallium (TMGa) and ammonia (NH3) precursors was examined in this range. Laser-induced fluorescence studies support the view that the d issociated intermediate fragments GaCH3 and NH are the reacting specie s in GaN film formation, irrespective of substrate temperature. It was found that two crystal phases coexist in films grown at substrate tem peratures below 500 degrees C, wurtzite crystal structure with (0002) orientation forms at substrate temperatures above 500 degrees C. The g rowth rate increases with both NH3/TMGa ratio, and TMGa flow rate, whi le the temperature dependence shows a thermal activation energy of 0.2 eV which is smaller by a factor of five than that of films prepared b y conventional thermal CVD. The large NH3/TMGa ratios needed to achiev e stoichiometry are interpreted in terms of the two-photon dissociatio n cross section of NH3.