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 been investigated. Ultraviolet (193 nm) photolytic decomposition of trimethylgallium (TMGa) and ammonia (NH3) precursors was examined in this range. Laser-induced fluorescence studies support the view that the dissociated intermediate fragments GaCH3 and NH are the reacting species in GaN film formation, irrespective of substrate temperature. It was found that two crystal phases coexist in films grown at substrate temperatures below 500 degrees C, wurtzite crystal structure with (0002) orientation forms at substrate temperatures above 500 degrees C. The growth rate increases with both NH3/TMGa ratio, and TMGa flow rate, while 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 by conventional thermal CVD. The large NH3/TMGa ratios needed to achieve stoichiometry are interpreted in terms of the two-photon dissociation cross section of NH3.