Gallium incorporation kinetics during gas source molecular beam epitaxy growth of GaN

The kinetics of gallium incorporation during gas source molecular beam epitaxy (GSMBE) growth of GaN are investigated for varying substrate temperature (T-s) and incident ammonia flux (J(NHj)). Incident Ga atoms eventually either (1) react with NH3 to form GaN, (2) accumulate on the film surface, or (3) desorb. For an incident Ga flux (J(Ga)) of 0.30 monolayer/s and an incident flux ratio of J(NH3)/J(Ga) similar to 10(3), desorption of Ga is significant for T-s above similar to 700 degrees C, and Ga surface accumulation is significant for T-s below similar to 750 degrees C. Two distinct activation energies for Ga desorption are observed: 1.4 +/- 0.1 eV for 625 degrees C < T-s < 740 degrees C and 0.4 +/- 0.1 eV for 740 degrees C < T-s < 825 degrees C. The rates of Ga desorption and Ga surface accumulation are both suppressed by increasing J(NH3). These effects combine to produce a GaN formation rate which increases with increasing J(NH3) and which peaks at T-s similar to 750 degrees C over the range of J(NH3) investigated. These results are consistent with a model based on the T-s-dependent reactivity of NH3, towards Ga and indicate the presence of a relatively complex surface chemistry, with a strong likelihood that hydrogen is playing an important role. (C) 1997 Elsevier Science Ltd.