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 atom s 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 1 0(3), desorption of Ga is significant for T-s above similar to 700 deg rees C, and Ga surface accumulation is significant for T-s below simil ar to 750 degrees C. Two distinct activation energies for Ga desorptio n 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 i ncreasing J(NH3). These effects combine to produce a GaN formation rat e which increases with increasing J(NH3) and which peaks at T-s simila r to 750 degrees C over the range of J(NH3) investigated. These result s are consistent with a model based on the T-s-dependent reactivity of NH3, towards Ga and indicate the presence of a relatively complex sur face chemistry, with a strong likelihood that hydrogen is playing an i mportant role. (C) 1997 Elsevier Science Ltd.