By exploiting non-destructive and non-invasive spectroscopic ellipsometry, we have significantly enhanced the understanding of metallic adatom surface kinetics. The metallic transition layer, i.e. buffering layer prior to the adatom incorporation to epilayer, is of a great importance in III-N semiconductor growth.
One of the main aspects of this thesis is to ascertain the ability of spectroscopic ellipsometry (SE) to investigate the kinetics of group-III atoms. The SE, which is a well-developed ex-situ multilayer analyzing technique, is affixed to a molecular beam epitaxy growth chamber for in-situ real-time monitoring. From the variation of optical characteristics during material growth, we found how the impinging adatoms adsorb, migrate, accommodate, and desorb on the surface with a single monolayer precision.
An experiment of gallium (Ga) adlayer on wurtzite gallium nitride (GaN) was conducted for three different orientations, (0001), (000-1) and (1-100). We found that the Ga adatoms form a 2-dimensional wetting layer, and continued Ga adatoms impingement beyond a corresponding critical thickness gives rise to 3-dimensional droplets, which act as a Ga reservoir. By comparing the adsorption and desorption characteristics, we verified that the desorption activation energy is caused by a chemical bonding, and the difference of adsorption and desorption for different orientations of the surface is caused by different sticking coefficients for adsorption, and vibration frequency for desorption process.
For indium on GaN (0001) surfaces, analysis of an indium surfactant and segregation effects are focused. In this work, we found that the In adlayer has two stable coverages due to the existence of a highly desorptive nucleation process prior to the lateral clustering to complete a monolayer. From the observation of flux threshold in the adsorption rates, it is apparent that the In wetting layer accommodation process follows two steps: nucleation formation and nucleation-mediated lateral clustering.
Kinetics of Al on AlN was investigated in a nitrogen ambient environment. By monitoring Fabry-Perot interference using SE, we found that the Al atoms supplied from Al droplets continuously fill the unoccupied sites in the wetting layer, and that only the Al atoms spread on the surface interact with nitrogen atoms yielding a continuous 2D AlN growth.