Relation between Ga Coverage and Surface Reconstructions

In the course of our previous work (27), we noticed that all smooth layers exhibited a complex RHEED pattern when the surface was quenched to about 150 C with a cooling rate of 40 C/min and the N flux shut off immediately at the termination of growth. The RHEED pattern observed exhibits a fourfold periodicity along all major azimuths, consistent with a (4 x 4) reconstruction with respect to the unit mesh of the GaN(liOO) surface. Note that this reconstruction is not identical to the pseudo-(4 x 5) reconstruction reported by Lee et al. [18] which gave rise to a fourfold pattern along the [0001] azimuth only. In the following, we establish a relation between the adsorbed Ga coverage and reconstructions of the surface. 

It is clear from these experiments that two stable Ga coverages exist, which condense at low temperatures into a (1 x 2)/(4 X 2) reconstruction at bUayer coverage and a (4 x 4) reconstruction at bilayer coverage. On surfaces with noninteger amounts of adsorbed Ga, both reconstructions may coexist (presumably in the form of domains), although we have no experimental evidence for such a coexistence. Furthermore, the recovery behavior of the surface at elevated temperatures provides the opportunity to study the adsorption/desorption kinetics of Ga in real time. 

we investigate Ga adsorption and desorption kinetics in vacuum. The sample vras held at a constant temperature of 740 °C. Ga adsorption/desorption isotherms were recorded by monitoring the intensity of the specularly 

To confirm the four-stage scenario mentioned earlier, we have developed a quantitative model. The model is required to account for adsorption and desorption of Ga, for the formation of excess Ga interacting with the Ga adlayer and for GaN growth in the case of the simultaneous presence of Ga and N. Our model, in units normalized to the maximum Ga-adlayer coverage, is described by the following equations  

Both the adlayer coverage 9 and the excess coverage n are assumed to damp the intensity exponentially. The ripening of excess Ga to clusters and droplets leads to a different rate with respect to the Ga-adlayer coverage as expressed by the prefactor b. 

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