Surface-diffusion and step-bunching mechanisms of metalorganic vapor-phase epitaxy studied by high-vacuum scanning tunneling microscopy

On a very flat GaAs surface we grow a 1/6 monolayer amount of GaAs by metalorganic vapor-phase epitaxy and observe two-dimensional (2D) nuclei by high-vacuum scanning tunneling microscopy. From the 2D nucleus densities we calculate the surface diffusion coefficient of 2×10−6 cm2/s at 530 °C. During growth, the bunched step (multistep) separation saturates and is independent of the substrate misorientation angle. These results can be explained by a mechanism that takes into account both 2D nuclei formation on a wider terrace and their coalescence on ascending steps. A step-bunching simulation based on our model reveals that the saturated multistep separation is proportional to the 2D nucleus separation, i.e., the inverse of the square root of the density.