Adatom Kinetics On and Below the Surface: The Existence of a New Diffusion Channel

Employing density-functional theory in combination with scanning tunneling microscopy, we demonstrate that a thin metallic film on a semiconductor surface may open an efficient and hitherto not expected diffusion channel for lateral adatom transport: adatoms may prefer diffusion within this metallic layer rather than on top of the surface. Based on this concept, we interpret recent experiments: We explain why and when In acts as a surfactant on GaN surfaces, why Ga acts as an autosurfactant, and how this mechanism can be used to optimize group-III nitride growth.

Figure 1

Phase diagram of possible equilibrium surface reconstructions of bare and In-covered GaN (0001) as a function of the In and Ga chemical potentials. Also shown is the schematic geometry (side view) of the relevant surface structures. Small, medium, and large sized filled circles mark the positions of N, Ga, and In atoms, respectively.

Figure 2

Cross-sectional view of the valence charge density for a nitrogen adatom at the stable subsurface site (left) and at the barrier configuration for subsurface diffusion (right). Grey and dark grey (online color: blue) filled circles mark the position of Ga and N atoms, respectively. The balls in the top layer mark In atoms. The numbers give the bond length in $\mathrm{\AA }$.

Figure 3

Potential energy surface for a subsurface N adatom at the In adlayer GaN (0001) surface. The line with arrows marks the lowest energy diffusion path. The large/small filled circles indicate In/N atoms.

Figure 4

Experimental results for In-terminated GaN films, comparing (0001) (left side) and $(000\overline{1})$ (right side) surfaces. (a),(b) STM images, for films terminated with a layer of In atoms. The feature “A” in (b) arises from a Ga atom in the top layer, with all other top layer atoms being In. The bright corrugation maxima in both images arise from In atoms in underlying layers. The images were acquired with sample voltages of $+1.25$ and $-1.0\mathrm{V}$, respectively, and are displayed with gray scale ranges of 0.5 and 0.8 nm, respectively. (c),(d) Side view models for the surface structures. (e),(f) AFM images of the surface morphology of InGaN films. The images are displayed with gray scale ranges of 15 and 23 nm, respectively.