Atomistic View of the Autosurfactant Effect during GaN Epitaxy

The Ga-N site exchange critical to the autosurfactant effect during GaN epitaxy is studied. On the GaN(0001) pseudo ($1\times 1$), the first site exchange results in N incorporation at the subsurface $T_1$ site, forming ghost islands. The second exchange that converts these islands to that of bilayer height can be triggered by continued scanning tunneling microscopy imaging, which involves electrons tunneling to or from localized states associated with the second layer Ga. The resulting electrostatic force sets off a chain reaction which frees these Ga atoms, allowing N to form covalent Ga-N-Ga bonds of a new GaN bilayer.

Figure 1

(a) STM image of (1) bright-edged, (2) center-depressed, (3) striped, and (4) flat islands. A center-depressed island both before (b) and after (c) conversion with corresponding line profiles. (The tilt observed in the line profiles is an artifact from the unprocessed images.) (a) $(200\times 200){\mathrm{nm}}^2$, sample bias $-0.1\mathrm{V}$, and tunneling current 0.1 nA; (b),(c) $(55\times 65){\mathrm{nm}}^2$, sample bias $-1.5\mathrm{V}$, and 0.1 nA.

Figure 2

STM images of bright-edged islands (a) before and (b) after conversion. Line profiles taken along the indicated arrows are also shown. (a) $(25\times 25){\mathrm{nm}}^2$, sample bias $-0.12\mathrm{V}$, 0.2 nA. (b) $(20\times 20){\mathrm{nm}}^2$, sample bias $-0.81\mathrm{V}$, 1.2 nA.

Figure 3

(a) STM and corresponding (b) STS images of both converted and unconverted islands. Image size: $(40\times 100){\mathrm{nm}}^2$, sample bias 0.1 V, and 0.1 nA.

Figure 4

Calculated total density of states (a) without and (b) with N incorporated at a $T_1$ site; (c),(d) the corresponding density of states at the Fermi level.

Figure 5

Ball-and-stick model illustrating the Ga-N exchanges involved in the growth of a GaN bilayer.