In-In pair correlations and In surface segregation in $\mathrm{In}{}_x\mathrm{Ga}{}_{1-x}\mathrm{As}$ alloys are studied by first-principles total-energy calculations. By calculating the substitution energy of a single In atom, we find that the near-surface energetics explains the observed In segregation on $\mathrm{InGaAs}\left(001\right)-\beta 2(2\times 4)$ surfaces. Indium surface segregation further enhances the In site selectivity, thus the long-range ordering. We find that the [110] and [001] In-In pair correlations are repulsive and nearly isotropic in bulk but are highly anisotropic near the (001) surface. The sign of the [110] In-In interaction energies vs the distance from the surface is oscillatory. These findings explain the recent puzzling cross-sectional $\times$-STM results.

• Received 28 May 1999

DOI:https://doi.org/10.1103/PhysRevLett.84.3654