Surface morphology and ionization potentials of polar semiconductors: The case of GaAs

Using an ab initio pseudopotential approach, we have studied the ionization potential of several surfaces of GaAs, with different orientation, reconstruction, and stoichiometry. In particular, we have examined the As-rich (100) $c-(4\mathrm{}\times 4),$ $(2\mathrm{}\times 4)2\beta ,$ and $(2\mathrm{}\times 4)\beta$ surfaces, the Ga-rich (100) $(4\mathrm{}\times 2)2\beta$ and $(4\mathrm{}\times 2)\zeta$ surfaces, as well as the (110) surface. The calculated variations of the ionization potential with surface morphology are as large as 0.7 eV. The largest ionization potential (5.70 eV) occurs for the As-rich $(2\mathrm{}\times 4)\beta$ surface, and the smallest (5.00 eV) for the Ga-rich $(4\mathrm{}\times 2)2\beta$ one. The results are compared with available experimental data, and are explained in terms of a model based on a superposition of neutral bulk charge-density units, carrying neither dipole nor quadrupole, and charge transfers between the units dictated by the electron counting rule.