Co on p-InAs(110): An island-induced two-dimensional electron system consisting of electron droplets

The coverage dependence of the Co-induced band bending on p-type InAs(110) ${(N}_A=4.6\mathrm{}\times {10}^{17}\hspace{0.5em}{\mathrm{cm}}^{-3})$ is investigated by angle-resolved photoelectron spectroscopy and interpreted in terms of the morphology of the adsorbate layer deduced from scanning-tunneling-microscopy (STM) images. After room temperature deposition Co forms small islands with a density close to the density of bulk Co. At low coverages each of these islands donates about one electron to the substrate leading to a laterally inhomogeneous band bending. At higher coverages less electrons per island are donated resulting at the highest coverage in a rather homogeneous surface band shift of 560 meV. Surprisingly, an occupied two-dimensional electron system is already found at a band shift of 420 meV (0.2 monolayer coverage). This is not expected in the one-dimensional band-bending model, which in this case predicts a subband energy 80 meV above the Fermi level. The discrepancy is explained by lateral inhomogeneities of the surface potential.