Optical response of reconstructed GaP(001) surfaces

GaP(001) surfaces were found to reconstruct with (2\ifmmode\times\else\texttimes\fi{}1) or (2\ifmmode\times\else\texttimes\fi{}4) symmetry under surface conditions corresponding to a high or low V/III surface stoichiometry ratio, respectively. These surface reconstructions, identified by reflection high-energy electron diffraction, have been prepared in a chemical beam epitaxy (CBE) system by varying the sample temperature. Reflectance anisotropy spectra (RAS) of these surfaces have been taken under both CBE and metal-organic vapor phase epitaxy conditions. Three different phases of (2\ifmmode\times\else\texttimes\fi{}4) symmetry have been distinguished according to their characteristic RAS response in agreement with recent theoretical predictions [A.M. Frisch, W.G. Schmidt, J. Bernholc, M. Pristovsek, N. Esser, and W. Richter, Phys. Rev B 60, 2488 (1999)]. For the (2\ifmmode\times\else\texttimes\fi{}1) reconstruction a line-shape analysis of the RAS signatures was performed and their temperature shifts have been compared to the respective shifts of the bulk critical points. These experiments indicate the GaP(001)-(2\ifmmode\times\else\texttimes\fi{}1) surface dielectric anisotropy originating from transitions between bulk states modified by the surface due to band-folding effects and anisotropic shifts of the ${E}_{1}$ and ${E}_{0}^{\ensuremath{'}}$ critical point energies.