Electronic structures of GaP(100) surface reconstructions probed with two-photon photoemission spectroscopy

Philipp Sippel, Oliver Supplie, Matthias M. May, Rainer Eichberger, and Thomas Hannappel

Phys. Rev. B 89, 165312 – Published 28 April 2014

Abstract

The electronic structures of two significant atomically well-defined (100)-surface reconstructions of gallium phosphide were investigated with two-photon-photoemission spectroscopy (2PPE). We allocated a series of occupied and unoccupied surface states and deduced the influence of each particular reconstruction on the electronic structure of the surface. Photoemission signals arising from bulk optical transitions were distinguished from surface-state related signals by studying the influence of oxygen exposure to the surfaces and by comparing our results to reflectance anisotropy spectroscopy (RAS) measurements. This revealed that the features in the RAS signal correlate with the transition energies between unoccupied and occupied surface-related states that were observed in the 2PPE measurements. An anisotropy around the $E_{0}^{'}$ critical point, previously known from RA spectra, was also investigated with 2PPE and is ascribed to modifications of the bulk electronic structure in the vicinity of the surface.

Received 18 October 2013
Revised 21 February 2014

DOI:https://doi.org/10.1103/PhysRevB.89.165312

Authors & Affiliations

Philipp Sippel^*, Oliver Supplie, Matthias M. May, and Rainer Eichberger

Helmholtz-Zentrum Berlin für Materialien und Energie, Institute for Solar Fuels, Hahn-Meitner-Platz 1, D-14109 Berlin, Germany

Thomas Hannappel

Technische Universität Ilmenau, Institut für Physik, Postfach 100565, D-98684 Ilmenau, Germany

^*philipp.sippel@helmholtz-berlin.de

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Issue

Vol. 89, Iss. 16 — 15 April 2014

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Images

Figure 1
RA spectra of the Ga-rich and the P-rich surface reconstruction, clean (black) and after $O_{2}$ exposure (red/grey). The insets show the atomic models of the Ga- and the P-rich surface reconstructions according to Frisch et al. [9] and Hahn et al. [13], respectively. Dashed lines indicate critical point transitions and vertical bars indicate energy differences between occupied and unoccupied surface states, as well as between occupied surface states and selected bulk states, as identified with 2PPE.
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Figure 2
Spectra of both surface reconstructions, indicated here with Ga and P, respectively, before and after oxygen exposure. The excitation energies were (a) $h ν = 4.59 eV$ and (b) $h ν = 4.96 eV$ . The spectra are normalized to the peak $B$ and shifted with respect to the $y$ axis for better visibility.
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Figure 3
Schematics of the surface-state energies relative to the bulk band edges on the left-hand side for the P-rich surface and on the right-hand side for the Ga-rich surface. Vertical arrows correspond to photons of $h ν \approx 4.86 eV$ and indicate transitions that lead to peaks in the spectra. The bulk band structure in the middle, sketched according to Ref. [44], illustrates the bulk transitions, discussed in the text. The curved arrow indicates a scattering process. Peaks $B$ and $C$ have been left out, since their origin could not be allocated unambiguously.
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Figure 4
2PPE spectra of the clean Ga-rich surface for different excitation energies. The energy scale represents the intermediate-state energy with respect to the CBM. The inset shows the multiple gauss fit with a monoexponential background, which was used to determine the positions of the peaks, exemplarily for the spectrum recorded with $h ν = 4.77 eV$ .
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Figure 5
Intermediate-state energy with respect to the CBM for different peaks of the Ga-rich (filled symbols) and P-rich (open symbols) surface in dependence of the excitation energies. The energy scale is calculated according to Eq. (3). Values for $A^{'}$ and $C$ are only plotted for excitation energies, where unambiguous extraction of the peak position was possible.
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Figure 6
(a) 2PPE spectra of the Ga-rich surface recorded with a photon energy of $h ν = 4.86 eV$ with $p$ and $s$ polarized laser light of same intensity, respectively, before oxygen exposure of the sample. (b) Anisotropy of the signal based on the assumption that peak $B$ is isotropic for the sample before oxygen exposure and (c) after oxygen exposure. The vertical dashed line marks the position of $D_{Ga}$ .
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