Averaged surface roughness of samples A1–C1. Each sample corresponds to a different set of etching and/or annealing conditions given in Table I .
(Color online) (a) AFM image of the germanium surface after the wet-etching treatment and subsequent annealing without plasma exposure; (b) AFM image of the same sample after the oxygen plasma cleaning and subsequent annealing. The germanium surface is free of any contamination and much smoother than before the plasma cleaning.
(Color online) RHEED image of the germanium sample along the  zone axis after the oxygen plasma cleaning showing intense and sharp half order spots, indicating a 2 × 1 reconstruction.
(Color online) (a) Ge 3 d spectrum shows the characteristic peak for pure germanium. The surface is free of any GeO2, since no features of any possibly remaining germanium oxides, which have higher binding energies, are detectable. (b) O 1 s spectrum shows the nonexistence of oxygen on the surface. The two broad features which can be seen are due to Ge L2M23M23 and L3M23M23 Auger electrons. (c) C 1 s spectrum shows no residue of carbon could be found on the cleaned surface, whose characteristic peak would have been at expected around 284.5 eV.
(Color online) ARPES spectra taken at 300 K with a photon energy of hν = 21.22 eV at various emission angles, θe, along the  crystallographic direction. Structures labeled with S are interpreted as surface-related features and those with A to a direct bulk transition. The sharpness of the photoemission spectra and the presence of the surface features are a clear indication of the surface quality.
Procedures for wet etching the germanium surface. Samples A1 and A2 illustrate the effect of in situ annealing right after wet etching. Samples B1–B3 are used to study the effect of acid concentration on the surface roughness. All samples (except A1 and C1) are annealed at 500 °C for 2.5 h and subsequently at 600 °C for 0.5 h (ramping rate: 5 °C/min) before the final annealing step which is shown in the table. The mean value and the standard deviation of the surface roughness were calculated using several 5 × 5 μm2 images of the sample's surface.
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