Chronoamperometric conditioning of float zone n-Si(111) in 2M NaOH solution in the potential range negative from open-circuit potential is performed in a combined electrochemistry/ultrahigh-vacuum surface analysis experiment. Synchrotron Radiation Photoelectron Spectroscopy measurements at the U49/2 beamline at Bessy II using the SoLiAs facility show formation of a ultrahigh-vacuum-stable permanent accumulation layer without junction formation. Comparison of the Thomas–Fermi screening potential and the mean inelastic scattering length λesc of photoelectrons at hν=150eV (λesc=4Å) and hν=585eV (λesc=15Å) indicates a surface electron concentration of 3×1018cm−3 for a bulk doping level of 1015cm−3. The observed shift of the Si2p3∕2 and 2p1∕2 core level with photon energy is in excellent agreement with the shifted onset of the x-ray photoelectron spectroscopy valence-band spectrum measured at hν=150eV.
The antiferromagnetic phase transition of octahedrally coordinated (Cd, Mn)S single crystals with Mn concentrations 0.8 _I 3 4 1~~ _I 1 is studied by EPR measurements in the temperature range 80 to 550 K. The width of the Mn2+ EPR line is found to increase with decreasing temperature nearly proportional to (T -T N ) -l due to a corresponding decrease of the spin-spin relaxation time. The NBel temperature T N for the transition from the paramagnetic into the (disordered) antiferromagnetic phase being determined from the linewidth divergence decreases strongly with decreasing z~~. The results are discussed in connection with actual EPR investigations of (Cd, Mn) and (Zn, Mn) chalcogenide mixed crystals with zincblende/wurtzite structure for lower Mn concentrations. The EPR signal below T N being observed in the case of polycrystalline thin films or powder samples is ascribed to surface near regions of the microcrystals.
Synchrotron radiation photoelectron spectroscopy was employed to investigate the chemical state of Si(111) surfaces upon anisotropic etching in concentrated NH4F solution. Minute amounts of oxidized silicon were detected and attributed to the fast Si–H–OH formation at atomic steps. Combining in situ optical and scanning probe techniques, consecutive chemical treatments were developed to achieve optimized morphological and chemical surface properties. Native oxides and a stressed SiO2/Si layer are removed by a two-step NH4F treatment leading to a terraced surface without triangular etch pits; subsequently, silicon in the Si1+/2+/3+ valence states is dissolved by HF (50%) while the surface topography is preserved.
The surface condition of electrochemically H-terminated Si is compared with the situation at the first photocurrent maximum in dilute acidic ammonium fluoride solution where the divalent dissolution converts into the four-valence process. The first high spectral-resolution photoelectron spectroscopy data using synchrotron radiation of electrochemically hydrogenated Si are presented. A combined electrochemistry/ultrahigh vacuum surface analysis system, attached to the U 49/2 beamline at the synchrotron Bessy II, is used for photoelectron spectroscopy ͑PES͒ of the electrochemically conditioned samples. We analyze the Si 2p, O 1s, and F 1s core levels. A comparison of a density functional theory calculation of the reaction sequence, proposed in the dissolution model of Gerischer and co-workers, with the PES results supports this model. The anodized sample is characterized by a residual H coverage of 0.35 monolayers evidenced by a surface core level shift, Si-OH and Si-F x species, F Ϫ , and a higher oxidized Si species. SiO 2 is not found. Even on very well H-terminated surfaces, we find residual Si-OH complexes, fluoride and water. In situ atomic force microscopy shows a roughening with a root mean square roughness parameter of 2.6 nm.Electrochemical surface conditioning of Si has a long history with important applications, using topography and structure influencing processes such as electropolishing, roughening in the divalent dissolution regime, and anodic oxidation, for instance. 1-3 Despite the industrial importance of such treatments, little is known about the microscopic details of the interfacial processes. For applications in photonic gap materials, porous Si formation and new light coupling structures for solar cells, 4-6 a better knowledge of the initial dissolution processes occuring during divalent dissolution in acidic fluoride containing solutions can lead to improved control of structure formation. Several dissolution models have been proposed. [7][8][9] The most elaborate model suggests various reaction intermediates which can, in principle, be analyzed by surface sensitive methods ͑in situ and ex situ͒ provided the species reside on the surface and are present in sufficient amounts ͑у0.1 monolayers, MLs͒ and live long enough. For detection in ex situ experiments, the bonding to the surface can be important. We use photoelectron spectroscopy ͑X-ray photoelectron spectroscopy XPS, and ultraviolet photoelectron spectroscopy UPS͒ as ex situ surface characterization methods employing synchrotron radiation for higher surface sensitivity in XPS and high spectral resolution. 10,11 In XPS core level shifts, the partial charge change on the Si surface atoms can, in principle, be followed. We compare these changes with theoretical calculations using density functional theory ͑DFT͒ of the species suggested in the preferred model. In situ atomic force microscopy ͑AFM͒ measurements are merely used to give evidence of observed structural changes. ExperimentalFor photoelectron spectroscopy ͑PES͒ analyses, electroc...
Experimental investigation of the electronic structure of Mo-doped BiVO4 high-quality single-crystals with synchrotron radiation-excited angle-resolved photoelectron spectroscopy (ARPES).
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