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Producción Científica, The luminescence of SrTiO3 depends on the sample type, either doped or stoichiometric, as grown or treated, the excitation conditions, and temperature. The origin of the luminescence emissions, blue, green, and infrared, remains controversial. In particular, the role played by defects, mainly oxygen vacancies, impurities, and self-trapped holes and electrons on the different emissions are far to be elucidated. We present a cathodoluminescence (CL) and photoluminescence (PL) study of undoped and Nb-doped samples. The different excitation conditions of CL and PL permit to distinguish the luminescence emission from the bulk (CL) and from a surface skin region (PL). Significant differences between both techniques are seen for the undoped sample, while the Nb-doped sample presents less differences, highlighting the role played by the surface defects and the doped electrons. The study is complemented by the temperature dependence of the luminescence spectra and the emission due to defects generated by plastic deformation., Junta de Castilla y León (project VA283P18), Ministerio de Economía, Industria y Competitividad (project ENE2017-89561-C4-3-R)

We have measured the polar scans from a cubic GaN(001) single crystal by X-ray photoelectron diffraction (XPD) with high angular resolution. The intensities were recorded within (100), (110) and (110) substrate planes from Mg Ka excited N 1s and Ga 3d core levels. The data reveal unexpected shapes in forward focusing peaks for the N is XPD patterns. Along [001] and [111] directions a large splitting in the structures is observed. The observed patterns are reproduced very well by a multiple scattering spherical wave cluster calculation but the single scattering approach is sufficient to qualitatively explain the remarkable features in the N Is profiles. In particular we demonstrate that the splitting of the structures along some low index crystallographic directions is caused by the predominance of interference phenomena due to atoms located in specific positions close to the dense atomic chains. As a result of the smaller forward scattering amplitude of the N atoms as compared to that of the Ga atoms, the interference effects are strongly reinforced when there are Ga atoms close to N atomic chains. In the Ga 3d XPD patterns these interference effects also appear but forward scattering still prevails along low index directions.

Ultra-thin Mn films have been evaporated in ultra-high vacuum on a Ag(001) substrate kept at temperatures between 210 and 340 K. A c(2×2) LEED (low-energy electron diffraction) diagram is obtained due to the formation of a two-layer-thick superficial Mn–Ag alloy. The intensity and width of the half integer spots are investigated as a function of coverage, deposition temperature, and time elapsed after Mn deposition. The brightest and sharpest diagram is obtained for ∼1.5 monolayers deposited at room temperature, which corresponds to a surface plane made of c(2×2) domains formed by Mn and Ag atoms in a checkerboard arrangement, while the subsurface plane is saturated in Mn. Depending on the preparation conditions, various c(2×2) surface alloys may be obtained that mainly differ in the MnxAg1−x composition of the second atomic plane. A comparison of the time evolution of the LEED spot intensity with ion scattering spectroscopy data reveals that two different thermally activated phenomena determine the formation and nature of the c(2×2) superstructure: on the one hand, part of the deposited Mn atoms must exchange with Ag atoms from the substrate first plane, and on the other hand, the Mn and Ag atoms of the film surface plane diffuse laterally so that nucleation and growth of the c(2×2) domains take place.

We observe a complex shape of the Mn2p core level photoemission spectra for Mn in structures of reduced dimensionality such as an ideal monolayer on Ag(001). The Mn2p 3/2 and Mn2p 1/2 spectral intensity is found to split into doublets with separation about 4 and 5 eV, respectively. The data indicate an interpretation in terms of a poorly screened and a well screened final state of the Mn with a core hole. In the monolayer range these two final states have comparable intensities and dominant contributions from atomic 2p 5 3d 5 and 2p 5 3d 6 configurations, respectively. Their intensity ratio is found to be extremely sensitive to the Mn atomic environment and directly reflects the degree of the Mn3d localization, electron correlation and reduction in charge fluctuations. Additional fine structure is clearly visible in high-resolution data for both final state 3d electrons counts and can be assigned to direct and exchange Coulomb interactions of the unfilled core and 3d shells. Slow 3d-charge fluctuations also result in a remarkable three-peak structure of the Mn3s line.

Mn films with a thickness of 2–15 monolayers (ML) have been deposited, in ultra-high vacuum, on a Ag(001) single crystal and analyzed by low energy electron diffraction (LEED) and X-ray photoelectron diffraction (XPD). When a 12 ML film is deposited at room temperature (RT), a ∼3 ML-thick diffuse interface is formed, and Ag segregation occurs on top of an epitaxial body centered tetragonal (bct) Mn layer. If the same amount of Mn is deposited at 80 K in order to block interdiffusion, the film grows in a polycrystalline α-Mn phase. Annealing the sample at 300 K does not markedly change its structure. However, when a 3 ML template of Mn is first deposited on the substrate kept at 80 K, and then 9 ML at 300 K, the data show that the film is pseudomorphic and adopts an epitaxial bct phase, and that the system exhibits an abrupt interface.

The growth mode and electronic properties of ultrathin films of silver deposited at room temperature (RT) on a 9 monolayers (ML) pseudomorphic Mn film on a Ag(001) surface have been studied by means of low energy electron diffraction (LEED), X-ray photoelectron diffraction (XPD) and angle-resolved ultraviolet photoemission (ARUPS). In particular ARUPS was used to investigate the valence band structure. Initially the data show some Ag segregation on top of the Mn film grown at RT. In the 0.5–0.9 ML coverage range the Ag adlayer exhibits d-band spectral behavior characteristic of two-dimensional dispersion. Further evaporation of Ag results in the formation of Ag bilayer platelets, and > 3 ML deposition the system exhibits a three-dimensional electronic structure converging towards a bulk Ag(001)-like system. Thus the growth mode of Ag on the Mn film is found to be nearly layer by layer up to 2 ML.

We have studied the structure of epitaxial Mn sandwiches made of ∼ 1 monolayer (ML) Ag/3 ML Mn/Ag(100) by using photoemission methods and low-energy electron diffraction (LEED). Epitaxial films of 3 ML Mn grown on Ag(100) at room temperature (RT) are found to have a body-centered-tetragonal (bct) structure. Yet, upon deposition of ∼ 1 ML Ag, remarkable changes occur in the Mn film. X-ray photoelectron diffraction data reveal that the Mn layer relaxes towards a face-centered-cubic (fcc) structure with the lattice parameter of Ag. This implies an exceptionally large atomic volume for a 3d transition metal as well as unusual electronic and magnetic properties. Actually, an evolution towards more atomic-like behaviour is clearly evidenced by valence band and Mn3s core level photoemission data.

The growth mode of Mn on Ag(100) in the monolayer range was studied using X-ray photoelectron diffraction (XPD), angle-resolved valence band photoemission and low energy electron diffraction. Our results show that MnAg intermixing takes place. The thickness of the intermixed films depends critically on the growth temperature. At room temperature the XPD data clearly indicate that Mn is located in the first and second topmost atomic layers. The growth at higher temperatures, i.e. for 1 ML deposited at 460 K, or 1 ML deposited at RT and then annealed at 460 K, results in enhanced interdiffusion and Ag segregation.

Thin Cu layers have been evaporated at room temperature on Cr films (~ 10 monolayers) deposited on a Cu(001) single crystal. The Crystallographic structure of these sandwiches is analysed by means of low-energy electron diffraction (LEED), angle-resolved ultra-violet photoemission spectroscopy (ARUPS) and X-ray photoemission spectroscopy. The data show that the attenuation of the Cr signal versus Cu deposition is much lower than expected for a uniform film growth. Moreover, the LEED pattern characteristic of the Cr film (made of Cr(110) domains with four different orientations) is visible for Cu coverages as high as 23 ML. This indicates a strong diffusion of the Cu atoms on the Cr film surface and the formation of Cu three-dimensional islands which present, according to ARUPS and X-ray photoelectron diffraction measurements, a highly disordered fcc Crystallographic structure. However the low reactivity of the sandwiches towards oxygen suggests that the whole Cr surface is wetted by at least 1 ML of Cu.

Thin Cr films (∼20 A) deposited in vacuum at 300 K on a Cu(001) single crystal have been annealed at various temperatures and analyzed by angle resolved ultraviolet photoemission, X-ray photoemission, X-ray photoelectron diffraction and low energy electron diffraction. The data show that Cu segregation occurs above 425 K, resulting in an atomic monolayer wetting the whole Cr film. At 775 K, the Cr overlayer collapses and the Cu(001) substrate surface becomes partly uncovered.

We have investigated the crystallographic structure of ultra-thin Mn films deposited at room temperature on a clean Ag(100) single crystal, using angle-resolved ultra-violet photoemission, X-ray photoelectron diffraction (XPD) and low-energy electron diffraction. The evolution of the Ag 4d valence band shows deviations from simple layer by layer growth. These results, combined with the XPD data, indicate that by ∼1–1.5 monolayers of Mn a two layers thick Mn-rich epitaxial MnAg alloy is formed that continues the face centered cubic lattice of the Ag substrate.