Ultraviolet Photoelectron Spectroscopy (UPS) and LEED Studies of BaTiO₃ (001) and SrTiO₃ (100) Surfaces

Abstract

Ultraviolet Photoelectron Spectroscopy (UPS) and LEED Studies of BaTiO, (001) and SrTiO, (100) SurfacesBy R. COURTHS UPS studies (21.2 and 40.8 eV) are performed onBaTiO, (001) and SrTiO, (100) surfaces. In both the compounds there is an enhanced emission a t the top of the valence band for hv = 21.2 eV, which cannot be explained with the existing theoretical bulk densities of states. From the results of angle-dependent UPS measurements i t is concluded that the top of the valence band emission is due to a n oxygen surface state. This interpretation is supported by band structure calculations for thin films of BaTiO, of Marathe e t al. -4 band gap state at 0.9 eV (BaTiO,) and 1.2 eV (SrTiO,) below the bottom of the conduction band can be detected on oxygen deficient surfaces and is interpreted as arising from Ti3+ species. In UHV annealed surfaces are 1 x 1 ordered. They show a 2 x 2 or two mutually perpendicular 2 x 1 LEED patterns in case of a higher concentration of Ti3+-0-vacancy defects due to defect ordering.UPS-Studien (21,2 and 40,s eV) werden an BaTi0,(001)-und SrTi03(100)-0berflachen durchgefiihrt. In beiden Verbindungen wird fur hv = 21,2 eV eine verstarkte Emission a n der Valenzbandoberkante beobachtet. die nicht mit existierenden theoretischen Volnmenzustandsdichten erklart werden kann. Aus den Ergebnissen yon winkelabhangigen UPS-Messungen wird geschlossen, daB diese Emission von einem Sauerstoffoberflachenzustand verursacht wird. Diese Interpretation wird gestutzt durch die Bandstrukturrechnungen a n diinnen BaTi0,-Filmen von Marathe et al. Ein Bandluckenzustand 0,9 eV (BaTiO,) bzw. 1.2 eV (SrTiO,) unterhalb der Leitungsbandunterkante wird in sauerstoffunterschissigen Oberflachen beobachtet und Ti3+-Spezies zugeordnet. I n UHV geheizte Oberflachen zeigen eine 1 x 1-Ordnung. I m Falle einer hoheren Konzentration der Ti3+-0-Fehlstellen-Defekte zeigen die Oberflachen ein 2 x 2-oder zwei zueinander senkrechte 2 x 1-LEED-Muster aufgrund von Defektordnung.

Ab initio study of rumpled relaxation and core-level shift of barium titanate surface

“…Barium titanate (BaTiO 3 ), as a typical ferroelectronic material with ABO 3 , has been studied extensively in the influence of surface effects on its functional properties. In detail, and experimentally, the electronic properties and the ion chemical states of various BaTiO 3 surfaces have been pursued using ultraviolet-photoemission (UPS) and electron-energyloss spectroscopy (EELS), low-energy electron diffraction (LEED) and Auger spectroscopy [5][6][7][8][9][10][11][12]. Interestingly, the two states of barium core level were observed using ultraviolet and X-ray photoelectron spectroscopy for the core-level electronic structure of BaTiO 3 (0 0 1) vacuumfractured [9][10][11][12].…”

Defect-mediated adsorption of methanol and carbon dioxide on BaTiO3(001)

“…For example, core-level photoemission spectroscopy suggests that several Ba-O phases exist in the ͑100͒ surface layer after annealing in UHV. 22,24 Ba surface enrichment has also been observed by AES and ion scattering spectroscopy on surfaces annealed to high temperatures with oxygen. 29 In addition, high resolution electron microscopy shows the presence of separate Ti-O and Ba-O phases on the surface, as a result of BaTiO 3 decomposition under electron irradiation.…”

“…In general, it has been found that single crystal BaTiO 3 can be chemically reduced by annealing in vacuum ͑T = 900-1200 K͒, as indicated by a progressive change in the sample color from light yellow to green, and eventually to blue and black. [22][23][24][25][26] The reduction in BaTiO 3 occurs through the loss of oxygen atoms, which makes the material n-type and pins the Fermi level near the bottom edge of the conduction band. 24 It has also been shown that surface oxygen vaa͒ Author to whom correspondence should be addressed; electronic mail: bonnell@lrsm.upenn.edu cancies can be generated by Ar-ion bombardment.…”

“…24 It has also been shown that surface oxygen vaa͒ Author to whom correspondence should be addressed; electronic mail: bonnell@lrsm.upenn.edu cancies can be generated by Ar-ion bombardment. 22,23 These defects can be detected by surface sensitive techniques such as photoelectron spectroscopy. For example, Courths used photoemission, low energy electron diffraction ͑LEED͒, and Auger electron spectroscopy ͑AES͒ to study the BaTiO 3 ͑001͒ surface under various conditions.…”

“…For example, Courths used photoemission, low energy electron diffraction ͑LEED͒, and Auger electron spectroscopy ͑AES͒ to study the BaTiO 3 ͑001͒ surface under various conditions. 22 A stable 1 ϫ 1 structure with a low intensity state in the band gap could be produced by Ar-or O-ion sputtering followed by annealing in ultrahigh vacuum ͑UHV͒ to 1100 K. The intensity of the band gap emission state is sensitive to sample annealing conditions: it is greatest when the surface is freshly sputtered or after prolonged vacuum annealing, and it is suppressed upon annealing in 5 ϫ 10 −5 torr O 2 or exposure to O 2 at room temperature. Based on these observations, the band gap state is attributed to Ti 3+ -O vacancy complexes on the surface.…”