Structural transitions of epitaxial ceria films on Si(111)

Abstract

The structural changes of a (111) oriented CeO2 film grown on a Si(111) substrate covered with a hex-Pr2O3(0001) interface layer due to post deposition annealing are investigated. X-ray photoelectron spectroscopy measurements revealing the near surface stoichiometry show that the film reduces continuously upon extended heat treatment. The film is not homogeneously reduced since several coexisting crystalline ceria phases are stabilized due to subsequent annealing at different temperatures as revealed by high resolution low energy electron diffraction and X-ray diffraction. The electron diffraction measurements show that after annealing at 660 °C the ι-phase (Ce7O12) is formed at the surface which exhibits a (√7 × √7)R19.1° structure. Furthermore, a (√27 × √27)R30° surface structure with a stoichiometry close to Ce2O3 is stabilized after annealing at 860 °C which cannot be attributed to any bulk phase of ceria stable at room temperature. In addition, it is shown that the fully reduced ceria (Ce2O3) film exhibits a bixbyite structure. Polycrystalline silicate (CeSi(x)O(y)) and crystalline silicide (CeSi1.67) are formed at 850 °C and detected at the surface after annealing above 900 °C.

Thermal stability and surface behaviors of CeO2/Si films during in-situ vacuum annealing

“…In order to investigate the surface behaviors of the CeO 2 /Si thin films, the samples were annealed from 400 K to 1000 K in step of 50 K, and the annealing time was manually controlled according to the spectra. It is found that the surface compositions are only strongly temperature dependent, which is great different from Iordanova's results [19] and the reduction temperature is slightly higher than Wilkens' value [13]. It could be induced by different reactions occurring in the surface and interface in the thick film system.…”

“…Ce4d spectra as function of O2 exposure and annealing temperature. thin films deposited on Si substrates [7,13,21], which resulted from the diffusion of silicon to the surface. Compared with that of SiO 2 [22], the peak shifts toward lower binding energy, and corresponds to Si 3+ [14,23], which could originate from the products of solidstate reactions, such as SiO x or/and solid solution with Ce Si O bond, Ce Si oxide formation and so on.…”

“…However, the reduction temperature varies with the film thickness because of the solid-state reaction in the interface [12]. Furthermore, different behaviors during reduction are revealed in the bulk and surface [13]. In the previous studies, a lot of works are focused on the interfacial reaction between CeO 2 films and substrates, in which a several-monolayer thin films were achieved for the convenience; however, few works were carried out for the surface behaviors of the system during annealing [17,18].…”

“…pulsed laser deposition [1], magnetron sputtering [2,3], electron beam evaporation [4,5], etc., and performances of cerium oxides films were widely examined [4,[6][7][8][9][10][11]. Interfacial reactions between CeO 2 films and substrates are also concerned during post-deposition annealing treatment [2,6,12,13], and intermediate sub-oxide layers are observed by TEM [3], XPS [4,6,12,14], AES depth profile [4] etc. When annealed in ultra-high vacuum (UHV) condition, CeO 2 thin films are identified to be reduced to Ce suboxide at a certain temperature [12,[14][15][16], and to be oxidized to CeO 2 readily under the condition of O 2 exposure [15].…”

Post deposition annealing of epitaxial Ce1−xPrxO2−δfilms grown on Si(111)

“…Several intermediate bulk phases exist in the phase diagrams of ceria and praseodymia 22 which can be stabilized in pure ceria thin film systems. 23,24 Hence, we assume that this structure can be attributed to a reduced crystalline phase of the mixed oxide with low order. The samples with higher praseodymium content do not show this structure.…”