Structural phase transition of ultra thin PrO₂films on Si(111)

Abstract

Ultra thin heteroepitaxial PrO(2) films on Si(111) were annealed under UHV conditions and investigated by x-ray diffraction (XRD), x-ray reflectometry (XRR) and spot profile analysis low energy electron diffraction (SPALEED) with regard to structural stability and phase transitions due to the high oxygen mobility of the oxide. This gives information about the manageability of the material and its application as a model catalyst system in surface science. While the samples are stable in UHV at room temperature, annealing at 300 °C exhibits a terminated phase transition from PrO(2) and PrO(2-Δ) to cub-Pr(2)O(3) with an increase in the silicate at the interface and a decrease in the crystalline praseodymia layer mainly due to atomic diffusion of silicon into the oxide film. Strain effects during the phase transition also cause mosaic formation at the surface. Further annealing up to 600 °C shows only little change in the film structure. This will finally lead to a model of the film structure during the annealing process.

Photoemission study of praseodymia in its highest oxidation state: The necessity of in situ plasma treatment

“…We like to mention, that in our previously published XRD experiments on praseodymia films oxidized by molecular oxygen at high pressure, 8,12,20 we observed an oxygendeficient PrO 2− phase. In the light of the praseodymia films oxidized by plasma this phase of the praseodymia films has clearly to be attributed to Pr 6 O 11 .…”

“…As discussed before, 8,20,23 the diffraction intensity close to the Si(111) Bragg peak contains the full information about both the vertical lattice constant and the film thickness of the praseodymia films as well as the interface distance and interface roughness due to the interference between substrate and film. Diffraction close to the Si(222) Bragg peak, however, which is forbidden according to kinematic diffraction theory, is only sensitive to the diffraction from the praseodymia film so that this diffraction condition is better suited to characterize quantitatively the praseodymia film itself and to avoid complications with respect to the interference between film and substrate.…”

“…Therefore, the praseodymia films of these samples split up into different oxide phases as reported previously. 8,12,20 The plasma-treated sample, however, exhibits only one Bragg peak and, therefore, has only one praseodymia phase. In addition, fringes due to the finite thickness of the praseodymia films are also clearly visible in Fig.…”

Effect of amorphous interface layers on crystalline thin-film x-ray diffraction

“…It was shown that after the UHV annealing process the oxide film was reduced to cubic Pr 2 O 3 . 12 In order to keep things simple, we chose the PrO 2 unit cell, which consists of only three atoms, to simulate the oxide layer. The difference to other praseodymia unit cells, e.g., Pr 2 O 3 , is negligible for the simulations shown in this work.…”

“…In order to keep things simple, we will stick with a simple one-column model here because the character of the interface turned out to be identical for both columns in Ref. 12. The first step of our XRD interface analysis is to reduce the amount of free parameters by incorporating XRD analysis results of the measurements at L Ϸ 2, where no significant interference between oxide and film occurs because the intensity of the symmetry forbidden Si͑002͒ S is very weak.…”