Structural Changes of Ultrathin Cub-PrO₂(111)/Si(111) Films Due to Thermally Induced Oxygen Desorption

Abstract

The structural changes of ultra thin PrO 2 (111) films grown on Si(111) due to thermal reduction in ultra high vacuum are investigated. Therefore, praseodymia films are treated with oxygen plasma to obtain completely oxidized PrO 2 films. The reduction of these films is studied with thermal desorption spectroscopy combined with ex-situ x-ray diffraction measurements. It is shown that the distinct desorption peaks of molecular oxygen are correlated with stabilization of several intermediate phases of praseodymia. The most intense signal of desorbing oxygen is observed at ≈ 650 K. It is coupled to the strongest impact on the crystalline structure since we observe the transition PrO 2 →Pr 5 O 9 . Further oxygen desorption leads to additional phase transitions with laterally coexisting phases. Grazing incidence x-ray diffraction measurements reveal that the initial pinning to the lateral lattice constant of hexagonal Pr 2 O 3 (0001) is not breached due to the plasma treatment. This hexagonal pinning is released during the oxygen desorption so that the crystalline structure of the praseodymia films does not only expand vertically but also laterally.

Controlling the physics and chemistry of binary and ternary praseodymium and cerium oxide systems

“…The reduction behaviour of PrO 2 (111) films has been the subject of several detailed studies. 94,96,107,137 As discussed in Section II.1, XPS measurements reveal that fully oxidised PrO 2 (111) films can only be achieved by oxygen plasma treatment. 94 The Pr 3d spectra shown in Fig.…”

“…Assuming that the praseodymia films are elastically deformed, one can calculate the cubic lattice constant of the undistorted praseodymia depending on the film stoichiometry. 107 Taking into account the Poisson ratio of n = 0.32 reported by Zaumseil et al, 108 The results from XRR studies show that the previously sharp interface between the hex-Pr 2 O 3 (0001) film and the Si (111) substrate is dissolved and a praseodymium silicate with increasing thickness evolves after annealing at temperatures above 500 1C. 91 The silicate interface layer consumes more and more of the praseodymia film with increasing annealing temperature.…”

“…24. 107 All intermediate phases of praseodymia are formed by vacancy generation in the oxygen sublattice of the fluorite structure of PrO 2 and, thus, these vacancies result in an expansion of the cubic unit cell due to the decreased average oxidation state of praseodymium. Following Vegard's law, 114 the film stoichiometry can be determined.…”

Reducible Oxides as Ultrathin Epitaxial Films

“…As for terbium oxide thin films, also in this case, an oxidizing treatment in oxygen plasma was shown to be necessary to oxidize the Pr 2 O 3 phase to the PrO 2 fluorite phase [107]. UHV thermal treatments to reduce the PrO 2 phase have been shown to give origin to a phase including a mixture of Pr 6 O 11 and Pr 5 O 9 , the latter being unstable in the bulk phase [109].…”

“…Praseodymium oxide has been studied only in the form of relatively thick films on Si(111) substrates [107][108][109]. As for terbium oxide thin films, also in this case, an oxidizing treatment in oxygen plasma was shown to be necessary to oxidize the Pr 2 O 3 phase to the PrO 2 fluorite phase [107].…”