Postdeposition annealing induced transition from hexagonal Pr2O3 to cubic PrO2 films on Si(111)

Abstract

Films of hexagonal praseodymium sesquioxide (h-Pr2O3) were deposited on Si(111) by molecular beam epitaxy and thereafter annealed in 1 atm oxygen at different temperatures, ranging from 100 to 700 °C. The films of the samples annealed at 300 °C or more were transformed to PrO2 with B-oriented Fm3¯m structure, while films annealed at lower temperatures kept the hexagonal structure. The films are composed of PrO2 and PrO2−δ species, which coexist laterally and are tetragonally distorted due to the interaction at the interface between oxide film and Si substrate. Compared to PrO2, PrO2−δ has the same cubic structure but with oxygen vacancies. The oxygen vacancies are partly ordered and increase the vertical lattice constant of the film, whereas the lateral lattice constant is almost identical for both species and on all samples. The latter lattice constant matches the lattice constant of the originally crystallized hexagonal praseodymium sesquioxide. That means that no long range reordering of the praseodymium atoms takes place during the phase transformation.

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

“…With increasing annealing temperature a gradual reduction of the oxide film and loss of Pr 4+ species is observed (spectrum e) up until annealing to 920 K leads to spectrum (f), which exhibits the typical shape observed for Pr 2 O 3 . 7,10,12,15 The emission from Pr 4+ has almost completely vanished. Residual intensity at the position of z is most likely due to surface induced valence changes as mentioned above.…”

“…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 .…”

“…[10][11][12][13][14] Spectrum (a) in Fig. 3 shows a typical Pr 3d spectrum of an ex situ prepared PrO 2 layer on Si(111) that we obtained in our own experiments.…”

“…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

“…Depending on temperature, oxygen pressure, and duration of the annealing process the stoichiometric composition can range from Pr 2 O 3 to PrO 2 . [5][6][7] Also, an amorphous interface layer is formed during the annealing process. 8 It consists of two sublayers.…”