Structure of oxygen-plasma-treated ultrathin praseodymia films on Si(111)

S. Gevers, T. Weisemoeller, A. Schaefer, V. Zielasek, M. Bäumer, J. Wollschläger
2011 Physical Review B  
Ultrathin praseodymia films, which have been oxidized by molecular oxygen, have been treated additionally with oxygen plasma to increase their oxidation state. The structure and morphology of the films have been investigated by x-ray diffraction and x-ray reflectometry. Thorough analysis of these measurements gives information regarding modifications of the oxide film structure (especially the vertical lattice constants) due to the oxygen content and interface silicate formation before and
more » ... ion before and after oxygen plasma treatment. Large parts of the plasma-treated samples exhibit a significantly higher oxygen content compared to the untreated samples; this is attributed to the formation of stoichiometric PrO 2 . The remaining film has only a small oxygen deficiency. Thus, a more homogeneous film structure is formed by exposure to oxygen plasma. Furthermore, no additional silicate interface formation can be detected. Rare earth oxides (REOs) are of interest for many applications in the fields of heterogeneous catalysis 1 and microelectronics. 2 For instance, REO films are of potential use to improve the performance and functionality of future semiconductor devices by integrating alternative semiconductor materials into the present Si technology. 3, 4 In this regard, single-crystalline REO films are under discussion as highly functional insulating buffer material to form so-called engineered Si wafers. The main research fields are integrated germanium-on-insulator systems aiming to boost the sub-45-nm complementary metal-oxide-semiconductor technologies and further to achieve the cost-effective monolithic integration of III-V optoelectronic materials (GaAs) on the Si wafer platform. Praseodymia is a promising candidate for these insulating buffer materials and previous studies show that crystalline Ge films grow epitaxially on praseodymia-silicon heterostructures. 4 Since as-grown praseodymia films initially have hexagonal structure (hex-Pr 2 O 3 ), it is necessary to transform the structure of these films to PrO 2 (fluorite structure) to obtain stacking-twin-free Ge films. 4,5 For this purpose hex-Pr 2 O 3 films are usually exposed to molecular oxygen at high pressure, but full transformation to PrO 2 (111) has not been achieved yet. 6-8 Instead, the films decompose into two laterally coexisting oxide species with different lattice parameters and significantly different stoichiometries (PrO 1.833 , PrO 2− ) as shown by detailed analysis of the praseodymia Bragg peaks. This is disadvantageous for the quality of the subsequently grown Ge film due to the imperfect oxygen sublattice. The thickness of an interface layer consisting of both species increases with higher annealing temperatures, too. This interface exhibits negative effects on the dielectric properties of the oxide film. 5 Schaefer et al. 8 recently established an alternative technique to oxidize praseodymia films on Si(111) by exposure to cold oxygen plasma. Here, x-ray photoelectron spectroscopy (XPS) supported by low-energy electron diffraction (LEED) and x-ray diffraction (XRD) demonstrated that the near-surface region of this praseodymia film has a higher oxidation state compared to the samples annealed in molecular oxygen. Our work, presented here, focuses on more detailed investigations of the structure of oxygen-plasma-treated praseodymia films by XRD and x-ray reflectometry (XRR). Particular features of the praseodymia film structure are obtained from our careful XRD analysis using kinematic diffraction theory combined with XRR analysis based on the Parratt algorithm. Thus, we determine the composition of the oxide films regarding different praseodymia phases (identified by the vertical lattice constants) and their ordering. For instance, we obtain information concerning not only the lateral phase separation but also the vertical stacking of the different phases. Ultrathin hex-Pr 2 O 3 films with 15 nm film thickness were deposited on clean boron-doped Si(111) substrates (ρ = 5-15 cm, off oriented by 0.35 • ± 0.15 • ). 9 These samples were annealed ex situ in 1 atm oxygen at 450 • C for 30 min to obtain heteroepitaxial praseodymia films with fluorite structure (Fm3m in Hermann-Mauguin notation) and exclusive B-type stacking. 10 The oxygen plasma treatment was performed afterward for 60 min at 9 mbar oxygen partial pressure and a gas flow of 15 sccm (standard cubic centimeters per minute) using an ultrahigh-vacuum-compatible, capacitively coupled rf plasma source 11 with a power of 30 W. After in situ XPS and LEED measurements, the samples were transferred under clean oxygen atmosphere to beamline BW2 at DESY (Hamburg, Germany) to determine the structure of both untreated and oxygen-plasma-treated oxide films. Here, ex situ XRD and XRR measurements were performed in -2 geometry using a six-circle diffractometer at photon energy of 10 keV. A one-dimensional x-ray photon detector provides additional lateral information via reciprocal space mapping (RSM). The XRR measurements and calculated intensities of the untreated and the oxygen-plasma-treated samples are presented in Fig. 1 . Both samples exhibit well-defined intensity oscillations (fringes), pointing to homogeneous film structures, but the fringes of the plasma-treated sample are more pronounced. Therefore, the roughness of the plasmatreated sample decreases with respect to the untreated sample, probably due to the previously reported cleaning of the film surface. 8 The model which was used to calculate the intensity 193408-1
doi:10.1103/physrevb.83.193408 fatcat:lskzisdjc5gdhlrbxiljfrtniy