Twin-free epitaxial cubic (111) praseodymium sesquioxide films were prepared on Si(111) by hexagonal-to-cubic phase transition. Synchrotron radiation grazing incidence x-ray diffraction and transmission electron microscopy were applied to characterize the phase transition and the film structure. As-deposited films grow single crystalline in the (0001)-oriented hexagonal high-temperature phase of praseodymium sesquioxide. In situ x-ray diffraction studies deduce an activation energy of 2.2eV for the hexagonal-to-cubic phase transition. Transmission electron microscopy shows that the phase transition is accompanied by an interface reaction at the oxide/Si(111) boundary. The resulting cubic (111) low-temperature praseodymium sesquioxide film is single crystalline and exclusively shows B-type stacking. The 180° rotation of the cubic oxide lattice with respect to the Si substrate results from a stacking fault at the substrate/oxide boundary.
Engineered wafer systems are an important materials science approach to achieve the global integration of single crystalline Ge layers on the Si platform. Here, we report the formation of single crystalline, fully relaxed Ge(111) films by molecular beam epitaxial overgrowth of cubic Pr oxide buffers on Si(111) substrates. Reflection high-energy electron diffraction, scanning electron microscopy, and x-ray reflectivity show that the Ge epilayer is closed, flat, and has a sharp interface with the underlying oxide template. Synchrotron radiation grazing incidence x-ray diffraction and transmission electron microscopy reveal the type-A/B/A epitaxial relationship of the Ge(111)/cubic Pr2O3(111)/Si(111) heterostructure, a result also corroborated by theoretical ab initio structure calculations. Secondary ion mass spectroscopy confirms the absence of Pr and Si impurities in the Ge(111) epilayer, even after an annealing at 825 °C.
The defect structure of Ge(111) epilayers grown by molecular beam epitaxy on cubic Pr2O3(111)/Si(111) support systems was investigated by means of transmission electron microscopy and laboratory-based x-ray diffraction techniques. Three main types of defects were identified, namely, rotation twins, microtwins, and stacking faults, and studied as a function of Ge film thickness and after annealing at 825 °C in ultrahigh vacuum. Rotation twins were found to be localized at the Ge(111)/cubic Pr2O3(111) interface and their amount could be lowered by the thermal treatment. Microtwins across {111¯} were detected only in closed Ge films, after Ge island coalescence. The fraction of Ge film volume affected by microtwinning is constant within the thickness range of ∼20–260 nm. Beyond 260 nm, the density of microtwins is clearly reduced, resulting in thick layers with a top part of higher crystalline quality. Microtwins resulted insensitive to the postdeposition annealing. Instead, the density of stacking faults across {111¯} planes decreases with the thermal treatment. In conclusion, the defect density was proved to diminish with increasing Ge thickness and after annealing. Moreover, it is noteworthy that the annealing generates a tetragonal distortion in the Ge films, which get in-plane tensely strained, probably due to thermal mismatch between Ge and Si.
The stoichiometry, structure, and defects of self-assembled heteroepitaxial Ge nanodots on twin-free type B oriented cubic Pr2O3(111) layers on Si(111) substrates are studied to shed light on the fundamental physics of nanocrystal based nonvolatile memory effects. X-ray photoelectron spectroscopy studies prove the high stoichiometric purity of the Ge nanodots on the cubic Pr2O3(111)∕Si(111) support system. Synchrotron based x-ray diffraction, including anomalous scattering techniques, was applied to determine the epitaxial relationship, showing that the heteroepitaxial Ge(111) nanodots crystallize in the cubic diamond structure with an exclusive type A stacking configuration with respect to Si(111). Grazing incidence small angle x-ray scattering was used in addition to analyze the average shape, size, and distance parameters of the single crystalline Ge nanocrystal ensemble. Furthermore, transmission electron micrographs report that partial dislocations are the prevailing extended defect structure in the Ge nanodots, mainly induced by surface roughness on the atomic scale of the cubic Pr2O3(111) support.
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