Self-assembled Ge nanocrystals on high-k cubic Pr2O3(111)∕Si(111) support systems

Abstract

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.

The influence of lattice oxygen on the initial growth behavior of heteroepitaxial Ge layers on single crystalline PrO2(111)∕Si(111) support systems

“…32 This feature is detected up to very high nominal Ge thicknesses ͓deposition steps ͑1͒-͑6͔͒, in line with the Volmer-Weber growth mode of Ge on Pr oxides recently reported by Schroeder et al 28 In spectrum ͑2͒, after 8 s of Ge deposition, the Ge 3d peak shows up at 32.4 eV. In the literature, the Ge 3d peak position of elemental Ge is reported at BEs typically between 28.9 and 30.2 eV.…”

“…In contrast, the O 1s photoelectron signal of the Pr oxide substrate at 529.4 eV is strongly attenuated but remains weakly visible even after 284 s of Ge deposition ͓spectrum ͑7͔͒, as can be explained by the reported VolmerWeber growth mode of Ge on Pr oxide surfaces. 28 The observed shift of the Pr oxide O 1s peak from 529 eV ͓spec-trum ͑1͔͒ to 529.4 eV ͓spectrum ͑7͔͒ probably results from a change in the film stoichiometry ͑from PrO 2 to Pr 2 O 3 ͒ due to oxygen depletion by the described interaction with Ge. In this context, it is interesting to note that a similar BE increment of the O 1s core line has been detected for cerium oxides when stoichiometry changes from CeO 2 to Ce 2 O 3 .…”

“…The extreme surface roughness can be attributed to the formation of single crystalline Ge͑111͒ islands on the Pr oxide surface as a result of a Volmer-Weber growth mode, recently reported for self-assembled Ge nanocluster structures on cubic Pr oxide films by a multitechnique thin film study. 28 Due to the presence of two ͑111͒ oriented cubic fcc-related crystal structures on the sample, namely, the Ge deposit and the Pr oxide support, image 3͑d͒ can be best described by the superposition of two similar types of spot patterns which however differ by their epitaxial orientation. The weaker spot pattern can be assigned to the Bragg peaks of uncovered Pr oxide regions which are arranged on the above described inclined line structures with the orientation depicted by the dashed line.…”

“…However, a recent anomalous SR-GIXRD study on the Volmer-Weber growth mode of Ge nanoclusters on Pr oxide supports confirmed the absence of type-B oriented Ge rotational twins during the initial deposition stages and, thus, fully supports the here given description. 28 Images 3͑e͒, 3͑f͒ and 3͑g͒ report the RHEED patterns after 56, 104, and 284 s of Ge deposition, respectively. These pictures demonstrate the further development of a rough Ge island structure on the Pr oxide surface and are characterized by the transition towards a pure type-A Ge Bragg peak RHEED pattern, as indicated in image 3͑g͒ by the dotted line.…”

“…Again, this experimental finding is best explained by the formation of a Ge film which does not completely cover the Pr oxide support. 28 Only after a long evaporation time, a closed Ge layer is formed and the Pr 3d 5/2 signal of the Pr oxide is not detected anymore ͓curve ͑8͔͒.…”