Yuanyuan Lei

Void-mediated formation of Sn quantum dots in a Si matrix

Atomic scale analysis of Sn quantum dots (QDs) formed during the molecular beam-epitaxy (MBE) growth of SnxSi1-x (0.05 less than or equal to x less than or equal to 0.1) multilayers in a Si matrix revealed a void-mediated formation mechanism. Voids below the Si surface are induced by the lattice mismatch strain between SnxSi1-x layers and Si, taking on their equilibrium tetrakaidecahedron shape. The diffusion of Sn atoms into these voids leads to an initial rapid coarsening of quantum dots during annealing. Since this formation process is not restricted to Sn, a method to grow QDs may be developed by controlling the formation of voids and the diffusion of materials into these voids during MBE growth.

Si0.85Ge0.15 oxynitridation in nitric oxide/nitrous oxide ambient

Low temperature, nitric oxide (NO)/nitrous oxide (N2O) aided, sub-35 A Si0.85Ge0.15 oxynitrides have been grown at 550 and 650 °C, while the oxynitridation feed gases have been preheated to 900 and 1000 °C, respectively, before entering the reaction zone. X-ray photoelectron spectroscopy and secondary ion mass spectroscopy (SIMS) data suggest that NO-assisted oxynitridation incorporates more nitrogen than the N2O-assisted one, while there is minimal Ge segregation towards the dielectric/substrate interface in both oxynitridation processes. Moreover, SIMS results suggest that nitrogen is distributed throughout the film in contrast to high temperature Si oxynitridation, where nitrogen incorporation takes place near the dielectric/substrate interface. Z-contrast imaging with scanning transmission electron microscopy shows that the oxynitride grown in NO at 650 °C has a sharp interface with the bulk Si0.85Ge0.15, while the roughness of the dielectric/Si0.85Ge0.15 substrate interface is less than 2 A. These re...

Structural transformations in self-assembled semiconductor quantum dots as inferred by transmission electron microscopy

Transmission electron microscopy studies in both the scanning and parallel illumination mode on samples of two generic types of self-assembled semiconductor quantum dots are reported. III-V and II-VI quantum dots as grown in the Stranski-Krastanow mode are typically alloyed and compressively strained to a few %, possess a more or less random distribution of the cations and/or anions over their respective sublattices, and have a spatially non-uniform chemical composition distribution. Sn quantum dots in Si as grown by temperature and growth rate modulated molecular beam epitaxy by means of two mechanisms possess the diamond structure and are compressively strained to the order of magnitude 10 %. These lattice mismatch strains are believed to trigger atomic rearrangements inside quantum dots of both generic types when they are stored at room temperature over time periods of a few years. The atomic rearrangements seem to result in long-range atomic order, phase separation, or phase transformations. While the results suggest that some semiconductor quantum dots may be structurally unstable and that devices based on them may fail over time, triggering and controlling structural transformations in self-assembled semiconductor quantum dots may also offer an opportunity of creating atomic arrangements that nature does not otherwise provide.

Endotaxial Growth Mechanisms of Sn Quantum Dots in Si Matrix

Two distinct mechanisms for the endotaxial growth of quantum dots in the Sn/Si system were observed by means of analytical transmission electron microcopy. Both mechanisms operate simultaneously during temperature and growth rate modulated molecular beam epitaxy combined with ex situ thermal treatments. One of the mechanisms involves the creation of voids in Si, which are subsequently filled by Sn, resulting in quantum dots that consist of pure α-Sn. The other mechanism involves phase separation and leads to substitutional solid solution quantum dots with a higher Sn content than the predecessor quantum well structures possess. In both cases, the resultant quantum dots possess the diamond structure and the shape of a tetrakaidecahedron. (Sn,Si) precipitates that are several times larger than the typical (Sn,Si) quantum dot possess an essentially octahedral shape.

Highly uniform (Cd, Mn, Zn)Se/(Zn, Mn)Se quantum dot array formation by means of thermal treatments

Thermal treatments of (Cd,Mn,Zn)Se/(Zn,Mn)Se multiquantum well heterostructures inside the electron microscope resulted in the formation of three-dimensional CdSe based quantum dots (QDs). The array uniformity of the QDs was investigated by means of the Z-contrast imaging technique in the scanning transmission electron microscope and found to be superior to that of Stranski–Krastanow grown CdSe based QDs. The outcome of the heating experiment demonstrated that thermal treatments might be considered as one of the ways in obtaining highly ordered QD arrays. Possible mechanisms of the QD formation by means of thermal treatments are also discussed.

Analysis of the Atomic-Scale Defect Chemistry at Interfaces in Fluorite Structured Oxides by Electron Energy Loss Spectroscopy

Abstract : Gd(3+) doped Ce oxides are a major candidate for use as the electrolyte in solid oxide fuel cells operating at ^ 500 deg C. Here, the effect of the atomic structure on the local electronic properties, i.e., oxygen coordination and cation valence, at grain boundaries in the fluorite structured Gd(0.2)Ce(0.8)O(2-x) ceramic electrolyte is investigated by a combination of atomic resolution Z-contrast imaging and electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM). In particular, EELS analyses from grain boundaries reveals a complex interaction between segregation of the dopant (Gd(3+)), oxygen vacancies and the valence state of Ce. These results are similar to observations from fluorite-structured Yttria-Stabilized Zirconium (YSZ) bicrystal grain boundaries.