T. Topuria

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.

MOVPE grown self-assembled and self-ordered InSb quantum dots in a GaSb matrix assessed by AFM, CTEM, HRTEM and PL

Self-assembled InSb quantum dots (QDs) were grown by metal-organic vapour phase epitaxy (MOVPE) in a GaSb matrix. Atomic force microscopy (AFM), conventional diffraction contrast transmission electron microscopy (CTEM), high resolution transmission electron microscopy (HRTEM), and photoluminescence (PL) were used for the assessment of the QDs. Reductions in the III :V ratios and growth rates resulted in a change of the morphology of the InSb islands from hillocks without facets, and a low level of order to dumbbell shaped islands with distinct facets and a higher level of order.

Magnetic CdSe-based quantum dots grown on Mn-passivated ZnSe

In this letter we describe the properties of self-assembled CdSe quantum dots (QDs) grown on Mn-passivated ZnSe buffers. We show that the Mn deposited on the ZnSe surface during the passivation process acts as a nucleating seed for self-assembled QD formation. For moderate amounts of Mn deposition, the dots grown in this way show a significant improvement in size uniformity compared to CdSe dots grown on ZnSe without Mn passivation. Using photoluminescence, we also show that the dots exhibit large Zeeman splitting, indicating that this growth method is suitable for fabricating magnetic QDs that exhibit strong spin polarization effects.

Atomic resolution scanning transmission electron microscopy

Recent developments in scanning transmission electron microscopy (STEM) now make it routinely possible to obtain direct images and spectra from interface and defects structures with atomic spatial resolution. Here we describe the experimental conditions required to set-up and align a 200 kV STEM/TEM microscope to perform this analysis. The various imaging and analysis techniques will be illustrated with examples from interfaces in II-VI and III-V quantum dot systems and dislocation cores in GaN.

Internal self-ordering in In(Sb,As), (In,Ga)Sb, and (Cd,Zn,Mn)Se nano-agglomerates/quantum dots

Nano-agglomerates of In(Sb,As) in InAs, (In,Ga)Sb in GaSb, and (Cd,Zn,Mn)Se in (Zn,Mn)Se are classified by transmission electron microscopy. In scanning transmission electron microscopy, atomic resolution Z-contrast images reveal different modes of internal compositional modulation on the atomic length scale, resulting for all three material systems in nano-agglomerates of an appropriate size that may constitute a new type of quantum dot. For other nano-agglomerates of In(Sb,As) in InAs and (In,Ga)Sb in GaSb, we observed a second type of nanoscale ordering that results in nano-agglomerates with an internal compositional modulation on a length scale of a few nm. Both types of compositional modulation are discussed as having arisen from a rather long-term structural response to a combination of internal and external strains.

Self-ordered CdSe quantum dots in ZnSe and (Zn, Mn)Se Matrices Assessed by transmission electron microscopy and photoluminescence spectroscopy

Single and multilayer sheets of self-assembled CdSe [quantum dots (QDs)] were grown by means of molecular beam epitaxy in both ZnSe and (Zn0.9Mn0.1)Se matrices. Both types of structure were assessed by means of transmission electron microscopy in the scanning, high-resolution, and diffraction-contrast modes. Complementary results from wider sample areas were obtained by means of photoluminescence spectroscopy. In one of the samples analyzed, a fractional monolayer of MnSe was deposited immediately before the CdSe deposition. A second structure grown under identical conditions, but without the MnSe fractional monolayer, was also analyzed. This comparison provides direct evidence for an enhanced size and shape homogeneity of 3D QDs caused by the presence of a tiny amount of MnSe at the interface. In the multilayer structure, we observed the co-existence of highly strained quasi-2D QDs and CdSe rich aggregates with compositional modulations on certain crystallographic planes in close proximity.

Characterization of ultrathin dopant segregation layers in nanoscale metal-oxide-semiconductor field effect transistors using scanning transmission electron microscopy

Silicide/Si source/drain interfaces (Co–silicide and Ti–silicide) in nanoscale metal–oxide–semiconductor field effect transistors (MOSFETs) were investigated using scanning transmission electron microscopy and electron energy loss spectroscopy. Z-contrast images of the N-type doped device show substitutional arsenic segregation on Si lattice sites with a very narrow profile precisely at the Co–silicide/Si interfaces. A detailed comparative electron energy loss study of As-doped and undoped devices reveals that arsenic remains electrically active and supplies additional charge carriers at the interface. These characteristics are desirable for optimum device performance with minimum contact resistance. A similar effect is also observed in MOSFETs with a Ti-silicided source/drain.

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.

Direct atomic scale characterization of interfaces and doping layers in field-effect transistors

Atomic-resolution Z-contrast imaging and electron energy loss spectroscopy combined with energy dispersive x-ray spectroscopy are used to investigate the structure-property relationships in an ideal metal–oxide–semiconductor device structure. Arsenic segregation with a very narrow profile occurring precisely at the silicide/Si interface was identified. Images show that the As is substitutional on the Si lattice sites, implying that it remains electrically active. These structural results imply desirable electronic properties for the device and are consistent with electrical measurements showing a decrease in contact resistance for these samples.

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.