N. Y. Jin-Phillipp

Long-range ordered lines of self-assembled Ge islands on a flat Si (001) surface

Self-assembled growth in combination with prepatterning yields ordered lines of Ge islands on a planar Si (001) surface. The self-assembled Ge nanostructures are grown on top of a 15-period Si/SiGe superlattice, which is deposited on a prepatterned Si substrate. The pattern consists of 10 nm deep trenches with a period of 250 nm. The superlattice translates the surface modulation of the substrate into a strain-field modulation at the growth front of the superlattice. This strain field modulation provides the template for the ordered nucleation of self-assembled Ge islands. Our method gives rise to the long-range ordering of perfectly passivated nanostructures and can in principle be applied to any other strained material system.

Structural and optical properties of vertically aligned InP quantum dots

Stacked layers of self-assembled InP quantum dots embedded in Ga0.52In0.48P have been prepared by solid source molecular beam epitaxy. Thereby the distance between the dot layers has been varied from 2 to 16 nm. Cross sectional transmission electron microscopy shows that the InP dots are aligned in the growth direction [100]. As the distance between the dot layers is reduced, each dot of the first dot layer is reproduced in the upper layers, and this leads to an improvement of the dot size homogeneity of the stacked InP dot system. This is confirmed by photoluminescence (PL) measurements, which demonstrate a very narrow linewidth of 26 meV for a triple layer with 2 nm separation between the dot layers in comparison with a linewidth of 41 meV for a single layer sample. At the same time, the PL peak of the dots is shifted by 72 meV to lower energies which is ascribed to a reduced strain and strong electrical coupling between the densely stacked InP dots.

Diameter scalability of rolled-up In(Ga)As/GaAs nanotubes

Free-standing nanotubes are formed by rolling-up InGaAs/GaAs bilayers on a GaAs substrate. We present a systematic study of the tube diameter as a function of bilayer thicknesses. In our study we take into account that 2–4 monolayers of the top GaAs layer are consumed due to oxidation during the overall tube formation process. We find that a macroscopic continuum mechanical model can well describe the diameter of the nanotubes from 80 nm to 600 nm for nearly symmetric layers and from 21 nm to 550 nm for asymmetric bilayers. For thin symmetric layers the diameter is slightly smaller than predicted by theory. We find that the growth temperature significantly influences the nanotube diameter.

Closely stacked InAs/GaAs quantum dots grown at low growth rate

We present a systematic study of closely stacked InAs/GaAs quantum dots (QDs) grown at low growth rates. Transmission electron microscopy reveals that for thin spacer layers vertically aligned QDs merge into one large QD. After capping the initial QD layer the GaAs surface is decorated with well-developed nanostructures, which act as nucleation centers for the QDs deposited in the second layer. Despite the size increase, photoluminescence (PL) experiments show a systematic blueshift up to 103 meV of the QD related signal with decreasing spacer thickness. We explicitly show that this significant blueshift cannot fully be ascribed to specific growth phenomena, but instead is caused by the actual presence of the second dot layer. We report a PL linewidth as narrow as 16 meV at low temperature for a sample with 5 nm spacer thickness.

Free-standing SiGe-based nanopipelines on Si (001) substrates

Thin solid films form nanopipelines if the films are released from a substrate and put back onto their own surface. We give a detailed description of free-standing SiGe-based nanopipelines created on Si (001) substrates. The initial layer sequence is grown by molecular beam epitaxy and comprises SiGe-based epitaxial layers grown on a Ge sacrificial layer. After selectively etching away the Ge sacrificial layer, SiGe nanopipelines have formed on the surface. Nanopipelines as long as 20 μm with diameters ranging from 50 to 530 nm are fabricated. We show that SiGe nanopipelines perform multiple revolutions if selective etching is carried out long enough. Adding carbon to Si epitaxial layers is proposed to extend the design freedom of Si-based nanopipelines and nanotubes.

Vertical alignment of laterally ordered InAs and InGaAs quantum dot arrays on patterned (001) GaAs substrates

We demonstrate vertical alignment of laterally ordered self-assembled quantum dot (QD) arrays stacked on artificially pre-patterned substrates with two-dimensional hole arrays. The initial InGaAs layer is directly grown on the periodically modulated surface in order to exactly control nucleation sites of QDs to be stacked. After growing three InGaAs dot layers with GaAs spacers as a buffer, laterally ordered InAs dots are grown as an optically active layer. The cross-sectional images of transmission electron microscopy reveal vertical alignment of the stacked QDs. Photoluminescence signal at room temperature is detected from the three-dimensional QD superlattice.

Radial superlattices and single nanoreactors

We investigate the wall structure and thermal stability of individual freestanding rolled-up nanotubes (RUNTs) using micro-Raman spectroscopy, transmission electron microscopy, and selected area electron diffraction. Our studies reveal that the walls of the InAs/GaAs RUNTs consist of a radial superlattice comprising alternating crystalline and noncrystalline layers. Furthermore, we locally heated individual RUNTs with a laser beam, and Raman spectroscopy was used in situ to monitor any structural changes. At about 300 °C the heated part of a RUNT starts to oxidize and eventually transforms into crystalline β-Ga2O3. This result shows that RUNTs can serve as nanoreactors that locally synthesize material at intentional places on a substrate surface.

Enhanced ionic conductivity and mesoscopic size effects in heterostructures of BaF2 and CaF2

Abstract Ionic heterolayers of CaF 2 /BaF 2 /CaF 2 have been prepared by molecular beam epitaxy (MBE). The spacing has been varied from ∼1 nm to ∼1 μm. In the range of 1 μm to 5 nm, the conductivity (measured effective conductivity parallel to the interface) increases progressively with the increased interfacial density. This is even true for spacings in the sub-Debye range. For comparatively large spacings (>50 nm), semi-infinite space charges provide a quantitative description, while the behavior at smaller spacings reveals nano-size anomalies. At spacings smaller than ∼5 nm, the conductivity decreases. The results clearly demonstrate the possibility to prepare artificial ion conductors and the potential of mesoscopic ion conduction. The paper describes transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray pole figure, secondary ion mass spectrometry (SIMS) and impedance spectroscopic characterization.

Reversible shape changes of Pd nanoparticles on MgO(100)

We studied the interaction of oxygen with MgO(100) supported Pd nanoparticles at 10(-5) mbar oxygen pressure and a sample temperature of 570 K. We employed high-resolution X-ray reciprocal space mapping, which allows us to resolve the average particle shape from the quantitative analysis of intensity diffraction rods running perpendicular to corresponding facet surfaces. We identified the oxygen induced formation of nanosized (112) facets which is reversible in a CO atmosphere. Our results give direct evidence for the microscopic evolution of the nanoparticle shape under reactant exposure, which is essential for an atomistic understanding of catalytic reactions on nanoparticles.

Structures of BaF2-CaF2 heterolayers and their influences on ionic conductivity.

Recently, artificial ion conductors have been prepared by growing epitaxial heterolayers consisting of BaF2-CaF2 using molecular beam epitaxy. The ionic conductivity of these heterolayers shows a strong dependence on the layer thickness [N. Sata, S. Eberman, K. Eberl, and J. Maier, Nature 408, 996 (2000)]. In this paper three such heterolayers with different spacings (sample A: 80 nm, sample B: 10 nm, sample C: 1 nm) are investigated by conventional transmission electron microscopy and high-resolution transmission electron microscopy. The spacings are chosen such that they fall into the three conductivity regimes observed in N. Sata et al. (l > 50 nm; 8 < l < 50 nm; l < 8 nm). In accordance with conductivity studies, the samples with spacings of 10 nm or greater (A,B) are epitaxial and continuous, whereas in the case of extremely small spacing (C) the continuity of the layers is destroyed by formation of a column-like structure. Analytical electron microscopy reveals that, instead of forming multilayers, Ca and Ba separate in different columns in sample C. The structure properties of sample A (large l) are quite ideal: Planar interfaces with regular arrays of misfit dislocations with their Burgers vectors on the interface are observed. In the case of sample B (medium l) the lattice misfit is accommodated, in addition, by wavy interfaces associated with dislocations characterized by a Burgers vector that makes a large angle to the interfaces. The (111) lattice spacing very close to the interfaces is markedly changed due to this novel relaxation mechanism in the multilayer. The influences of the crystallographic defects on the ionic conductivity are also discussed.