T. H. Metzger

Analysis of strain and stacking faults in single nanowires using Bragg coherent diffraction imaging

Coherent diffraction imaging (CDI) on Bragg reflections is a promising technique for the study of three-dimensional (3D) composition and strain fields in nanostructures, which can be recovered directly from the coherent diffraction data recorded on single objects. In this paper, we report results obtained for single homogeneous and heterogeneous nanowires with a diameter smaller than 100 nm, for which we used CDI to retrieve information about deformation and faults existing in these wires. We also discuss the influence of stacking faults, which can create artefacts during the reconstruction of the nanowire shape and deformation.

Coherent x-ray wavefront reconstruction of a partially illuminated Fresnel zone plate

A detailed characterization of the coherent x-ray wavefront produced by a partially illuminated Fresnel zone plate is presented. We show, by numerical and experimental approaches, how the beam size and the focal depth are strongly influenced by the illumination conditions, while the phase of the focal spot remains constant. These results confirm that the partial illumination can be used for coherent diffraction experiments. Finally, we demonstrate the possibility of reconstructing the complex-valued illumination function by simple measurement of the far field intensity in the specific case of partial illumination.

Composition and strain in SiGe/Si(001) nanorings revealed by combined x-ray and selective wet chemical etching methods

The authors used x-ray diffraction to investigate strain and composition in SiGe nanorings formed during partial Si capping of self-assembled SiGe/Si(001) islands. The obtained results are corroborated with selective wet chemical etching experiments. Clear evidence is provided that rings are composed of a Ge rich core surrounded by Si richer ridges indicating that a substantial material redistribution occurs during the shape transformation from SiGe islands to rings. The results suggest that SiGe ring formation is driven by strain relief.

Pseudoepitaxial transrotational structures in 14 nm-thick NiSi layers on (001) silicon

In a system consisting of two different lattices, structural stability is ensured when an epitaxial relationship occurs between them and allows the system to retain the stress whilst avoiding the formation of a polycrystalline film. The phenomenon occurs if the film thickness does not exceed a critical value. Here we show that in spite of its orthorhombic structure, a 14 nm-thick NiSi layer can three-dimensionally adapt to the cubic Si lattice by forming transrotational domains. Each domain arises by the continuous bending of the NiSi lattice, maintaining a close relationship with the substrate structure. The presence of transrotational domains does not cause a roughening of the layer, but instead it improves the structural and electrical stability of the silicide in comparison with a 24 nm-thick layer formed using the same annealing process. These results have relevant implications for the thickness scaling of NiSi layers which are currently used as metallizations of electronic devices.

Anomalous x-ray diffraction on InAs/GaAs quantum dot systems

Free-standing InAs quantum dots on a GaAs (001) substrate have been investigated using grazing incidence x-ray diffraction. To suppress the strong scattering contribution from the GaAs substrate, we performed anomalous diffraction experiments at the superstructure (200) reflection, showing that the relative intensities from the dots and the substrate undergo a significant change with the x-ray energy below and above the As K edge. Since the signal from the substrate material can essentially be suppressed, this method is ideally suited for the investigation of strain, shape, and interdiffusion of buried quantum dots and quantum dots embedded in heteroepitaxial multilayers. In addition, we show that it can be used as a tool for studying wetting layers.

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

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 ...

Structure and ordering of GaN quantum dot multilayers

Grazing incidence x-ray techniques are used to characterize the structure of multilayered GaN quantum dots in an AlN matrix. For a dot lateral size of 170 A, the values of the interdot vertical and lateral correlation lengths are 1500 and 250 A, respectively. The presence of smaller quantum dots is observed only in the layers deposited first. The strain distribution in the multilayer is also investigated as a function of depth. Along the dot columns, the crystal lattice remains coherent, with elastic relaxation from the bottom to the top of the multilayer.

Strain states in a quantum well embedded into a rolled-up microtube: X-ray and photoluminescence studies

The shift in the optical response of a quantum well (QW) embedded in the wall of a rolled-up microtube along the tube axis is examined. The microtube is investigated by x-ray microdiffraction to deduce the strain state of the rolled-up heterostructure. Using these results, the optical response of the QW is calculated. A good agreement of the lattice parameters determined by diffraction and photoluminescence is found, if a double shell tube is assumed for certain parts of the tube. Our results indicate that a rolled-up multiwalled microtube experiences different strains on different windings at the same lateral position.

In situ observation of the elastic deformation of a single epitaxial SiGe crystal by combining atomic force microscopy and micro x-ray diffraction

An in situ combination of atomic force microscopy and micro x-ray diffraction was developed to study the elastic behavior of nanosized objects. This technique offers the means to locally access the Young elastic moduli and Poisson ratios of individual nanostructures. Here, we investigated the elastic behavior of a single self-assembled 450 nm high SiGe island. As pressure was applied on the island, the resonance frequency of the atomic force microscope tuning fork was tracked together with the x-ray diffraction stemming from this individual crystal. The change in the tip-island contact stiffness could be derived from the variation in the resonance frequency of the tuning fork, whereas the island mean lattice parameter was inferred from the center of mass of the island’s Bragg scattering. From this information, the reduced elastic modulus of the tip-island system could be directly determined, which is in very good agreement with literature values. The pressure needed to compress the island lattice to the S...

Zero-strain GaAs quantum dot molecules as investigated by x-ray diffuse scattering

The authors report on x-ray diffuse scattering at nominally strain-free GaAs(001) quantum dot molecules (QDMs). Al0.3Ga0.7As deposited by molecular beam epitaxy on GaAs(001) acts as barrier layer between the GaAs(001) substrate and subsequently grown QDMs; the adjusted thickness of 50nm preserves the in-plane lattice parameter. Pairs of lenselike quantum dots are created with preferential orientation along [11¯0] placed on shallow hills. Grazing incidence diffraction along with kinematical scattering simulations indicate completely strain-free QDs which prove a strongly suppressed intermixing between QDMs and the underlying AlGaAs barrier layer.