Tailoring of magnetism & electron transport of manganate thin films by controlling the Mn-O-Mn bond angles via strain engineering.

Strain engineering beyond substrate limitation of colossal magnetoresistant thin (La0.6Pr0.4)0.7Ca0.3MnO3 (LPCMO) films on LaAlO3-buffered SrTiO3 (LAO/STO) substrates has been demonstrated using metalorganic aerosol deposition technique. By growing partially relaxed 7-27 nm thick heteroepitaxial LAO buffer layers on STO a perfect lattice matching to the LPCMO has been achieved. As a result, strain-free heteroepitaxial 10-20 nm thick LPCMO/LAO/STO films with bulk-like ferromagnetic metallic ground state were obtained. Without buffer the coherently strained thin LPCMO/STO and LPCMO/LAO films were insulating and weakly magnetic. The reason for the optimized magnetotransport in strain-free LPCMO films was found to be a large octahedral Mn-O-Mn bond angle φOOR ~ 166-168° as compared to the significantly smaller one of φOOR ~ 152-156° determined for the tensile (LPCMO/STO) and compressively (LPCMO/LAO) strained films.

Reconstruction of Angstrom resolution exit-waves by the application of drift-corrected phase-shifting off-axis electron holography.

Phase-shifting electron holography is an excellent method to reveal electron wave phase information with very high phase sensitivity over a large range of spatial frequencies. It circumvents the limiting trade-off between fringe spacing and visibility of standard off-axis holography. Previous implementations have been limited by the independent drift of biprism and sample. We demonstrate here an advanced drift correction scheme for the hologram series that exploits the presence of an interface of the TEM specimen to the vacuum area in the hologram. It allows to obtain reliable phase information up to 2π/452 at the 1 Å information limit of the Titan 80-300 kV environmental transmission electron microscope used, by applying a moderate voltage of 250 V to a single biprism for a fringe spacing of 1 Å. The obtained phase and amplitude information is validated at a thin Pt sample by use of multislice image simulation with the frozen lattice approximation and shows excellent agreement. The presented method is applicable in any TEM equipped with at least one electron biprism and thus enables achieving high resolution off-axis holography in various instruments including those for in-situ applications. A software implementation for the acquisition, calibration and reconstruction is provided.

Mesoscopic properties of interfacial ordering in amorphous germanium on Si(111) determined by quantitative digital image series matching.

For the quantitative characterization of atomic ordering in the transition region between crystalline and amorphous materials we have previously described a method based on averaging HREM images along the interface, simulation of averaged images with the use of the averaged projected potential approximation and determination of the atom arrangement by means of an iterative matching procedure for high-resolution focus series. In order to study mesoscopic properties of crystal induced ordering a fully quantitative procedure is developed in this work. For this purpose, the width of the averaging region is defined as a compromise providing necessary accuracy of calculations and desirable locality of characterization of the atom distribution. Fluctuations of the obtained atom distribution on the amorphous side of the interface are estimated by means a of special Monte-Carlo simulation technique. As a result, distribution functions obtained from different regions can be quantitatively compared and statistically significant differences can be identified and related to the atomic structure. The method is applied to investigate the near interfacial atom order at the interface between atomically flat crystalline Si(111) and amorphous Ge. It is shown that significant variations in the atomic density distribution occur on a 5-10nm scale for germanium atoms in the second and third atomic layer lying parallel to the interface.

Structure and elemental distribution of (Ga,Mn)N nanowires.

(Ga,Mn)N nanowires were grown by plasma-assisted molecular beam epitaxy on p-type Si(111) substrates. Chemical composition and elemental distribution of single nanowires were analyzed by energy dispersive X-ray spectroscopy revealing an inhomogeneous Mn distribution decreasing from the surface of the nanowires toward the inner core region. The average Mn concentration within the nanowires is found to be below 1%. High-resolution transmission electron microscopy shows the presence of planar defects perpendicular to the growth direction in undoped and Mn-doped GaN nanowires. The density of planar defects dramatically increases under Mn supply.

Localization and preparation of recombination-active extended defects for transmission electron microscopy analysis.

Recombination-active extended defects in semiconductors frequently occur at a low density which makes their structural and chemical analysis by transmission electron microscopy (TEM) techniques virtually impossible. Here an approach is described that uses in situ electron beam induced current (EBIC) in a focused ion beam machine to localize such defects for TEM lamella preparation. As an example, a defect complex occurring in block-cast multicrystalline silicon with a density of less than 10(4) cm(-3) has been prepared and analyzed by TEM. The chemical sensitivity of the technique is estimated to be about 10(13) atoms cm(-2) which is comparable to synchrotron-based x-ray techniques. The localization accuracy of the TEM lamella is shown to be better than 50 nm when low-energy EBIC is used.

Disturbance of tunneling coherence by oxygen vacancy in epitaxial Fe/MgO/Fe magnetic tunnel junctions.

Oxygen vacancies in the MgO barriers of epitaxial Fe/MgO/Fe magnetic tunnel junctions are observed to introduce symmetry-breaking scatterings and hence open up channels for noncoherent tunneling processes that follow the normal WKB approximation. The evanescent waves inside the MgO barrier thus experience two-step tunneling, the coherent followed by the noncoherent process, and lead to lower tunnel magnetoresistance, higher junction resistance, as well as increased bias and temperature dependence. The characteristic length of the symmetry scattering process is determined to be about 1.6 nm.

Regular dislocation networks in silicon as a tool for nanostructure devices used in optics, biology, and electronics.

Well-controlled fabrication of dislocation networks in Si using direct wafer bonding opens broad possibilities for nanotechnology applications. Concepts of dislocation-network-based light emitters, manipulators of biomolecules, gettering and insulating layers, and three-dimensional buried conductive channels are presented and discussed. A prototype of a Si-based light emitter working at a wavelength of about 1.5 microm with an efficiency potential estimated at 1% is demonstrated.

Analysis of high resolution transmission electron microscope images of crystalline-amorphous interfaces.

For the analysis of images of homogeneous crystalline-amorphous interfaces we propose to average them along the interface obtaining the averaged interface image or the averaged intensity profile. Due to averaging, contrast components with the periodicity of the crystalline area of the image are extracted. Thus, the contrast features originating from the random overlap of the projected potentials of atoms in the amorphous layer are suppressed. It is shown that averaged images can be simulated by the multi-slice method using the novel approach to model the near interfacial amorphous structure by its mean atomic density distribution in front of the crystalline boundary. The crystalline structure is represented by its known atomic positions. We apply the proposed method to the investigation of the near interfacial short-range order in the c-Si/ a-Ge crystalline-amorphous interface.