M.R. McCartney

Direct observation of potential distribution across Si/Si p-n junctions using off-axis electron holography

Off‐axis electron holography was used to observe the potential distribution across a 2×1018/cm3 p‐ and n‐doped Si/Si p‐n junction. With digital image recording and processing, and a novel method for thickness determination, we have successfully extracted two‐dimensional maps of the depletion region potential. For a defect‐free region, we measured relatively abrupt changes in potential in the range 1.0–1.5 V across lateral distances of 20–30 nm. Although influenced by instrumental and sample limitations, these values are reasonably consistent with expected Si junction parameters and thus establish the promise of this technique for measuring potential distributions across device junctions and interfaces.

Magnetic interactions within patterned cobalt nanostructures using off-axis electron holography

The magnetization reversal behavior of submicron-sized patterned Co elements has been studied using off-axis electron holography. Magnetic fields were applied to the samples in situ to determine both the hysteresis loops of individual elements and the interactions between closely spaced elements. The observations show that an understanding of the formation of remanent states is likely to be of particular importance for the design of magnetic elements for device applications.

The electron-beam-induced reduction of transition metal oxide surfaces to metallic lower oxides

Abstract The clean surfaces of several maximally valent transition metal oxides, namely TiO 2 , Nb 2 O 5 , V 2 O 5 and WO 3 , have been reduced to the corresponding binary oxide during extended observation inside a 400 keV high-resolution electron microscope. Selected-area electron diffraction patterns and optical transforms direct from the high-resolution lattice images were used to identify the oxide, while electron energy loss spectra from a sample of V 2 O 5 eliminated the possibility that the isostructural carbide had been formed. These reduced binary oxides have a defective rocksalt structure, and they are metallic in nature , which may thereby stifle further electron-stimulated desorption of oxygen.

Correlation of coercivity and microstructure of thin CoFe films

The magnetic and structural properties of sputtered Co50Fe50(CoFe) films were examined. CoFe films 300 A thick deposited on Si substrates at room temperature showed large coercive fields of 140 Oe. When similar thickness films were deposited at 100 °C, the coercivity dropped to 90 Oe, and when they were deposited on CoO, the coercivity was reduced to 12 Oe. Cross-sectional imaging with transmission electron microscopy revealed that the CoO underlayer had induced a columnar grain structure in the CoFe, with grain diameters ranging from 50 to 150 A. CoFe films grown on Si contained larger grains of 200–350 A in diameter with fewer distinct vertical grain boundaries. Lorentz microscopy showed that domain walls in the hard CoFe film formed complex, fixed patterns in fields less than the coercivity, whereas walls in the CoFe/CoO sample were more conformal and mobile in response to changing fields. Possible structural origins for the wide variation in coercivity obtained with different substrates, deposition te...

Magnetic tunnel junctions thermally stable to above 300°C

Magnetic tunnel junctions formed from sandwiches of magnetically hard Co75Pt12Cr13 and magnetically soft Co88Pt12 ferromagnetic layers separated by thin alumina tunnel barriers are shown to be thermally stable to temperatures in excess of 300 °C. A comparison of cross-section transmission electron micrographs of an untreated sample and a similar one annealed at 350 °C indicates that the thickness of the amorphous tunnel barrier is slightly decreased after annealing. The resistance and magnetoresistance are only slightly affected by annealing at temperatures of up to ∼300 °C but then decrease monotonically at higher annealing temperatures. Interaction of the alumina layer with the adjacent ferromagnetic layers is the likely cause of this decrease.

Off-axis electron holography of epitaxial FePt films

Off-axis electron holography at the nanometer level has been used to investigate the magnetic microstructure of thin epitaxial FexPt1−x (x∼0.5) ordered alloy films. High-resolution electron microscopy in cross section showed high quality epitaxial growth but also revealed some widely spaced regions with structural defects. Lorentz microscopy and off-axis electron holography in field-free conditions established conclusively that similar defective areas were associated with local perturbations of the in-plane magnetic field within the thin films. Further holographic observations with the FePt[001] axis parallel to the film normal revealed variations in phase shifts in the vacuum outside the sample which indicated flux leakage along the film normal. Overall, the results demonstrate that off-axis electron holography is a highly useful technique for mapping local variations of the in-plane magnetic structure associated with defective thin films.

Measurement of lattice-fringe vectors from digital HREM images: experimental precision

Abstract The practical measurement of lattice-fringe spacings and angles recorded in digital high-resolution electron micrographs is evaluated experimentally. The method is based on a statistical estimation procedure and involves computer analysis of reciprocal-space parameters. This work concentrates on the analysis of images recorded with slow-scan CCD cameras, but alternative methods of image pick-up are also briefly considered. The method has been successfully applied to images recorded with electron doses smaller than 1eA 2 and for sample dimensions as small as 8Aacross. The practical precision depends on specimen characteristics, electron dose and the size of the measurement area and is in the range of 0.001–0.05Afor lattice spacings, and 0.1°–0.5° for lattice-plane angles. Finally, the technique is demonstrated in studies of a catalyst system, of a reduced surface oxide phase and of TiO 2 and TiN particles.

Studies of electron irradiation and annealing effects on TiO2 surfaces in ultrahigh vacuum using high-resolution electron microscopy

Electron-beam-induced surface damage and subsequent annealing processes in rutile crystals have been investigated with a 300 keV high-resolution electron microscope which has been modified for ultrahigh vacuum and equipped with a specimen heating holder. Room temperature irradiation produced a reduction sequence identical to that observed previously in conventional microscope vacuum, except for some evidence of enhanced sputtering. Irradiation at higher temperatures (200–400°C) increased the damage rate resulting in approximately the same amount of the surface TiO phase but producing larger areas of the intermediate TiO2-II phase. Sputter pits in both rutile and TiO2-II phases became markedly more facetted as the temperature was increased. Irradiation at temperatures greater than 500°C resulted in the formation of well-facetted holes in the rutile, but no accumulation of crystalline reduced phases was observed. Annealing of the damaged areas in the absence of the electron beam, as shown by the reformation of crystalline rutile, revealed a temperature dependence for the restoration of the stoichiometry of the reduced areas, while complete removal of the sometimes extensive sputter pits was observed at temperatures in excess of 200 ° C.

Epitaxial relationships in electron-stimulated desorption processes at transition metal oxide surfaces

Abstract The electron-stimulated desorption of oxygen from the surfaces of maximally-valent transition-metal oxides during electron microscope observation leads to the formation of the corresponding monoxide. The technique of surface profile imaging was used to follow the development of the monoxide, and revealed that these (metallic) phases invariably grew with a well-defined three-dimensional epitaxial relationship with the original oxide. Computer-drawn structural models enabled insight to be gained about the atomic rearrangement which took place during the monoxide growth.

Electron microscopy study of Ni/Ni3Al diffusion-couple interface—I. Microstructural observation and microchemical analysis

Abstract The Ni/Ni3Al diffusion-couple interface has been examined using the techniques of analytical electron microscopy and high-resolution electron microscopy. It is shown that the Ni-rich solid solution phase (γ phase) is transformed from the Ni3Al-based intermetallic phase (γ′ phase) and that it extends into the γ′ phase during diffusion annealing. The orientations of the γ and γ′ phases remain the same. Coherency exists across the γ/γ′ phase boundary. The position of the boundary is not well defined but it is in an equilibrium condition in terms of composition.