[show abstract][hide abstract] ABSTRACT: Observation of an unusual, negatively-charged antiphase boundary in (Bi0.85Nd0.15)(Ti0.1Fe0.9)O3 is reported. Aberration corrected scanning transmission electron microscopy is used to establish the full three dimensional structure of this boundary including O-ion positions to ∼±10 pm. The charged antiphase boundary stabilises tetragonally distorted regions with a strong polar ordering to either side of the boundary, with a characteristic length scale determined by the excess charge trapped at the boundary. Far away from the boundary the crystal relaxes into the well-known Nd-stabilised antiferroelectric phase.
[show abstract][hide abstract] ABSTRACT: Nanometric inclusions filled with nitrogen, located adjacent to FenN (n¼3 or 4) nanocrystals within (Ga,Fe)N layers, are identified and characterized using scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS). High-resolution STEM images reveal a truncation of the Fe-N nanocrystals at their boundaries with the nitrogen-containing inclusions. A controlled electron beam hole drilling experiment is used to release nitrogen gas from an inclusion in situ in the electron microscope. The density of nitrogen in an individual inclusion is measured to be 1.460.3 g/cm3. These observations provide an explanation for the location of surplus nitrogen in the (Ga,Fe)N layers, which is liberated by the nucleation of FenN (n>1) nanocrystals during growth.
Journal of Applied Physics 07/2013; 114(3):033530. · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: Hafnium oxide and silicate materials are now incorporated into working CMOS devices; however, the crystallization mechanism is still poorly understood. In particular, addition of SiO2 to HfO2 has been shown to increase the crystallization temperature of HfO2, hence, allowing it to remain amorphous under current processing conditions. Building on earlier study, we herein investigate bulk HfxSi(1-x)O2 samples to determine the effect of SiO2 on the crystallization pathway. Techniques, such as XRD, HTXRD, thermal analysis techniques and TEM are used. It is found that the addition of SiO2 has very little affect on the crystallization path at temperatures below 900°C, but at higher temperatures, a second t-HfO2 phase nucleates and is stabilized due to the strain of the surrounding amorphous SiO2 material. With an increase in SiO2 content, the temperature at which this nucleation and stabilization occurs is increased. The effect of strain has implications for inhibiting the crystallization of the high-k layer, reduction of grain boundaries and hence diffusion, reduction of formation of interface layers and the possibility of stabilizing t-HfO2 rather than m-HfO2, hence, increasing the dielectric of the layer.
Journal of the American Ceramic Society 12/2012; 95(12):3985. · 2.11 Impact Factor
[show abstract][hide abstract] ABSTRACT: The discovery of unusual nanorod precipitates in bismuth ferrite doped with Nd and Ti is reported. The atomic structure and chemistry of the nanorods are determined using a combination of high angle annular dark field imaging, electron energy loss spectroscopy, and density functional calculations. It is found that the structure of the BiFeO3 matrix is strongly modified adjacent to the precipitates; the readiness of BiFeO3 to adopt different structural allotropes in turn explains why such a large axial ratio, uncommon in precipitates, is stabilized. In addition, a correlation is found between the alignment of the rods and the orientation of ferroelastic domains in the matrix, which is consistent with the system's attempt to minimize its internal strain. Density functional calculations indicate a finite density of electronic states at the Fermi energy within the rods, suggesting enhanced electrical conductivity along the rod axes, and motivating future investigations of nanorod functionalities.
[show abstract][hide abstract] ABSTRACT: High angle annular dark field (HAADF) image simulations were performed on a series of AlAs/GaAs interfacial models using the frozen-phonon multislice method. Three general types of models were considered-perfect, vicinal/sawtooth and diffusion. These were chosen to demonstrate how HAADF image measurements are influenced by different interfacial structures in the technologically important III-V semiconductor system. For each model, interfacial sharpness was calculated as a function of depth and compared to aberration-corrected HAADF experiments of two types of AlAs/GaAs interfaces. The results show that the sharpness measured from HAADF imaging changes in a complicated manner with thickness for complex interfacial structures. For vicinal structures, it was revealed that the type of material that the probe projects through first of all has a significant effect on the measured sharpness. An increase in the vicinal angle was also shown to generate a wider interface in the random step model. The Moison diffusion model produced an increase in the interface width with depth which closely matched the experimental results of the AlAs-on-GaAs interface. In contrast, the interface width decreased as a function of depth in the linear diffusion model. Only in the case of the perfect model was it possible to ascertain the underlying structure directly from HAADF image analysis.
[show abstract][hide abstract] ABSTRACT: The image processing technique of column ratio mapping was applied to aberration-corrected high angle annular dark field (HAADF) images of short period MBE (molecular beam epitaxy) grown InAs/GaAs superlattices. This method allowed the Indium distribution to be mapped and a more detailed assessment of interfacial quality to be made. Frozen-phonon multislice simulations were also employed to provide a better understanding of the experimental column ratio values. It was established that ultra-thin InAs/GaAs layers can be grown sufficiently well by MBE. This is despite the fact that the Indium segregated over 3-4 monolayers. Furthermore, the effect of the growth temperature on the quality of the layers was also investigated. It was demonstrated that the higher growth temperature resulted in a better quality superlattice structure.
[show abstract][hide abstract] ABSTRACT: High resolution scanning transmission electron microscope (HRSTEM)
imaging and electron energy loss spectroscopy spectrum imaging (EELS SI)
methods have been successfully applied to the analysis of
(BNFT) ceramics. The atomic scale structural and chemistry of defects in
these ceramics have been determined, leading to the discovery of a new
class of Nd-rich nanoprecipitates in these ceramics.
Journal of Physics Conference Series 01/2012; 471:012034.
[show abstract][hide abstract] ABSTRACT: Using EELS spectrum imaging, an HfN or Hf(O,N) reaction layer has been identified at the TiN/HfO2 interface in a metal inserted high-k gate stack. The reaction layer has a mean thickness of 0.45nm over an 18nm length of the interface. This reaction layer formed in the original HfO2. By binning the data ×4 along the interface, a variation of the width from 0.35nm to 0.65nm along the interface can be seen. The 10%-90% widths of the elemental profiles can also be found but the binning required is greater (×10). The profile widths are approximately constant along the interface but the values differ from element to element. Thus the reaction has not formed a conformal layer of uniform thickness. An efficient way of processing the data at different levels of binning is described.
Journal of Physics Conference Series 01/2012; 371(1).
[show abstract][hide abstract] ABSTRACT: Aberration corrected scanning transmission electron microscopy revealed that Bi0.85Nd0.15Fe0.9Ti0.1O3 ceramics contain coherent Nd-rich precipitates distributed throughout the perovskite lattice, implying charge compensation is obtained by the creation of VNd/// and not VBi///. At low concentrations, therefore, Ti4+ replace Fe2+ with the creation of 2/3VNd///, and at higher concentrations (when Fe2+ have been eliminated and the conductivity suppressed), Fe3+ with the creation of 1/3VNd///. The switch in ionic compensation mechanism from 2/3VNd/// at low Ti concentrations (∼1%) to 1/3VNd/// at higher concentrations (>1%) results in a decrease in the magnitude of ΔTC/Δx, as the disruption of long range anti-polar coupling declines.
[show abstract][hide abstract] ABSTRACT: High resolution scanning transmission electron microscope (HRSTEM) and electron energy loss spectroscopy spectrum imaging (EELS SI) methods have been successfully applied to the analysis of planar antiphase boundaries in (Bi0.85Nd0.15)(Fe0.9Ti0.1)O3 (BNFT) ceramics. Two distinct structures for these boundaries have been identified, one of which has never previously been reported for perovskites. This latter form has a characteristic ladder structure in the core, and comes in stepped and unstepped forms. Atomic resolution spectrum imaging reveals that excess Ti is key to the formation of these ladder-structured antiphase boundaries.
Journal of Physics Conference Series 01/2012; 371:012036.
[show abstract][hide abstract] ABSTRACT: Aberration-corrected high angle annular dark field (HAADF) imaging in scanning transmission electron microscopy (STEM) can now be performed at atomic-resolution. This is an important tool for the characterisation of the latest semiconductor devices that require individual layers to be grown to an accuracy of a few atomic layers. However, the actual quantification of interfacial sharpness at the atomic-scale can be a complicated matter. For instance, it is not clear how the use of the total, atomic column or background HAADF signals can affect the measured sharpness or individual layer widths. Moreover, a reliable and consistent method of measurement is necessary. To highlight these issues, two types of AlAs/GaAs interfaces were studied in-depth by atomic-resolution HAADF imaging. A method of analysis was developed in order to map the various HAADF signals across an image and to reliably determine interfacial sharpness. The results demonstrated that the level of perceived interfacial sharpness can vary significantly with specimen thickness and the choice of HAADF signal. Individual layer widths were also shown to have some dependence on the choice of HAADF signal. Hence, it is crucial to have an awareness of which part of the HAADF signal is chosen for analysis along with possible specimen thickness effects for future HAADF studies performed at the scale of a few atomic layers.
[show abstract][hide abstract] ABSTRACT: We have performed a detailed study of the lattice distortions of InP wurtzite nanowires containing an axial screw dislocation. Eshelby predicted that this kind of system should show a crystal rotation due to the dislocation induced torque. We have measured the twisting rate and the dislocation Burgers vector on individual wires, revealing that nanowires with a 10-nm radius have a twist up to 100% larger than estimated from elasticity theory. The strain induced by the deformation has a Mexican-hat-like geometry, which may create a tube-like potential well for carriers.
[show abstract][hide abstract] ABSTRACT: We have attempted to measure the O/Ni inner-shell cross-section ratios from a sample of NiO using EELS in an electron microscope. The results lay within roughly 10% of the predictions of both the hydrogenic and Hartree-Slater theories. However, the data showed a spread of around 20% which may have been caused by artefacts introduced during the specimen preparation.
[show abstract][hide abstract] ABSTRACT: The large range of signal intensities found in electron energy loss spectroscopy causes difficulty when trying to record the spectrum accurately. This paper describes a system which records the data serially as simultaneous pulse and analogue signals without the need for changing the gain of the photomultiplier tube during the scan. The effect of deadtime and dark current on the recorded signals is discussed and procedures for correcting the data for these effects are described. At the same time, it is possible to correct the data for stray signal in the spectrometer and non-linearity in the photomultiplier head-amplifier at high output signal levels. In addition to its use for electron energy loss spectroscopy, this system is ideal for recording other signals which have a wide dynamic range and change rapidly from point to point, e.g. energy filtered diffraction patterns from thin amorphous films.
[show abstract][hide abstract] ABSTRACT: This paper presents the shapes of the electron energy-loss near-edges structure (ELNES) on the N K-edge of the group IVA (Ti, Zr, Hf) and group VA (V, Nb, Ta) transition metal mononitrides close to stoichiometry. With the exceptions of NbN and TaN, these compounds have the rock-salt (B1) structure when close to stoichiometry. NbN exists with both the rock-salt structure and a hexagonal structure. Two distinct ELNES shapes were observed from it, one of which corresponds closely with previously published data from the rock-salt structure. Under normal conditions, TaN is considered to exist only in the hexagonal form, the rock-salt form being a high-temperature/high-pressure phase although it has been reported as the result of plasma jet heating of the hexagonal form. Again two distinct ELNES shapes were observed, one of which appeared to fit into the pattern of the shapes from the other compounds with the rock-salt structure. The systematic changes of shape observed are very similar to those observed in the equivalent carbides and qualitatively follow the behaviour expected from theoretical band structures. The change in the chemical shift of the N K-edge on going from a group IVA nitride to a group VA nitride is ∼-0·8 eV while that on going from a group IVA carbide to a group VA carbide is ∼+0·8 eV. This difference in behaviour is explained as the result of differences in the densities of states at the Fermi levels of the compounds. The position of the first peak in the ELNES also shows a systematic change in its energy relative to the core state as the number of valence electrons in the compound increases and also as the transition series of the metal species changes. The energies, Er, of the peaks in the ELNES relative to the threshold follow a relationship similar to that predicted by Natoli, i.e. (Er - V)a = const. where V is the ‘muffin tin’ potential and a is the lattice parameter. The first peak gives a negative constant in the relationship. The value of constant increases for each subsequent peak up to the sixth becoming positive for the fourth and higher peaks but drops slightly on going from the sixth to the seventh peak. Each peak gives a different value of V in the relationship. The data sets for the carbides and the nitrides are systematically different in a similar way for each peak and there are deviations from linearity within each set. The systematic difference is minimized and the linearity significantly improved if the difference in the energies of two prominent peaks is used instead of Er. This systematic variation of peak energy with lattice parameter can be used to predict the lattice parameter. If both the nitride and the carbide data for the energy of a prominent peak relative to the threshold are used, this results in a maximum deviation of 4% (or ∼0·02 nm). However, if the differences in the energies of two prominent peaks are used and the data for the carbides and the nitrides are treated independently, the maximum deviation drops to 0·4% (or ∼0·002 nm). At this level, uncertainties in the lattice parameters themselves come into play and better characterized materials are required to set true limits to the accuracy of the predictions. Finally some applications in the microanalysis of materials are outlined briefly.
[show abstract][hide abstract] ABSTRACT: The correlation that exists between elemental concentration ratios determined using EDX and EELS is examined. EELS and EDX were recorded simultaneously from the second phase carbides of a ferritic steel and the Cr/Fe and Mn/Fe concentration ratios were determined from each spectrum. For this system, it was found that the Cr/Fe ratios could be determined by EDX to within an accuracy of 4% and these were then used to provide a check on the EELS analysis procedures. The effects of using different partial ionization cross-section models and methods of applying these models were examined. It was found that an overall agreement of 3% between EDX and EELS was obtained in this particular experiment. Much poorer agreement was found to exist between the two techniques for the Mn/Fe ratios. The discrepancy is attributed mainly to the inadequacies of the procedure used for extracting the Mn characteristic signal in the EELS spectra.
[show abstract][hide abstract] ABSTRACT: Hafnium-containing compounds are of great importance to the semiconductor industry as a high-κ gate dielectric to replace silicon oxynitrides. Here, the crystallization processes and chemistry of bulk hafnia powders are investigated, which will aid in interpretation of reactions and crystallization events occurring in thin films used as gate dielectrics. Amorphous hafnia powder was prepared via a sol–gel route using the precursor HfOCl2·H2O. The powders were subjected to various heat treatments and analyzed using X-ray diffraction and thermal analysis techniques. A large change in the crystallization pathway was found to occur when the sample was heated in an inert environment compared with air. Instead of the expected monoclinic phase, tetragonal hafnia also formed under these conditions and was observed up to temperatures of ∼760°C. The tetragonal particles eventually transform into monoclinic hafnia on further heating. Possible mechanisms for the crystallization of tetragonal hafnia are discussed. It is proposed that, in an inert environment, tetragonal hafnia is stabilized due to the presence of oxygen vacancies, formed by the reduction of HfIV to HfIII. As the temperature increases the crystal grows until there are too few oxygen vacancies left in the structure to continue stabilizing the tetragonal phase, and hence transformation to monoclinic hafnia occurs.
Journal of the American Ceramic Society 02/2011; 94(3):886 - 894. · 2.11 Impact Factor
[show abstract][hide abstract] ABSTRACT: A method for extracting core and shell spectra from core-shell particles with varying core to shell volume fractions is described. The method extracts the information from a single EELS spectrum image of the particle. The distribution of O and N was correctly reproduced for a nanoparticle with a TiN core and Ti-oxide shell. In addition, the O distribution from a nanoparticle with a Cu core and a Cu-oxide shell was obtained, and the extracted Cu L(2,3)-core and shell spectra showed the required change in EELS near edge fine structure. The extracted spectra can be used for multiple linear least squares fitting to the raw data in the spectrum image. The effect of certain approximations on numerical accuracy, such as treating the nanoparticle as a perfect sphere, as well as the intrinsic detection limits of the technique have also been explored. The technique is most suitable for qualitative, rather than quantitative, work.
[show abstract][hide abstract] ABSTRACT: Reactions at the interfaces can occur in metal inserted high-k gate stacks and are likely to evolve during device processing. Such reactions may affect the electrical properties of the stack and hence these could change during processing. The key interfaces are often not atomically flat and characterising the reaction layers on the near atomic scale required is a challenge. Aberration corrected scanning transmission electron microscopy (STEM) and spectrum imaging (SI) using electron energy loss spectroscopy (EELS) is used to characterise an HfN or Hf(O,N) reaction layer, ∼0.25nm wide, between HfO2 and TiN. This demonstrates the very significant advances in high spatial resolution characterisation made in recent years.