Conventional and back-side focused ion beam milling for off-axis electron holography of electrostatic potentials in transistors

Quantitative electric field mapping of a p-n junction by DPC STEM

Local electromagnetic fields in a specimen is measured at high spatial resolutions using differential phase contrast (DPC) imaging in scanning transmission electron microscopy (STEM). According to previous studies, DPC signals can be quantified by measuring the center of mass of the diffraction pattern intensity and/or performing a deconvolution method based on a phase contrast transfer function (PCTF). However, when using a segmented detector, the field strength has been considerably underestimated for a very thick specimen. The main cause of the underestimation is assumed to be inelastic scattering, mainly bulk plasmon scattering. In this study, we develop a method to remove this inelastic scattering effect from segmented detector DPC signals by modifying the PCTF deconvolution method. Field quantification results using this new technique are compared with those using pixelated detector DPC and electron holography, and all results indicated good agreement within an error margin.

Three-Dimensional Imaging of Beam-Induced Biasing of InP/GaInP Tunnel Diodes

Electron holographic tomography was used to obtain three-dimensional reconstructions of the morphology and electrostatic potential gradient of axial GaInP/InP nanowire tunnel diodes. Crystal growth was carried out in two opposite directions: GaInP-Zn/InP-S and InP-Sn/GaInP-Zn, using Zn as the p-type dopant in the GaInP but with changes to the n-type dopant (S or Sn) in the InP. Secondary electron and electron beam-induced current images obtained using scanning electron microscopy indicated the presence of p-n junctions in both cases and current-voltage characteristics measured via lithographic contacts showed the negative differential resistance, characteristic of band-to-band tunneling, for both diodes. Electron holographic tomography measurements confirmed a short depletion width in both cases (21 ± 3 nm) but different built-in potentials, V_bi, of 1.0 V for the p-type (Zn) to n-type (S) transition, and 0.4 V for both were lower than the expected 1.5 V for these junctions if degenerately doped. Charging induced by the electron beam was evident in phase images which showed nonlinearity in the surrounding vacuum, most severe in the case of the nanowire grounded at the p-type Au contact. We attribute their lower V_bi to asymmetric secondary electron emission, beam-induced current biasing, and poor grounding contacts.

Quantitative measurement of nanoscale electrostatic potentials and charges using off-axis electron holography: Developments and opportunities

Off-axis electron holography has evolved into a powerful electron-microscopy-based technique for characterizing electromagnetic fields with nanometer-scale resolution. In this paper, we present a review of the application of off-axis electron holography to the quantitative measurement of electrostatic potentials and charge density distributions. We begin with a short overview of the theoretical and experimental basis of the technique. Practical aspects of phase imaging, sample preparation and microscope operation are outlined briefly. Applications of off-axis electron holography to a wide range of materials are then described in more detail. Finally, challenges and future opportunities for electron holography investigations of electrostatic fields and charge density distributions are presented.

Manipulation of dipolar magnetism in low-dimensional iron oxide nanoparticle assemblies

Trench-patterned iron oxide nanoparticles are switched between a superspin glass and a superferromagnetic state.

In situ transmission electron microscopy of ionic conductivity and reaction mechanisms in ultrathin solid oxide fuel cells

Solid oxide fuel cells (SOFCs) are promising candidates for use in alternative energy technologies. A full understanding of the reaction mechanisms in these dynamic material systems is required to optimize device performance and overcome present limitations. Here, we show that in situ transmission electron microscopy (TEM) can be used to study redox reactions and ionic conductivity in SOFCs in a gas environment at elevated temperature. We examine model ultrathin half and complete cells in two environmental TEMs using off-axis electron holography and electron energy-loss spectroscopy. Our results from the model cells provide insight into the essential phenomena that are important for the operation of commercial devices. Changes in the activities of dopant cations in the solid electrolyte are detected during oxygen anion conduction, demonstrating the key role of dopants in electrolyte architecture in SOFCs.

Convenient preparation of high-quality specimens for annealing experiments in the transmission electron microscope

A procedure based on focused ion beam milling and in situ lift-out is introduced for the preparation of high-quality specimens for in situ annealing experiments in the transmission electron microscope. The procedure allows an electron-transparent lamella to be cleaned directly on a heating chip using a low ion energy and back-side milling in order to minimize redeposition and damage. The approach is illustrated through the preparation of an Al-Mn-Fe complex metallic alloy specimen.

Microstructural characterization of an Al-li-mg-cu alloy by correlative electron tomography and atom probe tomography

Correlative electron tomography and atom probe tomography have been carried out successfully on the same region of a commercial 8090 aluminum alloy (Al-Li-Mg-Cu). The combination of the two techniques allows accurate geometric reconstruction of the atom probe tomography data verified by crystallographic information retrieved from the reconstruction. Quantitative analysis of the precipitate phase compositions and volume fractions of each phase have been obtained from the atom probe tomography and electron tomography at various scales, showing strong agreement between both techniques.

Reduction of electrical damage in specimens prepared using focused ion beam milling for dopant profiling using off-axis electron holography

GaAs specimens containing p-n junctions have been prepared using focused ion beam (FIB) milling for examination using off-axis electron holography. By lowering the FIB operating voltage from 30 to 8 kV, we have shown a systematic reduction of the electrically 'inactive' thickness from 220 to 100 nm, resulting in a significant increase in the step in phase measured across the junctions as well as an improvement in the signal-to-noise ratio. We also show that the step in phase measured across the junctions can be influenced by the intensity of the electron beam.

Nanotomography in the chemical, biological and materials sciences

Nanotomography is a technique of growing importance in the investigation of the shape, size, distribution and elemental composition of a wide variety of materials that are of central interest to investigators in the physical and biological sciences. Nanospatial factors often hold the key to a deeper understanding of the properties of matter at the nanoscale level. With recent advances in tomography, it is possible to achieve experimental resolution in the nanometre range, and to determine with elemental specificity the three-dimensional distribution of materials. This critical review deals principally with electron tomography, but it also outlines the power and future potential of transmission X-ray tomography, and alludes to other related techniques.