On specimen tilt for electron holography of semiconductor devices

Oxide Two-Dimensional Electron Gas with High Mobility at Room-Temperature

The prospect of 2-dimensional electron gases (2DEGs) possessing high mobility at room temperature in wide-bandgap perovskite stannates is enticing for oxide electronics, particularly to realize transparent and high-electron mobility transistors. Nonetheless only a small number of studies to date report 2DEGs in BaSnO3 -based heterostructures. Here, 2DEG formation at the LaScO3 /BaSnO3 (LSO/BSO) interface with a room-temperature mobility of 60 cm2  V-1  s-1 at a carrier concentration of 1.7 × 1013  cm-2 is reported. This is an order of magnitude higher mobility at room temperature than achieved in SrTiO3 -based 2DEGs. This is achieved by combining a thick BSO buffer layer with an ex situ high-temperature treatment, which not only reduces the dislocation density but also produces a SnO2 -terminated atomically flat surface, followed by the growth of an overlying BSO/LSO interface. Using weak beam dark-field transmission electron microscopy imaging and in-line electron holography technique, a reduction of the threading dislocation density is revealed, and direct evidence for the spatial confinement of a 2DEG at the BSO/LSO interface is provided. This work opens a new pathway to explore the exciting physics of stannate-based 2DEGs at application-relevant temperatures for oxide nanoelectronics.

Analysis of Local Charges at Hetero-interfaces by Electron Holography - A Comparative Study of Different Techniques

Interface charges confined within a few nanometers of hetero-interface can be characterized by measuring the phase shift of the transmitted beam using different electron holography techniques. However, reliable measurement of the electrostatic potential arising from the interface charges is challenging as the mean inner potential difference (ΔV₀) between two adjoining materials as well as local variation of the sample thickness affect the phase shift. In the present study, we show how electron holography can be used to characterize the confined charges at an oxide hetero-interface and evaluate the applicability of different techniques for this purpose. The model system chosen for this study is a LaAlO₃/SrTiO₃ (LAO/STO) (111) hetero-interface featuring a two-dimensional electron gas (2DEG), where the ΔV₀ between LAO and STO is about 2 eV, which is unignorably large and dominates the net potential variation across the interface. For transmission electron microscopy specimens prepared by focused ion beam we applied three different variants of electron holography techniques: off-axis, inline and hybrid electron holography; and compare the results obtained by these approaches in terms of the information transfer in the spatial frequency domain, and the signal-to-noise ratio of the electric field and charge density maps. To correctly assess the information pertinent to the interface-confined charges, we calculate the electrostatic potential and electric field distribution based on a charge model with taking account of the ΔV₀ between LAO and STO and compared the calculated profiles with the experimental results after calibrating the local thickness variation across the LAO/STO interface. The results show that hybrid electron holography recovers the information across a wide range of spatial frequencies, and as a result, delivers the most reliable charge density information, albeit convoluted with the unavoidable effects arising from ΔV₀.

Practical implementation of high-resolution electron ptychography and comparison with off-axis electron holography

Ptychography is a coherent diffractive imaging technique that can determine how an electron wave is transmitted through an object by probing it in many small overlapping regions and processing the diffraction data obtained at each point. The resulting electron transmission model describes both phase and amplitude changes to the electron wave. Ptychography has been adopted in transmission electron microscopy in recent years following advances in high-speed direct electron detectors and computer algorithms which now make the technique suitable for practical applications. Its ability to retrieve quantitative phase information at high spatial resolution makes it a plausible alternative or complement to electron holography. Furthermore, unlike off-axis electron holography, it can provide phase information without an electron bi-prism assembly or the requirement of a minimally structured region adjacent to the region of interest in the object. However, it does require a well-calibrated scanning transmission electron microscope and a well-managed workflow to manage the calibration, data acquisition and reconstruction process to yield a practical technique. Here we detail this workflow and highlight how this is greatly assisted by acquisition management software. Through experimental data and modelling we also explore the similarities and differences between high-resolution ptychography and electron holography. Both techniques show a dependence of the recovered phase on the crystalline orientation of the material which is attributable to dynamical scattering. However, the exact nature of the variation differs reflecting fundamental expectations, but nonetheless equally useful information is obtained from electron holography and the ptychographically determined object transmission function.

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.

Prospects of linear reconstruction in atomic resolution electron holographic tomography

Tomography commonly requires a linear relation between the measured signal and the underlying specimen property; for Electron Holographic Tomography this is given by the Phase Grating Approximation (PGA). While largely valid at medium resolution, discrepancies arise at high resolution imaging conditions. We set out to investigate the artefacts that are produced if the reconstruction still assumes the PGA even with an atomic resolution tilt series. To forego experimental difficulties the holographic tilt series was simulated. The reconstructed electric potential clearly shows peaks at the positions of the atoms. These peaks have characterisitic deformations, which can be traced back to the defocus a particular atom has in the holograms of the tilt series. Exchanging an atom for one of a different atomic number results in a significant change in the reconstructed potential that is well contained within the atom's peak.

Electron holography for fields in solids: problems and progress

Electron holography initially was invented by Dennis Gabor for solving the problems raised by the aberrations of electron lenses in Transmission Electron Microscopy. Nowadays, after hardware correction of aberrations allows true atomic resolution of the structure, for comprehensive understanding of solids, determination of electric and magnetic nanofields is the most challenging task. Since fields are phase objects in the TEM, electron holography is the unrivaled method of choice. After more than 40 years of experimental realization and steady improvement, holography is increasingly contributing to these highly sophisticated and essential questions in materials science, as well to the understanding of electron waves and their interaction with matter.