RESUMEN
Self-assembled quantum dots (QDs) are potential candidates for photoelectric and photovoltaic devices, because of their discrete energy levels. The characterization of QDs at the atomic level using a multimodal approach is crucial to improving device performance because QDs are nanostructures with highly correlated structural parameters. In this study, scanning transmission electron microscopy, geometric phase analysis, and atom probe tomography were employed to characterize structural parameters such as the shape, strain, and composition of self-assembled InAs-QDs with InGaAlAs spacer layers. The measurements revealed characteristic AlAs-rich regions above the QDs and InAs-rich regions surrounding the QD columns, which can be explained by the relationship between the effect of strain and surface curvature around the QD. The methodology described in this study accelerates the development of future QD devices because its multiple perspectives reveal phenomena such as atomic-scale segregations and allow for more detailed discussions of the mechanisms of these phenomena.
RESUMEN
Mobile charge carriers are essential components in high-performance, nano-engineered semiconductor devices. Employing charge carriers confined to heterointerfaces, the so-called two-dimensional electron gas, is essential for improving device performance. The real-space visualization of a two-dimensional electron gas at the nanometre scale is desirable. However, it is challenging to accomplish by means of electron microscopy due to an unavoidable strong diffraction contrast formation at the heterointerfaces. We performed direct, nanoscale electric field imaging across a GaN-based semiconductor heterointerface using differential phase contrast scanning transmission electron microscopy by suppressing diffraction contrasts. For both nearly the lattice-matched GaN/Al0.81In0.19N interface and pseudomorphic GaN/Al0.88In0.12N interface, the extracted quantitative electric field profiles show excellent agreement with profiles predicted using Poisson simulation. Furthermore, we used the electric field profiles to quantify the density and distribution of the two-dimensional electron gas across the heterointerfaces with nanometre precision. This study is expected to guide the real-space characterization of local charge carrier density and distribution in semiconductor devices.
RESUMEN
The compositional and structural investigations of threading dislocations (TDs) in InGaN/GaN multiple quantum wells were carried out using correlative transmission electron microscopy (TEM) and atom probe tomography (APT). The correlative TEM/APT analysis on the same TD reveals that the indium atoms are diffused along the TD and its concentration decreases with distance from the InGaN layer. On the basis of the results, we directly observed that the indium atoms originating from the InGaN layer diffuse toward the epitaxial GaN surface through the TD, and it is considered to have occurred via the pipe diffusion mechanism induced by strain energy relaxation.
RESUMEN
Differential phase contrast (DPC) in scanning transmission electron microscopy can be used to visualize electric field distributions within specimens in real space. However, for electric field mapping in crystalline specimens, the concomitant diffraction contrast is seriously problematic. In particular, for heterostructures with large lattice distortions, such as GaN-based semiconductor devices, the diffraction contrast cannot be reduced using conventional methods such as DPC image acquisition under off-axis conditions. In the present study, the electric field imaging of heterostructures is shown to suppress the diffraction contrast by averaging multiple DPC signals, obtained under various beam-tilt conditions near the zone axis. The remaining diffraction contrast was quantitatively estimated through simulations. This technique was demonstrated to enable the quantitative evaluation of electric field distributions across GaN/AlGaN multi-heterostructures, with errors possibly attributed to the residual diffraction contrast.
RESUMEN
Probing the charge density distributions in materials at atomic scale remains an extremely demanding task, particularly in real space. However, recent advances in differential phase contrast-scanning transmission electron microscopy (DPC-STEM) bring this possibility closer by directly visualizing the atomic electric field. DPC-STEM at atomic resolutions measures how a sub-angstrom electron probe passing through a material is affected by the atomic electric field, the field between the nucleus and the surrounding electrons. Here, we perform a fully quantitative analysis which allows us to probe the charge density distributions inside atoms, including both the positive nuclear and the screening electronic charges, with subatomic resolution and in real space. By combining state-of-the-art DPC-STEM experiments with advanced electron scattering simulations we are able to map the spatial distribution of the electron cloud within individual atomic columns. This work constitutes a crucial step toward the direct atomic scale determination of the local charge redistributions and modulations taking place in materials systems.
RESUMEN
The doping characteristics and carrier transport in a GaAs p-n junction were visualized with a approximately 10 nm spatial resolution, using light-modulated scanning tunneling spectroscopy. The dynamics of minority carriers under operating conditions, such as recombination, diffusion, and electric field induced drift, which had previously been analyzed on the basis of empirical electric properties, were successfully examined on the nanoscale. These results provide a solid basis for elucidating the mechanism of the carrier transport properties predicted by using the macroscopic analysis.
