Characterization of Dilute Mn Implantation in Bi2Se3 Films via 4D-STEM and EELS

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Advanced electron microscopy techniques, such as in-situ electron energy loss spectroscopy (EELS) and 4D-Scanning Transmission Electron Microscopy (4D-STEM), have become indispensable for investigating the atomic-scale effects of dopants in functional materials. These methods are particularly crucial for studying doped topological insulators, where the precise location and structural impact of dopant ions are critical for tuning electronic properties. In this work, we leverage low-dose, multiframe EELS and multislice ptychography to study Mn-doped Bi2Se3 thin films, a promising platform for realizing the quantum anomalous Hall effect (QAHE). By combining these techniques with pixelated cameras and advanced signal-averaging approaches, we have been able to overcome the significant challenges of characterizing the subtle structural and electronic effects of low dopant concentrations. Mn doping in Bi2Se3 thin films is an essential step toward breaking time-reversal symmetry and opening a gap at the Dirac point of its topological surface states, a requirement for QAHE [1-6]. However, excessive or uncontrolled doping can end up as magnetic impurities in the system. These can disrupt these surface states and suppress ferromagnetic ordering due to the formation of Mn-Mn antiferromagnetic dimers [7] or paramagnetic/superparamagnetic Mn clusters [8]. These challenges necessitate precise control over dopant placement, which is achieved by using P-NAME, a focused ion beam with co-incident SEM for localized doped areas ranging from single-ion implantation to 1x1019 ions cm-3 with sub-10 nm precision [9]. Furthermore, Mn doping introduces localized magnetic moments that interact with ultrafast carrier relaxation processes, affecting charge transport and quantum device functionality [10]. Understanding these effects demands techniques with sufficient sensitivity and resolution, particularly for low doping concentrations where conventional characterization tools often fall short. Initial traditional STEM atomic resolution imaging as in Figure 1, reveals changes in spacing in the layered Bi2Se3 structure between the Mn implanted region regions (highlighted with purple arrow) and non-implanted region (highlighted with yellow arrow). STEM Using correlative in-situ low dose, multiframe EELS and 4D-STEM, we characterize the spatial distribution and electronic fingerprints of Mn dopants while also examining the effects of lattice distortions and strain on the film due to implantation via the P-NAME system. While this provides an excellent overview of the distribution of Mn dopants in the film, it does not provide 3D depth information about the dopants within the film. This is critical as QAHE-suppressing Mn-Mn dimer and cluster formations may occur along atomic columns and this information would not be present with standard EELS or 4D-STEM measurements. Therefore, we explore the use of multislice ptychography to deliver depth sectioning of the film at the same Mn-dopant sites. This should provide us with a full 3D survey of the Mn-dopant sites, which is critical to advance these systems to device-ready applications. The combination of low-dose multiframe EELS, 4D-STEM strain mapping and multislice ptychography enables comprehensive insight into dopant behavior, revealing both structural and electronic changes with sub-angstrom precision. These methods allow us to address fundamental questions about Mn doping, including its impact on carrier dynamics, lattice distortions, and surface states, while also highlighting the capabilities of advanced ion implantation systems to precisely tailor material properties. This study establishes a pathway for the rational design of qubit-dense, QAHE-based devices and illustrates the vital role of cutting-edge electron microscopy techniques in materials discovery [11]. STEM imaging of Bi2Se3 thin film implanted locally with Mn ions.