Nanoscale Cathodoluminescence and Conductive Mode Scanning Electron Microscopy of van der Waals Heterostructures

Abstract

van der Waals heterostructures (vdW-HSs) integrate dissimilar materials to form complex devices. These rely on the manipulation of charges at multiple interfaces. However, at present, submicrometer variations in strain, doping, or electrical breakages may exist undetected within a device, adversely affecting macroscale performance. Here, we use conductive mode and cathodoluminescence scanning electron microscopy (CM-SEM and SEM-CL) to investigate these phenomena. As a model system, we use a monolayer WSe2 (1L-WSe2) encapsulated in hexagonal boron nitride (hBN). CM-SEM allows for quantification of the flow of electrons during the SEM measurements. During electron irradiation at 5 keV, up to 70% of beam electrons are deposited into the vdW-HS and can subsequently migrate to the 1L-WSe2. This accumulation of charge leads to dynamic doping of 1L-WSe2, reducing its CL efficiency by up to 30% over 30 s. By providing a path for excess electrons to leave the sample, near full restoration of the initial CL signal can be achieved. These results indicate that the trapping of charges in vdW-HSs during electron irradiation must be considered, in order to obtain and maintain optimal performance of vdW-HS devices during processes such as e-beam lithography or SEM. Thus, CM-SEM and SEM-CL form a toolkit through which nanoscale characterization of vdW-HS devices can be performed, allowing electrical and optical properties to be correlated.

Metrology for 2D materials: a perspective review from the international roadmap for devices and systems

“…The addition of spectroscopic capabilities to SEM instruments, such as cathodoluminescence (CL) enables in situ correlation of morphological, structural, and compositional imaging with electronic properties of 2D layers and heterostructures with high spatial and spectral resolution. [18][19][20] STEM and SPM enable direct visualization and identication of disorder in the atomic arrangement of 2D materials crystals, 21,22 while TEM-based analysis such as electron energy loss spectroscopy (EELS) and nanobeam electron diffraction (NBED) have been used to demonstrate the spectral response dependence of 2D materials to temperature, composition, number of layers, and to map strain. [23][24][25][26] In addition, the limited scattering of the e-beam with atomically thin layers enables sub-angstrom resolution at low beam energies through ptychographic reconstruction in these materials.…”

Cathodoluminescence Nanoscopy: State of the Art and Beyond

“…Carrier diffusion length is another important factor that may limit the effective spatial resolution in incoherent CL measurements, which can be improved by lower diffusion coefficients and a shorter carrier lifetime. , However, for the material where the injected carriers experience difficulty diffusing away from the point of excitation, there can be an accumulation of charge, causing the deflection of the incident electron beam and image drift that will compromise the spatial resolution. This accumulation also leads to the dynamic doping of materials that may reduce the CL efficiency . Performing incoherent CL measurements in STEM , is a scheme worth noting, for the fact that the interaction volume is reduced to a minimum due to the ultrathin sample.…”

“…Especially in 1995, Christen et al made the hyperspectral maps in CL on GaAs quantum wells and visualized individual quantum dot positions . With the advances in material science and nanofabrication technologies, CL has provided a powerful platform to tackle emerging materials, including two-dimensional (2D) materials (Figure d), ,− single-photon emitters in nanodiamonds, ,− defects in boron nitride, ,− quantum dots, , hybrid perovskites, − and ultraviolet-light-emitting quantum wells. − In a recent study, CL was successfully employed for thermal property measurement in semiconductor nanowires . By extracting shifts in bandgap emission (Figure e), incoherent CL can be used to measure temperatures in situ and study thermal transport where the electron beam acts as a delta-function-like heat source, providing an enticing method called CL thermometry for rapid, in situ , and high-resolution thermal property characterization of integrated circuits and semiconductor nanodevices.…”

“…This accumulation also leads to the dynamic doping of materials that may reduce the CL efficiency. 57 Performing incoherent CL measurements in STEM 58,59 is a scheme worth noting, for the fact that the interaction volume is reduced to a minimum due to the ultrathin sample. Therefore, in the aspect of incoherent CL, STEM-CL allows a "cleaner" interaction (less than one bulk plasmon per electron) and provides a higher spatial resolution.…”