Imaging Atomically Thin Semiconductors Beneath Dielectrics via Deep Ultraviolet Photoemission Electron Microscopy

Morgann Berg, Fangze Liu, Sean Smith, R. Guild Copeland, Calvin K. Chan, Aditya D. Mohite, Thomas E. Beechem, and Taisuke Ohta

Phys. Rev. Applied 12, 064064 – Published 31 December 2019

Abstract

Imaging of fabricated nanostructures or nanomaterials covered by dielectrics is highly sought after for diagnostics of optoelectronics components. We show imaging of atomically thin ${Mo S}_{2}$ flakes grown on ${Si O}_{2}$ -covered $Si$ substrates and buried beneath ${Hf O}_{2}$ overlayers up to 120 nm in thickness using photoemission electron microscopy with deep-UV photoexcitation. Comparison of photoemission yield (PEY) to modeled optical absorption evinced the formation of optical standing waves in the dielectric stacks (i.e., cavity resonances of ${Hf O}_{2}$ and ${Si O}_{2}$ layers on $Si$ ). The presence of atomically thin ${Mo S}_{2}$ flakes modifies the optical properties of the dielectric stack locally. Accordingly, the cavity resonance condition varies between the sample locations over buried ${Mo S}_{2}$ and surrounding areas, resulting in image contrast with submicron lateral resolution. This subsurface sensitivity underscores the role of optical effects in photoemission imaging with low-energy photons. This approach can be extended to nondestructive imaging of buried interfaces and subsurface features needed for analysis of microelectronic circuits and nanomaterial integration into optoelectronic devices.

Received 19 November 2018
Revised 20 November 2019

DOI:https://doi.org/10.1103/PhysRevApplied.12.064064

Physics Subject Headings (PhySH)

Microstructure Optical & microwave phenomena

2-dimensional systems Insulators Multilayer thin films Nanostructures Semiconductors

Photoelectron emission microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Morgann Berg^1,2, Fangze Liu³, Sean Smith¹, R. Guild Copeland¹, Calvin K. Chan¹, Aditya D. Mohite⁴, Thomas E. Beechem¹, and Taisuke Ohta ^1,2,*

¹Sandia National Laboratories, Albuquerque, New Mexico 87185, USA
²Center for Integrated Nanotechnologies, Albuquerque, New Mexico 87185, USA
³Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
⁴Department of Chemical and Biomolecular Engineering, Rice University, Houston, Texas 77005, USA

^*tohta@sandia.gov

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Vol. 12, Iss. 6 — December 2019

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Images

Figure 1
PEEM images of ${Mo S}_{2}$ flakes covered with ${Hf O}_{2}$ films on 100-nm-thick ${Si O}_{2}$ underlayers, and illustrations of sample geometry and standing-wave formation within the dielectric stack. (a)–(d) show kinetic-energy-integrated photoemission yield images (i.e., the photoemission intensity is integrated across the entire electron kinetic energy range, and then converted to the unit of photoemission yield; see Ref. [40] for details). The thicknesses of ${Hf O}_{2}$ films (0, 5, 61, and 100 nm ${Hf O}_{2}$ ) are labeled at the top of image. (e)–(j) show the photon wavelength dependence of the kinetic-energy-integrated PEY images for 21-nm-thick ${Hf O}_{2}$ overlayer. Scale bars for photoemission yield, shown at the bottom of (a)–(j), represent higher yield as dark gray. The photon wavelength used for these measurements are labeled in each image. The arrow in (a) indicates the in-plane direction of the incident wave vector of deep-UV sample illumination. Purple waves and red triangular islands in (k) and (l) depict deep-UV illumination of buried ${Mo S}_{2}$ flakes with an incident angle of 73° off normal to the sample surface defined by the instrument's geometry. When the deep-UV wavelength matches the cavity's resonance condition, a standing wave forms in the dielectric, as illustrated by the color gradation (purple) in the dielectric films in (k). The nonresonance case is illustrated in (l).
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Figure 2
Photoemission yield spectra and the modeled optical absorption of the dielectric cavity. (a) PEY spectra as a function of photon energy obtained at sample locations over ${Mo S}_{2}$ flakes (dotted lines) and ${Si O}_{2}$ (solid lines), for dielectric stacks with 100-nm-thick ${Si O}_{2}$ and different ${Hf O}_{2}$ overlayer thicknesses. Spectra except for black ones are shifted along the vertical axis for clarity. Filled circles indicate peak positions in PEY spectra, while open circles denote the positions of low-intensity peaks. We determine all peak positions from PEY spectra over ${Si O}_{2}$ (solid lines). (b) Total optical absorption (1 − R) calculated for dielectric cavities comprised of stacks of ${Hf O}_{2}$ films with varying thickness atop 100-nm-thick ${Si O}_{2}$ . The filled and open circles overlaid on calculated absorption spectra in (b) depict the positions of resonance peaks in PEY spectra (a). (c) PEY spectra similar to (a) for dielectric stacks with nominally 12-nm-thick ${Si O}_{2}$ and different ${Hf O}_{2}$ overlayer thicknesses. Again, spectra except for black ones are shifted along the vertical axis for clarity. (d) Total optical absorption (1 − R) calculated for dielectric cavities comprised of stacks of ${Hf O}_{2}$ films with varying thickness atop 15-nm-thick ${Si O}_{2}$ .
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Figure 3
Spatial mapping of cavity mode resonance peak wavelengths [(a), (c), and (e)] and total peak areas [(b), (d), and (f)] for samples with 21-nm- and 61-nm-thick ${Hf O}_{2}$ overlayers on 100-nm-thick ${Si O}_{2}$ , and 117-nm-thick ${Hf O}_{2}$ overlayers on nominally 12-nm-thick ${Si O}_{2}$ . The unit of total peak area is proportional to the wavelength times the photoemission yield. The arrows in (a), (c), and (e) indicate the in-plane direction of the incident wave vector of deep-UV sample illumination. Ovals highlight the areas with contrast asymmetries (see the main text for details).
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Figure 4
PEEM images of ${Mo S}_{2}$ flakes covered with various dielectric materials. (a) ${Mo S}_{2}$ flakes sandwiched between 100-nm-thick ${Si O}_{2}$ (underlayer) and 76-nm-thick ${Al}_{2} O_{3}$ (overlayer). The cw light source is used for this measurement. See “Photoemission electron microscopy measurement” section of the Supplemental Material [35] for details. (b) ${Mo S}_{2}$ flakes sandwiched between a 100-nm-thick ${Si O}_{2}$ (underlayer) and a 21-nm-thick ${Si O}_{2}$ (overlayer). We use the deep-UV laser light source for this measurement. The photon energy used for measurements in (a) and (b) is 5.9 eV (λ = 210 nm).
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