Spectroscopic Characterization of Buried 2D-3D Interfaces Enabled by Novel Polymer-Free Transfer Technique

Two-dimensional (2D) materials have been studied for use in a variety of emerging electronic, optoelectronic, and photonic applications. Device fabrication often involves depositing various three-dimensional (3D) materials, such as metals or dielectrics, onto the 2D material, which creates a buried interface. Interfacing materials significantly affect the nature and homogeneity of the electronic and phononic structure in 2D materials. To date, spectroscopic studies of the doping and strain in supported 2D materials have mostly been limited to unrealistic cases where the 2D material is transferred or exfoliated onto a substrate of interest [1,2], with a few exceptions [3]. A polymer-free transfer technique capable of exposing any buried 2D-3D material interface would enable direct physical characterization of realistic heterojunction properties.<br/>In this presentation, we present a new polymer-free transfer technique that allows direct characterization of the buried interface in almost any 2D-3D heterostructure. The method is enabled by employing a water soluble GeO₂ substrate and wafer bonding process. We demonstrate the diversity of the technique by fabricating various heterostructures on monolayer MoS₂, including inert and reactive metals and oxide dielectrics, and expose the initially buried interfaces for direct characterization. We also fabricate the same heterostructures by exfoliating MoS₂ onto metals and oxides to directly compare strain, doping, and chemistry in MoS₂ across fabrication techniques. We employ Raman spectroscopy to measure the strain and doping profile in MoS₂ heterostructures. The surface chemistry and band alignment (where applicable) are measured using X-ray photoelectron spectroscopy (XPS).<br/>Reactively sputtered GeO₂ films exhibit a 2:1 O:Ge ratio according to XPS. After heterostructure fabrication and wafer bonding, the GeO₂ layer is completely dissolved in deionized H₂O within a few hours, exposing the buried interface. In general, peak shifts in MoS₂ Raman spectra indicate the degree of strain and doping imparted on MoS₂ significantly depends on the interfacing material and the fabrication method. When the interfacing material is directly deposited on MoS₂, the strain imparted on the MoS₂ is more significant than the degree of doping. Reactions between MoS₂ and reactive metals (Ti, Ni) are significant when the heterostructure is fabricated by direct deposition according to XPS and Raman spectroscopy. In contrast, when heterostructures are fabricated by exfoliating MoS₂ onto a substrate of interest, doping dominates over strain. Our novel method enables direct characterization of buried 2D-3D interfaces and expanded control over the strain profile in 2D materials, which could lead to new discoveries and functionalities.<br/>This work was supported by a Laboratory Directed Research & Development program at Sandia National Laboratories. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the article do not necessarily represent the views of the U.S. DOE or the United States Government. This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science.<br/>[1] B. G. Shin et al. Adv. Mater. 2016, 28 (42), 9378-9384<br/>[2] Y. Chen et al. ACS Nano 2018, 12, 2569-2579<br/>[3] K. Jo et al. ACS Nano 2021, 15, 5618-5630