High Optical Quality TMD Heterostructures Obtained from MBE Growth and Subsequent Transfer onto SiO₂/Si Wafers

The current preparation of 2D materials for use in devices mostly focuses on Chemical Vapor Deposition (CVD) and micro-exfoliation from bulk crystals. While CVD offers good scalability and big coverage of the sample it induces many defects during growth. Micro-exfoliation, on the other hand, yields monolayers with excellent optical and electronic properties but lacks user-friendliness and scalability for application.<br/>Molecular Beam Epitaxy (MBE) in ultra-high vacuum (UHV) offers unparalleled control of growth parameters and scalable growth of defect-free 2D materials with extremely good quality. [1] One of its current limitations is the reliance on metallic Ir(111) or Au(111) substrates which quench the optical response of grown materials, making them unsuitable for optoelectronic devices.<br/>In this work, we demonstrate a method to transfer an MBE grown transition metal dichalcogenide (TMD) heterostructure (HS) to a SiO₂/Si wafer that is a more suitable substrate for the investigation of optical properties and device fabrication. The original sample was prepared by heteroepitaxial growth of TMDs under UHV conditions. Temperature programmed growth followed by a CVD step results in a full layer of graphene on Ir(111) substrate. Then we performed a one-step MBE growth to prepare homogeneous regular MoS₂ islands during which individual Mo atoms were intercalated below graphene. The resulting HS MoS₂/Gr/Mo/Ir(111) is then used as a heterogeneous vdW substrate for growing WS₂ with the two-step method. The resulting sample consists of lateral and vertical quasi-freestanding heterostructures WS₂/MoS₂/Graphene/Mo on Ir(111), with TMD islands of lateral size below 100nm. Scanning tunneling spectroscopy reveals type-II band alignment for the lateral and a symmetric upward bend bending for the vertical HS. [2]<br/>The large, continuous graphene layer beneath the TMD HSs allows for the transfer from Ir(111) to a SiO₂/Si wafer. Electrochemical delamination from the Ir(111) crystal was used with PMMA as a supporting layer on top of the sample, which was then placed on the SiO₂/Si wafer. AFM of the transferred HS reveals mostly uniform height across the sample with some bi- and three-layer folding from the transfer process. Micro-Raman spectroscopy confirms the presence of the TMD HS on SiO₂/Si wafer and fluorescence spectroscopy reveals the characteristic exciton resonances which blueshift approaching the low temperature (4.2 K) limit, confirming its semiconducting nature [3].<br/>Wide-field hyperspectral fluorescence microscopy yields spatially and spectrally resolved images of the sample revealing spectrally homogeneous areas over hundreds of microns. Some sample areas show a different fluorescence instead of the typical TMD HS signature. This emission may originate from chemical complexes that could result from a reaction with graphene dangling bonds during the electrochemical delamination process.<br/>The combination of MBE growth and electrochemical delamination transfer technique provides a controllable and scalable new method of preparing 2D materials on any desirable surface suitable for application in electronic and optoelectronic devices.<br/>References<br/>[1] Joshua Hall et al 2018 2D Mater. 5 025005<br/>[2] B. Pielić et al 2021 “Electronic structure of quasi-freestanding WS2/MoS2 heterostructures” ACS Appl. Mater. Interfaces 2021, XXXX, accepted, 10.1021/acsami.1c15412<br/>[3] K. P. O’Donnel, X. Chen, Appl. Phys. Lett. 58, 2924 (1991);