Cl₂-Based Selective Etching of Transition Metal Dichalcogenides

Two-dimensional semiconducting transition metal dichalcogenides (TMDs) are highly promising candidates for advanced electronic and optoelectronic devices. However, current stage-of-the-art deposition approaches for growing TMD monolayers suffer from undesired nucleation and growth of additional layers. Such bilayer islands negatively impact electrostatic control in device structures, resulting in increased variability in key parameters of interest, such as threshold voltage and subthreshold swing [1]. One possible solution is to selectively remove such bilayer islands after growth using a selective etch. Prior work has demonstrated that chlorine gas (Cl₂) can selectively etch and remove bilayer islands from monolayer MoS₂, resulting in improved device performance and reduced variability. However, the effect of different process parameters was not investigated and the impact on the remaining monolayer was not studied extensively [2].<br/>Here we present a Cl₂-based etch process to selectively etch superficial bilayer MoS₂ islands on a continuous MoS₂ monolayer on sapphire. Prior to the etch process, single-crystalline MoS₂ was grown by metal-organic chemical vapor epitaxy from molybdenum hexacarbonyl and hydrogen sulfide precursors on c-plane sapphire substrates using an industry-standard 200 mm horizontal and hot-wall CVD reactor. The etch process was conducted on a similar 300 mm reactor. When exposed to Cl₂at temperatures between 400°C and 550°C, the bilayer islands are selectively removed without noticeable etching of the underlying monolayer as observed with scanning electron microscopy (SEM) and atomic force microscopy (AFM). Complete bilayer removal was achieved within 10 minutes of exposure at 500°C. When the temperature is increased to 600°C and above, the Cl₂ reacts with the MoS₂ monolayer, with complete etching and removal occurring within 10 minutes at 750°C.<br/>The selectivity of the etch can be additionally tuned by co-injecting S-containing species. Preliminary thermodynamic calculations suggest that H₂S and Cl₂ can react to form sulfur chlorides. Such reactions may have the dual effect of depleting Cl₂ and generating etching products, which should shift the thermodynamic equilibrium to disfavor etching. Indeed, the inclusion of H₂S suppresses bilayer etching, with bilayer islands being present even after treatment at 700°C. In contrast, co-injection of H₂ has a more limited impact, which thermodynamic calculations suggest is due to gas phase reactions between the H₂ and Cl₂ to produce less reactive HCl. Furthermore, thermodynamic calculations suggest that it is more favorable to etch WS₂ compared to MoS₂, which may enable preferential etching of specific transition metals within heterostructures.<br/>In addition to morphological changes, the Cl₂ etching caused changes in the Raman and photoluminescence (PL) response. The MoS₂ Raman modes narrowed after Cl₂ exposure; however, this effect was reduced with the co-injection of H₂S and is likely in part due to the removal of scattering sites along the edge of bilayer islands. The room temperature photoluminescence response strongly increases with treatment, while the cryogenic spectra suggest that Cl₂ exposure causes n-type doping of the MoS₂. Electronic device statistics will be presented as well.<br/><br/>[1] Q. Smets <i>et al</i>., "Sources of variability in scaled MoS FETs," <i>2020 IEEE International Electron Devices Meeting (IEDM)</i>, San Francisco, CA, USA, 2020, pp. 3.1.1-3.1.4, doi: 10.1109/IEDM13553.2020.9371890.<br/><br/>[2] Y. Shi et al., "Superior electrostatic control in uniform monolayer MoS₂ scaled transistors via in-situ surface smoothening," 2021 IEEE International Electron Devices Meeting (IEDM), San Francisco, CA, USA, 2021, pp. 37.1.1-37.1.4, doi: 10.1109/IEDM19574.2021.9720676.