Conformal Growth of Monolayer MoS₂ and WSe₂ on High Aspect Ratio Trenches

Two-dimensional (2D) materials have garnered significant attention for the role they could play in advancing nanoelectronics. In particular, the transition metal dichalcogenides (TMDs), a class of 2D semiconductors, have shown good electron and hole mobilities at sub-nm channel thicknesses compared to silicon, owed in part to their layered structure which avoids increased surface roughness scattering [1]. Additionally, their large band gaps (&gt; 2 eV) lead to transistor off-state leakage currents that are orders of magnitude lower than in silicon [2-4]. This makes them especially promising for memory applications such as dynamic random access memory (DRAM), where leakage current in access transistors limits performance, power consumption, and scaling [4]. Scaling of DRAM in recent decades has relied on the introduction of increasingly-complex 3D structures for both the capacitor and silicon access transistor to limit these leakage currents [5]. The demonstration of growth of monolayer TMDs on these kinds of non-planar 3D substrates, however, is limited. In addition, while layer-transfers can be achieved on planar substrates, they are significantly more difficult on 3D high-aspect ratio (high-AR) structures.<br/><br/>Here we demonstrate the direct growth of monolayer TMDs (MoS₂ and WSe₂) on high-AR trenches via chemical vapor deposition (CVD) at atmospheric pressure. This is achieved using solid source precursors in a quartz tube furnace in conjunction with substrates treated with perlyene-3,4,9,10 tetracarboxylic acid tetrapotassium salt (PTAS) as a seeding molecule [6,7]. We find that grains of monolayer TMDs nucleate and grow laterally, following the 3D contours of the SiO₂/Si substrate, including along sidewalls and into trenches. The grains are triangular under optical microscopy, which is seen in single crystals of TMDs grown on planar substrates, revealing the continuity of grains down into and over multiple trenches. Raman spectroscopy and photoluminescence (PL) measurements indicate monolayers for both TMDs.<br/><br/>Scanning electron microscopy (SEM) imaging shows that grains which nucleate on planar regions are able to continue growing along sidewalls. Conformal monolayer growth in trenches with AR ~ 2:1 is also observed for both TMDs examined here. Cross-sectional transmission electron microscopy (TEM) reveals that conformal growth of the 2D material is achievable even in trenches with high AR ~ 13:1, including at high radius of curvature corners and the bottoms of trenches (down to ~ 12 nm in width). This is further verified by electron energy-loss spectroscopy (EELS) analysis. The lack of bridging, clustering, or voids in the TMD films grown in these high-AR trenches is especially promising, and would be difficult or impossible to achieve with conventional sputtering, CVD, or layer transfers of other materials. Additionally, our grown films have grain sizes much larger (&gt; 10 μm) than the height of a single sidewall or trench, which could allow for reduced device-to-device variability in large-scale and extremely dense 3D circuits.<br/><br/>In summary, we have demonstrated direct, conformal growth of monolayer TMDs onto trenches with ARs as high as 13:1. The ability for TMDs to be grown on arbitrary 3D structures and trenches could open up new design windows which, when combined with their superior performance and leakage at the monolayer limit, could propel the continued scaling and evolution of high-density 3D memory systems.<br/><br/>[1] C. English <i>et al</i>., <i>Nano Lett</i>. 16, 3824 (2016)<br/>[2] C. Bailey <i>et al.</i>, <i>MRS-EMC</i> 125 (2019)<br/>[3] C. Bailey <i>et al</i>., <i>MRS Spring Meeting</i> (2020)<br/>[4] C. Kshirsagar <i>et al., ACS Nano</i> <b>10</b>, 8457 (2016)<br/>[5] A. Spessot and H. Oh, <i>IEEE Trans. Elec. Dev</i> <b>67</b>, 1382 (2020)<br/>[6] K.K.H. Smithe <i>et al</i>., <i>2D Materials</i> <b>4</b>, 011009 (2017)<br/>[7] J. Chen <i>et al</i>., <i>ACS Photonics</i> <b>6</b>, 787 (2019)