Ferecrystals: Non-Epitaxial Multilayer Growth of 2D Materials by Atomic Layer Deposition

Atomic layer deposition is a very versatile technology for the deposition of thin films with precise thickness control on large areas, non-planar surfaces and 3D objects. The chemical reaction is surface limited, well defined and works in most cases at low temperatures (RT to 150 °C). For a number of classical van der Waals 2D materials, there have been reports on ALD of transition metal dichalcogenide (TMDC) of MoS₂, SnS₂, WS₂ and WSe₂, which also included the electronic characterization as a field effect transistor (FET).<br/><br/>In this work, we have fabricated by atomic layer deposition (ALD) multilayers of layered materials based on topological insulators and van der Waals materials, called <i>ferecrystals. </i>These ferecrystals can be tailored to exhibit unusual properties such as high electrical conductivity or low thermal conductivity or magnetic properties. A detailed ferecrystal study was performed on ferecrytals of Sb₂Te₃ and SbO_x, which has been grown at the same temperature as single layers of Sb₂Te₃. Without post-annealing, the electrical and thermoelectric characterisation of the highly ordered samples have been performed with the ZT-chip setup. In general, the carrier mobility is very high &gt;150 Vs²/cm² and is even improved when the thickness of the Sb₂Te₃ layers is reduced and the number of SbO_x layers (typically 2 nm thickness) is increased. Detailed XRD investigations have been performed and an enhanced crystalline order is observed in the ferecrystal system compared to individual layers of Sb₂Te₃. We have grown ferecrystals based on Sb₂Te₃ and Sb₂Se₃ with tetrahedral and orthorhombic crystal structure, respectively. The p-type hole carrier concentration of Sb₂Te₃ films can be enhanced through the sublayer doping of Sb₂Se₃. The highest carrier concentration achieved was 2.5×10¹⁹ cm^-2 when the thickness ratio of Sb₂Te₃ to Sb₂Se₃ was (4 nm:2 nm). Further reduction of the Sb₂Te₃ thickness resulted in a high Seebeck coefficient of 172 μV/K at room temperature.<br/><br/>Reference: Jun Yang, Samik Mukherjee, Sebastian Lehmann, Fabian Krahl, Xiaoyu Wang, Pavel Potapov, Axel Lubk, Tobias Ritschel, Jochen Geck, Kornelius Nielsch, "Low-Temperature ALD of SbO<i>_x</i>/Sb₂Te₃ Multilayers with Boosted Thermoelectric Performance", Small 20, 2306350 (2024) https://doi.org/10.1002/smll.202306350