Nanostructuring of Hexagonal Boron Nitride: Capabilities & Limitations of Patterning with Thermal Scanning-Probe Lithography
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Hexagonal boron nitride (hBN) is a layered semiconducting material that is interesting for photonic and electronic nanodevices due to its favourable properties (e.g. high thermo-chemical stability, an atomically smooth surface and a strong emission in the UV). Recently, novel phenomena and functionalities have been reported accomplished by the nanostructuring of hBN containing structures, like localised quantum emission and band structure engineering of graphene. However, current lithography techniques used for patterning of hBN limit device architecture, as they are only able to make binary patterns (i.e. patterns with only two depth levels). Thermal scanning-probe lithography (tSPL) could overcome this limitation as it is capable of greyscale patterning (patterning with more than two depth levels). tSPL locally evaporates an organic resist with an atomically sharp hot probe tip after which the pattern is transferred into the underlying surface via reactive ion etching. In this work, we employ tSPL and explore its capabilities and limitations for the nanostructuring of hBN. We demonstrate that tSPL can be successfully used for patterning of hBN and confirm the possibility of greyscale patterning of hBN with tSPL. We obtained patterns with a minimum feature size with a full width at half maximum of 40 nm (pattern depth 40 nm), resolutions down to ~100 nm (pattern depths of 45 and 65 nm) and we reproduced the targeted pattern with a root-mean-square error as low as 10 nm (pattern depth 45 nm). Upon decreasing the size of the targeted features and increasing the pattern depth the quality of the patterns decreased, indicating the tip shape as the possible limiting factor for the quality of the pattern. Further research to determine which pattern step is limiting for the quality of patterning hBN, will enable optimisation of the method. Future research should focus its efforts on the capabilities and limitations of employing tSPL for greyscale patterning of hBN specifically. Nevertheless, tSPL was already shown to offer a successful route towards greyscale patterning of hBN. Therefore, an increased freedom in the device architecture of hBN containing nanodevices is enabled.