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Quantifying Electron Mobilities in MOCVD-Grown 2D Materials via THz-TDS

Dr. Markus Stein1, Mr. Daniel Anders1, Mr. Michael Zengel2, Mr. Robin G√ľnkel3, Dr. Thilo Hepp3, Mr. Nils Langlotz3, Prof. Sangam Chatterjee1, Prof. Kerstin Volz3
1Justus-Liebig-University Giessen, Giessen, Germany. 2Justus-Liebig-University Giessen, Giessen, USA. 3Philipps-Universität Marburg, Marburg, Germany

Abstract

In the relentless quest for enhanced properties and reduced dimensionalities in optoelectronic and high-end electronic devices, 2D materials, specifically atomically thin semiconductors, emerge as highly promising candidates. However, a significant hurdle resides in achieving large-scale production of high-quality 2D materials to meet the scalability demands for technological applications. Typically, 2D materials are derived through top-down synthesis methods, such as mechanical exfoliation, which only produces small flakes on a micrometer scale. Bottom-up growth techniques like MOCVD may enable wafer-scale 2D flake growth scalable to large-scale production. However, achieving seamless growth of large, continuous layers without compromising the high quality associated with mechanical exfoliation poses a formidable challenge. This study introduces MOCVD-grown large-area 2D layers of InSe and GaS on a sapphire substrate. Bulk InSe, which, in contrast to the competing transition metal dichalcogenides, is known for its high mobility of 2400 cm2V-1s-1, addresses the widespread need for high mobility devices. Conversely, GaS, with its ultraviolet bandgap, holds promise for solar blind photodiodes and light-emitting diode applications. The electrical properties of MOCVD-grown thin-film InSe and GaS layers are non-invasively analyzed using THz-TDS. Remarkably, thin-film InSe exhibits ultrahigh mobilities surpassing those of bulk InSe. The analysis of different thicknesses reveals a comparable THz response as soon as a closed layer of InSe is present. This implies the observation of a two-dimensional conductivity that is unaffected by the thickness of the thin layer. In contrast, GaS shows a negligible THz response, attributed to low mobility or charge carrier density.