Unconventional conductivity increase in multilayer black phosphorus

Multilayers of so-called 2D van der Waals materials have gained considerable attention as active components of next-generation electronic and optoelectronic technologies, with semiconducting black phosphorus (BP) regarded as one of the most promising systems. The applicability and performance limits of BP in both stand-alone and heterostructure-based multilayer devices are determined by individual flake charge transport properties, which synergistically depend on the number of layers and the strength of interlayer coupling between those. In this work, we study the DC electrical transport characteristics of high-quality BP field-effect devices within a wide range of flake thicknesses at room temperature. The experimental data show a non-trivial increase in conductivity and hole density with a reduced number of layers while maintaining constant field-effect mobility due to the prevalence of electron-phonon scattering. Based on the solution of the 1D Schrodinger-Poisson equation, we find that the observed phenomena are a direct consequence of non-negligible interlayer coupling, which in turn causes a local redistribution of free charge carriers towards the central layers. Our data show that due to the electrostatic conditions at the flake surfaces, a naturally protected 2D hole gas can be encapsulated in flakes as high as 10 nm, which preserves the bulk-like bandgap and effective carrier masses due to the electrostatic environment.

Automated summary

In this work, the DC electrical transport characteristics of high-quality black phosphorus (BP) field-effect devices with different thicknesses were studied at room temperature. The experimental data showed that the conductivity and hole density increase non-trivially with a reduced number of layers while maintaining constant field-effect mobility due to electron-phonon scattering. The observed phenomena were found to be a direct consequence of significant interlayer coupling, resulting in a local redistribution of free charge carriers towards the central layers. It was also discovered that a naturally protected 2D hole gas can be encapsulated in BP flakes as thick as 10 nm due to the electrostatic environment.