Modulation doping and charge density wave transition in layered PbSe-VSe ferecrystal heterostructures.

Controlling charge carrier concentrations remains a major challenge in the application of quasi-two-dimensional materials. A promising approach is the modulation doping of transport channels charge transfer from neighboring layers in stacked heterostructures. Ferecrystals, which are metastable layered structures created from artificial elemental precursors, are a perfect model system to investigate modulation doping, as they offer unparalleled freedom in the combination of different constituents and variable layering sequences. In this work, differently stacked combinations of rock-salt structured PbSe and VSe were investigated using X-ray photoelectron spectroscopy. The PbSe layers act as electron donors in all heterostructures, with about 0.1 to 0.3 donated electrons per VSe unit cell. While they initially retain their inherent semiconducting behavior, they themselves become metallic when combined with a larger number of VSe layers, as evidenced by a change of the XPS core level lineshape. Additional analysis of the valence band structure was performed for selected stacking orders at different sample temperatures to investigate a predicted charge density wave (CDW) transition. While there appear to be hints of a gap opening, the data so far is inconclusive and the application of spatially resolved techniques such as scanning tunneling microscopy is encouraged for further studies.