Extrinsic n-type semiconductor transition in ZrSe2 with the metallic character through hafnium substitution

Two-dimensional (2D) layered materials exhibit versatile electronic properties in their different phases. The intrinsic electronic properties of these materials can be modulated through doping or intercalation. In this study, we investigated the electronic properties of Hf-doped ZrSe2 single crystals using angle-resolved photoemission spectroscopy (ARPES) combined with first-principles density functional theory (DFT) calculations. It is observed that the valence band maxima of ZrSe2, located below the Fermi level, undergo a significant change with the introduction of Hf substitution. Hf can introduce extra charges into the conduction band, rather than making a mixed structure of HfSe2 and ZrSe2 band structure, which can cross the Fermi level. Compared to the semiconducting band structure of ZrSe2, we observed that the conduction band crosses the Fermi level at the high symmetry M point in Hf-doped ZrSe2. This suggests an increase of electron-type carriers around the Fermi level, resulting in an extrinsic charge carrier density in the conduction band, which can form a metallic behaviour. It can be noticed that the Hf cations can create disorder in the form of excess atoms of Zr, which yields more carriers in the conduction band in the shape of “smeared bands”. The tails of the “smeared band” occupied the d-orbitals extended into the Fermi level and left the d-band below. Similarly, the electrical resistance measurements further confirm the metallic-like character of Hf-doped ZrSe2 compared to the semiconductor ZrSe2, indicating increased carriers. This metallic-like behavior is suggested to be predisposed by the extrinsic electrons induced by the substitutional disorder. This study further demonstrates the possibility of band gap engineering through heavy metal doping in 2D materials.