HfO₂/HfS₂ hybrid heterostructure fabricated via controllable chemical conversion of two-dimensional HfS₂

While preparing uniform dielectric layers on two-dimensional (2D) materials is a key device architecture requirement to achieve next-generation 2D devices, conventional deposition or transfer approaches have been so far limited by their high cost, fabrication complexity, and especially poor dielectric/2D material interface quality. Here, we demonstrate that HfO₂, a high-K dielectric, can be prepared on the top surface of 2D HfS₂ through plasma oxidation, which results in a heterostructure composed of a 2D van der Waals semiconductor and its insulating native oxide. A highly uniform dielectric layer with a controlled thickness can be prepared; the possibility of unlimited layer-by-layer oxidation further differentiates our work from previous attempts on other 2D semiconducting materials, which exhibit self-limited oxidation up to only a few layers. High resolution transmission electron microscopy was used to show that the converted HfO₂/HfS₂ hybrid structure is of high quality with an atomically abrupt, impurity- and defect-free interface. Density functional theory calculations show that the unlimited layer-by-layer oxidation occurs because oxygen atoms can barrierlessly penetrate into the HfS₂ surface and the extracted sulfur atoms are absorbed into the oxygen vacancy sites within HfO₂ under O-rich conditions. A top-gated field-effect transistor fabricated with the converted HfO₂/HfS₂ hybrid structure was found to exhibit a low interface trap density D_it of 6 × 10¹¹ cm^-2 eV^-1 between the HfS₂ channel and the converted HfO₂ dielectric, and a high on/off current ratio above 10⁷. Our approach provides a low cost, simple, and ultraclean manufacturing technique for integrating 2D material into device applications.