Nanoscale Investigation of Defects and Oxidation of HfSe2

HfSe 2 is a very good candidate for a transition metal dichalcogenide-based field-effect transistor owing to its moderate band gap of about 1 eV and its high-κ dielectric native oxide. Unfortunately, the experimentally determined charge carrier mobility is about 3 orders of magnitude lower than the theoretically predicted value. This strong deviation calls for a detailed investigation of the physical and electronic properties of HfSe 2 . Here, we have studied the structure, density, and density of states of several types of defects that are abundant on the HfSe 2 surface using scanning tunneling microscopy and spectroscopy. Compared to MoS 2 and WSe 2 , HfSe 2 exhibits similar type of defects, albeit with a substantially higher density of 9 × 10 11 cm -2 . The most abundant defect is a subsurface defect, which shows up as a dim feature in scanning tunneling microscopy images. These dim dark defects have a substantially larger band gap (1.25 eV) than the pristine surface (1 eV), suggesting a substitution of the Hf atom by another atom. The high density of defects on the HfSe 2 surface leads to very low Schottky barrier heights. Conductive atomic force microscopy measurements reveal a very small dependence of the Schottky barrier height on the work function of the metals, suggesting a strong Fermi-level pinning. We attribute the observed Fermi-level pinning (pinning factor ∼0.1) to surface distortions and Se/Hf defects. In addition, we have also studied the HfSe 2 surface after the exposure to air by scanning tunneling microscopy and conductive atomic force microscopy. Partly oxidized layers with band gaps of 2 eV and Schottky barrier heights of ∼0.6 eV were readily found on the surface. Our experiments reveal that HfSe 2 is very air-sensitive, implying that capping or encapsulating of HfSe 2 , in order to protect it against oxidation, is a necessity for technological applications.