Nanoscale Investigation of Defects and Oxidation of HfSe2.

Abstract

HfSe2 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 HfSe2. Here, we have studied the structure, density, and density of states of several types of defects that are abundant on the HfSe2 surface using scanning tunneling microscopy and spectroscopy. Compared to MoS2 and WSe2, HfSe2 exhibits similar type of defects, albeit with a substantially higher density of 9 × 1011 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 HfSe2 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 HfSe2 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 HfSe2 is very air-sensitive, implying that capping or encapsulating of HfSe2, in order to protect it against oxidation, is a necessity for technological applications.