Yao Qirong, Zhang Lijie, Bampoulis Pantelis, Zandvliet Harold J W
Physics of Interfaces and Nanomaterials, MESA Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500AE Enschede, The Netherlands.
School of Physics and Electronics, Hunan University, 410082 Changsha, China.
J Phys Chem C Nanomater Interfaces. 2018 Nov 8;122(44):25498-25505. doi: 10.1021/acs.jpcc.8b08713. Epub 2018 Oct 18.
HfSe 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. Here, we have studied the structure, density, and density of states of several types of defects that are abundant on the HfSe surface using scanning tunneling microscopy and spectroscopy. Compared to MoS and WSe, HfSe exhibits similar type of defects, albeit with a substantially higher density of 9 × 10 cm. 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 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 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 is very air-sensitive, implying that capping or encapsulating of HfSe, in order to protect it against oxidation, is a necessity for technological applications.
由于HfSe具有约1 eV的适度带隙和高κ介电常数的原生氧化物，它是基于过渡金属二硫属化物的场效应晶体管的一个非常好的候选材料。不幸的是，实验测定的电荷载流子迁移率比理论预测值低约3个数量级。这种强烈的偏差需要对HfSe的物理和电子性质进行详细研究。在这里，我们使用扫描隧道显微镜和光谱学研究了HfSe表面大量存在的几种类型缺陷的结构、密度和态密度。与MoS和WSe相比，HfSe表现出类似类型的缺陷，尽管其密度高达9×10¹² cm⁻²。最丰富的缺陷是一种次表面缺陷，在扫描隧道显微镜图像中表现为暗淡特征。这些暗淡的暗缺陷具有比原始表面（1 eV）大得多的带隙（1.25 eV），表明Hf原子被另一个原子取代。HfSe表面的高密度缺陷导致非常低的肖特基势垒高度。导电原子力显微镜测量表明，肖特基势垒高度对金属功函数的依赖性非常小，表明存在强烈的费米能级钉扎。我们将观察到的费米能级钉扎（钉扎因子约为0.1）归因于表面畸变和Se/Hf缺陷。此外，我们还通过扫描隧道显微镜和导电原子力显微镜研究了暴露于空气中后的HfSe表面。在表面很容易发现带隙为2 eV且肖特基势垒高度约为0.6 eV的部分氧化层。我们的实验表明，HfSe对空气非常敏感，这意味着为了保护其免受氧化，对HfSe进行覆盖或封装对于技术应用来说是必要的。
