探討半金屬基板和二維材料之間的介面電子耦合效應

Abstract

近年來高摻雜濃度的二維過渡金屬硫化物半導體所展現出新穎材料物質表現，如：超導（superconductivity）、電荷密度波（charge density wave, CDW）或是相變（phase transition）引人研究高度興趣。其中，以半金屬作為基板並與二維半導體材料接合後，因為半金屬的費米能階很靠近二維半導體材料的導帶最低能量態，因趨往低能量的考量，使得半金屬基板的電子轉移電子到二維半導體材料。半金屬基板與半導體二維材料平衡後的費米能階位置，也影響二維半導體在此異質系統中的載子傳輸特性。然而，以半金屬作為基板，探討半金屬與二維材料單一原子層之間的耦合效應，仍是個未知且重要的課題。 本研究工作使用低溫掃描穿隧顯微鏡和掃描穿隧能譜（LT-STM/S）量測原子級單層二硫化鉬（MoS2）受到半金屬鉍（Bismuth）基板層間耦合效應後導致的電性變化。藉由實驗上的能態密度的電性量測和準粒子干涉(quasi-particle interference, QPI)表現，發現到二硫化鉬的導帶Q點能谷低於費米能階。推得此二維材料／半金屬（MoS2/Bi）系統的層間耦合效應，使得二硫化鉬的導帶邊界能態會比費米能階低約0.12eV。 再者，在高摻雜濃度的二維材料系統中，低溫的量測情況之下，電子與電洞間的庫倫靜電作用導致的庫倫能隙（Coulomb gap）和電子與聲子耦合導致原子位移所產生的能帶變化則顯得相對重要，使得費米能階處的能態密度因電子與電洞以及電子與聲子間的效應而發生顯著變化。實驗數據分析上，透過不同能量下的傅立葉影像分析以及非接觸原子力顯微技術(non-contact atomic force microscope, nc-AFM)，實驗結果發現：透過半金屬鉍提供電子摻雜到二硫化鉬後，此異質結構系統的二硫化鉬產生（2×1）的原子形貌重構影響費米能階處的能量狀態密度電子結構。

The two-dimensional (2D) semiconductor/semimetal junction exhibits an intriguing interfacial electronic property compared to the normal 2D semiconductor/metal junction. Because the Fermi-level of semimetals is very close to the conduction band (CB) of 2D semiconductors, the 2D transition metal dichalcogenides (TMDs) will be doped by the electrons transferring from the semimetal substrate in this system. In the equilibrium condition, however, the energetic position of the Fermi-level in the 2D semiconductor/semimetal junction dominates the carrier transport properties. Therefore, providing a complete understanding of the electronic behavior and insight into the interfacial electronic coupling in the TMDs on the semimetal substrate system becomes a key to future two-dimensional TMDs-based technologies. In this work, we use low-temperature scanning tunneling microscopy/spectroscopy (LT-STM/S) to figure out the electron doping behavior on the monolayer molybdenum disulfide (MoS2) by the bismuth substrate. The experimental results show the electronic behavior in this system, confirming the position of the Fermi level in the MoS2/Bi system is energetically higher than the MoS2 conduction band minimum of about 0.12 V by dI/dV measurements, and the quasi-particle interference (QPI) technique. Further, in such high electron-doped samples and at the low-temperature experimental conditions, the electron-hole Coulomb interaction and electron-phonon interaction are significant, lead to the modification of the density of states (DOS) at the Fermi level. Through the Fourier Transform (FT) analysis at different energies and non-contact atomic force microscopy (nc-AFM), the electron-doped on monolayer MoS2 by the semimetal bismuth substrate induces a (2×1) structural reconstruction, resulting in the modulation of the DOS electronic structure of MoS2.