探討與二維材料接觸之金屬接面成分以改善電荷載子的注入

Abstract

摩爾定律預測同一晶片上的晶體管數量將每1.5年增加一倍，自1965年提出以來，至今依然有效。然而，隨著矽基元件規模達到奈米級大小，已經達到了許多物理本身的理論限制。石墨烯和二硫化鉬(MoS2)等二維材料擁有許多新穎的性質，例如其原子級厚度，使其具有替代矽的潛力。然而，金屬電極與MoS2等二維材料之間的接觸電阻遠遠高於三維傳統的矽半導體。高接觸電阻的問題顯著抑制了金屬電極和二維半導體或石墨烯通道之間的電荷注入，因此阻礙了二維半導體元件未來的發展。以二硫化鉬為例，表面的電偶極生成、缺陷和金屬引起之間隙能階(metal-induced gap states)，造成費米能階被釘住，使的蕭基能障高度(Schottky barrier height)無法隨著金屬接觸之功函數調整。 我們開發了一種新的蒸鍍金屬電極的技術，使我們能夠在一個樣品上製造由兩種不同金屬，以不同比例組成的電極。接下來我們可以對這些合金電極和二維材料的接觸進行電性量測，以比較何種金屬有較低的接觸電阻，或找出是否有任何特定組成的合金可以改善電荷注入並降低接觸電阻。在接下來的實驗中，我們成功地進行了鈦和銀對於石墨烯的接觸電阻的優劣比較，以及發現了一種特定的銦和鉍金屬元素組成的合金電極，可以改善MoS2和電極的電荷注入效率。

Moore’s Law, which predicts that the numbers of transistors on the same chip will increase by 2-fold every 1.5 years, has remained valid since it was proposed in 1965. However, as the scale of the Silicon based devices reaches a few nanometers, many theoretical limits have also been reached. 2D materials, such as graphene and MoS2, have the potential of replacing Silicon-based devices due to their atomic-scale thickness and other interesting properties. However, the contact resistance between metal contacts and 2D materials such as MoS2 is way higher than that of the Silicon bulk counterpart. This problem of high contact resistance significantly suppresses the charge injection between the metal electrode and the 2D-semiconductor channel and therefore limits future developments of 2D-devices. Take an example from MoS2, the surface dipole formation, defects, and metal-induced gap states (MIGS) cause fermi level pinning, which means that we can no longer adjust the Schottky barrier height by changing the work function of our metal contacts.

We developed a new deposition technique which allows us to produce a matrix of electrodes made by different compositions of two metals on a single sample. We can then characterize these alloy contacts either to compare the contact resistance of the two metals, or to find out if there is any specific composition of alloy that can improve the charge injection and lower the contact resistance. In the following experiments, we successfully compared the contact resistance of graphene between Titanium and Silver, and identified a specific composition of Indium and Bismuth which improves the charge injection between MoS2 and our contacts.