二碲化鉬電晶體之異相介面工程

Abstract

由於過渡金屬硫屬化物的金屬接觸表面沒有懸鍵，使得過大的金屬接面接觸阻抗成為過渡金屬硫屬化物元件迫切需要解決的問題。在眾多解決辦法之中，邊緣接觸異相結構具有解決接觸阻抗問題的潛力。二碲化鉬的2H與1T’相晶格擁有相似基態能量，因此在形成異相結構上，二碲化鉬為六族過渡金屬硫屬化物中較合適的材料選擇。本研究成功地在氧化矽基板上，製作選擇性合成的邊緣接觸異相二碲化鉬背閘極電晶體元件，並提供多種1T’相二碲化鉬的製程條件，例如:合成溫度法、大氣暴露法、氧化環境法、水浸泡法與氫氟酸浸泡法。 本研究採用氫氟酸浸泡法，在二碲化鉬背閘極電晶體上選擇性形成1T’相之源極與射極以及2H相之通道區域，源極與射極電極選用電子槍蒸鍍系統沉積鈀與鎳金屬。該結構形成兩個材料接面。鈀與1T’相二碲化鉬接面由於同屬金屬特性，預期該接面為歐姆接觸，並不是主要的接觸阻抗來源，而異相二碲化鉬接面則被認為是主要的阻抗接面區。為了調查邊緣接觸異相結構的行為與原理，我們用第一原理商用計算程式(VASP)進行密度泛函理論計算。分別對傳統金屬接觸結構與邊緣接觸異相結構進行蕭特基能障分析與穿隧屏障分析。本研究利用兩種修正的層態密度法來分析蕭特基能障，並使用面平均電位法分析穿隧屏障。傳統金屬接觸結構在接觸介面上具有極大的穿隧屏障。反之，邊緣接觸異相結構具有較低的穿隧屏障以及與傳統金屬接觸結構相似的蕭特基能障表現。因此我們認為較低的穿隧屏障是降低邊緣接觸異相結構中接面接觸阻抗的關鍵。

Because of lack of dangling bonds at the contact interface, transition metal dichalcogenides (TMDs) encounter a significant challenge of reducing contact resistance. Heterophase edge-contacted structures, which form chemical bonds at the lateral interface, are promising solutions for the contact problem of TMD materials. Because of the similar ground-state energy of 2H and 1T’ phases of MoTe2, MoTe2 with a smaller phase transition barrier is considered as a highly potential candidate for realizing TMD devices with a heterophase contact. In this thesis, we successfully demonstrated heterophase edge-contacted MoTe2 back-gate transistors on a SiO2 substrate. We provided several methods for synthesizing 1T’-phase MoTe2 by sputtering amorphous MoTe2 films on the SiO2 substrate, such as control of synthesis temperature, atmosphere treatment, distilled water immersion, oxygen treatment and diluted HF treatment. By using the diluted HF treatment method, 1T’-phase MoTe2 was selectively formed at designated source/drain region treated by diluted HF while 2H-phase MoTe2 was formed at the channel region without treatment. Palladium (Pd) and nickel (Ni) was deposited by electron beam evaporation as the contact metal. 1T’-MoTe2/ Pd interface is an ohmic-like contact because of their metallic properties. The major contact barrier that influences devices performance is the heterophase interface. To investigate the properties of heterophase interface of MoTe2, both the traditional metal top-contacted structure and heterophase edge-contacted structure were simulated by the density functional theory (DFT) using the Vienna Ab initio simulation package (VASP). Schottky barrier height was extracted by two improved layer-decomposed density of states methods. The in-plane averaged electric potential method was used to evaluate the tunnel barrier at the interface. The traditional metal top-contacted structures have large tunnel barriers at the interface. By contrast, the heterophase edge-contacted structure shows a lower tunnel barrier and a comparable Schottky barrier height. The results suggest that the reduction of tunnel barrier at the interface is the main advantage of using the heterophase edge-contacted structure.