二硫化鉬電晶體之接觸阻抗改善工程

Abstract

元件持續微縮下，後莫爾定律的時代已經開啟，使用原子尺度的二維材料將能夠有效的抑止短通道效應，二維材料-過渡金屬硫屬化物中，以二硫化鉬為通道層的電晶體受到廣泛的研究，因為二硫化鉬優越的特性使之能廣泛應用於電子及光學元件上，但其接觸阻抗過大的問題也受到考驗。本篇論文使用化學氣相沉積法在藍寶石基板上生成二硫化鉬，並將二硫化鉬轉移到二氧化鉿介電層基板上，研究源/汲極的金屬與二硫化鉬間的接面，期能找到穩定降低接觸阻抗的辦法。 本篇論文利用氫氣電漿處理產生硫缺，由光學分析及第一原理模擬可以發現硫缺能夠降低能隙，而如果只有在源/汲極間做氫氣電漿處理，將能有效降低源/汲極的金屬與二硫化鉬之間的蕭特基能障，進而降低接觸阻抗；採用不同電漿處理，發現氟及氯電漿處理可以在二硫化鉬形成p型摻雜，並成功改善了p-型操作的二硒化鎢電晶體特性。利用覆蓋二氧化矽在二硫化鉬上，也能夠有效避免製程電晶體時，對二硫化鉬造成的汙染及傷害，而覆蓋二氧化矽也會影響源/汲極的金屬與二硫化鉬的接面，導致電荷移轉至二硫化鉬。置換較高功函數且與二硫化鉬較少交互作用的源/汲極的金屬-鈀(Pd)，可以發現鈀的蕭特基能障雖然比較大，但加較大的閘極偏壓時，能夠有較大的能帶彎曲，推測Pd與二硫化鉬間較少的交互作用有關，使電子能夠有效穿隧能障，進而降低接觸阻抗。

The atomic thickness of 2D materials effectively mitigates the short channel effect, and opens up new possibilities to scale device and extend the Moore’s law. Among 2D materials, transition metal dichalcogenides-MoS2 has attracted many attentions as the channel material of the transistor and prompted new developments of numerous electric and optical devices. However, large contact resistance limited the MoS2 transistor performance. We synthesize monolayer MoS2 on sapphire substrate by chemical vapor deposition method, and then transfer MoS2 film to HfO2 dielectric substrate. We carefully engineer the interface between source/drain (S/D) metal and MoS2 to find stable methods for reducing contact resistance. In this thesis, sulfur vacancies generated by H2 plasma treatment was found to result in band gap narrowing from optical analysis and VASP simulation. If only S/D region received the H2 plasma treatment, the band gap narrowing reduced the Schottky barrier between S/D metal and MoS2 and therefore reduced contact resistance. F- and Cl-based plasma treatment induced p-type doping in MoS2 transistors, and improved the p-type conduction of WSe2 transistors after the Cl2 plasma treatment. A SiO2 capping layer was utilized to suppress the potential damage of the device fabrication process on monolayer MoS2. Furthermore, the SiO2 capping layer influences the contact between S/D metal and MoS2 due to charge transfer. Replacing S/D metal from Mo to Pd, which has a higher work function and less interaction with MoS2, shows reduced contact resistance. Although the Schottky barrier is higher, a large band bending can be achieved by applying gate voltage. We attributed this to the less interaction between Pd and MoS2.