基板材質對於石墨烯/金屬接面電阻影響之探討

Abstract

在現今的半導體發展中，一直遵循著摩爾定律的節奏發展，然而先進製程遇到的問題越來越嚴苛，因此各家大廠期望從各個面向解決問題，石墨烯由於優異的材料特性，便受到熱烈的研究，而在本文的研究中，主要是針對金屬/石墨烯接面的接觸電阻做探討。影響石墨烯接觸電阻的面向很多，然而最主要影響接面仔子傳輸的便是石墨烯的功函數，而本文主要觀察基板的置換對石墨烯的功函數的調變。 第一部分，本文利用最廣為使用量測接觸電阻的方法傳輸線法(TLM)進行量測，以300nm SiO2為基準基板，量測出的結果特徵接觸電阻率為〖10〗^3~〖10〗^4Ω*um附近，與參考文獻的數值吻合，另外並針對石墨烯的退化(Degradation)、背閘極偏壓(Back-Gate Bias)、以及量測時做造成的誤差進行探討。本文發現石墨烯會隨著時間的經過，材料內的碳原子與大氣中的分子產生鍵結，使得石墨烯的電阻率受到影響，造成接觸電阻值的上升。而石墨烯也會因給予不同的背閘極偏壓，費米能接的位置不同，有不同的電阻率，因此會得到不同的接觸電阻值，以及在量測時因下針的位置或角度，會有些微量測上的誤差。 第二部分是針對石墨烯轉印至不同基板時，由於石墨烯厚度非常薄，因此環境因素會容易影響到石墨烯的材料特性，我們也根據KPFM去量測石墨烯在不同基板上的表面電位，並推得功函數，得到在Si3N4、HfO2、TiO2上，皆有功函數被調變的結果，推測原因為石墨烯在與基板分子接觸時，會因為極性的關係，導致原子之間發生載子流動的現象，進而造成石墨烯功函數會有變化。而本文的最後，也探討出若欲以基板面向去改善石墨烯的接觸電阻，應選用對石墨烯產生散射較小的基板。

When the device scale down , short channel effect will more heavier because the weak gate control . Every foundry fab are try to slove the problem from many aspects . 2D material is popular because its well-performed material characteristic . In this study , we are focus on the contact resistance between graphene and metal contact . And graphene work function is the most affecttive factor of contact resistance . So this work, we observe the graphene work function on different substrate . First , we used the TLM measurement method to measure graphene/titanium contact resistance on 300nm SiO2 substrate . Specific contact resistance we measurement is about 〖10〗^3~〖10〗^4Ω*um . It’s matching to other reference . And we also discuss the degradation , back-gate , and the measurement error effect on the graphene contact resistance . We find that graphene will degrade as time goes on because the carbon atom is interact with hydrogen and oxygen atom . It will turn graphene into graphene oxide . And different back-gate bias will let graphene have different density of state(DOS) , it will cause variation conductivity . Measurement result also have error because the measurement angle and position . We also transfer graphen on different substrate , and used KPFM to measure the graphene work function . We find that graphene work function on Si3N4 , HfO2 and TiO2 substrate are get more n-type doping result . We assume that because the polarity will cause charge transfer between metal and graphene . And it will let graphene work function changing . Eventurally , if we want to improve the contact resistance form contact substrate . High mobility substrate is best choice.