二茂鐵在少數層二硫化鉬表面所產生之電子傳輸與場效電晶體特性變化

Abstract

本論文主旨為探討當有機金屬化合物-二茂鐵 (ferrocene)存在於少數層二硫化鉬表面時所產生的電子傳輸與場效電晶體特性變化。本實驗將利用機械撥離法於矽基板以及300 nm厚的二氧化矽層上製作少數層的二硫化鉬，並以電子束微影與熱蒸鍍製作源極與汲極，最後使用爐管熱退火去除二硫化鉬的缺陷及降低元件的接觸電阻。在電性量測方面，將利用兩點量測法測量二硫化鉬元件源極與汲極的電流電壓關係圖，以及不同閘極電壓與元件電流的關係圖，並使用原子力顯微鏡確認二硫化鉬表面形貌與高度。之後將使用微量滴管將二茂鐵溶液吸出，滴上二茂鐵溶液後進行退火去除多餘溶劑，重複進行上述量測之後與背景值比較，藉此探討當二茂鐵存在於少數層二硫化鉬時所產生的電子傳輸與場效電晶體特性變化。在常溫的實驗中，確認了二硫化鉬為n型半導體。在量測閘極偏壓與元件電流的實驗中，被偏極化的二茂鐵分子的電偶極將會在二硫化鉬表面形成內電場，此內電場將抵銷部分外加閘極偏壓的影響，因此發現元件在閘極偏壓小於臨界偏壓的區域內 (off-state)，電流會隨著二茂鐵分子數量在二硫化鉬表面上增多而上升。在不同環境溫度下的量測實驗中觀察到，在溫度200 K-100 K的區間範圍內，電阻與溫度的關係可以由二維變程跳躍傳輸的理論擬合，若二茂鐵分子存在於二硫化鉬表面上時會造成特徵溫度 (characteristic temperature)上升以及載子遷移率 (carrier mobility)下降的現象，此結果顯示當二茂鐵分子存在於二硫化鉬表面時，將會導致導致二硫化鉬內部無序性 (disorder)增加。

In this thesis, we will discuss the changes of electron transport and field-effect behavior when the organometallic compound-ferrocene is adsorbed on the surface of few-layer molybdenum disulfide. We used mechanical exfoliation to make few-layer molybdenum disulfide (MoS2) flakes on silicon substrate, capped with 300-nm thick silicon dioxide layer. The standard electron beam lithography and thermal evaporation were used to deposit source and drain electrodes on few-layer MoS2 flakes. The MoS2 devices were annealed in a high vacuum in order to fix structural defects of MoS2 and to lower the contact resistivity. Two-probe measurement is used to measure the source-drain current voltage (I-V) curve and gate voltage-drain current (〖V_g-I〗_(SD )) curve. An atomic force microscopy was used to measure the topography and the thickness of few-layer MoS2. After the electrical properties of bare MoS2 devices were characterized, the ferrocene molecules were deposited on surface of the MoS2 by using micro tube. The ferrocene deposited MoS2 devices were annealed in a high vacuum to remove the solvent (chloroform). We then compared the electrical properties with that of bare MoS2 devices. The few-layer MoS2 devices are n-type semiconductors at room temperature. When the ferrocene molecules are deposited, the electric dipole of ferrocene gives internal electric fields that decrease electric fields in MoS2 supplied by the back-gate voltage. It results in the increasing of off-state current as more and more ferrocene molecules are adsorbed on the MoS2 surface. On the other hand, the temperature dependent resistance of ferrocene on MoS2 is well described by the theory of two-dimensional (2D) variable range hopping (VRH) transport in the temperature range from 200 to 100 K. The mobility reduces and the characteristic temperature extracted from fitting to the 2D VRH theory increases after ferrocene deposition. These results indicate that the existence of ferrocene introduces a disorder of stray electric fields in few-layer MoS2.