原子力顯微鏡及表面電位顯微鏡於單層二硒化鎢-二硒化鉬異質結構之研究

Abstract

二維過渡金屬二硫族化物在近年來引起了廣大的注意，主因來自於它們對於新一代電子或光電元件有相當大的應用潛力。而單層過渡金屬二硫族化物橫向異質接面的研究也已經被發表，這種結構可能是未來原子級電子與光電元件的基石。本論文中，我們先利用原子力顯微鏡量測二硒化鎢-二硒化鉬異質結構的表面形貌，確認其為單層的結構，並透過原子力顯微鏡中的相位模式量測二硒化鎢與二硒化鉬的分界。此量測除了比光學顯微鏡具有更高的空間解析度外，且可透過相位差異推論出二硒化鎢的表面能量大於二硒化鉬。接下來，我們使用表面電位顯微鏡來量測二硒化鎢-二硒化鉬異質接面的電子特性，此異質材料是經由化學氣相沉積法成長於藍寶石基板上並轉印到高定向熱解石墨基板上。我們發現量測到的表面電位值受石墨基板的影響會有所變化，此係來自於樣品與基板間的電荷轉移所致。而表面電位顯微鏡除了表面電位外也能量測異質接面的空乏區寬度，我們發現定出的能帶偏移與空乏區寬度與過去文獻上的推論極為一致。

Recently, two-dimension semiconducting transition metal dichalcogenides (TMDs) have attracted wide interest because of their potential applications in new electronic and optoelectronic devices. Monolayer TMD lateral heterojunctions have been demonstrated to exhibit p-n junction characteristics, which could be a fundamental building block for future atomically-thin electronics and optoelectronics. In this work, we first use atomic force microscopy (AFM) to measure the surface morphology of WSe2-MoSe2 heterostructures, demonstrating the monolayer structure. In addition, the boundary of WSe2-MoSe2 interface can be clearly identified by the phase mode. Besides the higher spatial resolution compared to optical microscopy (OM) images, the phase shift further demonstrates the different surface energies between WSe2 and MoSe2. Next, we have investigated the surface potential of the lateral heterojunction between monolayer WSe2 and MoSe2 by scanning Kelvin probe microscopy (SKPM). The monolayer WSe2-MoSe2 lateral heterojunctions were grown on sapphire substrates by chemical vapor deposition (CVD) and transferred onto highly oriented pyrolytic graphite (HOPG) substrates. We observed variations in surface potential of the heterojunction caused by the charge transfer between TMD and HOPG. SKPM mapping also resolves the depletion width of the lateral heterojunction. We found the measured band offset and depletion width for WSe2-MoSe2 lateral heterojunctions are in consistent with past literature reports.