利用掃描穿隧顯微鏡探測在二硫化鉬上表面缺陷的電子特性

Abstract

在N型半導體，缺陷通常扮演著捕捉電子的角色，因為其表面缺陷的能態大部分落在能隙中，導致導帶電子會掉到缺陷的表面能態裡，也就是電子被捕捉了，所以N型半導體在表面上都是電子空乏的。然而根據文獻指出，在N型半導體中也有些特例如InN，其表面的缺陷能態會提供電子出來，因缺陷的表面態位於導帶之上，導致缺陷的電子就提供到材料表面，所以表面更為導電，造成電子聚集在表面的現象，稱為表面載子累積，此現象進而影響到表面的電導率。由研究指出，在二硫化鉬厚度減少的情況下，電導率上升，雖然在電導值的量測上已經有一些相關的證據，但還是缺乏了一個直接的證據，說明缺陷的能態密度是對表面有影響的。因此，本實驗利用掃描穿隧顯微鏡，直接觀察於表面缺陷的電子特性，並透過機械剝離法，探討缺陷能態密度的變化。本實驗量測結果發現，在靠近導帶的dI/dV曲線特徵峰值，主要由鉬的懸鍵上的未配對電子所貢獻，而另一靠近價帶的dI/dV曲線，較微弱的特徵峰值是由硫缺所貢獻，在機械剝離法後發現，硫缺的能態密度會因為氧氣分子的吸收進而降低能態密度。

In n-type semiconductors, defects often play a role of capturing electrons. Because most energy states of defects are located in the band gap, electrons of the conduction band drop into the surface states of defects which means electrons are captured, so that electrons vacancy happened on the surface of n-type semiconductors. According to the previous study, the energy states of defects provide electrons to surface because the surface states of defects are above the conduction band, which makes the surface more conductive and electrons accumulating on the surface. This phenomenon is called surface accumulation. From the previous research, conductivity arises when the thickness of conductivity decreases. Even though there are some studies provide the measurement of conductance, direct evidence is still lacked toprove the affects of the energy states of defects on the surface. In this work, scanning tunneling microscopy was utilized to direct observe the electronic properties of defects at MoS2 surface at nano resolution, with and without mechanical exfoliation. A defect state close to the conduction band edge, arising from the dangling bonds of the Molybdenum due to their unsaturated charges. The other is a shallow state close to the valence band edge as a result of sulfur vacancy. After mechanical exfoliation, Oxygen adsorption suppresses the density of states values of sulfur vacancy defect.