本研究主要探討以化學氣相傳輸法成長的層狀半導體二硒化鉬(MoSe2)之二維電傳輸特性。首先我們分別用拉曼光譜(Raman spectrum)和X光繞射(X-ray diffraction)量測，證實MoSe2晶體具有高品質的單晶特性。以機械剝離法和聚焦離子束技術，將MoSe2製作成具有原始表面(non-fresh surface)和新撕表面(fresh surface)之奈米薄片元件。發現奈米薄片之電導率與塊材相比高出兩個數量級，電導率皆與厚度成反比。由transfer length method (TLM)的模型分析，可定義奈米元件接觸電阻的影響，進一步發現電子傳輸行為並非遵循傳統三維的傳輸行為，而是二維的電傳輸特性。經由掃描穿隧顯微鏡(scanning tunneling microscopy)和角度解析光電子能譜(angle-resolved photoemission spectroscopy)分析晶體表面，證實了原始表面和新撕表面所存在異常高的導電性，是由於表面的電子聚集。另外新撕表面似乎存在結構的不均勻性，導致量測到兩種本質特性，而本質上就有電子聚集的機率較高。利用場效電晶體(field-effect transistor)量測，可發現新撕表面表現出較低的電子遷移率和較高的電子濃度，這可能是因為新撕表面存在較強的表面散射機制。最後透過變溫的FET量測，證實電子遷移率是由雜質散射所主導，進一步證實了表面存在相當高的缺陷與表面態密度。

We report on the two-dimensional (2D) electronic transport in layered semiconductor of molybdenum diselenide (MoSe2) grown by chemical vapor transport. The single-crystalline quality of MoSe2 was examined by the Raman scattering and X-ray diffraction (XRD) measurements. The two-terminal devices of MoSe2 nanosheets with pristine (non-fresh) and fresh surface were fabricated by the mechanical exfoliation and focused-ion beam (FIB) technique. A thickness-dependent electrical conductivity was observed in which the nanosheets exhibit nearly two orders of magnitude higher conductivity than the bulk counterparts. According to the transfer-length method (TLM) analysis, we observed that the MoSe2 nanostructures follow a 2D transport behavior rather than the conventional 3D mode. Presence of surface electron accumulation (SEA), which was confirmed by the scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES), was found to be physical origin of the 2D electronic transport and relatively high conductive nature in MoSe2. The field-effect transistor (FET) measurement indicates that the nanosheets with the fresh surface exhibit lower mobility and higher electron concentration compared to those with the non-fresh surface. The temperature-dependent FET measurement indicates that the mobility is dominated by impurity scattering, further supports the statement of the presence of the SEA induced by the high-density surface states in MoSe2.

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