探討低層數二硫化鉬的金屬-絕緣體相變化

Abstract

二維材料相較於過去的三維材料，有許多新穎的物理特性與應用，舉凡奈米電子學、光學性質，皆有不同的研究發現。而二維材料中，除了石墨烯(Graphene)外，其他具有層狀結構之半導體二維材料如二硫化鉬(MoS2)、二硒化鎢(WSe2)的研究日漸受到重視。因為二硫化鉬為有能隙的半導體，使得二硫化鉬作為場效電晶體的材料時，其開關比能夠到達106-108，為好的場效電晶體元件，但其物理機制尚不清楚。 本實驗研究二硫化鉬的電性傳輸機制，二硫化鉬一般在實際情況下，電子傳輸並非有序系統下的彈道傳輸機制，須考慮半導體材料中，因聲子碰撞導致能量交換的散射問題。我們並討論二硫化鉬之厚度(層數多寡)對其電性傳輸特性的影響，包含載子遷移率、載子濃度、電導率以及電子侷限長度等。本實驗以機械剝離法、電子束微影、熱蒸鍍等標準製程，將少數層的二硫化鉬薄片製作成具場效電晶體結構之奈米元件，用低溫探測平台測量其電性、電場效應，計算其載子遷移率，並做變溫量測。我們觀察到二硫化鉬薄片之電阻隨著溫度下降而上升，呈現半導體傳輸性質，並可用Mott變程跳躍理論解釋。此外，我們發現在溫度約210 K以下，二硫化鉬有明顯的金屬-絕緣之相變化，推測與電場在某臨界值時的載子濃度有關，利用背閘極電壓控制載子濃度而發現臨界電場隨載子濃度變化。

Comparing to three-dimensional bulks, two-dimensional materials exhibit many novel physical properties and show excellently their applications in nano- and opto-electronics. In addition to graphene, other two-dimensional materials, having layered structure of semiconductor material such as molybdenum disulfide (MoS2) and tungsten diselenide (WSe2), draw much attention because of the presence of indirect bandgap. The MoS2 can be used for making field-effect transistors (FETs) to give a high on/off ratio of above 106. In this work, the electron transport in MoS2 is studied by two terminal devices with a back gating electrode. We use mechanical exfoliation, electron beam lithography and thermal evaporation to make few-layer MoS2 FET devices. From the device characterizations, we estimate the mobility, carrier concentration and localization length at different temperature range from 80 to 200 K and for MoS2 having different thickness. At a temperature higher than 200 K, the MoS2 shows an insulating to metallic phase transition. The transition could be attributed to the thermally excited carriers. On the other hand, the insulating phase appears at temperatures lower than 200 K and the insulating phase can be converted to the metallic phase by applying a back-gate voltage as well. The back gating voltage is used to increase the carrier concentration. The metallic phase was observed and the transition occurred at a conductivity very close to the ideal value of e2/h. At last, we show that the metallic phase can also be induced by applying a high electric field at zero gating voltage.