少數層二硫化鉬材料中觀察高溫下電子聲子交互作用所導致的金屬絕緣體相變化

Abstract

本實驗元件使用機械剝離法、電子束微影、熱蒸鍍與真空退火等標準製程，將少數層的二硫化鉬製作成場效電晶體奈米元件，再使用探針系統量測元件電性。先用兩點測量確認樣品室溫電阻在合理範圍，再變溫量測電性，溫度範圍在80 K到600 K之間，主要觀察電導與載子遷移率隨溫度變化，並與理論方程式擬合，在不同溫度範圍下探討二硫化鉬的電性傳輸機制。 實驗中從汲極-源極之電壓與電流關係，在室溫無閘極偏壓條件下，觀察到二硫化鉬樣品電阻值在105-107 Ω間。當施加背向閘極偏壓達到調控載子濃度時，可觀察到二硫化鉬的n型半導體特性，且電流開關比為106-108倍，從閘極電壓變化圖計算出室溫下二硫化鉬電子遷移率約5-150 cm2V-1s-1。 從二硫化鉬元件的電阻對溫度變化數據分析得知，在200 K以下二硫化鉬的傳輸行為符合二維變程跳躍理論，且有觀察到金屬-絕緣體相變化。當溫度高於200 K，熱擾動造成部分侷域化載子脫離束縛態，熱活化能傳輸取代二維變程跳躍傳輸。在450 K以下低溫範圍，無電場提高載子濃度情況下，二硫化鉬的電阻-溫度變化展現出半導體行為。當溫度高過450 K到600K，二硫化鉬電阻比低溫時電阻下降了3-4個數量級，此時電阻與溫度關係出現轉折而呈現金屬行為，電阻溫度係數約為0.016 K-1，推測此高溫下金屬-絕緣體相變化，可能為溫度高於二硫化鉬的德拜溫度，劇烈的聲子-電子交互作用所造成。

In this study, we exfoliate mechanically few-layer molybdenum disulfide (MoS2) flakes on silicon substrate capped with 300-nm thick silicon dioxide layer. The standard methods electron beam lithography and thermal evaporation were used to make a pattern of Au electrodes on MoS2 flakes. The patterned devices of MoS2 field effect transistor (FET) were then annealed in a high vacuum to reduce the contact resistance. Through electrical characterizations, we have studied the mobility and conductivity in a wide temperature range from 80 K to 600 K. The mobility of our MoS2 FET device is in the range of 5-150 cm2V-1s-1 at room temperature. In addition, their on/off ratios are of 106-108. As the temperature is lower than 200 K, the MoS2 FET devices show an insulating to metallic phase transition when the carrier concentration is increased by a positive back-gate voltage. The metal-to-insulator transition occurs at the device conductivity very close to the ideal value of e2/h. The transition could be attributed to strong electron-electron interaction in this special two dimensional material. The electron transport of MoS2, at temperatures between 80 and 200 K, is well described by the theory of two-dimensional variable range hopping, whereas that at temperatures between 250 and 350 K is well described by thermally activated transport. At zero back-gate voltage, the few-layer MoS2 exhibits a temperature behavior like semiconductor. When the temperature is higher than 450 K, the few-layer MoS2 changes its semiconducting behavior to metallic behavior. It shows another transition of an insulating to a metallic phase. In this temperature range, the resistance of few-layer MoS2 FET devices rises linearly with increasing temperature and the temperature coefficient of resistance is 0.016 K-1. This metal-to-insulator transition could be attributed to electron-phonon interaction.