探索少數層二硒化錸之電性傳輸行為及各向異性性質

Abstract

本實驗以機械剝離法、電子束微影、熱蒸鍍、高溫熱退火等製成方法，製作二硒化錸場效電晶體元件，並使用探針系統量測元件電性。固定源極及汲極之間的電壓，調控背向閘極偏壓，量測汲極電流來描繪二硒化錸場效電晶體元件的電特性。在室溫下二硒化錸元件為n型半導體，且電流開關比為106。我們改變環境溫度從80 K到600 K，觀察二硒化錸之電導隨溫度變化，並用理論模型擬合數據，來探討不同溫度區間之傳輸機制。 溫度在200 K以下時，電導與溫度關係符合二維Mott變程跳躍傳輸機制，且在不同元件上還觀察到金屬-絕緣體相變化。溫度在200 K至300 K之間時，載子同時受到多種因素影響使得電導無規律變化，無法分析傳輸行為。當溫度由300 K升至500 K時，部分侷域化載子因熱擾動脫離束縛態，電導與溫度關係符合熱活化傳輸機制。最後在溫度高於500 K時，電阻與溫度關係又再次發生轉折，由原本的半導體行為轉變成金屬性，電阻溫度係數為0.017 K-1，我們推測此時的金屬-絕緣體相變化是由電子與聲子之交互作用所造成。 二硒化錸具有各向異性，我們在元件同時設計多方向的電極測量不同角度下的電傳輸行為。在室溫下，我們定義電子遷移率最大之方向為0°角，而量測到電子遷移率最小之角度恰為90°。我們持續做各向異性的變溫量測，發現各向異性只影響電導值大小而不影響二硒化錸之傳輸機制。

In this thesis, we used mechanical exfoliation to produce few-layers rhenium selenide (ReSe2) flakes on a silicon wafer covered by 300-nm thick silicon dioxide. Standard electron-beam lithography and thermal evaporation were used to pattern gold (Au) electrodes on ReSe2 flakes. The as-fabricated field effect transistor (FET) devices of ReSe2 were annealed in a high vacuum to improve the metal contact and to reduce the contact resistance. The devices were then electrically characterized. At room temperature, ReSe2 reveals itself as a n-type semiconductor, showing an on-off ratio of current up to 106. We studied electron transport in the ReSe2 FET devices in a wide temperature range from 80 K to 600 K. At a temperature below 200 K, electron transport in ReSe2 is well described by the theory of two-dimensional Mott’s variable range hopping. In this temperature range, we have discovered a phase transition from an insulator to a metallic state with increasing carrier concentration, adjusted by the back-gating voltage. As a temperature in the range between 300 K and 500 K, electron transport in ReSe2 is well described by thermally activated transport. When the temperature is higher than 500 K, the few-layers ReSe2 flakes changes from semiconducting to metallic behaviors once more. It reveals another insulator-to-metal transition at such a high temperature. This transition could originates from the electron-phonon interactions. ReSe2 is known for its anisotropic electrical property. Here we make multiple probes to measure electrical properties in different lattice orientations. We measured transfer characteristics of ReSe2 FET devices in several different orientations. As a first step, we set the orientation of 0° as the direction showing the highest mobility. We found that the direction of the lowest mobility is always exhibiting at 90°. In addition, we studied the anisotropy of electron transport at temperatures from 80 to 300 K. We learned that the anisotropy effect in ReSe2 only gives a different magnitude of conductance in different orientation while the mechanism of electron transport is the same in all directions.