利用少數層二硒化鎢蕭特基結電晶體探索金屬絕緣相變化

Abstract

本實驗以機械剝離法在矽基板上製備少數層二硒化鎢薄片，利用電子束微影技術、熱蒸鍍鍍膜製作出二硒化鎢場效電晶體，再利用高真空熱退火提升元件功能特性。元件上的二硒化鎢薄片的厚度是用原子力顯微鏡量測，電晶體元件製放入探針系統，用電錶量測汲極-源極電壓與汲極電流之I-V曲線關係。室溫下，二硒化鎢薄片元件之電阻值約在〖10〗^7 至〖10〗^10 Ω，其電壓電流關係圖呈現非線性的關係，由此推斷電極與二硒化鎢薄片之間存在著金屬-半導體之蕭特基位障接面。本實驗中利用背向閘極電壓來調控二硒化鎢薄片通道之載子濃度，觀察到二硒化鎢通道之場效行為為雙極性，電子遷移率約為 20〖 cm〗^2 V^(-1) s^(-1)，而電流開關比可達〖10〗^7。 對於電極與二硒化鎢薄片之間金屬-半導體接觸的電性行為部分，我們用熱離子放射理論來分析，並估計出等效蕭特基位障值，且描繪出此蕭特基位障隨電晶體通道內之電場與載子濃度改變而變化的關係圖。當通道內電場大於5×〖10〗^5 V/m，且載子濃度高於7.18×〖10〗^12 cm^(-2)時，載子通過金半接面之蕭特基位障接近0 meV，反之載子傳輸電場小於1×〖10〗^6 V/m且載子濃度低於4.31×〖10〗^12 cm^(-2)時，載子通過金半接面需要克服約200 meV以上的蕭特基位障，因此可藉由調控汲極-源極電場與背向閘極電壓來改變蕭特基位障的大小。 我們觀察到二硒化鎢薄片的金屬絕緣相變化，當通道內電場為〖10〗^5 V/m，且載子濃度為5.75×〖10〗^12 cm^(-2)時，二硒化鎢電阻隨著溫度上升而下降，表現出半導體特性，此偏壓下的等效蕭特基位障約60 meV；然而，當通道電場增加至3×〖10〗^6 V/m時，二硒化鎢電阻隨著溫度上升而增加，轉變為金屬特性，此偏壓下的等效蕭特基位障趨近於0 meV。我們對電阻和溫度關係作圖，可觀察到二硒化鎢在200 K附近發生金屬絕緣相變化。最後，我們將等效蕭特基位障調控至0 meV時，分析低溫下電性傳輸機制，發現二硒化鎢在80K 至180 K之間符合二維Mott變程跳躍理論。因此，我們證實在等效蕭特基位障趨近於零時，可得到二硒化鎢本質的電性。

We have made few-layer tungsten diselenide (WSe2) field-effect transistors by using mechanical exfoliation with electron beam lithography, thermal evaporation, and post-annealing. The thickness of WSe2 flakes is determined using atomic force microscope. The device of WSe2 transistor is placed in a probe station where the electrical behavior of source-drain current-voltage (I-V) relation is measured. The I-V curves are nonlinear and it indicates back-to-back Schottky-contact features. The carrier-concentration of few-layer WSe2 flakes is modulated by applying a voltage on the back gate electrode. The WSe2 devices show ambipolar behavior. Their electron mobility is about 20〖 cm〗^2 V^(-1) s^(-1), and their on/off ratios are as high as 107. The thermionic emission theory is used to analyze the I-V curves and the effective Schottky barrier height is evaluated. We study the variation of effective Schottky barrier height as a function of carrier concentration and electric field in the WSe2 channel. When the electric field is lower than 1×〖10〗^6 V/m and the carrier concentration is lower than 4.31×〖10〗^12 cm^(-2), the effective Schottky barrier reaches 200 meV. On the other hand, the Schottky barrier vanishes when the electrical field is higher than 5×〖10〗^5 V/m and carrier concentration is higher than 7.18×〖10〗^12 cm^(-2). Thus the effective Schottky barrier height of the few-layer WSe2 transistors can be modulated by the source-drain and the back gate voltage. We observed metal-insulator transition in few-layer WSe2 transistors. When the electric field is 〖10〗^5 V/m and the carrier concentration is 5.75×〖10〗^12 cm^(-2). Resistance of WSe2 decreases when temperature rises, and WSe2 shows semiconductor characteristic. The effective Schottky barrier is about 60 meV in this bias. However, when the electric field increases to 〖3×10〗^6 V/m. Resistance of WSe2 increases when temperature rises, and WSe2 shows metal characteristic. The effective Schottky barrier approachs to 0 meV in this bias. Then we found the relation between the resistance and temperature and observed metal-insulator transition of WSe2 occurs at 200 K. Finally, We analyzed WSe2 1electrical transport at low Schottky barrier height, confirming that WSe2 follows two-dimension Mott’s variable range hopping in the temperature range between 80 K and 180 K. As a result, we infered that decreasing the Schottky barrier heigh can make WSe2 electrical properties more clearly.