鉻摻雜與純二硒化鎢二維材料之電子傳輸與記憶體效應之研究

Abstract

本實驗以機械剝離法將摻雜不同鉻濃度之二硒化鎢從原塊中分離成少數層，並利用電子束微影系統、熱蒸鍍系統和高溫熱退火系統依序將材料製作成場效電晶體元件及退火降低元件接觸電組，接著使用電性量測系統測量摻雜不同鉻濃度之二硒化鎢元件電性，藉由改變背向閘極偏壓及溫度(溫度範圍為80 K到300 K)來觀察不同鉻摻雜濃度元件的電導與載子遷移率變化。 室溫下純二硒化鎢表現出n型半導體行為，電流開關比達107，隨著鉻摻雜濃度增加，電流開關比逐漸減小且電晶體表現由n型半導體行為逐漸轉變為雙極性行為。當鉻摻雜濃度為20％時，電流開關比約降至為102。觀察背向閘極偏壓20 V - 70 V電導隨溫度之變化，純二硒化鎢在260 K以下呈現金屬性質， 260 K以上則呈現半導體性質。而鉻摻雜元件，隨著摻雜濃度增加電導下降，在200 K以下呈半導體性質，而200 K至260 K分成兩個部分，背向閘極偏壓50 V以下，電導隨溫度增加而下降且電導下降幅度隨鉻摻雜濃度增加而變大，背向閘極偏壓50 V以上，電導隨著溫度上升而緩慢增加且與鉻摻雜濃度無關，260 K以上電導隨溫度上升而增加且電導增加幅度隨鉻摻雜濃度增加而變大。 接著研究鉻摻雜元件的電性傳輸機制，於溫度區間80 K至190 K符合變程跳躍理論，然而特徵溫度不隨背向閘極偏壓增加而降低及不隨鉻摻雜濃度增加而上升，因此推測除Mott二維變程跳躍傳輸之外還有其他傳輸機制存在。另外探討電子遷移率與溫度變化，純二硒化鎢元件電子遷移率散射機制由聲子散射主導，電子遷移率隨溫度增加而變小，而鉻摻雜元件則由雜質散射主導，電子遷移率隨溫度增加而變大。 最後研究二硒化鎢的記憶體特性，元件操作藉由調控閘極偏壓來改變元件狀態，以及量測抹去 (Erase)狀態電流 (IE)與寫入 (Program)狀態電流 (IP)之比值 (IE/IP)，判斷抹去狀態及寫入狀態分開的程度，隨著摻雜濃度的增加，兩狀態分開程度減少，且記憶窗口大小變小，純二硒化鎢、1％及20％鉻摻雜濃度二硒化鎢IE/IP分別為100、12.77及1.96。

WSe2, a two-dimensional and layered semiconducting material, attracts much attention in recent years. Scientists argued that single-layer WSe2 may have high potential for electronic and optoelectronic applications due to its high on/off ratio (as high as 106) and its direct band gap of 1.6 eV. Therefore, many research groups investigated deeply the electrical and photoelectric properties of WSe2. On the other hand, electrical properties of Cr doped WSe2 were not well studied yet. Here we will compare electrical properties, including electron transport, mobility scattering mechanisms and memory effects, of pure and Cr-doped WSe2. The field effect transistor (FET) devices of pure WSe2 show a typically n-type conducting behavior with a high current on/off ratio as high as 107. However, after doped with Cr ions, the conducting behavior changes from a n-type to an ambipolar behavior and the current on/off ratio decreases to be about 102. In addition, after Cr doping, the mobility decreases from 7 cm2/V-s to 9.7×10-3 cm2/V-s with increasing doping concentrations. The temperature dependent mobility presents a transition from a phonon scattering to an impurity scattering mechanism for heavily Cr-doped WSe2. Electron transport of pure WSe2 and Cr-doped WSe2 is studied in the temperature range from 80 to 300 K. For pure WSe2, it shows a metal-insulator transition at back-gate voltages from 20 to 70 V at temperature below 260 K. For the Cr-doped WSe2, two different conducting behaviors are observed in the temperature range from 200 to 260 K. At temperature below 200 K, the Cr-doped WSe2 show an insulating behavior and electron transport is described by Mott’s two-dimensional variable range hoping (2D-VRH). However, the characteristic temperature (T0), estimated from fitting with the 2D-VRH theory, as a function of carrier concentration does not show a rational behavior. Finally, we investigate the memory effect in either pure WSe2 or Cr-doped WSe2. For the memory device operation, we demonstrate two states of “Program” and “Erase” and present the switching between the two states by applying a back-gate voltage for a short period of 0.1 s. Both of the “Program” and the “Erase” state can be maintained for at least 2000 s. We define IE as the reading current of the “Erase” state and IP as the reading current of the “Program” state. The IE/IP ratio is used to identify the separation of the “Program” and the “Erase” states. The IE/IP ratio is reduced from a factor of 100 to 1.96 when the Cr-doping concentration is increased from 0 to 20%.