探索二維系統之非局部效應與金屬-絕緣體相變化

Abstract

二維層狀材料是近幾年最受矚目的材料，從幾乎零能隙的半金屬材料石墨烯到有限能隙半導體材料二硫化鉬，其獨特的物理特性與電子傳輸行為都將對未來科學探索或是工業應用造成巨大的影響，因此本實驗將利用機械剝離法製備單層石墨烯和二硫化鉬場效電晶體元件分別探索長距離電子-電子庫侖交互作用 (非局部效應)與電性傳輸機制。 第一部份為石墨烯非局部效應之探索，本研究成功利用四根電極之石墨烯元件觀察到因長距離電子-電子庫侖交互作用所造成之非局部效應，非局部效應是指在未施加磁場下，於石墨烯元件兩端施加水平方向固定電流會造成橫向電壓差，實驗中發現非局部效應不只與長距離庫侖交互作用有關也與幾何形狀效應有關，另外藉由比較不同樣品之最小通道電導率 (σ_Dirac)與非局部效應之關係，也發現非局部效應只能存在σ_Dirac=7-11 e^2⁄h。理論上，非局部效應只會發生在狄拉克點附近，隨著通道載子濃度或是系統雜質濃度增加都會使非局部效應消失，但是本實驗發現到適當的無序性 (雜質濃度)可以有效使非局部效應不受屏蔽效應影響，另外實驗中也探討量測橫向電壓之電極位置與非局部效應之關係，其實驗結果顯示當量測訊號電極位置靠近汲極時，非局部效應會受電極摻雜效應影響而消失，最後發現非局部效應隨著溫度降低會有明顯增強之趨勢。 第二部分則探討少數層二硫化鉬於T=80-600 K之電性傳輸機制，其電性傳輸機制可分為三個溫度區間作探討，首先是溫度區間為80-240 K，於該溫度區間二硫化鉬之導電型態為絕緣體態，其電子傳輸行為可用二維變程跳躍傳輸機制描述，隨著調控通道載子濃度或是通道電場到達臨界值時，二硫化鉬則會由絕緣體態轉變成金屬態，且金屬-絕緣體相變化發生於通道電導率為〖1 e〗^2⁄h時，另外本實驗也探討金屬-絕緣體相變化，特徵溫度大小與通道電場、通道載子濃度與特徵溫度之關係，而溫度區間為300-430 K之電子傳輸行為主要是以熱活化傳輸機制為主，最後探討430-600 K之電性傳輸機制，該區段二硫化鉬導電行為會再次由絕緣體轉變成金屬傳輸行為，且樣品電阻隨溫度變化皆呈線性關係，藉由電阻與溫度之關係可得二硫化鉬之電阻溫度係數，而電阻溫度係數會隨載子濃度和樣品厚度改變。

Two-dimensional layered materials such as semi-metal of graphene and semiconductor of MoS2 have attracted attention in recent years. Both of them show the exclusive physical characteristics and the interesting electron transport behaviors that will cause high impact to the academic research and the industry application in the future. In this study, the mechanical exfoliation of graphene and few-layer MoS2 were used to fabricate field effect transistor devices and explore long-range electron-electron Coulomb interaction and electrical transport properties. The first part, the graphene devices with four electrodes were used to investigate the non-local effect, which is attributed to the long-range electron-electron Coulomb interaction. The non-local effect represents that the transverse voltage can be measured while applying a constant horizontal current without applying an external magnetic field. We observed that the non-local effect induced by not only long-range electron-electron Coulomb interaction but also geometry effect. In addition, we also observed that the non-local effect exists at minimum conductivity of 7－11 e2/h. Theoretically, the non-local effect only exists near the charge-neutral Dirac point, the high carrier concentration and impurity carrier will eliminate the non-local effect. However, we observed that the appropriate disorder (impurity carrier) can prevent the non-local effect screened by high carrier concentration. Additionally, the relation between position of voltage-probe electrodes and the non-local effect was also discussed, the result indicates that the non-local effect disappears while the voltage-probe electrodes close to the source-drain electrodes. Finally, the temperature dependence of non-local effect was also carried out and the result shows that the non-local effect at low temperature is stronger than room temperature. The second part, we studied the electrical transport properties of few-layer MoS2 in the temperature range between 80 and 600 K and divided the temperature behavior of conductance into three different temperature regimes to discuss. At temperatures below 240 K, the electrical properties of MoS2 show the insulating behavior which can be descried by two-dimensional variable range hopping transport (2D-VRH). When the carrier concentration and source-drain electric field are higher than critical values, the electron transport behavior changes from insulating to metallic phase and the transition occurs at conductivity of 〖1 e〗^2⁄h. In addition, we also observed that metal-insulator transition which is associated with carrier concentration, source-drain electric field and characteristic temperature (T0). In the temperature range from 300 to 410 K, the electrical transport changes from 2D-VRH to thermal activated transport. At temperatures above 430 K, the electron transport behavior changes from insulating to metallic phase again and the temperature behavior of resistance can be described by the equation of R∝T. Moreover, the layered-dependent and carrier-dependent temperature coefficient of resistance (TCR) have been also investigated.