探討黑磷之金屬-絕緣體相變化與電性傳輸特性

Abstract

本研究以機械剝離法撕貼少層數黑磷於矽基板，使用電子束微影與熱蒸鍍標準程序製作金電極，經由熱退火改善材料缺陷與元件之接觸電阻，以兩點量測確認樣品的電性行為，再透過原子力顯微鏡確定元件厚度。 黑磷呈現p型半導體特性，電流開關比隨厚度減少而增加，且於厚度小於7 nm的電流開關比可高達10^5倍。變溫量測實驗中，透過閘極偏壓控制載子濃度可觀察到少數層黑磷存在明顯的金屬-絕緣體相變化，其相變化之載子臨界濃度大小皆為1012 cm^-2左右，並計算相變化載子臨界濃度之電子-電子交互作用能量和費米能比值，證實符合二維電子氣系統中金屬-絕緣體相變化為電子間的庫倫交互作用導致 。 載子傳輸機制透過Mott變程跳躍機制與熱活化傳輸機制的並聯擬合，以不同閘極偏壓的無序程度和熱活化能，解釋兩者傳輸現象的貢獻。 依據Ioffe-Regel 判別法，我們以電子的平均自由路徑相對於電子的費米波長比值，得知電子處於金屬態(metallic phase)與絕緣體態(insulating phase)的局域性程度，估算出載子局域化長度(localization length)。並透過遷移率，分析載子濃度對於量子傳輸的散射行為，其電導率為e^2/h劃分金屬性與絕緣體性呈現截然不同的散射趨勢。

In this study, we used mechanical exfoliation method to transfer thin flakes from black phosphorus (BP) crystal onto heavily doped silicon, covered with a 300-nm thick SiO2 layer, and we deposited Au electrodes on BP flakes by standard electron-beam lithography and thermal evaporation. The devices were annealed in vacuum at 200 oC for 30 minutes to improve the surface roughness and electrical contact properties. The electrical properties of BP was measured by a two-probe measurement and the thickness was determined by an atomic force microscope. For few-layered BP transistors, the gating behaviors (Ids-Vg curves) exhibited an ambipolar behavior whereas the hole doping behavior is more prominent. The on-off ratio of current increases with a decrease of BP flake thickness and it can be as high as 10^5 at a thickness less than 7 nm. We observed a metal-insulator transition (MIT) at a high carrier concentration in few-layer BP. The critical carrier concentration for the observation of the MIT is about 1012 cm-2. The transitional conductivity of the MIT is very close to an ideal value of e^2/h. The estimated ratio between Coulomb energy and Fermi energy is bigger than 1, implying that the MIT originates from a strong Coulomb interactions in the two-dimensional electron gas system. We investigated electron transport in BP flakes by fitting the temperature dependent conductivity with Mott’s two-dimensional variable range hopping transport and thermally activated transport. On the other hand, we applied the Ioffe-Regel criterion to check electron transport in BP flakes. We calculated the Fermi wave vector (kF) and the mean free path (le) to estimate localization length from an insulating phase to a metallic phase at different carrier concentrations. We discovered different scattering mechanisms from a clear evidence of temperature dependent mobility when the system is in either a metallic or an insulating phase.