單晶鍺奈米線合成及特性量測

Abstract

本研究利用熱蒸鍍法配合氣液固(Vapor-Liquid-Solid, VLS)機制進行單晶鍺奈米線的合成，討論載流氣體、基板溫度、腔體壓力及矽基板方位的改變對鍺奈米線成長的影響。另外，本研究也成功利用低溫熱蒸鍍法，在大幅降低熱預算的情況下，有效進行單晶鍺奈米線及奈米串珠結構的合成。本研究利用X光繞射分析儀(X-ray diffractometer, XRD)及高解析穿透式電子顯微鏡(High Resolution transmission Electron Microscope, HRTEM)進行奈米線及奈米串珠的結構鑑定，由XRD量測可觀察到鑽石立方結構的鍺，由HRTEM影像及傅立葉轉換繞射圖形確定鍺奈米線為單晶無缺陷且沿[111]方向成長。 本研究將鍺奈米線散佈於電性量測基板，利用電子束微影進行電路的定位，在完成鎳的沉積及光阻的去除後即完成元件製備。並利用快速退火系統對元件進行加熱，使其反應成為鍺化物/鍺/鍺化物的異質結構，異質接面的形成能有效的降低接觸電阻，並形成良好的歐姆接觸;另外我們利用統計的方式，對不同直徑奈米線做I-V量測，觀察到鍺奈米線的電阻率與直徑有正相關的趨勢。由HRTEM影像及傅立葉轉換繞射圖形確定鍺化物為斜方晶結構的Ni2Ge，且觀察到Ge與Ni2Ge的磊晶關係為Ge[110]//Ni2Ge[110] 及 Ge(-1 1 -1)//Ni2Ge(1 -1 -2)。另外本研究也對此異質接面進行臨場加熱觀察，在500 ℃的加熱溫度下，觀察到Ni2Ge以1.29 nm/s的速率進行成長。我們再對鍺化物/鍺/鍺化物的異質結構奈米線進行場效電晶體特性量測，觀察到本研究合成的本質鍺奈米線具有P型半導體特性，且其場效電洞遷移率可達44.3 cm2/Vs。由發光特性量測得知鍺奈米線表面存在許多氧缺陷，進而引發鍺奈米線的光感測特性，並在光感測特性的量測觀察到兩階段的電流釋放。

Intrinsic single-crystalline Ge nanowires and nanosphere chains have been synthesized on Au-coated Si substrates through thermal evaporation and vapor–liquid–solid mechanism. The influence of different growth parameters on Ge nanowires was systematically studied. The size, morphology and density of Ge NWs can be controlled by adjusting carrier gas, growth temperature, and chamber pressure. Single-crystalline Ge nanowire and nanosphere chains grown along [111] were determined by HRTEM and FFT diffraction pattern. Diamond-cubic crystal structure of Ge was also observed from XRD spectrum. Global back-gate intrinsic Ge nanowire field-effect transistors on the Si3N4 dielectric were fabricated through successive steps of e-beam lithography, nickel deposition and lift off. After RTA process, Ni2Ge/Ge/Ni2Ge heterostructure was formed. The electrical transport property was effectively improved by the heterojuction. Orthorhombic Ni2Ge was determined by HRTEM and FFT. The epitaxial relationship between Ge and Ni2Ge were Ge[110]//Ni2Ge[110] and Ge(-1 1 -1)//Ni2Ge(1 -1 -2). The formation process of Ni2Ge was observed through in situ TEM. Ni2Ge grew at a speed of 1.29 nm/s at 550 ℃. The linear growth behavior of the Ni2Ge nanowire along the Ge nanowire indicated that the growth may be interface reaction controlled. From electrical transport properties measurement, we found that the resistivity exhibited a linear relationship with diameter of nanowire. The resistivity of Ge nanowire was much lower than bulk Ge material. From FET property measurement, intrinsic Ge NW showed p-type behavior and field effect hole mobility of 44.3 cm2/Vs. Room temperature photoluminescence spectra of Ge nanowires possessed a broad blue emission around 462 nm, which was attributed to the oxide related defect states. Due to the existence of oxide defects, Ge nanowires were able to detect visible light.