單根奈米線場效電晶體之光電特性研究

Abstract

本篇論文主要探討的是單根奈米線的元件製作，並對此元件進行在場效電晶體特性及光感應特性上之研究。本研究改變一般傳統場效電晶體結構的作法,以單根奈米線當作載子通道,將載子通道移至氧化層上方,並將矽基板當作背部閘極加以偏壓來對奈米線引起電場效應, 使得有效載子通道的導電載子濃度變化,進而改變通道電阻及載子漂移率。 有鑑於此，我們先以汽相-液相-固相成長機制及有機金屬化學氣相沉積方法長出三種不同材料的奈米線：二氧化鈦奈米線、氧化鋅奈米線、氧化鋅/硒化鋅-核/殼奈米線，再運用光學微影製程技術與電子束微影技術來成功連接單根奈米線兩端,形成單根奈米線場效電晶體。除此之外，我們以模擬太陽光垂直照射於場效電晶體的單根奈米線上，研究功率強度與不同照射時間後的電流效應影響。 在單根二氧化鈦奈米線元件部分，在閘極偏壓的控制下，確實可使有效載子濃度變大，導致輸出電流強度會增強，為n型半導體，且電流情形並非呈現原點對稱；其對光的開與關電流比例可以高達1000，而且光功率強度比起照射時間對二氧化鈦奈米線的影響更是顯著。在氧化鋅奈米線部分，我們也發現到，氧化鋅奈米線也是屬於n型載子通道；在對光感應方面，開/關的電流比值最高可達到300倍(光功率為100 毫瓦特、外加電壓為1伏特時)，具有良好的感光性質。此外，當我們在氧化鋅奈米線外層包覆成長硒化鋅後，與氧化鋅奈米線比較可明顯看出硒化鋅/氧化鋅奈米線的光感應的確比氧化鋅奈米線光感應的效果好，光電流強度可再大幅增加200倍。

Fabrication of single nanowire devices and their optoelectronic properties and field effect transistor (FET) characteristics are reported in this thesis. Vapor-liquid-solid growth (VLS) and metal organic chemical vapor deposition (MOCVD) were used to grow three different nanowires: TiO2 nanowires, ,ZnO nanowires, and ZnO / ZnSe core / shell nanowires. Optical lithography and electron beam lithography technology were then used to fabricate single nanowire field effect transistors. For TiO2 single nanowire FET we found that the carrier concentration in the nanowire can be increased by gate bias, and from its FET characteristic we conclude that the TiO2 nanowire grown with ours method is a n-type semiconductor. We also found that the TiO2 single nanowire FET is very sensitive to the light exposure. The on-off ratio of the device current can reach 1000 when the incident light power is 30 mW. From the electric characteristic of ZnO nanowires FET, we found that ZnO nanowires are n-type semiconductors. The on / off current ratio of the ZnO nanowires FET can reach 300 at an incident optical power of 100 mW at an applied voltage of 1 V. After the ZnO wire was coated with ZnSe shell to form ZnO-ZnSe cor-shell nanowire, the light-sensitive of the device increase quite dramatically. We found the current intensity in the ZnO/ZnSe nanowire can increase by 200-fold as compared with the ZnO nanowires in the same light exposure condition. This dramatic increase of the light–induced current is attributed to the charge separation effect in of the type-II band alignment of the ZnO/ZnSe heterostructure.