奈米碳管記憶體之電荷儲存效應研究

Abstract

本論文研究奈米碳管在電場下的電荷儲存效應，並將此概念實際應用在現有的記憶體元件上。元件的電性量測、材料分析以及數值模擬的結果都將會在論文中被探討。 本論文中的記憶體元件是採用一基本的金氧半場效電晶體結構。利用旋塗法將碳管溶液均勻地散佈在晶片表面。奈米碳管以介電層包覆，形成電荷儲存層。掃描式電子顯微鏡的觀察結果發現，奈米碳管在氧化鋁表面的散佈情況極佳，大幅改善旋塗製程在氧化矽表面的碳管分布不均問題。碳管散佈在這兩種表面都被實際做成元件以探討其電性。沒有碳管塗佈的元件對照組則沒有表現出任何的記憶特性。 掃描式電容顯微鏡在本論文中被用來對介電層底下的奈米碳管作更進一步的分析。有埋藏碳管的區域與沒有碳管的區域表現出截然不同的掃描電容訊號。利用金屬探針在介電質表面施加電壓並且觀察隨時間變化的掃描電容二維圖形，更進一步的確認奈米碳管就是造成電荷儲存效應的原因，並且發現電荷是以局部而非整根碳管的方式儲存。這表示，造成記憶效應的的電荷改變是較局部性的機制所造成，而不是儲存在碳管中的自由載子。 另外，本論文亦利用元件模擬軟體，討論通道上方帶電的單根奈米碳管其產生的電場對元件電性有何影響。結果發現，如果此帶電區域平行於通道方向，則對於N型場效電晶體而言，正電荷儲存會很容易造成電性上的變化：一條狹窄的正電帶即可提前導通其下方的通道，並產生可觀的導通電流；如果是負電荷儲存在此狹窄的帶電區域，則其對通道的調變效果將很難被觀察到。反之，如果帶電區域垂直於通道方向，則負電造成的能障將會對通道產生調變，而正電則無法造成影響。

This thesis studies the electrical field-induced charge storage effect of carbon nanotubes (CNTs). The storage property is further applied in practical memory device implementation. Electrical characterization, material analysis, and computer modulation of the devices will all be discussed. Memory device in this thesis took a typical metal-oxide-semiconductor field effect transistor (MOSFET) structure. By spin-coating method, carbon nanotubes were uniformly spread onto wafer surface. Nanotubes were covered by dielectric materials and become a trapping layer. Scanning electron microscopy (SEM) images show that carbon nanotubes have well dispersion quality on Al2O3 surface. This property substantially solves the segregation and uneven distribution problems of coating nanotubes on SiO2 surface. CNTs coated on these two surfaces were both made into devices. Control sample without carbon nanotube coating were also prepared and verified. In that case, no memory effect was observed. Scanning capacitance microscopy (SCM) is used to perform a further study on carbon nanotubes under dielectric layer. Completely different SCM signals were observed from the regions with and without nanotube underneath. By scanning again and again, a time-variant image was obtained and more details of the charging effect were revealed. First, the carbon nanotube was proved to be the essential reason for the memory effect. Moreover, the charging effect appeared locally on nanotubes instead of a whole-tube phenomenon. That also means charges are stored in the local sites around the nanotubes, not as free carriers in CNTs. Numerical simulation works in this thesis discussed the situation of a single charged nanotube parallel or perpendicular to the channel (source to drain) direction, and to determine the channel modulation under these conditions. The results show that if the charged region is parallel to channel, positive charges would have strong effect on nMOSFET’s characteristics. A narrow line of positive charge can turn on the channel beneath it and causes a considerable on-current. But if the tube is negatively charged, its modulation effect would be hard to detect. On the other hand, if the tube is perpendicular to the channel, negative charges would easily modulate electrical characteristics while positive charges cannot.