奈米碳管接觸阻抗與電場效應研究

Abstract

奈米碳管在未來有可能成為新一代的奈米元件，但由於奈米碳管與金屬之接觸阻抗會對於奈米元件的電性有極大影響，本論文即著重於改善奈米碳管與金屬接觸阻抗的技術與奈米碳管電場效應的相關研究。奈米碳管與Pt有很低的接觸阻抗，但與基板附著力不佳。我們利用Pt摻雜Ti或Ta形成合金，成功開發出不但可以與碳管形成低接觸阻抗而且良好附著力的金屬連線製程。在接觸阻抗量測方面，採用三端點方式量測前端阻抗及後端阻抗間接求得接觸阻抗，並且改良佈局結構解決量測上的誤差。在量測系統的精確度之下，得知Pt合金的接觸阻抗應在kΩ以下。而Ti與碳管的接觸阻抗很高，經由電性與化性分析知道600℃退火可形成金屬碳化物而降低接觸阻抗，400℃則無此改善現象。我們也開發出利用Pt合金當作電極並具有電流開/關的CNTFET結構，而具有高接觸阻抗的Ti電極結構，難以找到明顯的電流調變現象。由於碳管的奈米尺度，碳管表面電場會大於一般微米尺度金屬線電場約一個數量級以上，而隨著氧化層深度快速遞減。因為碳管表面的強電場，電荷會從碳管注入到氧化層形成電壓電流特性的遲滯效應。我們可以利用遲滯效應的原理製作奈米碳管非揮發性記憶體，目前的寫入速度可以在100ms以下。水氣的存在產生電流的回復效應，對非揮發性記憶體應用是一個傷害因數。

Carbon nanotubes (CNTs) have been proposed as candidate for nanoelectronics application in the future. Since contact resistance (Rc) between metal and CNT has significant influence on the electrical performance of CNT devices, we focus on contact resistance and electric field effect in the thesis. CNTs have low Rc with Pt electrode, however, Pt deposited on SiO2 surface shows poor adhesion. Composed of Pt-Ti or Pt-Ta alloys, we successfully develop not only low Rc but also good adhesion metal-CNT contact technology. About the measurement, we approach Rc by the way of measuring the front and end resistances (Rf, Re) with 3-terminal structure. Within the limit of the measurement system, Rc with Pt electrode should be lower than kΩ. Although Rc with Ti electrode is very high, electrical and chemical analyses reveal that CNTs react with Ti to form carbide and Rc is reduced significantly after annealing at 600℃. However, annealing at 400℃ fails to reduce Rc. We fabricate CNTFETs with very good ON/OFF performance with Pt-alloy electrode. Owing to the nano-scale of CNTs, the magnitude of electric field on the surface of CNTs is one-order greater than micro-scale metal-line. The electric field decreases quickly away from CNTs. Because of the strong electric field, charges can inject into oxide layer at suitable VG and ID-VG curves show hysteresis characteristic. Using the hysteresis effect, we demonstrate the non-volatile CNT-memory dunction and the write-speed can be faster than 100ms. Retention effect will be induced by water moisture and cause an obstacle to non-volatile memory application.