「金屬電極−碳管−金屬電極」之電性量測元件的 接觸電阻值及製程優化

Abstract

實驗室的目標是製作以單壁奈米碳管作為電極的單分子電性量測平台。我們透過共價鍵將分子與碳管連接形成「碳管−分子−碳管」結構，並利用熱敏式掃描探針微影技術(thermal scanning probe lithography, t-SPL)將其與外部電路連接，建構「金屬電極−碳管−分子−碳管−金屬電極」電性量測平台。儘管實驗室過去有製作「碳管−分子−碳管」結構，但由於此結構電阻值過高而無法獲得分子的非彈性電子穿隧能譜(inelastic electron tunneling spectroscopy, IETS)，因此無法證實分子有橋接在碳管之間。此外，在相同的製程條件下製作「金屬電極−碳管−金屬電極」元件時，10個元件之間的電阻值差異達到四個數量級且平均電阻值為236 (±285) MΩ。 元件電阻值來源為金屬電極的固有電阻、碳管與金屬間的接觸電阻以及碳管的通道電阻。為了降低金屬電極的固有電阻對元件電阻值的影響，本研究將金屬材料由鉑替換為金，並且對濺鍍速率、沉積溫度以及金屬厚度進行調整與優化，使「金電極−金電極−金電極」平均電阻值為88.5 (±6.6) Ω。為了降低接觸電阻，本研究以優化後的沉積參數製作「金電極−碳管−金電極」元件，並加入退火步驟，同時對退火溫度與時間進行優化。退火後元件的平均電阻值從過去的236 (±285) MΩ降低至654 (±504) kΩ，若剔除離群值則為410 (±198) kΩ，與許多文獻報導的碳管元件電阻值10^4 Ω只相差一個數量級，並且將過去10^4~10^8 Ω四個數量級的電阻值範圍改善到近一個數量級。最後透過搭配拉曼光譜的指認，在「金電極−碳管−金電極」元件的IETS中觀察到碳管特有的振動模式，為未來「碳管−分子−碳管」結構的IETS量測提供重要基礎。

Our laboratory aims to develop single-molecule electrical measurement platforms using carbon nanotubes (CNTs) as electrodes. We create a CNT−molecule−CNT structure by covalently bonding molecules to carbon nanotubes and connect it to external circuits using thermal scanning probe lithography (t-SPL) to construct metal−CNT−molecule−CNT−metal electrical measurement platforms. Although the laboratory had previously fabricated CNT−molecule−CNT structures, the high resistance prevented obtaining inelastic electron tunneling spectroscopy (IETS), making it impossible to verify molecular bridging between carbon nanotubes. Furthermore, when fabricating metal−CNT−metal devices under identical processing conditions, the resistance values among 10 devices varied by four orders of magnitude, with an average resistance of 236 (±285) MΩ. Device resistance stems from the intrinsic resistance of metal electrodes, contact resistance between carbon nanotubes and metals, and channel resistance from carbon nanotubes. To reduce the impact of metal electrode resistance, this study replaced platinum with gold as the electrode material and optimized sputtering rate, deposition temperature, and metal thickness parameters, achieving an average Au−Au−Au resistance of 88.5 (±6.6) Ω. To decrease contact resistance, we fabricated Au−CNT−Au devices using optimized deposition parameters and introduced an annealing step, optimizing both temperature and time. After annealing, the average resistance of devices decreased from 236 (±285) MΩ to 654 (±504) kΩ, and it becomes 410 (±198) kΩ if outliers are excluded, approaching within one order of magnitude of the 10^4 Ω reported in many literature studies for carbon nanotube devices. The resistance variation among devices was significantly reduced, decreasing from a four-order-of-magnitude range (10^4~10^8 Ω) to within one order of magnitude Finally, through Raman spectroscopy identification, we observed specific vibration modes of carbon nanotube in the IETS of Au−CNT−Au devices, providing an important foundation for future IETS measurements of CNT−molecule−CNT structures.