氧化物金屬奈米線及其奈米線接點之電子傳輸特性

Abstract

毫無疑問，瞭解電子在奈米線內部的運動行為是應用奈米線製作新穎奈米電子元件一個相當重要的步驟，然而，要能實際達成應用的目的，僅止於這樣的瞭解卻是不夠的。我們知道，任何奈米線，不論是作為元件本身或是作為傳遞訊號的導線，它都必須與外界有所接觸，這個接觸可能是元件與元件之間的連結，也可能是元件與巨觀世界之間的連結。由於相關奈米製程的步驟繁複，一個非理想的接點顯然十分容易因為污染或製程條件的些微差異而形成，而這樣一個非理想的接點極可能會改變電子的運動方式，並進而影響元件的正常操作模式。因此，徹底掌握電子在奈米線內部與在奈米線接點附近的傳輸特性是兩個同等重要的研究課題。 本篇論文即是針對上述的主題，以氧化釕與氧化銥兩種極具應用價值的氧化物金屬奈米線為主體，進行一完整的研究。氧化釕與氧化銥是具有相同結構、高度化學與熱穩定性，以及擁有相當於一般金屬導電率的過渡金屬氧化物。利用標準的電子束微影製程，以及運用不同數目的量測電極，我們不僅釐清了它們本身從室溫到液態氦溫度的電子傳輸特性，也成功地描述了電子在奈米線接點附近從室溫到液態氦溫度的運動行為。 關於氧化釕與氧化銥奈米線本身的電子傳輸特性，我們發現，與它們的單晶塊材相同，其奈米線的電阻率從室溫到液態氦溫度的變化仍舊可以用Boltzmann傳輸理論來成功描述。然而，對於氧化釕奈米線而言，我們發現，從理論擬合得到的Debye溫度卻會隨著奈米線直徑縮小而大幅度的變小。對於直徑約40奈米的氧化釕奈米線而言，其內的Debye溫度已變小至只有其單晶塊材Debye溫度的大約一半大小。對於我們所觀察到的結果，我們提出並探討了一些可能引發這種現象的物理機制，我們歸納後認為，這樣的現象應該是來自於晶格點與晶格點之間的等效鍵結力(亦即晶體的楊格彈性係數)隨著奈米線直徑變小而減弱所導致。另外，在此篇論文裡，我們將僅報告較大直徑氧化銥奈米線的測量結果，對於較小直徑氧化銥奈米線的進一步測量則正在進行當中。 對於電子通過氧化釕與氧化銥奈米線接點的傳輸行為，我們發現，對於具有較高接觸電阻的接點而言，不同材質的奈米線，其接觸電阻隨溫度變化的行為分別可以用不同的物理模型(函數關係)來成功解釋。在氧化釕奈米線方面，我們發現，其較高阻值的接觸電阻隨溫度的變化行為可以用thermally fluctuation-induced tunneling的物理模型來解釋，這主要是由於在接點附近形成一個等效的絕緣層，導致電子通過接點時是以tunneling的方式來傳遞。在氧化銥奈米線方面，我們發現，在溫度約100 K以下，其較高阻值的接觸電阻與溫度的相依關係則是遵從了logR正比於T的負1/2次方之關係式，這主要是因為在接點附近形成了一個由顆粒狀金屬所組成的區域，使得電子通過接點時是以hopping的方式來傳輸。我們認為，不同材質奈米線的接觸電阻具有不同的溫度相依關係，極可能只是一個隨機的結果，亦即，接點結構乃是從兩種可能的結構當中(絕緣層或顆粒層)隨機形成，然而，這樣的結構差異也很可能是肇因於金屬電極材料(本篇論文使用Cr/Au作為電極材料)在不同材質的奈米線上具有不同的表面應力。更進一步的釐清我們則正在進行當中。

Fair understanding of the intrinsic electronic transport properties of individ- ual nanowires (NWs) is certainly the key step for numerous nanoelectronic applications. Quantitative knowledge about the relevant electronic contacts is also very crucial in correctly interpreting the experimental results. In this work, we have studied the intrinsic electronic transport properties of indi- vidual single-crystalline RuO2 and IrO2 NWs, which belong to the family of transition metal oxides that have advantages of being chemically stable while possessing comparatively high conductivities. With the help of the standard electron-beam lithographic technique, individual NWs are contacted by submi- cron metal electrodes from above. By applying di®erent probe con‾gurations to our measurements, not only the intrinsic electronic transport properties of the individual as-grown NWs but also the temperature behaviours of high- resistance electronic contacts, Rc(T), have been determined down to liquid- helium temperatures. Two main results have been obtained. First, the measured temperature dependent resistivity of the NWs is found to agree well with the current theo- retical understanding of these materials. Although they can be well described by the existing theory, we found that the Debye temperature in RuO2 NWs is signi‾cantly reduced to only one half of its bulk value when the diameter of the NW decreases down to ¼ 40 nm. (Comparable experiments on IrO2 NWs with diameters down to this scale have not been performed.) Possible mechanisms accounting for this observation have been discussed. It is conjec- tured that the chemical binding in the NWs may be gradually weakened as the diameter decreases. Second, for high-resistance electronic contacts, the measured Rc(T) reveals semiconducting or insulating behaviour, i.e., it increases rapidly with decreas- ing temperature. However, di®erent temperature dependence has been found for di®erent kind of NW. For RuO2 NWs, the temperature behavior of Rc can be satisfactorily explained in terms of the thermally °uctuation-induced tunneling through a junction formed at the interface between the electrode and the NW. On the other hand, for IrO2 NWs, a power law of the form logRc / T¡1=2 over a very wide temperature range from ¼ 100 K down to liquid-helium temperatures has been observed. This later conduction process is ascribed to the hopping of electrons through nanoscale metal (Cr) granules incidentally formed at the contact region during the thermal evaporation of the submicron Cr/Au electrodes. Although such a di®erence may arise from the di®erent surface conditions of di®erent kinds of NWs (such as di®erent surface stresses), we believe that either mechanism could occur even for the same kind of NW; they just appear randomly. Unfortunately, direct evidences supporting this viewpoint are not obtained in this work.