接點電阻與不同退火條件下氧化鋅奈米元件電性研究

Abstract

元件尺寸的縮小，接觸面積逐漸減少，介面系統所造成的接點電阻逐漸影響元件的電性。在本實驗中，我們選擇直徑約50 nm的氧化鋅奈米線，利用電子束微影與熱蒸鍍的技術製成二點與四點的奈米元件，進一步使用採用熱退火處理後的氧化鋅奈米線製成元件，熱退火的環境為高真空，約10-6 Torr，溫度為500 ℃，時間為6或24小時，則可研究接點電阻與本質氧化鋅奈米線的電性傳輸。 鈦氧化物的生成，使得電極系統內的無序程度增加，導致接點電阻變大，採用變程式跳躍傳輸理論，描速電子經過此系統時的傳輸行為。我們推論隨著接點電阻率增加，電極內部鈦氧化物增多，系統無序程度增大，電子將由低維度拓展至高維度傳輸。 另一方面，在氧化鋅奈米線的電性研究，在高溫下，電子的行為將由熱活化傳輸所主導，隨著溫度下降，較多的電子將轉成三維之變程式跳躍理論來傳輸。並且發現隨著熱退火時間的增加，氧化鋅奈米線的電子濃度增加，活化能Ea逐漸變小，我們推論氧化鋅內的氧空缺會隨著熱退火時間的增加而增多，造成淺層的雜質能階增多，導致活化能Ea逐漸變小，而熱活化傳輸主導的溫度範圍越廣，電子的傳輸會在越低溫處轉變成三維之變程式跳躍傳輸所主導。

The interface problems in nanowire-based electronics play important roles due to the reason that the reduced contact area in nanoelectronics multiplies enormously the contribution of electrical contact properties. In this study, zinc oxide (ZnO) nanowires were employed in fabricating two-probe and four-probe nanowire devices for electrical studies of nanowire-electrode contact and intrinsic nanowire properties. The ZnO nanowires with a circular cross-section and a diameter of ~50 nm were synthesized on quartz substrates by a thermal evaporation and were dispersed on silicon substrates, which were capped with a 150-nm thick SiO2 layer and were photolithographically patterned with Ti/Au (~10/60 nm in thickness) micron electrodes. Prior to the electron beam lithography process, some of the dispersed ZnO nanowires were annealed in a high vacuum (~10-6 torr) at 500 0C for 6 or 24 h to increase native oxygen defects as well as the nanowire conductivity. The electron beam lithography technique was then used to generate nanometer leads and to connect the nanowires and micron electrodes. In either two- or four-probe nanowire devices, the separation distance between two nearest-neighbor nanometer leads was kept constant to be ~ 1 μm or 500 nm. Through a study of temperature dependent electrical properties in two- and four-probe nanowire devices, the electrical contact properties and intrinsic transport of ZnO nanowires can be determined. Electron transport in the nanocontact system follows Mott variable range hopping theory of the form . The exponential parameter, p, rises from 2 to 4 with an increase of specific contact resistivity, implying a change from one to three dimensional hopping. On the other hand, the intrinsic electrical properties of ZnO nanowires exhibit thermally activated transport and three-dimensional Mott variable range hopping at high and low temperatures, respectively. In addition, it was observed that, after annealing in a high vacuum for 24 h, the resistance (resistivity) of ZnO nanowires is reduced to be hundred times smaller in comparison with those without annealing.