氧化鋅奈米線電子元件之奈米接點電性研究

Abstract

兩點量測之氧化鋅奈米元件的電性至今已被廣泛地研究，但由於接觸面積的縮小增強了接點對奈米元件的影響，也因此在氧化鋅奈米元件中，接點的電性應該加以考慮及深入探討，才可使得兩點量測之氧化鋅奈米元件的電性傳輸更加清楚及透徹。本實驗利用電子束微影的方式，將直徑為40奈米之圓柱型氧化鋅奈米線，製成多組兩點量測之單根氧化鋅奈米線元件，量測其變溫之電流-電壓特性並分析實驗結果，發現可將元件依其室溫電阻大小分成兩類。 第一類型的元件，其室溫電阻值皆大於1 MΩ，而元件之電性深受奈米接點的影響。此類型元件之電流-電壓曲線呈現非線性及非完美對稱之特性，而電性符合熱離子發射及蕭特基效應，因此推測元件之奈米接點由蕭特基接點構成，且主導了元件的電性行為，故整體元件可用背對背之蕭特基二極體的等效電路來表示，而元件隨著溫度下降之電阻行為，在低溫下與變程式跳躍傳輸相符，推論其原因在於氧化鋅奈米線和鈦電極間可能存在非晶或無序的奈米接點；第二類型的元件，其室溫電阻約略為15 kΩ，而元件的電性呈現本質氧化鋅奈米線之特徵。此類型之電流-電壓曲線在小偏壓下呈線性行為，而電阻隨著溫度變化的行為與熱活化傳輸相符，因此推論奈米接點由歐姆接點構成，整體元件可用一歐姆電阻之等效電路加以表示。

Electrical properties of two-probe ZnO nanowire devices have been studied recently. Since the reduced contact area could magnify electrical contributions, the electrical properties at the nanocontacts of the nanowire devices should be investigated thoroughly. In this work, we adopted standard e-beam lithography and used 40-nm diameter ZnO nanowires to fabricate two-contact devices. Current-voltage (I-V) behaviors of these nanowire devices were obtained at various temperatures. We found that the devices could be categorized into two types according to their room-temperature resistances.

For type I devices, they have a room-temperature resistance much larger than 1 MΩ and display electrical properties at the nanocontacts. The I-V curves show nonlinear and asymmetric features, and they can be fitted with thermionic-emission theory implying Schottky contact effects. The devices could be modeled as two back-to-back connected Schottky nanocontacts. In addition, we found that temperature dependent resistances could be fitted well with the variable-range hopping theory so we proposed disorder and noncrystalline nanocontact model between the Ti electrode and the ZnO nanowire to explain our data. For type II devices, they usually have a low resistance at room temperature and they could exhibit intrinsic electrical properties of the ZnO nanowires. The I-V curves reveal a linear manner in low voltage. The temperature dependent resistances of the type II devices show thermal activated transport in ZnO nanowires. We argued that the type II devices could be modeled as a single ZnO nanowire with two Ohmic contacts connecting to electrodes.