氧化鋅薄膜之低溫電性傳輸行為研究

Abstract

在氧化鋅材料中，載子的傳輸性質一直是一項極重要且基礎的研究課題。在本論文中，我們藉由觀察室溫到液態氦整個溫度範圍的電阻率跟溫度的關係來揭示基本的氧化鋅薄膜傳導機制。 我們對一系列由射頻濺鍍時通純氬氣或通氬氣和氧氣的混合氣體沈積的氧化鋅薄膜執行四點直流電阻量測，發現氧化鋅薄膜的室溫電阻率會隨通過氧氣流量的減少而變小。這暗示著在氧氣不足情況下所濺鍍出來的氧化鋅薄膜可能會因為被視為施體雜質的氧空缺增加而使導電載子濃度變大。我們也由電阻率跟溫度關係知道在低於100 K溫度下，氧化鋅薄膜的導電機制是由變程跳躍( Variable Range Hopping )機制所主導而非近程跳躍( Nearest Neighbor Hopping )機制主導。 除此之外，在濺鍍沈積時通過混合氣體(氬氣和氧氣)的氧化鋅薄膜會有從Mott VRH 過渡到 Efros-Shklovskii VRH的現象發生。而且我們的實驗數據還能被Mott VRH與ES VRH的公式在所對應的不同溫區中擬合的相當好。經由擬合結果，我們可以得到VRH的特徵溫度。然後由已知的特徵溫度，我們就可以估算其它VRH的參數值如載子的平均跳躍距離、平均跳躍能量和庫倫能隙等。最後，我們討論這些得到的值是否符合VRH規範。

The electrical transport property is always the important and fundamental studies in both the pure ( undoped ) and doped zinc oxide thin films. In this work, we revealed the underlying conduction mechanisms in ZnO thin films by observing the temperature behavior of electrical resistivity, from room temperature down to liquid-helium temperature. We have performed the four-probe dc resistance measurements on a series of zinc oxide thin films deposited by rf sputtering in pure argon gas or in mixture of argon and oxygen atmosphere. Room temperature electrical resistivity of these films has been found to decrease with decreasing the flux of oxygen gas. It could imply the higher carrier concentration in oxygen deficient films owing to increasing of the donor impurities like oxygen vacancies or zinc interstitials. The conduction mechanism in the temperature below 100 K was dominated by Variable Range Hopping ( VRH ) mechanism instead of the Nearest Neighbor Hopping ( NNH ) mechanism.Moreover, a clear crossover from Mott variable range hopping to Efros-Shklovskii variable range hopping phenomenon occurs in the zinc oxide thin films depositing in mixture gas (argon-oxygen). The experimental data was fitted very well with the equations of Mott VRH and ES VRH in different temperature regimes, respectively. We can extract characteristic temperature of VRH from results of fitting and estimate other parameters, such as average hopping distance, average hopping energy, and coulomb gap for a given characteristic temperatur. Finally, we discussed if the quantities satisfied the VRH criterions.