在單根金屬氧化物奈米線中的電子傳輸過程

Abstract

在本篇論文中，我們研究了單根的氧化銦錫、氧化鋅、以及摻雜銦或鉛的氧化鋅奈米線的本徵導電性質。這些單晶的奈米線可以經由熱蒸鍍或雷射輔助的化學氣相沈積的方式合成。鈦/金或鉻/金微電極可利用電子束微影技術製作於單根奈米線上。利用四點量測的方式，300 K到0.25 K之間的電阻，以及70 K到0.25 K之間的磁電阻，可以被有系統的量測與分析。 氧化銦錫奈米線的值經約為110 ~ 220 奈米。從300 K到1.5 K的電阻率隨溫度的變化可以用Bloch–Grüneisen 定律加上一個低溫區的修正項來描述。這個低溫修正項反映著動態的點缺陷的存在，以及電子被這些缺陷所散射導致了電阻率隨溫度下降而有較劇烈的上升變化。 氧化鋅奈米線的值經約為90 ~ 200 奈米。雖然我們沒有特意對它們做摻雜，但成長時所自然生成的缺陷卻可以提供載子，而導致不同的電阻率。電阻率較大的樣品我們稱之為半導體性的。它們的電阻率和溫度的關係可以用三個□□i-1exp(-Ei/kBT) 項的相加來描述 (i=1, 2, 3)。其中Ei是特徵能量，且E1 > E2 > E3。電子從費米面被熱激發到導帶，以及一個分裂的雜質能帶(upper D- band)上的過程分別對應到E1以及E2項。而費米面附近的最近鄰跳躍導電則對應到E3項。電阻率最小的一個樣品則已經呈現金屬的某些特性，並且其導電是和表面相關的。 摻雜銦以及鉛的氧化鋅奈米線（直徑約70 ~ 90 nm）已經表現出簡併費米氣體(degenerate Fermi gas)的特性。我們發現70 K到0.25 K之間的中間溫區的磁電阻必須要用二維的弱局域效應才能合理的分析。而在這個溫區的兩端的低溫區以及高溫區，則分別要以一維和三維的弱局域效應來描述。在比較中間溫區的垂直磁場以及平行磁場的磁電阻後，我們可以定量分析出一個特徵厚度t，它對應到奈米線中的某種二維的結構。因此我們假設奈米線有類似中心核-外殼(core- shell)的結構。t就對應到殼的厚度。因此，當相位同調長度L□□和厚度t接近時，就會發生弱局域效應的維度的轉變。類似的情形也發生在另一個量子干涉效應，電子-電子交互作用(EEI)上。當EEI的特徵長度，熱擴散長度LT，和厚度t接近時，也會導致EEI的維度改變。由磁電阻的分析可以得到同調相位破壞率□□-1和溫度的關係（□□-1(T) ~ Tp, 1< p <1.5）。這樣的p值可能是來自於大能量交換的電子-電子散射，或是聲子維度受限時的電子-聲子散射。另外，我們發現其中一個摻雜鉛的氧化鋅奈米線的I-V曲線在低偏壓區有明顯的非線性存在。我們把它歸因於某種穿越中心核和殼的邊界的導電過程。

The intrinsic electrical transport properties of individual nanowires (NWs), including ITO, ZnO, and (In,Pb)-doped ZnO NWs, are studied in this thesis. These single-crystalline NWs were synthesized by either the thermal evaporation method or the laser-assisted chemical vapor deposition (CVD) method. Four-probe Ti/Au or Cr/Au electrodes were fabricated by the electron-beam lithography technique. The resistances between 300 and 0.25 K and the magnetoresistances (MRs) between 70 and 0.25 K of these NWs have been systematically studied. The temperature dependent resistivities, □(T), of four ITO NWs with diameters of 110 to 220 nm and lengths of a few □m long have been measured. The results indicate that the as-grown ITO NWs are metallic, but disordered. The overall temperature behavior between 300 and 1.5 K can be described by the Bloch–Grüneisen law plus a low-temperature correction due to the scattering of electrons off dynamic point defects. This observation suggests the existence of numerous dynamic point defects in as-grown ITO NWs. For the nominally undoped ZnO NWs with diameters of 90 to 200 nm, the temperature behavior of □(T) between 300 and 5 K reveals that the electrical-transport mechanisms are due to a combination of the thermal activation conduction and the nearest-neighbor hopping conduction processes. Three distinct activation and hopping contributions with discrete characteristic activation energies are observed. Above about 100 K, the charge transport mechanism is dominated by the thermal activation of electrons from the Fermi level, □, to the conduction band. Between approximately 20 and 100 K, the charge transport mechanism is due to the activation of electrons from □ to the upper impurity (D−) band. Between approximately 5 and 20 K, the charge transport mechanism arises from the nearest-neighbor hopping conduction within the lower impurity (D) band. Such unique electrical conduction behaviors can be explained in terms of the intricate material properties (in particular, the presence of moderately high concentrations of n-type defects accompanied with a slight self-compensation) in natively doped ZnO NWs. In one heavily doped NW, a surface-related conduction process manifesting the two-dimensional attributes of quantum-interference transport phenomena is observed. The carrier concentrations in our NWs have been estimated, and they were found to lie close to the critical concentration for the Mott metal–insulator transition. The indium- and lead-doped ZnO NWs with diameters of 70 to 90 nm showed behavior of degenerate Fermi gas of their resistivities, □(T). We have measured the MRs of several doped ZnO NWs between 0.25 and 70 K in magnetic fields with directions both perpendicular and parallel to the wire axes. Our quantitative analysis showed that we have to utilize the weak-localization (WL) effects of different dimensionalities to explain the MRs in different ranges of temperature. Otherwise, the MRs can not be satisfactorily described. A characteristic length, named the effective wire width, a, extracted from one-dimensional (1D) WL effect has been introduced. From the perpendicular and parallel MRs, another characteristic length, named the effective film thicknesses, t, was extracted under the framework of the two-dimensional (2D) WL effect. Hence, a core-shell-like structure inside individual nanowires is suggested. Within this model, as the electron phase-coherent length, L□, decreases with increasing temperature, a 1D-to-2D dimensional crossover of the WL effect occurs around the characteristic temperature where L□□~ a, and also a 2D-to-3D dimensional crossover occurs around another characteristic temperature where L□□~ t. The exponent of temperature, p, of the electron dephasing rate, □□-1, has been determined. The result suggests that the dephasing mechanisms could be due to the electron-electron (e-e) scattering with large energy transfer or the electron-phonon (e-ph) scattering with reduced phonon dimensionality. In addition, the core-shell-like structure has been verified from the temperature behaviors of low-temperature resistivities in a moderately high magnetic field, which demonstrated the dominating electron-electron interaction (EEI) effect. A dimensional crossover of EEI was also observed under the condition that the thermal diffusion length, LT, became close to the shell thickness t. In a lead-doped ZnO NW, the nonlinearity of the I-V curves around zero-bias is attributed to the 2D property of relatively small shell thickness and the electron motion across the core-shell interface.