Transport in chemically reduced graphene oxide and GaP1-xSbx

本篇論文主要在研究奈米結構中之電性及載子傳輸機制，內容包含下列兩個主題： 1. 還原氧化石墨烯之傳輸特性研究： 本論文第一個主題為研究經過化學過程還原的氧化石墨烯中之載子在4.5 K 到296 K的溫度範圍間所展現的傳輸性質。在以往的研究中還原氧化石墨烯之載子傳輸行為可能隨著樣品有variable range hopping及thermally-activated transport兩種方式。我們所研究之樣品是經長時間的化學過程和熱過程充足還原，在此種經充足還原之氧化石墨烯，我們觀察到其載子是遵守Arrhenius-type的thermally-activated transport行為而不是variable range hopping行為。 這個工作中的另一個部份是比較不同厚度的還原氧化石墨烯其電阻(R)隨著厚度不同有何變化。我們使用一個相當於並聯電阻的模型來解釋其層數不同所造成的電阻值差異。另外，我們也作了不同厚度的樣本其電阻對溫度(R-T)關係之比較。讓R-T特性不同的原因，推測是由存在還原氧化石墨烯層與層之間的電子-電子及電子-聲子交互作用所造成。 雖然經化學還原之氧化石墨烯其結構相較於石墨烯已有許多雜質與缺陷，但仍然保留了一些石墨烯特殊的性質。經過充足還原的還原氧化石墨烯樣本在真空中也出現了石墨烯特殊的雙極性現象。在電阻對閘極電壓的關係圖中，電中性點稍微往正閘極電壓處偏移，顯示了在還原氧化石墨烯中電荷雜質的存在。 2. 銻磷化鎵之傳輸特性研究： 本論文的第二部分則是研究III-V族合金銻磷化鎵在溫度4.2 K至80 K間的電性及傳輸性質，由R-T資料的分析顯示了本銻磷化鎵樣品在溫度低於33 K時的載子傳導方式為variable range hopping；在經過34 K到48 K的一個過渡區間後，當溫度上升至超過49 K，主要傳輸方式轉變成了thermally-activated transport。上述分析結果可由載子濃度對溫度的變化趨勢驗證，當溫度低於50 K時，載子濃度約略是隨著溫度下降而變大，此為hopping transport regime才有的行為；在溫度升高至50 K到80 K之間後，載子濃度則暫時不再隨溫度變化，顯示其可能進入了thermally-activated transport regime。This thesis is focused on the electronic properties and the transport mechanism of carriers in nanostructures. It consists of the following two parts: 1. The transport in chemically reduced graphene oxide (rGO): The first part of this study was focused on the transport behavior of carriers in rGO films over a temperature (T) range between 4.5 K and 296 K. Both variable range hopping (VRH) and thermally-activated transports were reported in previous studies. An Arrhenius-type electron transport behavior rather than a VRH one is discovered from our rGO samples almost fully-reduced by the chemical and thermal processes. The resistance (R) dependence on the thickness of rGO layers, i.e. on the number of the graphene oxide layers, was also investigated. A parallel resistance configuration was suggested to explain the observed R dependence on the film thickness. Furthermore, the comparison of the R-T features from three rGO samples with different thicknesses was conducted. The possible causes giving rise to the R-T characteristics may be ascribed to the electron-electron and electron-phonon interactions existing between the interlayers and at the interface between the rGO film and the SiO2/Si substrate. The unique ambipolar characteristic from a pristine graphene was also observed on our rGO films subject to a small gate voltage (Vg) between -0.3 V and 0.3 V under vacuum. A shift of the charge neutrality point to a positive gate voltage in the resistance-gate-voltage (R-Vg) relation indicated the existence of the charged impurities in the rGO films. 2. The transport in gallium phosphide antimonide (GaP1-xSbx): The second part of this thesis was the study of the electronic and transport properties of the III-V-based alloy GaP0.71Sb0.29 . The analysis of the R-T data below 49 K indicated the transport mechanism was dominated by hopping conduction. When the temperature increased to above 49 K, the thermal energy is high enough to activate the localized carriers, leading to a thermal-activation-dominant transport behavior. The relation between carrier concentration (n) and temperature also pointed to the same result as the analysis of the R-T data. The carrier concentration increased with the decreasing temperature below 50 K, revealing the feature of the hopping transport regime. However, the temperature independence of n beyond 50 K probably indicated a transition to the thermally-activated transport regime