探討硒化鉛奈米顆粒陣列之熱擾穿隧現象

Abstract

由於奈米晶體在合成的過程中，為了使晶體能夠穩定的成長而不互相影響，在晶體的外圍通常有一層絕緣的有機包覆層，這層包覆層會嚴重影響奈米晶體的電性。而本次實驗的目的主要是探討包覆層對硒化鉛奈米晶體電荷傳輸行為的影響。 本實驗中所使用的硒化鉛奈米晶體是利用化學方法合成，形狀為八面體、直徑約為80 nm，外面有一層包覆層材料為TOP，使得材料的電性呈現一絕緣體，元件室溫電阻約10 GΩ以上，我們將材料藉由真空熱處理的方式來去除絕緣包覆層使晶體靠近可降低元件的電阻，並藉由控制熱處理的溫度與時間可以調控顆粒間距，發現在不同的間距下硒化鉛的電荷傳導行為並不相同。在此我們引入熱擾動導致穿隧傳輸理論(fluctuation-induced tunneling, FIT)來解釋大場下以及小場下硒化鉛奈米晶體的傳輸行為，在小場下其電阻率與溫度的關係式為 ，ρ為電阻率、T為溫度，其中 ， 為顆粒平均的間距，從實驗數據我們知道硒化鉛奈米晶體的電阻率與顆粒間距成一指數關係 ；在大場下FIT解釋了電流與電壓的關係，且擬合得到的參數與小場下所得到的結果一致。然而，最後當顆粒間距足夠接近的時候，我們發現元件的電荷傳輸行為會從半導體的特性變成為金屬的特性。

The electrical transport of PbSe nanocrystal solids (nanostructured bulks) with different inter-particle distances have been studied by systematic analysis of I-V and R-T data. To fine tune the inter-particle distance, we employ the thermal and chemical treatments. It is found that the thermal treatment is more reproducible and easier to control the inter-particle distance. In the as-deposited sample, the nanocrystal solid is an insulator. After annealing at 225 oC and 250 oC, the R-T relation exhibits a semiconductor-like behavior and, at low temperature, a clear asymptotic behavior is observed. Fluctuation induced tunneling (FIT) model is used to explain the low-field and high-field behaviors quantitatively. In low-field region, R-T relation can be well fitted by the relation, . Through analyzing the fitting parameters, the inter-particle distances were estimated and it was in good agreement with the experimental configuration. The distribution of the resistivity of the samples can be explained by the inter-particle distance variation which had a relation of and it is consistent with the theoretical prediction. Additionally, the high-field behavior can be fitted well by the FIT model as well, and the fitting parameters are in agreement with those extracted from the low-field results. Furthermore, as the inter-particle distance become shorter, the samples make a transition to a metallic behavior. An insulator-to-metal transition is observed.