改善矽量子點埋入式氧化鋅多層膜與特性研究

Abstract

量子點近年來被廣泛地研究及運用在各式各樣的電子元件、光電元件之中，而矽量子點大幅被運用在記憶體、發光二極體、光感測器與光伏元件中，而在眾多材料裡我們選用矽量子點，因為矽元素在地球上含量高、對環境友善及擁有其餘量子點的優點。而目前較多的研究是將矽量子點埋入到矽相關的介電材料中，雖然在光學特性上有達到上述的優點，但在電子的傳輸上往往受到這些介電材料的高電阻特性而備受限制，而使整體的元件特性無法達到預期。而我們使用氧化鋅來取代這些高阻值的介電材料，氧化鋅的寬能隙不僅用來侷限量子點同時寬能隙也增強了短波段的吸收，故這樣的結構具有極大潛力來改善現今太陽能電池的效率。在2011年，我們提出將矽量子點埋入到氧化鋅薄膜，而使用氧化鋅與矽多層膜的結構，但是在退火後薄膜產生局部突起的現象，經由實驗發現矽量子點從非晶轉變成結晶態時內部的應力增加而造成氧化鋅薄膜的突起。 此研究利用共濺鍍方式調製出三種多層膜結構以消除薄膜局部突起的現象。第一種結構，在矽與氧化鋅堆疊的結構內在矽沉積時共濺鍍二氧化矽(SiO2)靶，提供適量的氧以形成矽過多氧化矽層(Si rich oxide, SRO)，而氧會在矽量子點周圍形成氧化矽SiOx做為緩衝層以降低應力所造成的影響。第二種結構，利用漸變式矽濃度氧化鋅薄膜(Gradient Silicon in ZnO multilayer，GSZO-ML)，以分散矽結晶時所產生的應力，而消除薄膜突起的問題。第三種結構，我們再使用電特性較好的低濃度矽摻雜氧化鋅(Lowly Si doped in ZnO，LSZO)取代氧化鋅，並根據第一部份的結果以過多矽濃度氧化鋅(Si Rich in ZnO，SRZO)取代原有的矽薄膜，兩種薄膜材料交互堆疊20個週期，再經過退火形成非晶矽量子點，研究此種多層膜光學特性與電特性並沉積於p型矽基板上研究其光伏特性，最後再使用混氣(N2: H2=95:5)退火使其光電特性更加提升。

Quantum dots are widely employed in many electric devices and photonic devices, such as memory, light emitting diodes, photodetectors and photovoltaic in recent decades. Among many materials, the Si quantum dot (Si QD) thin films are potential candidates due to the abundance, environment friendly and keeping the advantages in another kinds quantum dots. In many researches, the Si QDs often are embedded in silicon based dielectric materials. Although the optical properties have advantages mentioned above in Si QD thin films, the transport of carriers are limited by the high resistivity of dielectric materials. The performances are not as expected. Therefore, zinc oxide (ZnO) is adopted to replace the high resistivity dielectric materials since the wide-bandgap (3.1-3.3eV) property which can not only confine the Si QDs but also enhance the light absorption of short wavelength so that these had great potential to improve efficiency of photovoltaic device. In 2011, we proposed Si QDs embedded in ZnO matrix by utilizing Si/ZnO multilayer thin film structure. However, the occurrence of local film prominences in ZnO thin films is observed, which is due to the increased interior film stress resulting from the phase transformation of a- to nc-Si QDs. In this study, we propose three multilayer structures by co-sputtering to eliminate the local film prominences. In first structure, the SiO2 is co-sputtered with Si to raise the content of oxygen and form the Si rich oxide (SRO) in the multilayers of ZnO and Si alternate deposition. The SiOx would form to be buffer layer to reduce the strain produced by c-Si QDs. In second structure, we used the gradient concentration of Si in ZnO and formed GSZO thin films to disperse the strain produced by c-Si QDs. In the third structure, we used the lowly Si doped ZnO (LSZO) to replace the ZnO due to the better characteristics than ZnO and Si Rich in ZnO (SRZO) to substitute the Si layer because of the result in first part. There were 20 periods alternate deposition by LSZO and HSZO in the multilayer. After annealing the amorphous Si QDs would form. We analyzed the optical and electrical properties in this multilayer and deposited it on the p-type Si substrate to measure the characteristics of photovoltaic device. Finally, we used the forming gas (N2: H2=95:5) annealing to enhance the photonic properties.