第二型態垂直耦合量子點太陽能電池

Type-II Vertically Aligned Quantum Dot Solar Cells

吳宏明(Hong-Ming Wu)

指導教授 ： 劉維昇

元智大學/電機通訊學院/光電工程學系/碩士(2011年)

https://doi.org/10.6838/YZU.2011.00336

摘要

中文摘要
本論文主要研究InAs量子點成長於GaAs基板上，設計其覆蓋層厚度為10nm以便形成量子點垂直耦合，並覆蓋不同Sb成份之批覆層與調製Sb含量製作出第二型態量子點太陽能電池，探討其光激發螢光(PL)、場發射電子顯微鏡(SEM)、原子力顯微鏡(AFM)、穿透式電子顯微鏡(TEM)量測結果，了解其量子點表面特性、橫切面、電壓電流、轉換效率和光特性。我們摻雜Sb於覆蓋層，觀察到Sb在量子點表面形態上可以減少In-Ga Intermixing並維持量子點高度，以及改善量子點在水平與垂直面的均勻性，而在光性方面，由於量子點大小均勻性改善與減少缺陷產生，因此提升了PL光強度與降低了半高寬(FWHM)，在低溫下量測為22 meV，並與各國先進實驗室相比毫不遜色。
根據上述實驗，將Sb摻雜於覆蓋層之材料特性彙整，結合了調製Sb含量多寡可調整能帶特性，並設計出新穎式第一型態(Sb~14%)與第二型態(Sb>20%)垂直耦合量子點。在低溫10 K下做變功率光激光譜量測，其中功率由50到100mW基態波峰藍移了14.1 meV，由能態彎曲效應使得能隙變大而證實了第二型態的存在，並隨著功率的增加能帶彎曲效應也更加明顯。由於我們成長的垂直耦合量子點，在垂直方面點與點間距不到10 nm，比起水平量子點點與點間距離的二分之一還要近，並且根據Gregor Wentzel 、Hendrik Anthony Kramers 和 Leon Brillouin 三位物理科學家以量子系統波函數重新整理為指數函數，並由此方法得到波函數近似解，一般將這方法簡稱WKB，以WKB計算電子穿隧時間為0.8 ns，而量子點電子與電洞輻射複合時間為0.5~2 ns，因此造成藍移的效應除了能態彎曲之外，研判電子穿隧效應是造成波長藍移之另一因素。
量子點太陽能電池電性量測的結果指出使用InAs/GaAsSb QDSC比起典型的GaAs電池增加了8.8%的短路電流，並且延伸了在1100nm以上的外部量子效應，研判是由量子點吸收低能的光子進而延長外部量子效應，此實驗結果顯示利用Sb元素摻雜於覆蓋層，提高了量子點均勻性並使得磊晶缺陷減少，因此提高了量子點太陽能電池之製作可行性。

關鍵字

量子點 ； 太陽能電池 ； 第二型態

並列摘要

Abstract
In this thesis, we study the InAs quantum dots (QD) which are grown on GaAs substrate. The thickness is designed as a 10nm of capping layer for the formation of vertical quantum dots. Quantum dots cover different antimony (Sb) compositions of capping layers. In order to form type-II band alignment thrugh modulating Sb content to produce quantum dot solar cell device (QDSC). We investigate the quality and density of quantum dots via Scanning Electron Microscopy (SEM) and Atomic Force Microscope (AFM) images. The formation of the vertical alignment is confirmed through the use of Transmission Electron Microscopy (TEM). In order to realize the optical properties of QD we use Photoluminescence (PL) measurements.
We dope Sb material in the GaAs capping layer. Then we observe the reduction of In-Ga intermixing on the surface and preserve the height of quantum dot via Sb action as a surfcant. Therefore, GaAsSb capping layer improves the uniformity of quantum dots. Moreover, GaAsSb capping layer enhances PL intensity and lowers the Full Width Half Maximum (FWHM) to 22meV at the set condition of 10K. Compared with the advanced laboratories around the world, our FWHM value is much better. Besides, we have adjusted the band characteristics by modulating the Sb composition and designed the band characteristics of Type-I (Sb ~14%) and Type-II (Sb~20%).
The power-dependent PL measurements at the low temperature of the 10K show an obvious blue shift of 14.1meV behavior of QD ground state when the exciting power increased from 50 mW to 100 mW. Therefore, we confirm the formation of Type-II by the blue-shift behavior because of the increased band bending. The vertical distance between the two 10nm dots is smaller than the horizontal one by a factor of 0.5. Consequently, according to the WKB calculations, the electron tunneling time is 0.8ns. However, the general radiative recombination time of quantum dots is 0.5~2ns. Therefore, we conclude that the electron tunneling effect is one of the reasons for a blue-shift behavior.
This study has successfully improved the dot size uniformity and QDSC performance by capping GaAsSb overgrown layer on InAs QDs. Based on experimental results, InAs/GaAsSb QDSC shows an increase of Jsc value by 8.8 %, more than what a typical GaAs cell can do, as well as demonstrates an extended response of the wavelength up to 1100 nm because of the absorption of lower energy photons by the QDs. The results of this study confirm the feasibility of enhancing QDSC through using InAs/GaAsSb columnar dots.

並列關鍵字

Quantum Dot ； Solar Cell ； Type-II

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