第二型態垂直耦合量子點覆蓋銻砷化鎵應力緩衝層之載子生命期研究

Carrier Lifetime Investigation in Type-II Vertical Aligned InAs Quantum Dot Structure with a GaAsSb Strain-Reducing Layer

王嬿婷(Yen-Ting Wang)

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(2012年)

摘要

由於天然資源的消耗與能源危機意識的抬頭，世界各國莫不致力於發展替代能源。而光電太陽能具有蘊藏豐富及乾淨無污染之優勢，因此如何有效率的開發利用此一能源，已是各先進實驗室當前最重要的發展目標。
本論文針對量子點覆蓋層之研究，並提出嶄新分離光激載子之構想。即於砷化銦/砷化鎵系統中，藉由銻元素摻雜於砷化鎵，使之成為銻砷化鎵覆蓋層，將量子點能態結構由第一型態轉為第二型結構(Type-II Structure)，藉此分別侷限光激電子與電洞於量子點與覆蓋層中，以減少載子輻射複合以及歐傑複合效應。
其中垂直耦合量子點結構可應用於不同光電元件，如量子點雷射以及太陽能電池。 但不同光電元件需有不同相對應之載子生命週期。有鑑於含銻量子點不同元件應用上，缺乏相對應之載子生命期研究；此外，Physical Review Letters, 78提出多層砷化銦量子點高耦合程度形成之中間能帶結構，將提升低能量光子之吸收效率。載子亦可透過中間能帶結構傳導至太陽能電池之P/N接面，增強量子點太陽能電池之轉換效率。其中影響太能能電池轉換效率之因素可分為p 層材料、n 層材料、載子生命期及擴散長度。

研究中我們發現，垂直耦合量子點結構的穿隧效應與第二型態能帶結構，皆能延伸量子點載子生命期，論文中並將兩種效應結合，使得將載子生命期得以再延伸。並結合使用變功率光激光譜(Power dependent Photoluminescence spectra, PD-PL)與時間解析光激光譜(Time-Resolved Photoluminescence, TRPL)量測，區別第一型態與第二型態能帶結構，本論文證實同時具備垂直耦合機制與第二型態能帶結構的量子點試片，可擁有最長之載子生命期。因此垂直耦合第二型結構之量子點結構可有效減少電子電洞波函數重疊機率之特性，非常具有學術研究價值，預期將可帶來嶄新的研究方向並改善提升太陽能電池光電轉換效率。

關鍵字

第二型態能帶結構 ； 垂直耦合量子點 ； 載子生命期

並列摘要

Due to the awareness of natural resources shortage and energy crisis, nations around the world all devoted in the development of alternative energy source. Solar power has the advantages of cleanness and abundance; therefore how to exploit this energy efficiently has become the most important development target. The novel idea of separated photo-excited carriers by modifying the energy band structure from type-I to type-II structure via antimony element is introduced. In type-II structure, the photo-excited electrons and holes are separate confined in QDs and overgrown layer, respectively. Therefore, this configuration can help reduce the carrier radiative recombination and Auger effect.
Besides, Physical Review Letters, 78 point out the intermediate band formation in vertical align quantum dot structure will increase absorption efficiency with low-energy photons. Intermediate band can also make carriers transfer to P/N junction quickly, result in the enhancement of solar cell conversion efficiency. The solar cells conversion efficiency influence factor can be separate into four parts：p layer、n layer、carrier lifetime and diffuse length. In this work, we focus on prolonging carrier lifetime to improve solar cell efficiency. Antonio Luque research group have already confirmed that long carrier lifetime reduce the opportunity of electron-hole recombination to improve solar cell efficiency.
Carrier dynamic of the vertically aligned InAs/GaAsSb quantum dot structure is analyzed by Time-Resolved Photoluminescence (TRPL) in this work. Structure of InAs columnar quantum dots with GaAsSb overgrown layer is first proposed for elongated carrier lifetime in photovoltaic application. Both the tunneling effect of columnar quantum dot structures and the type-II energy band alignment can increase the carrier lifetime in this novel structure. Power dependent PL and TRPL measurement are employed to characterize the optical properties of typical InAs/GaAs type-I and InAs/GaAsSb type-II vertically aligned quantum dot structure. The longest carrier lifetime is demonstrated in the columnar InAs/GaAsSb type-II band structure.

並列關鍵字

Type-II band alignment ； Vertical align quantum dot ； Carrier lifetime

參考文獻

[39] 吳宏明, 元智大學光電工程研究所碩士論文, “第二型態垂直耦合量子點太陽能電池” (2011).

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