Researchers at the University of California, Los Angeles, have come up a with a new and simple way to control the optical properties of buried indium arsenide (InAs) quantum dots by inserting gallium arsenide antimonide (GaAs(Sb)) cladding layers above and below the dots. The technique allows both the shape and size of the dots to be controlled as well as the wavelength of light they absorb and emit – results that will be important for next-generation solar-cell applications.
“We decided to study a new system in our lab: InAs QDs buried within AlAsSb barrier layers,” said team member Meng Sun. “This semiconductor nanostructure could be ideal for making intermediate-band solar cells (IBSCs) because it is highly efficient at converting solar energy into electricity, according to theoretical calculations. The fact that we are able to fine-tune the optical properties of the QDs within these solar cells by simply adding GaAs(Sb) cladding layers means that we now have a whole new level of control over these materials.”
These QDs should also have a type-II band structure (where only one of the electronic charge carriers is actually confined within the QD) – a prediction that the UCLA has now confirmed using power-dependent and time-resolved photoluminescence measurements. Such a band structure is especially exciting for IBSCs, says Sun, because the longer carrier lifetimes therein can further help increase solar efficiency.
Best cladding layers
The researchers obtained their results using atomic force and tunnelling electron microscopy to study the structural properties of the QDs with differing configurations of cladding layers. They then characterized the optical properties of the dots using spectroscopic techniques. The measurements revealed that the ground state transition energy of the QDs increases linearly with the cube root of the light excitation intensity – behaviour that is typical of a type-II band structure. The experiments allowed the UCLA team to identify what types of GaAs(Sb) cladding layers are best for optimizing the InAs quantum dots’ light-emitting properties.
The semiconductor nanostructures the team created were all grown using molecular beam epitaxy, a technique that allows for incredible control over the thickness and composition of the growing layers. “We are able to interrupt the usual growth of our InAs/AlAsSb structures just before and after we grow the InAs QDs and insert the GaAs(Sb) cladding layers,” explained Sun. “The GaAs(Sb) layers are just 1.4 nm thick, but we found that this is big enough to induce large changes in the behaviour of the QDs.
“The original motivation behind this study was our desire to create next-generation solar cells in the form of IBSCs,” he told nanotechweb.org. We have now not only confirmed that it is possible to actually create InAs/AlAsSb QDs, but that we can enhance their optical performance using GaAs(Sb) cladding layers.”
Sun says that he and his colleagues will now start building IBSC devices using their new material system and look at how these perform.
The current work is detailed in Applied Physics Letters.
About the author
Belle Dumé is contributing editor at nanotechweb.org.
美國研究人員發現，在砷化銦(InAs)量
子點的上下方插入銻砷化鎵(GaAsSb)包覆
層(cladding layer)，能控制該量子點的光學
性質。這項簡單的新技術使研究人員不僅能
控制量子點的形狀與大小，亦可操縱量子點
的吸收與發光波長。這項成果對於次世代太
陽電池應用的發展而言相當重要。
這項由加州大學洛杉磯分校(UCLA)的
Meng Sun 等人所完成的研究，對象為埋嵌
於砷銻化鋁(AlAsSb)阻障層內的 InAs 量子
點。Sun 指出，這種半導體奈米結構適合用
來製作中間能帶太陽能電池
(intermediate-band solar cell, IBSC)，因為理
論預測此結構能高效率地將太陽能轉換為
電能。該團隊僅加入GaAs(Sb)包覆層，便能
微調太陽電池內量子點的光學性質，顯示對
於這些材料的性質擁有全新層次的控制能
力。
UCLA 團隊透過用功率相依及時間解
析光致發光實驗，證實這些量子點具有第二
型(type-II)能帶結構（即只有一種電荷載子
被侷限在量子點內）。此類結構由於載子生
命週期較長，有助於提升太陽電池效率，特
別適合 IBSC 的應用。他們以原子力顯微鏡
和穿遂式電子顯微鏡研究搭配不同包覆層
之量子點的結構特性，並以光譜技術探究其
光學特性。測量結果顯示，量子點的基態躍
遷能量與光激發強度的立方根成正比，符合
第二型能帶結構的行為。
該團隊也透過此實驗確認何種GaAs(Sb)
包覆層能最佳優化 InAs 量子點的發光性
質。他們使用分子束磊晶成長此半導體奈米
結構，該技術可精準控制層厚與組成。Sun
表示，他們能中斷 InAs/AlAsSb 基板的成長
過程，在生長InAs量子點前後插入GaAs(Sb)
包覆層，包覆層的厚度僅 1.4 nm，但已足以
改變量子點的行為。
Sun 表示，該研究動機原先為求開發下
一代 IBSC 太陽電池，而他們現在已能製作
InAs/AlAsSb 量子點，並且可藉由GaAs(Sb)
包覆層加強其光學表現。該團隊目前計畫使
用此新材料系統打造 IBSC 裝置並測試其效
能。詳見 Appl. Phys. Lett. 102, 023107
(2013)。
原始網站：
http://nanotechweb.org/cws/article/tech/52198\
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