太空用砷化鎵/鍺太陽電池之研究 --以原子層磊晶法生長非活性化砷化鎵/鍺介面

Abstract

因為GaAs太陽電池有高轉換效率、對溫度變化與宇宙射線有良好抵抗力等 優點，所以非常適合用於人造衛星上。然而GaAs太陽電池也有一些缺點， 例如GaAs基板太貴，太重及太脆。因此便宜、有相似晶格常數的Ge晶片成 為最佳的代用品。將在GaAs晶片上的 P/N接面和在 N型Ge晶片上的GaAs 的 P/N接面相比，若在GaAs/Ge介面上可以得到額外的光電壓，則稱這是 一個活性化 Ge晶片。然而這一個活性化Ge晶片的結構，不會提供任何額 外的能量輸出，反而使得轉換效率下降。又因為Ge是非極性材料，而GaAs 是極性材料，所以在Ge晶片上生長GaAs磊晶層時，常常會得到一個表面很 糟的複晶結構。為了得到一個非活性化Ge晶片及良好的GaAs磊晶層，本研 究採用ALE法在Ge晶片上生長一層GaAs的緩衝層。並且改變不同的磊晶溫 度及不同的反應前導物（AsH3及TBAs）。結果發現 ALE緩衝層可以消除Ge 晶片活性化的問題，並且也能改善GaAs磊晶層的表面平整度。 For spacecraft power generation, GaAs solar cells have the advantage of high conversion efficiency and high radiation and temperature tolerance. But GaAs solar cells have several drawbacks such as material cost ,fragility and weight. Germanium has been proposed as an alternative, robust and cheap substrate for MOVPE. In fact , Ge provides a good lattice match with GaAs and large-area slices are available. Extra photovoltage can be generated at the GaAs/Ge interface ( called active-Ge ), in cascade with the GaAs p/n junction, when the GaAs p/n junction is grown on an n-type Ge substrate. However, this active-Ge structure does not provide any extra power output and , in fact, reduces the total efficiency . Since Ge is nonpolar material and GaAs is polar material, when we grow GaAs epilayers on Ge substrates, the GaAs epilayers are often in polycrystaline form and the morphology is poor. In order to get a passive GaAs/Ge interface and good GaAs epilayers,this study used the ALE method to grow GaAs buffer layers on Ge substrates.Different growth temperatures and As precursors(AsH3 and TBAs) were used. It is found that the ALE buffer layers can eliminate the active Ge substrate and reduce the dislocations on GaAs/Ge interfaces. And the morphology of GaAs epilayers are very good.