偏角度基板對穿隧層之效應與成長砷化鎵於鍺/矽基板上對低成本高效率多接面三五族太陽電池之應用

Abstract

為了提升三五族太陽能電池在此領域之競爭力，首先必須要增加其光電轉換效率並降低其發電成本。在此論文中，我們證明偏角度基板對三五族多接面太陽能電池中砷化鋁鎵/砷化鎵穿隧層(AlGaAs/GaAs tunnel diodes, TDs)材料特性有一定影響。當砷化鋁鎵/砷化鎵穿隧層成長於10度砷化鎵基板上時，此時穿隧層表面粗糙度可降至0.154nm並產生最佳界面狀況。實驗結果也證實此基板可有效降低砷化鎵層中氧等不純物濃度；也可減少砷化鋁鎵層中非等向性位置(anisotropic sites)，進而減少此層在成長過程中不純物含量以提升三五族太陽能電池光電轉換效率。同時，我們也證實磷化銦鎵/砷化鎵(InGaP/GaAs)雙接面太陽電池中使用砷化鋁鎵/砷化鎵穿隧層(成長於10度砷化鎵基板上)可得到較高光電轉換效率(~20%)與外部量子效率(InGaP 子電池: 82% ; GaAs子電池: 85%)。由高聚光測試(~185×)結果發現，使用砷化鋁鎵/砷化鎵穿隧層之磷化銦鎵/砷化鎵雙接面太陽電池之光電特性較使用磷化銦鎵/砷化鎵穿隧層之雙接面太陽電池來的高。此外，如果能將砷化鎵等三五族材料成長於矽基板上取代傳統以鍺為基板的模式，將可大大降低三五族太陽能電池發電成本。本研究發現將變溫砷界面層成長於經過650度熱退火之鍺/矽基板可有效改善隨後成長之砷化鎵層磊晶品質(表面粗糙度:1.1nm；缺陷密度: ~2x107cm-2)。本研究亦證實由於砷-鍺、鎵-鍺間鍵節能量之不同與低砷流量之緣故，使砷化鎵/鍺/矽異質結構間之砷與鍺原子內部擴散可有效被抑制。此實驗結果證明變溫砷界面層成長於鍺/矽基板上對未來欲成長三五族微電子元件或光電元件於矽基板上將具有極大潛力。

To further promote the competitiveness of III-V multijunction solar cells in the field, the conversion efficiency of solar cells has to increase while their cost must be reduced. In this thesis, we have demonstrated that the material properties of the P++-AlGaAs/N++-GaAs tunnel diodes (TDs) could be affected by misoriented GaAs substrates for III-V multijunction solar cell applications. The best surface morphology (0.154nm) and interface sharpness for the TDs were obtained on the (100) tilted 10° off toward [111] GaAs substrate. TD materials grown on this substrate can efficiently reduce oxygen-incorporation in N++-GaAs layer, and also reduce the anisotropic sites for oxygen-incorporation in the P++-AlGaAs layers. We also proved that InGaP/GaAs dual junction solar cells with a P++-AlGaAs/N++-GaAs TD grown on 10°off GaAs substrates exhibit superior photovoltaic conversion efficiency (~20%) when operated at one sun, and produces higher EQE (~82% for InGaP top cell and 85% for GaAs bottom cell) as compared to the P++-GaAs/N++-InGaP TD. The cell design with a P++-AlGaAs/N++-GaAs TD grown on 10°off GaAs substrates also displays superior I-V characteristics when these solar cell devices were operated at higher concentration ratios (~185×). On the other hand, production cost of III-V multijunction solar cells can be largely reduced while III-V materials, such as GaAs epitaxy, are grown on Si substrates instead of traditional Ge substrates. We have demonstrated that the As prelayer grown using graded-temperature technique on the Ge/Si substrate annealed at 650°C effectively improves the epitaxial quality of GaAs epitaxy (roughness: 1.1 nm, dislocation density: ~2x107cm-2). Furthermore, the interdiffusion of Ge and As atoms in the GaAs/Ge/Si heterostructure can be effectively suppressed by the graded-temperature As prelayer because of the difference in energies between As-Ge and Ga-Ge bonds and low As flux. These results suggest that the graded-temperature As prelayer grown on Ge/Si substrate has great potential for use in the growth of III-V nanoelectronic devices and optoelectronic devices on the Si substrate.