本論文之研究目的著重於改善成核層之磊晶條件以達到改善以鍺材料為主的太陽能電池所常發現之問題，如APDs 和鍺原子的擴散問題。因此我們可以將這些調整好地磊晶參數應用於未來多街面太陽能電池中以提升整體效率。在研究中，我們利用原子力顯微鏡,高解析X射線繞射儀以及光激發螢光光譜等量測設備來探討所成長的磊晶品質以及組成。
首先，我們利用有機金屬氣相沉積法成長砷化鎵成核層於Ge/Si 及Ge基板上。接著我們藉由調整五三比，成長溫度，成核層厚度及基板傾角來改善成核層的磊晶品質。我們發現在低五三比的情形下，其水平成長速率會大於垂直成長速率，使得其成長為一平坦表面，經由實驗證實，再過低的五三比情形下，會有碳汙染的情形發生，在本實驗中得到最佳的五三比為5，我們也發現在低成長溫度的環境中，因為低溫造成其原子之動能也較低，使得其原子擴散長度也會隨之減少，也因此解決了擴散的問題，但經由實驗也發現，當溫度過低的時候，原子會沒有足夠的能量去排列，使得表面形貌變差，而在本實驗中測得之最佳成長溫度為370度。在成核層厚度的實驗中，我們透過分析儀器得知最佳成核層厚度為23.5 nm，最後我們也比較了不同傾角之影響，我們發現當傾角增大時，會使得其成長模式為梯流動成長模式(step-flow)，進而使得APDs沒辦法生成。經過實驗比較得知，6度傾角的磊晶品質是較好的。
我們也研究了另一個成核層的材料，磷化銦鎵。文獻中顯示當磷化銦鎵之鎵原子組成變多時，會導致箭頭型之缺陷增多。所以我們將其組成調整成In0.503Ga0.497P。而我們也對這個材料做一系列分析，透過實驗我們發現，其成長溫度過低會造成其成長源磷化氫分解不完全，使得其表面形貌變糟糕，因此我們使用高溫成長，在高溫的環境下，其材料有序度也會呈現偏向無序化，這也會使得其電特性變好，而透過實驗得知其最佳之成長溫度為675度。而在成長五三比變化的實驗中，我們發現在此成長溫度下其有序度變化幅度很小，因此我們最後選用其表面形貌及磊晶品質較佳之五三比500.4。從變化成核層厚度之實驗中也發現成核層厚度為25 nm之試片，其磊晶品質及光特性皆較佳。最後我們比較兩種成核層材料，透過實驗結果發現，由於磷原子相對於砷原子有較短的擴散長度，能夠解決擴散問題，且成長磷化銦鎵材料在鍺基板上時，也能夠有效的改善APDs的問題，因此我們推論成長磷化銦鎵在鍺基板上，會得到較佳的光特性與磊晶品質，並期望能使之後成長之太陽能電池效能提升。
我們接著利用砷化鎵最佳成核層條件成長太陽能電池，我們發現其試片表面並非鏡面般的表面，因此我們推論其太陽能電池之效能並非太好，我們希望能透過增加銦原子的含量以達到晶格匹配的效果，使得其太陽能電池之光電流能得以提升，並透過實驗結果得知，當我們增加0.5%的銦原子時，其磊晶品質為最佳的，所以我們將先前成長之太陽能電池增加0.5%的銦原子，所測得之短路電流從7.19 mA/cm2提升至8.116 mA/cm2，而其效率也從2.02%提升至3.02%。我們也期望改變預處理之方式來增加太陽能電池之效能，透過實驗得知，當我們使用先通入鎵原子再通入砷原子之預處理方式，能使得其填充因子從53.196%大幅提升至68.718%，效率也因此從3.02%微幅提升至3.5%。
未來我們將會使用磷化銦鎵取代砷化鎵作為成核層之材料，成長為一太陽能電池，並試著將其基板置換為6度矽基板，以達到減少基板成本之目的，且接著將最佳化磷化銦鎵子電池，最後完成多接面太陽能電池之製作。

In this thesis, the main purpose of our research is focused on tuning the epitaxial conditions of nucleation layer to improve the problems of Ge-based solar cells such as APDs and Ge outdiffusion. Therefore, we could tune the epitaxial conditions, and we can apply these parameters to form the high efficiency tandem solar cells in the future. Plenty of material characterization techniques such as atomic force microscopy (AFM), high resolution X-ray diffraction (HR-XRD), and photoluminescence (PL) system have been used to study the quality and characteristics.
First, we tried to grow the GaAs nucleation layer on Ge/Si and Ge substrate by MOCVD. Then, we adjusted the V/III ratio, growth temperature, buffer thickness, and misorientation angle to improve the buffer quality. We could observe that when the growth in the low V/III ratio ambience, the lateral growth rate was higher than the vertical growth rate, which would form the flat plane. From the experiments results, if the V/III ratio was too low, it would have the carbon pollution problem and the best V/III ratio was 5. We also observe that when the epitaxial process was in low temperature ambience, the Kinetic energy would be decreased and further cause the atoms diffusion length decreased. But if the growth temperature was too low, the atoms would have not enough energy, so that they could not migrate to find their appropriate sites. And the surface morphology would be worse. We concluded that the appropriate growth temperature was 370℃. From the experiments of varying the buffer thickness, we know the best buffer thickness was 23.5nm through the analysis systems. Finally, we also compared the misorientation angle. From the analysis, we could observe that the 6 degree substrate could let the growth mode become step-flow mode, and the number of APDs would be eliminated. From the results, we could know that the 6 degree substrate was better than the other.
We have also studied another nucleation layer material, InGaP. According to the literature, when the layer composition of InGaP got more Ga-rich, the number of arrowhead defects would be increased. So we tuned the layer composition of InGaP to In0.503Ga0.497P. We also have done a series of researches about the InGaP. Through the experiments results, we could observe that when we decreased the growth temperature, the roughness would be increased. It was due to the reduction of PH3 decomposition. Therefore, we adopted the high growth temperature to grow the samples. The material would have the disorder tendency in the high growth temperature ambience. It would also let the solar cell response enhanced by increased minority carrier properties. From the above analysis, we concluded the growth temperature 675℃ was the appropriate growth temperature. Then, we tried to tune the V/III ratio of InGaP material. From the above experiments, we could observe that all the degree of order of InGaP was relatively low. The suitable V/III ratio was 500.4 to achieve the better roughness and crystal quality. In the experiments of varying the buffer thickness, we could observe that the sample with 25 nm buffer layer was the best for both the enhancements of epitaxial and optical characteristics. Finally, because of the phosphorus diffusion length was shorter than arsenic, the InGaP buffer could prevent the Ge diffusion problem. When we used the InGaP buffer, it could eliminate the APDs problem. Therefore, we concluded that the growth of the InGaP nucleation layer on Ge substrate, the optical and crystalline quality were all be enhanced. And we also hope it could enhance the cell performance.
In the following, we used the best parameters of GaAs nucleation layer to grow a solar cell, but we could observe that the surface was not mirror-like. The reason of that might be the worse cell performance. Hence, we incorporate 0.5% indium into GaAs base layer and emitter layer to solve the lattice mismatch problem, so that the short circuit current would be increased from 7.19 mA/cm2 to 8.116 mA/cm2, and the efficiency would also be enhanced from 2.02% to 3.02%. We also tried to change the preflow style to improve the diffusion problem, and enhance the cell performance. When we utilized the preflow Ga ML and As ML style, the Ga vacancies of GaAs film would be decreased, and the diffusion length would also be decreased. Because the adoption of this preflow style, the fill-factor would be increased from 53.196% to 68.718%, and the efficiency would also be increased from 3.02% to 3.5%.
In the future, we will use the InGaP material instead of GaAs material as the nucleation layer material. Then, the growth of a InGaP buffer solar cell will be continued. And we can further use the 6∘Si substrate to grow a solar cell in order to achieve the purpose of reduce the cost of solar cells. And then we will optimize the InGaP subcell. Finally, we will grow a tandem solar cell.

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[1]X. Yu, L. Scaccabarozzi, A. Lin, M. Fejer, and J. Harris, "Growth of GaAs with orientation-patterned structures for nonlinear optics," Journal of Crystal Growth, vol. 301-302, pp. 163-167, 2007.
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Bibliography-Chapter5
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