負型砷化鎵含銀多層歐姆接觸結構及其應用於太陽能電池之研究

Study of Multi-layered Silver-contained Ohmic Contact Structure on N-type GaAs and Its Application to Solar Cell

張立群(Li-Chun Chang)

指導教授 ： 廖森茂 ； 吳志宏

中原大學/工學院/電子工程研究所/碩士(2008年)

https://doi.org/10.6840/cycu200800289

摘要

摘要
在本論文中，我們使用鎳/鍺/銀在負型砷化鎵上形成未含金之歐姆接觸結構，並且運用傳輸線模型法計算出歐姆接觸結構之特徵接觸電阻值，以評估其優劣。實驗結果顯示，鍺/銀系列金屬化結構能夠展現出歐姆接觸特性，並且發現鍺(20nm)/銀(150nm)在425℃一分鐘的回火條件下，能夠形成此系列之最佳特徵接觸電阻值ρc ~ 4.81×10-6Ωcm2。然後，我們添加鎳層於鍺層與銀層之下，由量測結果得知鎳/鍺/銀系列金屬化結構亦能夠呈現出歐姆接觸特性，並在鎳(10nm)/鍺(20nm)/銀(150nm)在425℃一分鐘的回火條件下可以形成本實驗最佳特徵接觸電阻值ρc ~ 4.15×10-6Ωcm2。然而，由實驗得知過多的鍺將無法形成歐姆接觸特性；而過多的鎳除了使得最佳之回火溫度提高，並且還會降低其歐姆接觸之品質。此外，我們發現在添加鎳層後，鎳(10nm)/鍺(20nm)/銀(150nm)金屬化結構之表面明顯較平坦於鍺(20nm)/銀(150nm)金屬化結構。
我們也進行上述提及之金屬化結構與傳統鎳/鍺/金/鎳/金金屬化結構在200℃氫氣流動環境下的熱穩定度探討。從實驗結果得知，鎳(10nm)/鍺(20nm)/銀(150nm)金屬化結構能夠呈現出優於傳統鎳/鍺/金/鎳/金金屬化結構之熱穩定度特性。
透過以上實驗結果，我們將最佳化歐姆接觸結構鎳(10nm)/鍺(20nm)/銀(150nm)應用至磷化銦鎵/砷化鎵/鍺三接面太陽能電池之正面電極上，並透過不同回火溫度評估此歐姆接觸電極之效能。我們分別在室溫下以一個標準太陽光以及聚光下分別量測出太陽能電池之電壓電流特性。在一個標準太陽光下之特性量測，我們發現在正面電極回火溫度425℃時，太陽能電池之轉換效率、填充因子以及最大輸出功率均有最佳之表現。此外，在聚光下的特性量測，太陽能電池之轉換效率皆能夠提升，並且其填充因子都能夠維持在相當不錯的數值~0.82。

關鍵字

銀 ； 砷化鎵 ； 歐姆接觸 ； 太陽能電池

並列摘要

Abstract
In this study, we proposed a gold-free metallization ohmic contact structure composed of nickel (Ni), germanium (Ge), and silver (Ag) deposited on n-GaAs. We investigated ohmic contact through the TLM method and calculated the specific contact resistance. Experimental results indicate that the Ge / Ag series metallurgical structure have an ohmic contact behavior, and a rather low specific contact resistance ρc ~ 4.81×10-6Ωcm2 is attained from Ge (20nm) / Ag (150nm) after annealing at 425℃ for 1 minute. Then we deposited Ni layer before Ge and Ag layers, the Ni / Ge / Ag series metallurgical structure also have an ohmic contact behavior, too, and an optimum metallurgical structure Ni (10nm) / Ge (20nm) / Ag (150nm) with the lowest specific contact resistance ρc ~ 4.15×10-6Ωcm2 can be obtained after annealing at 425℃ for 1 minute. However, with the increasing Ge thickness, the characteristics of ohmic contact would be more difficult to be displayed; for the increasing thickness of Ni layer, the best annealing temperature is increased, and the quality of ohmic contact is dropping. In addition, the surface morphology of Ni (10nm) / Ge (20nm) / Ag (150nm) annealed at 425℃ for 1 min is as smoother as that of Ge (20nm) / Ag (150nm) by incorporating with an additional nickel (10nm).
We also studied the thermal stability of the as mentioned before metallurgical structures and conventional Ni / Ge / Au / Ni / Au metallurgical structure under environmental temperature 200℃ in H2 ambient. From the experimental results, it is found that Ni (10nm) / Ge (20nm) / Ag (150nm) metal-contact has better thermal stability than the conventional Ni / Ge / Au / Ni / Au metal contact.
After the TLM experiment, the optimum metallurgical structure Ni (10nm) / Ge (20nm) / Ag (150nm) is applied to n-GaAs ohmic contact that the front electrode of the InGaP / GaAs / Ge triple-junction (TJ) solar cells with three annealing temperatures. The current-volatge (I-V) characteristics are measured under one-sun and multi-suns at room temperature. By the one-sun measurement results, we get the better solar cell performance such as conversion efficiency (EFF), fill factor (FF), maximum output power (Pmax) at the annealing temperature 425℃. Furthermore, by the multi-suns measurement results, the conversion efficiency (η) were all improved, and the fill factor (FF) still kept at a relative good values ~0.82 in these three annealing temperatures.

並列關鍵字

GaAs ； Solar cell ； Ohmic contact ； Silver

參考文獻

[5] Alan L. Fahrenbruch, Richard H. Bube, “Fundamentals of solar cells”, Academic Press, Inc., 1983, p200.

[6] W. D. Edwards, W. A. Hartman, and A. B. Torrens, “Specific contact resistance of ohmic contacts to gallium arsenide,” Solid State Electronics, 15 (1972) p387.

[7] N. Braslau, “Alloyed ohmic contacts to GaAs,” J. Vac. Sci. Technol., 19, (1981) p803

[8] H. R. Kawata, T. Oku, A. Otsuki, M. Murakami, “NiGe-based ohmic contacts to n-type GaAs. II. Effects of Au addition,” J. Appl. Phys., 75 (1994) p2530.

[9] F. Ren, A. Y. Cho, D. L. Sivco, S. J. Pearton, C. R. Abernathy, “Use of Sn-doped GaAs for non-alloyed ohmic contacts to HEMTs,” Electron. Letter., 30 (1994) p912.