以分子束磊晶法成長氮銻砷化鎵材料及應用

Molecular Beam Epitaxy of GaAsSbN and Its Applications

馬大鈞(Ta-Chun Ma)

指導教授 ： 林浩雄

國立臺灣大學/電機資訊學院/電子工程學研究所/博士(2010年)

https://doi.org/10.6342/NTU.2010.03074

摘要

本論文以具有射頻電漿源之氣態源分子束磊晶系統進行氮銻砷化鎵於砷化鎵基板上的磊晶研究；內容包括電漿源特性的研究、以不同氮源成長氮銻砷化鎵的嵌入研究、氮銻砷化鎵光學特性的研究以及砷化鎵/氮銻砷化鎵異質接面太陽電池元件的特性研究。
首先我們研究射頻電漿源操作條件與氮源粒子種類之關係。透過光譜以及質譜分析，我們認為電漿源功率對氮氣之裂解以及氮原子之激發作用甚鉅，而受激態之氮分子通量對流入電漿源之氮氣流量變化較敏感。然而無論在任何操作條件都會讓電漿源同時產生原子和分子形式的氮源粒子，故我們將電漿源通入高氮氣流量，並置一檔板於電漿源口處，強迫電漿源所產出的氮源粒子經過多次碰撞後回復到基態狀態後再進行磊晶，利用此方法我們可使用原子形式之氮源進行磊晶研究。在氮銻砷化鎵的磊晶研究中，我們觀察到以氮原子為氮源時，氮會與銻競爭嵌入，與大部分使用亞穩態氮分子源文獻報導的氮銻嵌入行為不同。且氮原子之嵌入行為可使用表面動力學解釋。而我們也在較低氮氣流量與較低功率的條件下，移開擋板，成長氮銻砷化鎵。在這種條件下，氮成分會隨著銻成分的增加而增加，與文獻報導的亞穩態氮分子源的嵌入行為相符。
當氮銻砷化鎵內的銻氮莫爾分率比為3.2~4.0時其晶格可與砷化鎵基板匹配。我們成功地使用原子氮源完成晶格匹配的氮銻砷化鎵，以吸收譜量測其能隙可低達0.8 eV。透過光激發螢光譜的分析，我們發現在低溫成長的樣品有較佳的均勻度與光學性質。故我們在低溫(420 oC)成長砷化鎵與氮銻砷化鎵異質接面太陽電池元件。由不同氮銻砷化鎵厚度(250、500、1000和2000 nm)的元件在不同退火條件下的光頻譜反應分析，我們獲知磊晶過程中會隨著厚度的增加逐漸產生缺陷，較佳的厚度在250~500 nm。但是此批樣品經過熱退火後改善有限，較佳的太陽光電轉換效率為3.6%。開路電壓過低造成轉換效率不理想，其原因為元件之表面復合電流。當我們使用二氧化矽和氮化矽材料對250 nm厚的氮銻砷化鎵太陽電池表面以及側壁作鈍化保護之後，開路電壓改善了將近50%，整體轉換效率提升了60%以上，目前最佳之轉換效率為5.7%.

關鍵字

分子束磊晶 ； 氮銻砷化鎵 ； 射頻電漿 ； 異質接面 ； 太陽電池

並列摘要

In this dissertation, we use a RF-plasma-assisted molecular beam epitaxy system to investigate the growth of GaAsSbN mixed-group V alloys. Our research topics include the characterization of plasma source, the incorporation behavior of the N in GaAsSbN, optical properties of lattice-matched GaAsSbN and the characteristic of GaAs/GaAsSbN heterojunctions on GaAs substrate.
Due to the complexity of the plasma source, we use optical emission spectroscopy and quadruple mass spectroscopy to identify the relation between the operation parameters and N-radical species. We placed a shutter in front of the plasma aperture, and obtained atomic N-radical source for GaAsSbN growth. We found that N incorporation in GaAsSbN grown with atomic N-radicals decreases with the increase of Sb flux, which is very different from that of samples grown with meta-stable molecular N-radicals. Through the study of plasma characterization, we also found that the condition with low RF power, low N2 flow rate, and without the blocking of shutter can provide a source dominated by meta-stable molecules. When meta-stable molecule source is used, the N incorporation in GaAsSbN grown with meta-stable molecule source increases with the increase of Sb flux, which is consistent with the reports in literature.
Based on the results of plasma characterization, we have successfully grown lattice-match GaAsSbN epilayers on GaAs substrates. The lowest absorption edge achieved in this study is as low as 0.8 eV. In addition, according to the results of PL measurement, we found that the samples grown at 420 oC have the strongest PL intensity resulting from better homogeneity, as compared with samples grown at other temperatures. Finally, we fabricated and characterized n+-GaAs/p-GaAsSbN/p+-GaAs heterojunctions. We found that when GaAsSbN exceeds a certain thickness, inhomogeneity related defects begin to be generated in the epilayer. After thermal annealing, the optimum GaAsSbN thickness is 500 nm and the conversion efficiency under AM1.5G solar simulator is 3.6%. The high forward-biased current of the heterojunctions results in low open-circuited voltage. After the junction passivation with SiN and SiO2, the open-circuited voltage increases by 50 % and the conversion efficiency increases by 60% to 5.7%.

並列關鍵字

molecular beam epitaxy ； GaAsSbN ； RF plasma ； heterojunction ； solar cell

參考文獻

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