太陽能多晶矽晶體生長的晶粒控制及晶粒競爭之研究

Grain Control and Grain Competition of Multicrytalline Silicon Crystal Growth for Photovoltaic Application

翁煜庭(Yu-Ting Weng)

指導教授 ： 藍崇文

國立臺灣大學/工學院/化學工程學研究所/碩士(2013年)

https://doi.org/10.6342/NTU.2013.02194

摘要

太陽能多晶矽的品質決定在長晶時的成核與晶粒控制，研究發現從未完全熔化之碎矽料長出的晶碇，初始晶粒多且小，且此晶碇錯排的增長速率較慢，可使得整體晶碇品質較好且均勻，所以，了解這種生長模式是主要的目標。在本論文中利用球型子晶作為成核層並模擬類似的生長模式，球型子晶為一種隨機晶向晶種，可提供晶粒在生長初期為公平競爭，吾人可藉此了解晶粒生長時的競爭機制及其生長癖好。在這個實驗中共觀察到三種晶粒競爭的方式，且發現初始的non-Σ晶界比例高達70 %。此外，吾人設計了三個實驗嘗試以更簡易的方式來控制晶粒，使底部初始的晶粒較小且獨立生長。在矽粉石英粉塗佈實驗中，最高可誘導出30 % 左右的non-Σ晶界，並從晶粒上可看出有較小的晶粒，可惜的是，仍可看到在塗佈區域有幾個較大晶粒生長，原因可能來自於過冷度大，晶粒成核時將熱吸走，使得成核點變少導致。在凹槽軟床的實驗中，non-Σ晶界的比例非常低，多為Σ3孿生晶界，其生長較類似於dendrite 模式。而在底部置入石英管的實驗中，可以在底部誘導出20 % 左右的non-Σ晶界，從石英管區域長出的晶粒較小，且含有許多孿生晶粒，其原因來自於石英管內空間狹小，晶粒易於側壁成核，這種生長方式會伴隨著孿生晶界。從三個實驗的初步結果發現藉由塗佈層控制成核的方式可在底部誘導出較高比例的non-Σ晶界，但和球型子晶向比仍顯不足，塗佈層的均勻性是未來可以改善的方向。

關鍵字

晶粒控制 ； 晶粒競爭 ； 多晶矽 ； 垂直固化

並列摘要

Nucleation and grain control decide the performance of mutilcrystalline silicon for photovoltaic application. It was found that mc-Si which is grown from unmelt feedstock layer have smaller grain size, and the dislocation propagation rate is lower. The ingot have good and more uniform quality in this growth mode. So, understand this kind of growth is important. In this study, we use spherical seeds as nucleation layer to imitate the growth mode. Sperical seeds provide random orientation and fair opportunity for grain competition. We can learn the grain competition mechanism and growth habits from the experiment. Three kinds of grain competition mechanism were found and the initial non-Σ grain boundary is about 70 %. In this study, we proposed three methods to obtain the same growth behavior, high percentage of non-Σ grain boundary and small grain size. First, the Si/SiO2 mixed coating was proposed. In this control, it can be obtained non-Σ grain boundary up to 30 % and with small grains. Unfortunately, there were still some big grains remained in the controlled region. The undercooling may be reason. Second, multi-notch pattern was used , but the percentage of non-Σ grain boundary were very low. The grains were almost (110), which is the same as notch crucible[1]. Third, in the SiO2 tubes experiment, the grains were small, and the non-Σ grain boundary were 20 %. Nucleation on the tube wall may be the reason why the percentage of twin boundaries were 60 %[2]. In our proposed methods, mixed coating can introduce more non-Σ grain boundary than others. To improve the non-Σ grain boundary percentage, enhance the uniformity of coating may be the key.

並列關鍵字

grain control ； grain competition ； multicrystalline silicon ； directional solidification

參考文獻

[1] T.F. Li, K.M. Yeh, W.C. Hsu, C.W. Lan, High-quality multi-crystalline silicon (mc-Si) grown by directional solidification using notched crucibles, Journal of Crystal Growth, 318 (2011) 219-223.

[2] T. Duffar, A. Nadri, On the twinning occurrence in bulk semiconductor crystal growth, Scripta Materialia, 62 (2010) 955-960.

[3] T. Taishi, T. Hoshikawa, M. Yamatani, K. Shirasawa, X. Huang, S. Uda, K. Hoshikawa, Influence of crystalline defects in Czochralski-grown Si multicrystal on minority carrier lifetime, Journal of Crystal Growth, 306 (2007) 452-457.

[4] C.W. Lan, W.C. Lan, T.F. Lee, A. Yu, Y.M. Yang, W.C. Hsu, B. Hsu, A. Yang, Grain control in directional solidification of photovoltaic silicon, Journal of Crystal Growth, 360 (2012) 68-75.

[5] K. Fujiwara, W. Pan, N. Usami, K. Sawada, M. Tokairin, Y. Nose, A. Nomura, T. Shishido, K. Nakajima, Growth of structure-controlled polycrystalline silicon ingots for solar cells by casting, Acta Materialia, 54 (2006) 3191-3197.