複晶矽之藍光飛秒雷射退火

Abstract

我們利用鈦藍寶石超快雷射倍頻(λ=400 nm)藉由掃描的方式對非晶矽進行藍光飛秒雷射退火結晶。由於超短脈衝雷射導致材料的非線性吸收效應在材料表面產生緻密的電漿，使得非晶矽發生非線性熔融。我們對藍光飛秒非晶矽晶退火再結晶形成複晶矽結構之最佳條件及樣品特性加以探討。 從實驗結果，我們發現在當達到是當能量密度條件下，複晶矽在脈衝重疊百分比達89.8%以上時會開始成長。當我們繼續增加脈衝重疊百分比時，將會有更多的能量被非晶矽所吸收，並激發價帶電子躍遷至導帶，這過程使得經藍光飛秒雷射退火之非晶矽轉變成為複晶矽，當我脈衝重疊百分比增加到臨界時，晶粒便開始遞減。 從實驗結果可觀察到，在室溫下儘管我們提高脈衝重疊百分比，晶粒的成長大小似乎有一個極限值存在，由實驗結果可知：在室溫時，最大平均晶粒約280 nm，此時所用能量密度為30 mJ/cm2，93.75□的脈衝重疊百分比。 最後我們從光子吸收係數與光穿透深度的觀點，將藍光飛秒雷射退火與近紅外飛秒雷射退火和準分子雷射退火加以比較、討論。

Amorphous silicon (a-Si) was crystallized by femtosecond laser annealing (FLA) using a blue (λ= 400 nm) ultrafast Ti:Sapphire laser system. The line-scan method was applied in the experiment. The intense ultrashort laser pulses lead to efficient nonlinear absorption and the generation of very dense photoexcited plasma in irradiated materials, enabling nonlinear melting on silicon materials. We also study the structural characteristics of recrystallized amorphous silicon films. The grains in the FLA-processed a-Si were found to grow larger as overlapping between irradiated areas was increased up to 89.8% at appropriate fluence. When we increase the overlapping at the same fluence, there will be more energy absorbed by amorphous silicon layer to excite the electrons from valence band to conduction band. Then the annealed amorphous silicon translates into polycrystalline silicon. After increasing the overlapping larger the threshold, the grain size decrease oppositely. We observe that there seems to be a grain size limitation in the room temperature condition even though we increase the overlapping condition. We get the largest grain size is around 280 nm as fluence is 30 mJ/cm2 and overlapping is 93.75□ at room temperature. At last we make comparisons with near-infrared femtosecond laser annealing and excimer laser annealing from the viewpoints of absorption coefficient and penetration depth.