利用光激發-兆赫波探測研究飛秒雷射退火非晶矽

Abstract

在本論文中，光激發-兆赫波探測技術被利用來研究複晶矽的光電特性如載子回復生命期、載子漂移速度。複晶矽是利用鈦藍寶石超快雷射對非晶矽進行雷射退火結晶而成。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。我們利用光激發-兆赫波探測技術和兆赫時域光譜技術去研究複晶矽結構特性。 實驗上，我們利用雙指數函數去尋找適合由於光激發所引起的兆赫脈衝振幅改變的鬆弛時間參數。對於具有較大晶粒(約500 nm)和較小晶粒(約50 nm) 的複晶矽而言，鬆弛時間分別是32.69皮秒和24.43皮秒。利用兆赫時域光譜技術，對於不同晶粒大小的複晶矽而言，Drude模型能提供吻合的趨勢線對於在兆赫波段下的導電率。同時藉由Drude模型得知，載子遷移率分別是175±19.4 cm2/Vs和94.5±20.2 cm2/Vs對於較大晶粒(約500 nm)和較小晶粒(約50 nm)。最後我們比較利用電性量測方式得到由複晶矽製成的薄膜電晶體元件後的載子遷移率和利用Drude model得到的載子遷移率。得知載子遷移率與複矽晶的晶粒大小呈現正相關的關係。

We use the optical-pump–terahertz-probe technique (OPTP) to study relaxation dynamics of photo excited carriers in polycrystalline silicon. The polycrystalline silicon is prepared by femtosecond laser annealing (FLA) using near-infrared (λ = 800 nm) ultrafast Ti:Sapphire laser system. The intense laser pulses make the generation of dense photoexcited plasma in the irradiated materials due to nonlinear absorption, enabling nonlinear melting on silicon materials. We study the structural characteristics of recrystallized amorphous silicon by OPTP and THz-TDS system. By using double exponential function to fit the pump-induced changes in transmission of the amplitude of the THz pulse, for large grain size (~500 nm) and small grain size (~50 nm), the relaxation time is 32.69 ps and 24.43 ps, respectively. We show that Drude model provides good fit to the transient terahertz conductivity in the two different grain sizes of polycrystalline silicon. The transient motilities of poly-Si for large grain size (~500 nm) and small grain size (~50 nm) are 175±19.4 cm2/Vs and 94.5±20.2 cm2/Vs, respectively. We also compare the mobility calculated by electrical characteristics measurement of poly-Si thin film transistor devices with the results calculated from OPTP measurement.