以飛秒光譜研究拓樸絕緣體單晶與薄膜之超快動力學

Abstract

本論文中，我們利用飛秒光譜探討拓樸絕緣體單晶及薄膜中載子帶間躍遷與帶內躍遷之動力學行為。其動力學特性將藉由可見光激發-可見光探測光譜、兆赫波時析光譜，以及可見光激發-中紅外光探測光譜進行研究。當中，溫度相依之能帶空缺效應與載子密度相依之表面態散射率抑制效應與時析角分辨光電子能譜具有良好之一致性。 首先，我們以脈衝雷射蒸鍍法製備磊晶碲化鉍薄膜與c軸摻銅碲硒化鉍薄膜。薄膜與單晶樣品之結構特性皆使用X光繞射譜以及拉曼光譜進行檢驗。其化學元素組成透過能量散失光譜進行確認。樣品之載子極性與濃度以標準霍爾量測方法取得。 通過量測摻雜與未摻雜之硒化鉍單晶，我們確認在短延遲時間內之瞬時反射率主要由能帶填充效應以及自由載子吸收效應貢獻，此兩者貢獻可藉由瞬時反射率變化之正負號判定。同調光頻聲子與同調聲頻聲子的貢獻可藉由特徵行為的時間尺度進行區分。在時間尺度約十皮秒左右之慢遲緩行為可歸因於載子擴散行為所導致。上述能帶填充效應之弛緩行為可歸因至能帶空缺效應。利用雙溫模型，我們由能帶填充效應之弛緩行為得到溫度相依電子聲子耦合常數。電子聲子耦合常數隨溫度減弱的行為抑制了表面態與塊材態導帶間的散射行為。 磊晶碲化鉍薄膜中之溫度相依載子傳輸特性以及電子-聲子耦合作用可藉由溫度可調兆赫波時析光譜進行研究。其中，雜質能階會因熱激發效應產生額外載子，使自由載子濃度增加。在溫度為200 K以下時，載子散射機制主要由電子-電子散射主導。薄膜兆赫光譜之居德權重展現了脈衝雷射沉積薄膜具備應用潛力。由於Fano效應所致，紅外聲子模態出現異常的紅移行為。其中，聲子能量的重整效應可歸因於電子-紅外聲子交互作用。 透過時間解析紅外光反射頻譜，我們對光激載子的帶內躍遷動力學行為進行探討。其中，居德權重的增加說明了導帶中電子占據率的增加。藉由載子密度相依實驗，我們觀察到能隙重整效應與熱聲子效應。表面態的金屬性響應行為因導帶電子的注入而受到抑制，導帶內高能量電子的散射率抑制行為也表現在時間解析紅外光反射頻譜中。同時，我們觀察到摻銅碲硒化鉍薄膜之低頻實部電導變化率出現低谷特徵。最後，我們提出迪拉克電子之電磁響應可利用光激發-中紅外光譜來研究。

In this dissertation, the interband and intraband carrier dynamics of the topological insulators (TI) single crystals and thin films are investigated by the femtosecond spectroscopy. The dynamic characteristics of the carrier relaxation process have been investigated by the degenerate pump-probe spectroscopy (OPOP), terahertz time-domain spectroscopy (THz-TDS), and optical pump-mid-infrared probe spectroscopy (OPMP). The temperature dependent band emptying effect and the density of state dependent suppression in surface state scattering are in good agreement with the results revealed in time- and angular-resolved photoemssion spectroscopy (trARPES). Epitaxial Bi2Te¬3 thin films and c-axis Cu0.1Bi2Te1.2Se1.43 (CBTS) thin films were fabricated by pulsed laser deposition (PLD). The structural characteristics of the TI crystals and films were examined by the X-ray diffraction spectroscopy and Raman spectroscopy. The chemical composition of all samples have been investigated by the Energy-dispersive X-ray spectroscopy (EDX). The carrier polarity and concentration are determined by standard Hall measurement. In the short timescale, the transient reflectivity change (∆R⁄R) obtained from the doped Bi2Se3 crystals shows that the band filling effect (BF) and the free carrier absorption (FCA) effect can be distinguished by the sign of the ∆R⁄R signal. The coherent longitudinal optical and acoustic phonon can be observed in the different timescale. The temperature dependent diffusion coefficient can be determined by the characteristic time of the slow decay at 10 ps timescale. The relaxation of the FCA effect can refer to the band-emptying (BE) effect. By the two temperature model, the temperature dependent electron-phonon coupling constant is determined. The suppression of the inelastic scattering of bulk conducting electrons can attribute to the decrease of the electron-phonon coupling constant. The temperature dependent THz-TDS revealed the carrier transport properties the electron-phonon coupling in the epitaxial Bi2Te3 film. The thermal excitation of carriers from the impurity band led to the increase of carrier density. The carrier scattering is dominated by electron-electron scattering when temperature below 200 K. The Drude weight of the film shows the application potential of PLD films. Further, the anomalous redshift of the IR phonon is attribute to the Fano effect. The renormalization of the phonon energy is attributed to the electron–IR phonon interaction. The temporally and spectrally resolved Mid-IR reflectivity reveals the intraband dynamics of photoecited carriers. The positive change of Drude weight indicates the photoinduced excess population in the conduction band. The band gap renormalization and the hot phonon effect are observed via the density dependent experiment. The reduction of the metallic surface state response is quenched by the injection of the bulk conducting carriers. The reduction of the scattering rate of the bulk conducting electrons has been observed. The low frequency dip of ΔG_1 has been observed in CBTS thin film. We conclude that the response of the Dirac electrons can be observed by the optical pump-mid-infrared probe spectroscopy.