氮化銦之兆赫輻射與二次諧波及其載子動力學

Abstract

我們利用兆赫波輻射光譜研究氮化銦的輻射機制，其包含瞬時光電流效應和光整流效應。我們計算出wurtzite結構晶體的二階非線性光學之塊體和表面電場感應的電偶響應，進而透過計算結果，兆赫波的入射角、方向角、極化角度相關趨勢得到良好的擬合結果。在低功率激發下，氮化銦的輻射機制主要由瞬時光電流所領導; 隨著激發強度提高, 非線性趨勢更加明顯。經由比對兆赫波的方向角、極化角之趨勢，我們推測出瞬時光電流和光整流對於氮化銦的兆赫輻射影響。 二次諧波與光整流同屬於二階非線性光學響應，我們利用先前計算結果成功驗證非對稱晶體的二次諧波主要由塊體電偶響應支配其他表面的響應。實驗上所得到的擬合係數與理論計算相符。 利用時間解析的光激發探測技術，我們研究氮化銦薄膜和其奈米柱的瞬時載子特性，並發現在奈米柱中的載子有較短的生命期，推測是因奈米柱的結構會產生較多的缺陷而使得載子有較快的捕捉時間。我們觀察到薄膜的生命期與載子濃度之間的反比趨勢，但因由缺陷濃度會影響載子的生命期，各樣品的缺陷濃度不一致使此反比趨勢些微誤差。

THz emission mechanisms for InN are investigated by using THz emission spectroscopy (TES). The second-order optical response of bulk and surface electric field for wurtzite crystal could successfully explain the incident angle-, azimuthal angle-, and pump polarization- dependence of emitted THz field. According to the azimuthal angle- and pump polarization- dependence of TES, we deduced the interplay of transient current and optical rectification in THz emission for InN. Second harmonic generation (SHG) is the second-order optical response similar to optical rectification. From the calculated results we successfully demonstrated that bulk electric-dipole contribution dominates any surface contribution in SHF for noncentrosymmetric crystal. The best fitting parameters to azimuthal angle dependent SHG experiment are consistent with the calculated results based on bulk electric-dipole contribution. We also investigated the transient carrier dynamics for InN epilayer and nanorods via time-resolved optical pump-probe technique. The time evolution of transient reflectivity changes of InN is the interplay of photoinduced absorption and band-filling. The pump fluence independent lifetime of InN epilayer is regarded as defect-related recombination time. We also observe an inverse relationship between carrier lifetime and carrier concentration.