氣相傳輸法成長氧化鋅奈米線之光學特性研究

Abstract

我們成功地利用氣相傳輸法成長氧化鋅奈米線，並研究其結構與光學特性. 六方形柱狀氧化鋅奈米線能夠選擇性地成長在預鍍多晶氧化鋅薄膜上。 當鋅蒸氣傳送到基板上時，會很快地由氣態轉為固態，並且氧化成氧化鋅。 而在薄膜上的奈米尺度的凹坑以及山丘可以提供一個很好的成核點因此成長出氧化鋅奈米線。 使用鍍金的多孔矽做為基板時，我們發現當多孔隙的孔矽率越高時，氧化鋅奈米線越有垂直基板成長的趨勢。 使用具有(0002)優選方向的氧化鋅薄膜作為基板時，氧化鋅奈米線可以垂直成長於基版上。 當氧化鋅奈米線成長於氧化鋅薄膜/(0001)藍寶石的基版時，不僅垂直基板成長，在水平方向也是有磊晶的關係。 當使用(0001)藍寶石基板時,我們發現氧化鋅奈米線的成長方向在水平方向的投影具有三重對稱性，造成的原因為氧化鋅與藍寶石基板有磊晶的關係。 在奈米線的光學特性研究方面，我們使用變溫光激發光光譜得到自由激子複合放光主導著室溫的光激發光光譜，在低溫時則為束縛激子複合放光所主導。 我們也得到了自由激子與束縛激子的束縛能。 另一方面， 從變溫光激發光光譜可得到氧化鋅奈米線的輻射躍遷與縱光聲子極化場之耦合強度。 利用激子極子的形成，我們可以解釋為什麼在氧化鋅奈米線裡，自由激子與縱光聲子的交互作用會遠大於其他的束縛激子之作用。 在氧化鋅奈米線裡很強的激子與聲子耦合不但影響Haung–Ray S 因子而且影響第一階縱聲子能量，這個結果可歸咎於激子極子的形成。 除此之外，我們也發現了位於光子能量2.5電子伏特的缺陷發光主要來自於氧化鋅奈米線體的侧壁。 我們也驗證了氧化鋅奈米線的光激發光的受激輻射放光以及雷射的產生。 雷射的產生是由於光子在被散射多次,則會誘發出更多的同調光子,進而達到了隨機雷射的產生。 最後，我們也成功的製備出氧化鋅為軸心而氧化鎂包覆於外的奈米線。 利用熱擴散的方法，可以將其光激發光光譜位置調變至更高能量。 除此之外，我們也驗證出氧化鎂鋅奈米線受激輻射放光的行為。

We successfully fabricate ZnO-based nanowires by vapor transport method. The optical properties of ZnO nanowires are also investigated. Hexagonal ZnO nanowires have been selectively synthesized via vapor-solid process without gold catalysis on a pre-coated ZnO buffer layer. The presence of nanometer-sized pits or hills on the surface of ZnO buffer layer provides nucleation sites to which the zinc vapor is transferred and condensed. Followed by immediate oxidation the ZnO nanowires were grown on the buffer layer. ZnO nanowires can be also synthesized on porous silicon substrates with different porosities via the vapor-liquid-solid method. The texture coefficient analyzed from the XRD spectra indicates that the nanowires are more highly orientated on the appropriate porosity of porous silicon substrate than on the smooth surface of silicon. Vertically well-aligned ZnO nanorods are synthesized without employing any metal catalysts on various substrates including glass, Si(111), and saphire(0001), which were pre-coated with c-oriented ZnO buffer layers, by simple chemical vapor deposition. The epitaxial relationship between ZnO nanowires and various substrates is discussed in detail. From the temperature dependent photoluminescence spectra, we deduce the activation energies of free and bound excitons. Besides a strong ultra-violet emission at 3.26 eV observed at room temperature, the coupling strength of the radiative transition to LO-phonon polarization field was deduced in use of the Huang-Rhys factor from low temperature photoluminescence spectra to show that single crystalline ZnO nanorods. The coupling strength of the radiative transition of hexagonal ZnO nanowires to the longitudinal optic (LO) phonon polarization field is deduced from temperature dependent photoluminescence spectra. An excitonic polaron formation is discussed to explain why the interaction of free excitons with LO phonons in ZnO nanowires is much stronger than that of bound excitons with LO phonons. The strong exciton-phonon coupling in ZnO nanowires affects not only the Haung-Ray S factor but also the FXA-1LO phonon energy spacing, which can be explained by the excitonic polaron formation. We report room-temperature ultraviolet stimulated emission and lasing from optically pumped high-quality ZnO nanowires. Emission due to the exciton-exciton scattering process shows apparent stimulated-emission behavior. Several sharp peaks associated with random laser action are seen under high pumping intensity. The mechanism of laser emission is attributed to coherent multiple scattering among the random-growth oriented nanowires. The characteristic cavity length is determined by the Fourier transform of the lasing spectrum. Finally, we demonstrate a simple method to achieve the bandgap engineering in core-shell ZnO-MgO nanowires by using Mg diffusion. Furthermore, we report the observation of stimulated emission (SE) from optically pumped ZnMgO nanowires.