不同成長溫度下高銦組成氮化銦鎵奈米點之光學特性研究

Abstract

本論文中，我們主要探討利用金屬有機化學氣相沉積法在不同成長溫度下之高銦組成氮化銦鎵奈米點的表面形貌、組成變化以及其光學特性研究。 由原子力顯微鏡影像顯示，當氮化銦鎵奈米點成長溫度由550提升至725 oC時，該奈米點所呈現出的平均密度由4.0×109降低至2.2×107 cm-2。關於此現象可歸因於銦原子本身具有較長的遷移長度，導致成長溫度上升時在氮化鎵表面形成的奈米點平均密度降低。此外，經由X光繞射光譜之譜峰位置結果，可推估三元合金氮化銦鎵奈米點的銦組成含量由85 %增加至99 %。我們發現此銦組成範圍所對應的能隙能量與光激螢光光譜之發光位置(0.77至0.92 電子伏特)互相吻合，進而可確認紅外光螢光訊號來自於氮化銦鎵奈米點，並且與銦組成含量有關。另外，由X光繞射光譜之半高寬與紅外光光激螢光光譜之積分強度比較顯示，較高溫成長之樣品具有良好的發光品質。 除此之外，由螢光光譜中發現，當樣品成長溫度高於650 oC亦同時存有可見光螢光訊號，其頻譜分佈在2.0至2.4電子伏特。我們藉由近場光學顯微系統檢驗發現，此可見光螢光訊號乃源自於氮化銦鎵奈米點外的區域。然而，可見光螢光訊號強度隨著改變激發能量漸高於氮化鎵能隙能量而增加，發光位置亦隨著激發功率增加而產生藍移現象。綜合以上兩結果，我們認為此可見光訊號的躍遷方式可能是樣品成長過程中形成氮化鎵缺陷帶，致使其中施子-授子之能階躍遷，亦或是自由電子至深層束縛授子能階的躍遷行為。在載子生命週期部分，我們觀察到兩段時間常數，並推測短時間來自於淺層授子能階載子以垂直躍遷且非經由聲子散射之輻射復合時間，長時間則屬於載子在深層侷域態或以非垂直躍遷以之輻射復合時間。

In this thesis, we investigated the surface morphologies, alloy composition, and optical properties of In-rich InxGa1-xN dots (x＞0.85), which were prepared at different growth temperature (Tg) by metalorganic chemical vapor deposition (MOCVD). The atomic force microscopy (AFM) images showed that the InGaN dots density decreased from 4.0×109 to 2.2×107 cm-2 as the Tg was increased from 550 to 725 oC. This can be attributed to the enhanced migration length of In adatoms that resulted in the formation of less dense dots. The X-ray diffraction peak of ternary InGaN shifted gradually toward the InN (0002) as Tg was increased until 725 °C, hence the In content increased from x = 0.85 to 0.99. The calculated band gap is consistent with the photoluminescence (PL) peak position ranging from 0.77 to 0.92 eV, which is attributed to the emission from InGaN dots with different In compositions. For Tg > 650 oC, another visible emission band around 2.0 - 2.4 eV was observed. By near-field scanning optical microscopy (NSOM) mapping, we found that the visible emission band emerged from the region outside the In-rich InGaN dots. The integrated PL intensity increased with tuning to above the band gap excitation that suggested GaN related-defect levels are involved for recombination of photo-generated carriers. Furthermore, a blueshift of the visible peak position with the increasing photoexcitation power density was observed. This can be interpreted by the donor–acceptor pair (DAP) or free to bound (FB) transition model. Time-resolved PL shows two decay components, in which the fast decay constant (τ1) represents radiative recombination time of carriers in shallower potentials or via vertical transitions without involving phonons scattering , while the slower one (τ2) was accounted for radiative recombination in deeper localized states or non-vertical transitions.