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標題: | 高鎵摻雜氧化鋅透明導電體生長及其應用研究 Growth of Highly Ga-doped ZnO Transparent Conductor and Its Applications |
作者: | Yu-Feng Yao 姚毓峰 |
指導教授: | 楊志忠(Chih-Chung Yang) |
關鍵字: | 氧化鋅,鎵摻雜,透明導電體, ZnO,Ga-doped,Transparent Conductor, |
出版年 : | 2015 |
學位: | 博士 |
摘要: | 在本論文中,首先我們報告藉由氣液固的成長模式,以銀奈米顆粒當作催化劑,利用分子束磊晶成長出高鎵摻雜的氧化鋅(氧化鎵鋅)奈米針。當成長基板的溫度足夠高時,部分的銀奈米顆粒熔化作為催化劑,可使氧化鎵鋅過飽和在殘存銀奈米顆粒上析出並形成氧化鎵鋅奈米針。我們所成長的氧化鎵鋅奈米針在場發射測試中得到文獻紀錄上最低的起始和臨界電場及最高的場增益因子。這些優越的場發射性能可以歸因於如下因素:(1)由於高鎵摻雜成長的氧化鋅奈米針具有低功函數和高導電率;(2)垂直排列氧化鎵鋅奈米針的尖銳幾何形狀;(3)生長奈米針時的銀摻雜可進一步減少奈米針的電阻率;(4)在奈米針頂端殘存小顆的銀可以使頂端導電率更高。
接著,我們在表面具有銀奈米顆粒的發光二極體上成長氧化鎵鋅透明導電層,此結構可以結合表面電漿子耦合、電流擴散,光萃取和減少接觸電阻的機制來增強發光效率。在相對高溫的成長溫度下(350 oC),熔化的銀奈米顆粒可以當作成長氧化鎵鋅奈米針的催化劑,此奈米針可以使光萃取效率增加。此時,埋在氧化鎵鋅層下殘存的銀奈米顆粒可以產生表面電漿子耦合效果。在較低的氧化鎵鋅成長溫度下(250 oC),可以保存所有的銀奈米顆粒而產生較強的表面電漿子耦合效果。在製作銀奈米顆粒前先於p-型氮化鎵上成長經過熱退火的薄氧化鎵鋅中間層,此中間層可以減少在氧化鎵鋅和p-型氮化鎵之間的接觸電阻率,因此元件的電阻可以降低。雖然利用此中間層會使得銀奈米顆粒的侷域表面電漿子共振波長藍移而遠離本研究中量子井的發光波長(535奈米),使得表面電漿子耦合變弱。但利用此中間層,當發光二極體發光波長較短時,可有效提升表面電漿子耦合強度。 此外,我們製作結構包含p-型氮化鎵層、氧化鎘鋅/氧化鋅量子井、高溫成長的氧化鋅層、氧化鎵鋅層的發光二極體。在順偏電壓下,由量子井結構在元件的邊緣發出黃綠光。在逆偏電壓下,p-型氮化鎵價電帶內的電子藉由量子井的協助,可以利用穿隧方式進入氧化鎵鋅的導電帶並與注入氧化鎵鋅的電洞復合發光,使整個元件均勻發出紅黃和紫外顏色的光,同時載子也會在p-型氮化鎵內復合發出藍色光。藉由適當設計高溫成長氧化鋅層的厚度以控制順偏電壓下的發光強度,在交流電操作下,可以使元件在順逆偏下由於發光在空間位置和波長上的互補,利用視覺暫留而形成白光。 In this dissertation, we first demonstrate the molecular beam epitaxy growth of highly-degenerate Ga-doped ZnO (GaZnO) nanoneedles (NNs) based on the vapor-liquid-solid (VLS) growth mode using Ag nanoparticles (NPs) as the growth catalyst. It is shown that when the growth substrate temperature is sufficiently high, a portion of an Ag NP can be melted for serving as the catalyst to precipitate GaZnO on the residual Ag NP and form a GaZnO NN. Record-low turn-on and threshold electric fields in the field emission test of the grown GaZnO NNs are observed. Also, a record-high field enhancement factor in field emission is calibrated. Such superior field emission performances are attributed to a few factors, including (1) the low work function and high conductivity of the grown GaZnO NNs due to highly degenerate Ga doping, (2) the sharp-pointed geometry of the vertically aligned GaZnO NNs, (3) the Ag doping in VLS precipitation of GaZnO for further reducing NN resistivity, and (4) the residual small Ag NP at the NN tip for making the tip even sharper and tip conductivity even higher. Then, the combined effects of a few mechanisms for emission efficiency enhancement produced in the overgrowth of the transparent conductor layer of Ga-doped ZnO (GaZnO) on a surface Ag-nanoparticle (NP) coated light-emitting diode (LED), including surface plasmon (SP) coupling, current spreading, light extraction, and contact resistivity reduction, are demonstrated. With a relatively higher GaZnO growth temperature (350 oC), melted Ag NPs can be used as catalyst for forming GaZnO NNs such that light extraction efficiency can be increased. Meanwhile, residual Ag NPs are buried in a simultaneously grown GaZnO layer for inducing SP coupling. With a relatively lower GaZnO growth temperature (250 oC), all the Ag NPs are preserved for generating a stronger SP coupling effect. By using a thin annealed GaZnO interlayer on p-GaN before Ag NP fabrication, the contact resistivity at the GaZnO/p-GaN interface and hence the overall device resistance can be reduced. Although the use of this interlayer blue-shifts the localized surface plasmon resonance peak of the fabricated Ag NPs from the quantum well emission wavelength of the current study (535 nm) such that the SP coupling effect becomes weaker, it is useful for enhancing the SP coupling effect in an LED with a shorter emission wavelength. Besides, an LED structure consisting of a p-GaN layer, a CdZnO/ZnO quantum-well (QW) structure, a high-temperature-grown ZnO layer, and a GaZnO layer is fabricated. Under forward bias, the device effectively emits green-yellow light from the QW structure at the rim of device mesa. Under reverse bias, electrons in the valence band of the p-GaN layer move into the conduction band of the GaZnO layer through a QW-state-assisted tunneling process to recombine with the injected holes in the GaZnO layer for emitting yellow-red and shallow ultraviolet lights over the whole mesa area. Also, carrier recombination in the p-GaN layer produces blue light. By properly designing the thickness of the high-temperature-grown ZnO layer, the emission intensity under forward bias can be controlled such that under alternating-current operation at 60 Hz, the spatial and spectral mixtures of the emitted lights of complementary colors under forward and reverse biases result in white light generation based on persistence of vision. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/54088 |
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顯示於系所單位: | 光電工程學研究所 |
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