氧化鋅奈米線應用於發光二極體之研製

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(1)國立臺灣師範大學機電科技學系碩士論文(初稿) 指導教授：楊啟榮博士氧化鋅奈米線應用於發光二極體之研製 Development of light-emitting diodes using well-aligned ZnO nanowire. 研究生：趙偉迪撰中華民國九十八年七月.

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(3) 謝. 誌. 此篇論文得以順利完成，首先要感謝我的指導教授楊啟榮老師的悉心指導，對於學術研究方面的啟發，讓我瞭解正確的研究方法與嚴謹的做事態度，並時時教導我待人處世的道理，使我在這些年中獲益匪淺。老師對學問的嚴謹更是我輩學習的典範。感謝旭明光電股份有限公司朱振甫博士、工業技術研究院電子與光電研究所劉君愷博士，與臺師大機電科技學系主任程金保教授，在論文口試時提供諸多指教與建議，使本論文更臻嚴謹與周延。本研究的進行，須使用到許多檢測設備，在這方面要感謝臺灣師範大學薄膜暨接合工程實驗室於掃描式電子顯微鏡、能量分散式光譜分析儀與四點探針等儀器的協助、臺灣師範大學顯微光譜實驗室於近紅外光/可見光/紫外光顯微光柵式分光光譜儀的協助、臺灣大學貴重儀器中心理學院穿透式電子顯微鏡的協助、國立中山大學奈米科技研發中心於光激發光螢光頻譜的協助、清華大學貴重儀器中心於X光薄膜繞射儀的協助、財團法人國家實驗研究院儀器科技研究中心於霍爾量測儀的協助，在此一併感謝。此外，也感謝實驗室的學長茂榕、龍吟、昶均、裕強、耀方、嘉佑、服賢、峻銘，以及前幾屆的學長姐，在我投入微機電領域的過程中，知識的建立、技術的培養與精神的鼓勵；也感謝同窗夥伴幸憲、莉菱、俊良、鉅程與飛祥，在生活中的照料與研究上的砥礪，使我的研究更加順利。也要感謝學弟妹建凱、博元、鎮南、學舜、彥翔、秀麗、憶祖、忠策與純芳等人的幫忙，在此一併向各位致謝。最後要感謝家人對我的關心與照顧，並在求學過程中給予支持與勉勵，提供向上進取的原動力，使我能夠完成學業，僅以此論文獻給最親愛的父親、母親、姊姊，以及所有關心我的師長、同學與朋友。.

(4) 摘. 要. 發光二極體被視為未來主要的照明光源，高功率發光二極體於技術上屢有突破，但現階段發光效率的不足，使發光二極體無法取代傳統光源作為照明燈源的主流，故發光二極體發光效率的提升，是目前技術發展的重點之一。過去的研究指出，將奈米線應用於發光二極體的結構製作，能有效提升其發光強度；而在各式成長奈米線的方法中，以水熱法製備之奈米線具有高品質順向成長與製程簡易的優點，故本論文將採用此法成長氧化鋅奈米線，並以射頻濺鍍法沉積 N 型氧化鋅鋁薄膜，P 型材料則選用氧化鋅與氮化鎵，藉以製備氧化鋅奈米線發光二極體，進行其特性之研究。在奈米線的部份，藉由水熱法成功製備氧化鋅奈米線，直徑 34 nm−200 nm，長度 1 μm−2 μm，密度 4 NWs/μm2−68.23 NWs/μm2。EDS 分析顯示氧化鋅奈米線的鋅、氧比接近 50 % : 50 %。XRD 分析僅於 34°存在繞射峰值，亦即於(002) 面有較強之繞射訊號。PL 顯示奈米線的發光峰值位於 378 nm，並具有微弱之可見光放射。從 HRTEM 可觀察到，奈米線內部的晶格結構良好，晶格條紋間距為 0.2629 nm，証實奈米線為 C 軸取向成長。 N 型氧化鋅鋁薄膜部份，其最佳電阻率為 3×10-3 Ω-cm，載子濃度為 1.72 × 1021 cm-3，載子遷移率為 0.0715 cm2/V-s，於可見光波段的平均穿透率大於 80 %，於波長 450 nm 之最佳穿透率為 87 %，於波長 380 nm 之最佳穿透率為 77 %。 P 型氧化鋅部份，分別使用氧化鋅及純鋅靶材，嘗試藉由製程氣體氧/氬比例的控制，用以製備 P 型氧化鋅，但目前薄膜均呈現 N 型半導體電性。未來將使用摻雜 P2O5 之氧化鋅靶材，繼續 P 型氧化鋅薄膜之試驗。發光二極體部份，目前已於 P 型氮化鎵(鎂摻雜，載子濃度約為 1017 cm-3)薄膜上，成功製備氧化鋅奈米線/N 型氧化鋅鋁薄膜結構，並完成發光二極體之晶. -I國立臺灣師範大學機電科技學系.

(5) 粒製作，其尺寸為 300 μm ×300 μm。在約大於 15 V 的操作電壓下，以長工作距離顯微鏡可觀察到，發光二極體晶粒的部份區域放射出藍光，且發光強度隨外加電壓而增加。發光二極體之 I-V 曲線顯示其串聯電阻相當大，未來將以快速熱退火進行後處理，以期提升其性能，並檢測發光頻譜等特性。關鍵詞：氧化鋅奈米線、發光二極體、水熱法、氧化鋅鋁薄膜、P 型氮化鎵. - II 國立臺灣師範大學機電科技學系.

(6) Abstract Light emitting diode (LED) is considered as the major next-generation luminescence technology, but nowadays insufficient light efficiency of high power LED limits its application for illumination lighting. Some research group have developed nanowrie-inserted LED structure, and the EL intensity shows that the novel LED structure can improve light efficiency effectively. ZnO nanowires grown by hydrothermal method have excellent properties such as single crystal, vertical alignment, broad area growth and simple process. Thus in this study hydrothermal method is adopted to fabricate ZnO nanowires, N-type material of LED is aluminum-doped ZnO film (AZO) deposited by RF sputtering, and P-type materials are ZnO and Mg-doped GaN film. Finally, the characteristics of N-type AZO/ZnO nanowire/P-type ZnO or P-type GaN structure LED will be studied. The ZnO nanowries grown by hydrothermal method are 34 nm−200 nm in diameter, 1 μm−2 μm in length, and 4 NWs/μm2−68.23 NWs/μm2 in density. EDS shows the atomic percentage of Zn and O is closed to 50 % : 50 %. XRD peak locates at 34° and shows (002) preferred crystallinity. PL indicates the luminescence peak of nanowries appears at 378 nm, and weak visible emission is observed. HRTEM image shows good crystal structure, and the spacing of lattice fringe is 0.2629 nm. For AZO film, the optimal resistivity is 3×10-3 Ω-cm, carrier concentration is 1.72 × 10. 21. cm-3 and mobility is 0.0715 cm2/V-s. The average transmittance of. visible light is above 80 %, the optimal transmittance at 450 nm is 87 %, and the optimal transmittance at 380 nm is 77 %. ZnO and pure zinc targets are used to deposit P-type ZnO by O2/Ar flow controlling, but unfortunately all fabricated samples still show N-type properties. P2O5-doped ZnO target will be used to fabricated P-type ZnO film continuously.. - III 國立臺灣師範大學機電科技學系.

(7) For LED study, ZnO nanowrie/N-type AZO film have been fabricated on Mg-doped P-GaN film (carrier concentration is about 1017 cm-3), the size of LED die is 300 μm × 300 μm. Blue emission is observed from partial area of LED die through long-distance microscope when forward-bias is above 15-20 V, and voltage raise leads to increasing of light intensity. In the next step, current-voltage relation and electroluminescence (EL) spectrum will be examined. Keywords：ZnO nanowrie, light-emitting diode, hydrothermal method, AZO, P-GaN. - IV 國立臺灣師範大學機電科技學系.

(8) 總摘要. 目. 錄. ........................................................................................................................ I. 總目錄 ...................................................................................................................... V VIII 圖目錄 ......................................................................................................................VI XVIII 表目錄 ....................................................................................................................... 第一章序論 ............................................................................................................... 1 1.1 前言 ........................................................................................................... 1 1.2 發光二極體之簡介 ....................................................................................5 1.2.1 發光二極體之原理 ..........................................................................5 1.2.2 發光二極體之發光效率 .................................................................... 7 1.2.3 同質與異質結構發光二極體 ............................................................ 9 1.3 氧化鋅材料 ............................................................................................. 18 1.3.1 氧化鋅之材料特性 ..................................................................... 18 1.3.2 氧化鋅之發光機制 ..................................................................... 18 1.4 量子侷限效應 ........................................................................................ 25 第二章文獻回顧 ................................................................................................ 33 2.1 氧化鋅奈米線/奈米柱之製備 ……………………………………………. 33 2.1.1 VLS 與 VS 成長機制 ………………………………………………. 33 2.1.2 氣相沉積法 ………………………………………………………36 2.1.3 有機金屬化學氣相沉積法 ………………………………………… 38 2.1.4 電漿輔助化學氣相沉積法 ………………………………………… 40 2.1.5 脈衝雷射蒸鍍法 …………………………………………………… 41 2.1.6 電化學沉積法 ……………………………………………………… 41 2.1.7 聲化學法 …………………………………………………………… 44 2.1.8 水熱法 ....................................................................................... -V國立臺灣師範大學機電科技學系. 45.

(9) 2.2 氧化鋅奈米線/奈米柱發光二極體之製作 ................................................. 83 2.2.1 有機金屬化學氣相沉積法 ................................................................ 83 2.2.2 化學氣相沉積法 ................................................................................ 86 2.2.3 氣相傳輸法 ........................................................................................ 87 2.2.4 電化學沉積法 .................................................................................... 87 2.2.5 水熱法 ................................................................................................ 89 2.3 研究動機與目的 ....................................................................................114 第三章實驗設計與規劃 ........................................................................................... 115 3.1 元件設計 ..................................................................................................... 115 3.1.1 發光二極體結構設計 ....................................................................... 115 3.1.2 光罩設計 ............................................................................................ 117 3.2 實驗規劃與製程 .......................................................................................... 121 3.2.1 氧化鋅奈米線之製備 ........................................................................ 121 3.2.2 同質結構氧化鋅奈米線發光二極體之製作 .................................... 122 3.2.3 異質結構氧化鋅奈米線發光二極體之製作. 124. 3.3 實驗設備 ...................................................................................................... 132 第四章實驗結果與探討 .................................................................................... 142 4.1 氧化鋅奈米線 ....................................................................................... 142 4.1.1 溶液濃度對氧化鋅奈米線之影響 ............................................. 142 4.1.2 反應時間對氧化鋅奈米線形貌之影響 ..................................... 155 4.1.3 X 光繞射分析 ............................................................................. 161 4.1.4 光激發光螢光頻譜分析 ............................................................. 166 4.1.5 穿透式電子顯微鏡分析 ............................................................. 169 4.2 氧化鋅鋁薄膜 ........................................................................................171 4.2.1 靶材距離對氧化鋅鋁薄膜電阻值之影響 ................................ 171 4.2.2 氣體流量對氧化鋅鋁薄膜電阻值之影響 .................................... 172 4.2.3 濺鍍功率對氧化鋅鋁薄膜電阻值之影響 ...................................173 4.2.4 快速熱退火對氧化鋅鋁電阻值之影響 .................................. 173 - VI 國立臺灣師範大學機電科技學系.

(10) 4.2.5 濺鍍參數對氧化鋅鋁薄膜穿透率之影響 .................................. 174 4.3 P 型氧化鋅薄膜 ................................................................................... 184. 4.4 氧化鋅奈米線異質結構發光二極體之製作 ...................................... 187 第五章. 結論與未來展望 ................................................................................... 194 結論 ...................................................................................................... 194. 5.2 未來展望 .............................................................................................. 196. 參考文獻 ............................................................................................................... 197. 5.1. - VII 國立臺灣師範大學機電科技學系.

(11) 圖. 目. 錄. Figure 1-1. History of all kinds of LED ………………………………………... 3. Figure 1-2. Total luminescence versus light efficiency plot of lighting technology .......................................................................................... 3. CREE relative luminous flux versus LED junction temperature ……………………………………………………........ 4. Figure 1-3 Figure 1-4. Typical LED structure ....................................................................... 12. Figure 1-5. Radiative and nonradiative recombination of electron hole pair ….. 12. Figure 1-6. Energy band diagram of unbiased and biased P−N+ LED ................ 13. Figure 1-7. Parabolic electron and hole dispersion relations ............................... 13. Figure 1-8. Ideal and real I-V characteristic curve of LED ................................. 14. Figure 1-9. Schematic diagram of lattice mismatch ............................................ 14. Figure 1-10 Double heterojunction LED consisting of one quantum well ........... 15 Figure 1-11 Carrier distribution of (a) homojunction; (b) heterojunction LED ... 15 Figure 1-12 Schematic diagram of a p-n homojunction ZnO LED ...................... 16 Figure 1-13 Cross-sectional TEM image of the region around the p-type ZnO/n-type ZnO interface ................................................................. 16 Figure 1-14 Schematic diagram and band diagram of ZnO heterojunction LED ........................................................................................................... 17 Figure 1-15 Comparison of EL spectra of the p–n homojunction ZnO LED and the ZnO LED with Mg0.1Zn0.9O layers ............................................. 17 Figure 1-16 Three-dimensional structure model of the unit cell of ZnO, and the corresponding projection along the a axis ........................................ 21 Figure 1-17 Energy band of ZnO. Eg is band gap, and Ex is exciton binding energy. (a) band-to-band emission; (b) exciton emission of UV light ................................................................................................... 22 Figure 1-18 Schematic energy-band diagram of a ZnO grain in cross section ..... 22 Figure 1-19 Intensity of the (a) 510 nm green emission peak; (b) free-carrier electron density for powder ZnO as a function of reduction anneal temperature ...................................................................................... 23 Figure 1-20 A schematic overview of the relaxation processes that take place upon photoexcitation of a ZnO particle ............................................ 24 - VIII 國立臺灣師範大學機電科技學系.

(12) Figure 1-21 The draft of the calculated defect’s levels in ZnO film ..................... 24. Figure 1-22 Confinements of photons and electrons in various dimensions ......... 29. Figure 1-23 One dimension infinite potential well ................................................ 29. Figure 1-24 Idealized density of states for one band of a semiconductor structure of 3, 2, 1, and 0 dimensions ............................................... 30 Figure 1-25 (a) Schematic of MQW nanorods consisting of 10 periods of Zn0.8Mg0.2O/ZnO on the tips of ZnO nanorods; (b) FESEM images of the MQW nanorods ....................................................................... 30 Figure 1-26 TEM images of Zn0.8 Mg0.2O/ZnO MQW nanorods .......................... 31. Figure 1-27 (a) Schematic and SEM image of InGaN/GaN MQW NRA LEDs; (b) light output power-forward current plot ...................................... 31 Figure 1-28 SEM and TEM images of In 0.25Ga 0.75N/GaN MQWs ....................... 32. Figure 2-1. (a) Schematic illustration of polycenter nucleation regime of the VLS mechanism; (b) the concentration profile across the liquid droplet-solid nanorod interface ......................................................... 49. Figure 2-2. ZnO nanorods with diameter 40–60 nm on Si and length of several 49 microns, Au droplet in tip of each nanorods is observed …………. Figure 2-3. Schematic illustration of nucleation and growth of ZnO nanorods by the VLS mechanism. .................................................................... 50. Figure 2-4. SEM images of ZnO nanostructure grown with different supply time of Ar gas current ....................................................................... 50. Figure 2-5. Optical and SEM image of a monolayer of self-assembled polystyrene submicron spheres on an alumina substrate .................. 51. Figure 2-6. (a) The honeycomb pattern produced by sputtering; (b) gold pattern produced by evaporation using the shadow provided by polystyrene spheres ........................................................................... 51. Figure 2-7. (a) Top; (b) (c) 30° view of aligned ZnO nanorods, where the hexagonal pattern is apparent ............................................................ 52. Figure 2-8. SEM images of ZnO nanorods synthesized by VS mechanism, using zinc powder and oxygen gas as source materials .................... 52. Figure 2-9. ZnO nanowire alignment dependence on seed layer thickness ......... 53. Figure 2-10 (a) PL spectra of ZnO nanowires; (b) XRD spectra of ZnO seed layer oxidized from varying Zn layer thickness ............................... 53. - IX 國立臺灣師範大學機電科技學系.

(13) Figure 2-11 (a) Nucleation sites on a surface with a step; (b) preferential condensation of growth species at the c-plane edges; (c) addition nucleation beyond the edges when steps and vacancies are found .... 54. Figure 2-12 Appearance of various ZnO nanotubes .............................................. 54. Figure 2-13 (a) Low magnification; (b) high magnification FESEM images of vertically aligned javelin-like ZnO nanorods .................................... 55. Figure 2-14 TEM and HRTEM images of the javelin-like ZnO nanorods and its SAED pattern ..................................................................................... 55. Figure 2-15 Schematic illustration of the growth mechanism for the formation of javelin-like ZnO nanorods grown by thermal evaporation ............ 56. Figure 2-16 SEM image of ultralong ZnO nanobelt with wurtzite structure ........ 56. Figure 2-17 TEM images of ZnO nanobelts showing their geometrical shape ..... 57. Figure 2-18 (a)−(e) SEM images of ZnO nanowire arrays grown on sapphire substrates.; (f) HRTEM image of nanowire showing (0001) growth direction ............................................................................................. 57. Figure 2-19 (a) SEM side image of the structure of the ZnO whisker; (b) the tip of the ZnO whisker for 500 minutes growing time ............................ 58. Figure 2-20 (a) SEM side image of the structure of the ZnO whisker; (b) the tip of the ZnO whisker for 2500 minutes growing time .......................... 58. Figure 2-21 Length and diameter of the ZnO whiskers as a function of growth duration .............................................................................................. 59. Figure 2-22 Schematic diagram of the MOCVD system ....................................... 59. Figure 2-23 FESEM images of columnar growth of ZnO on (a) c-Al2O3; (b) epi GaN; (c) fused silica; (d) SiO/Si substrates ....................................... 60. Figure 2-24 Dark field TEM image and electron diffraction image of a single ZnO nanotip ....................................................................................... 60. Figure 2-25 SEM images of the ZnO nanorods grown on the various substrates: (a) (b) Si (111); (c) (d) Si (001); (e) (f) Al2O3 (0001); (g) (h) GaN/Al2O3 (0001); (i) (j) ZnO (0001) ............................................... 61. Figure 2-26 (a) SEM image; (b) EDX spectra of ZnMgO nanorods ..................... 62. Figure 2-27 ZnO nanorods grown on different substrates: (a) (b) c-plane sapphire; (c) (d) (111) textured Pt film; (e) (f) Si substrates .............. 63. Figure 2-28 (a) TEM micrograph of ZnO nanorods. Inset is the electron diffraction pattern of the nanorod; (b) HRTEM of a ZnO nanorod ... 64. -X國立臺灣師範大學機電科技學系.

(14) Figure 2-29 SEM images of 1% Ga doped ZnO on c-plane GaN: (a) top view; (b) tilted view; (c) cross-section view ................................................ 64. Figure 2-30 SEM images of ZnO with different Ga-doping levels of 0−5 % ....... 65. Figure 2-31 HRTEM images for ZnO nanorods on sapphire substrate: (a) interface; (b) nanorod; (c) superimposed diffraction pattern of Al2O3 and ZnO ................................................................................... 66. Figure 2-32 Schematic image of electric-field-assisted growth for ZnO nanorods ............................................................................................. 67. Figure 2-33 SEM images of ZnO nanorods electrodeposited under different growth temperatures ........................................................................... 67. Figure 2-34 SEM images of the electrodeposited ZnO nanorods and the ZnO nanotubes by selective dissolution of the rods in 0.1 M EDA aqueous solution ................................................................................. 68. Figure 2-35 Schematic illustration of the selective dissolution formation of the ZnO nanotubes ................................................................................... 69. Figure 2-36 Schematic illustration, SEM images and HRTEM of a sonochemical route for ZnO nanorods growth. ................................. 69. Figure 2-37 Fabrication process of ZnO nanorods by sonochemical method ....... 70. Figure 2-38 TEM images of ethanolic ZnO colloids (a) fresh colloid sample;(b) aged colloid sample; (c) ZnO monolayer after firing at 300 °C ........ 71. Figure 2-39 Aggregation and Ostwald ripening in growth of the ZnO crystals .... 72. Figure 2-40 TEM images of ZnO: (a) starting sol; (b) after one day of reflux of the concentrated sol ............................................................................ 72. Figure 2-41 TEM images of crystalline ZnO samples. Zn2+:OH is (a) 1:3; (b) 1:10; (c) 1:20; (d) 1:30 ....................................................................... 73. Figure 2-42 SEM image of the flowerlike ZnO nanostructures ............................ 73. Figure 2-43 (a) Growth sketch of larger ZnO crystal; (b) growth schematic diagram of ZnO nanostructures with and without CTAB .................. 74. Figure 2-44 FEG-SEM micrographs of ZnO microrod array grown onto transparent conducting tin oxide glass substrate ................................ 74. Figure 2-45 XRD pattern (Cu Kα) of the ZnO microrod array on F-SnO2 glass substrate ............................................................................................. 75. Figrue 2-46 Scheme of solution growth of highly oriented microtubular array of ZnO onto substrates ....................................................................... 75. - XI 國立臺灣師範大學機電科技學系.

(15) Figure 2-47 FEG-SEM micrographs of ZnO oriented microtube array chemically grown onto a TCO glass substrate ................................... 76. Figure 2-48 Indexed X-ray diffraction pattern of a highly oriented ZnO microtube array grown on a F-SnO2 substrate ................................... 76. Figure 2-49 FESEM images of ZnO nanorod array grown by the aqueous chemical method (a) onto a silicon wafer; (b) onto a ZnO nanostructured thin film ..................................................................... 77. Figure 2-50 (a) FESEM images; (b) HRTEM images of various ZnO nanowiress grown by aqueous chemical method ............................... 77. Figure 2-51 A typical zincite crystal with various planes marked ......................... 77. Figure 2-52 Illustration of the possible role of hexamine in the anisotropy of the obtained ZnO nanoparticles ......................................................... 78. Figure 2-53 ZnO nanowire array on a four-inch silicon wafer. At the center is a photograph of a coated wafer, surrounded by SEM images .............. 78. Figure 2-54 SEM images of ZnO nanorods grown on Zn/Si substrate:. Center photograph demonstrates uniform ZnO nanorod growth on a four-inch Si wafer .............................................................................. 79. Figure 2-55 Schematic diagram of selective growth process ................................ 79. Figure 2-56 SEM images of (a) as-deposited Zn metal; (b) converted ZnO seed layer; (c)-(f) patterned ZnO nanorod arrays ....................................... 80. Figure 2-57 SEM images of multistep grown ZnO nanorods (a) as-grown; (b) first step growth; (c) second step; (d) third step; (e) average diameter vs length .............................................................................. 80. Figure 2-58 SEM images of ZnO rod arrays grown with different dopants concentration in reactive solutions ..................................................... 81. Figure 2-59 Room temperature PL spectrum of different Ni-doped samples ....... 82. Figure 2-60 Schematic illustration and FESEM images of a n-ZnO nanorod/p-GaN film heterostructure device ...................................... 92. Figure 2-61 RT EL spectra of n-ZnO nanorod/p-GaN film structure .................... 92. Figure 2-62 I-V characteristic and current-EL intensity of the n-ZnO nanorod/p-GaN film structure ............................................................ 93. Figure 2-63 (a) Schematic diagram; (b) FESEM image of the n-ZnO film/ZnO nanowire array/p-GaN film heterojunction LED structure ................ 93. Figure 2-64 EL spectra of (a) the as-fabricated n-ZnO film/ZnO nanowire array/p-GaN film heterojunction diode; (b) the hydrogen-treated - XII 國立臺灣師範大學機電科技學系.

(16) LED .................................................................................................... 94. Figure 2-65 (a) I-V curves; (b) the relation between I-V curve and EL peak intensity of the as-fabricated diode, and (c) the relation between I-V curve and EL peak intensity of the hydrogen-treated heterojunction diode ........................................................................... 94. Figure 2-66 (a) A 45°-tilted SEM image and (b) schematic illustration of the Al-doped ZnO film/ZnO nanowire array/Mg-doped GaN film structures ............................................................................................ 95. Figure 2-67 The as-fabricated ZnO nanowires showed a sharp tip-end and the grain size of AZO increased on increasing the deposition time ......... 95. Figure 2-68 EL spectra of (a) film-based n-ZnO film/p-GaN film heterojunction diodes; (b) ZnO-nanowire-inserted GaN/ZnO heterojunction diodes ................................................................................................. 96. Figure 2-69 I-V curves of the film-based and ZnO-nanowire-inserted GaN/ZnO heterojunction diodes ......................................................................... 96. Figure 2-70 FE-SEM images of the as-grown ZnO nanorods and a homoepitaxially grown ZnO film on ZnO nanorods ......................... 97. Figure 2-71 I-V curve of an n-ZnO nanorods/p-GaN heterostructure at RT ......... 97. Figure 2-72 (a) Blue light emitting from the n-ZnO nanorods/p-GaN heterojunction; (b) EL spectrum from the heterostructure ................. 98. Figure 2-73 SEM and XRD results of the multidimensional n-ZnO/p-Si LED structure with a film-like top layer grown at different temperatures.. 98. Figure 2-74 Schematic structure and I-V curve of the multidimensional n-ZnO/p-Si LED ................................................................................ 99. Figure 2-75 SEM image of vertical ZnO nanowires and cross sectional schematic of ZnO nanowire LED ...................................................... 99. Figure 2-76 (a) I-V curve of ZnO NWs/PEDOT diode; (b) light intensity and voltage as functions of current of ZnO NWs/PEDOT LED ............. 100 Figure 2-77 PL of vertical ZnO nanowires and EL of n-ZnO NW/ p-polymer .... 100 Figure 2-78 Schematic diagram of n-ZnO nanorods/p-CuAlO2 film heterostructure LED .......................................................................... 101 Figure 2-79 Schematic diagram of proposed EL emission for n-ZnO nanorods/p-CuAlO2 film/p+-Si LED ................................................. 101 Figure 2-80 (a) PL spectrum of ZnO NRs; (b) EL and the EL photo of n-ZnO NR/p-CuAlO2/p+-Si LED and (c) EL and the EL photo of n-ZnO NR/p+-Si LED ................................................................................... 102 - XIII 國立臺灣師範大學機電科技學系.

(17) Figure 2-81 Schematic diagram of nanowire LED arrangement with insulator filling the space between wires and a very thin film covering the wire tips ............................................................................................. 103 Figure 2-82 SEM images of electrodeposited ZnO nanowires on polycrystalline SnO2 films on glass and nanowires embedded in PS ........................ 103 Figure 2-83 (a) I-V characteristics of the LED structure and (b) PL and EL spectra obtained at room temperature in air ...................................... 104 Figure 2-84 SEM images of electrodeposited ZnO nanorods on ITO glass and ZnO nanorods covered by electrophoretic paint ............................... 104 Figure 2-85 Room temperature electroluminescence spectra of the ZnO/PMT heterojunction at different applied voltages ...................................... 105 Figure 2-86 I-V characteristics of the (a) ITO/ZnO/Al and (b) ITO/ZnO/PMT/Al structure LED .................................................. 105 Figure 2-87 (a) Design scheme for a flexible LED structure; (b) nanowires on a bent Au film with curvature radius of < 10 μm ................................ 106 Figure 2-88 SEM images of (a) ZnO nanowires grown in electrodeposition on a planar transparent substrate; (b) (c) nanowires spin-coated with polystyrene ........................................................................................ 106 Figure 2-89 Optical and electrical performance of the flexible LED ................... 107 Figure 2-90 Representative SEM images of ZnO nanorods grown by hydrothermal methods: (a) top view; (b) side view .......................... 107 Figure 2-91 (a) EL spectra for heterojunction LED with different annealing conditions; (b) EL spectra for 600 °C O2 annealed LED .................. 108 Figure 2-92 I-V characteristics of the as-grown and annealed n-ZnO nanorods/p-GaN heterojunction diode .............................................. 108 Figure 2-93 Schematic diagram of an EL device with an n-ZnO nanorods/p-Si heterojunction ................................................................................... 109 Figure 2-94 SEM images of the ZnO NRs (a) before spin coating; (b) after spin coating and baking ............................................................................ 109 Figure 2-95 (a) The I-V characteristics of the EL device; (b) EL spectra of the obtained n-ZnO/p-Si heterojunction ................................................. 109 Figure 2-96 Schematic view of the solution-grown-ZnO nanowire LED device and location of electroluminescence ................................................. 110 Figure 2-97 SEM Images of (a) solution-grown ZnO nanowire arrays; (b) ZnO nanowires after spin coating of a PMMA insulating layer ............... 110 - XIV 國立臺灣師範大學機電科技學系.

(18) Figure 2-98 Solution-grown-ZnO nanowire LED device: (a) I-V curves; (b) electroluminescence spectra .............................................................. 111 Figure 2-99 (a) schematic view of waveguide experimental; (b) EL spectra at various angles from vertical orientation of the nanowire array ........ 111 Figure 2-100 Schematic diagram of the p-ZnO film/ZnO nanowire quasiarray homojunction LED structure ............................................................. 112 Figure 2-101 (a) Schematic diagram of the ZnO nanowires/p-ZnO film homojunction LED structure; (b) SEM micrograph of N-In codoped ZnO film ............................................................................. 112 Figure 2-102 (a) I-V characteristic of the ZnO homojunction LED; (b) RT EL spectra from ZnO homojunction LED under different forward biases ................................................................................................. 113 Figure 3-1. Schematic diagram of ZnO nanowire homostructure LED ............... 119. Figure 3-2. Schematic diagram of ZnO nanowire heterostructure LED .............. 119. Figure 3-3. Mask for homostructure LED die: (a) first lithography; (b) second lithography ........................................................................................ 120. Figure 3-4. Mask for heterostructure LED die: (a) first lithography; (b) second lithography; (c) third lithography ...................................................... 120. Figure 3-5. Flow chart for ZnO nanowires growth .............................................. 127. Figure 3-6. Flow chart for ZnO nanowires LED ................................................. 128. Figure 3-7. Fabrication process of ZnO homostructure LED .............................. 129. Figure 3-8. Fabrication process of ZnO heterostructure LED ............................. 130. Figure 3-9. (a) LPCVD system; (b) UV mask aligner; (c) spin coater; (d) hot plate; (e) precise balance; (f) optical microscope …...................... 138. Figure 3-10 (a) Ultrasonicator; (b) thermal evaporator; (c) DC & RF sputter;(d) surface profiler; (e) four-point probe; (f) SEM & EDS system ........ 139 Figure 3-11 (a) Thin film XRD system; (b) photoluminescence system; (c) TEM system; (d) HRTEM system; (e) NIR/VIS/UV micro-spot grating monochrometer; (f) Hall measurement system .................... 140 Figure 3-12 Semiconductor parameter analyzer ................................................... 141 Figure 3-13 Electroluminescence system ............................................................. 141 Figure 4-1. SEM images of hydrothermal result at 5 mM concentration ............ 146. Figure 4-2. SEM images of ZnO nanowires at 15 mM concentration ................. 147. - XV 國立臺灣師範大學機電科技學系.

(19) Figure 4-3. SEM images of ZnO nanowires at 35 mM concentration ................. 148. Figure 4-4. SEM images of ZnO nanowires at 40 mM concentration ................. 149. Figure 4-5. SEM images of ZnO nanowires at 55 mM concentration ................. 150. Figure 4-6. SEM images of ZnO nanowires at 75 mM concentration ................. 151. Figure 4-7. SEM images of ZnO nanowires at 95 mM concentration ................. 152. Figure 4-8. SEM images of ZnO nanowires at 105 mM concentration ............... 153. Figure 4-9. Plot of diameter and density versus concentration ........................... 154. Figure 4-10 SEM images of ZnO nanowires growing for different time: (a)-(j) are 0.5, 0.75, 1, 2, 3, 4, 5, 6, 7 hours, respectively ........................... 158 Figure 4-11 Plot of diameter and density versus growing time ............................ 159 Figure 4-12 PCPDFWIN ZnO XRD database file ............................................... 163 Figure 4-13 XRD spectrum of ZnO nanowires grown by 35 mM concentration . 164 Figure 4-13 XRD spectrum of ZnO nanowires grown by 95 mM concentration . 164 Figure 4-15. (a) SEM image of ZnO nanorods grown on ITO glass; (b) XRD analysisof ZnO nanorods grown by electrochemical deposition .... 165. Figure 4-16 PL spectrum of ZnO nanowires at different concentrations (a) 15 mM; (b) 35 mM; (c) 55 mM; (d) 75 mM; (e) 95 mM ; (f) 105 mM, respectively ....................................................................................... 167 Figure 4-17 UV peak of ZnO nanowires at different concentration ..................... 168 Figure 4-18 TEM and HRTEM images of ZnO nanowires .................................. 170 Figure 4-19 Sheet resistance and resistivity of AZO film at different spacing during sputtering process .................................................................. 176 Figure 4-20 Sheet resistance and resistivity of AZO film with different Ar flow during sputtering process .................................................................. 177 Figure 4-21 Sheet resistance and resistivity of AZO film with different RF power during sputtering process ....................................................... 178 Figure 4-22 Sheet resistance and resistivity of AZO film under different RTA temperatures ...................................................................................... 179 Figure 4-23 Optical transmittance of glass substrate ............................................ 180 Figure 4-24 UV/visible/IR light transmittance of AZO film at different sputtering spacing ............................................................................. 181 Figure 4-25 UV/visible/IR light transmittance of AZO film under different Ar - XVI 國立臺灣師範大學機電科技學系.

(20) flow during sputtering process .......................................................... 182 Figure 4-26 UV/visible/IR light transmittance of AZO film under different RF power during sputtering process ....................................................... 183 Figure 4-27 Fabrication process of LED die array (OM image): (a) Broad area AZO/ZnO nanowire/p-GaN; (b) after first lithography; (c) after etching; (d) after second lithography; (e) after Al evaporation; (f) after first lift-off; (g) after third lithography; (h) after Ni/Au evaporation and second lift-off, the finished LED die ...................... 190 Figure 4-28 Tilted SEM images of (a)(b) ZnO nanowires; (c)(d) AZO film/ZnO nanowire hybrid structure ................................................................. 191 Figure 4-29 (a)(b) LED testing by microprobe system, and (c)-(f) light emission of the fabricated LED die at forward bias 30 V, 40 V, 50 V, 60 V, respectively .............................................................................. 192 Figure 4-30 I-V curve of the n-ZnO film/ZnO nanowire/p-GaN film heterostructure LED .......................................................................... 193. - XVII 國立臺灣師範大學機電科技學系.

(21) 表. 目. 錄 4. Table 1-1. High-efficiency LEDs …………………………………………….... Table 1-2. Basic material properties of Zinc oxide............................................. 21. Table 2-1. Comparison on various researches of ZnO nanowire LED .............. 91. Table 3-1. List of experimental chemicals used in this study ............................ 131. Table 3-2. Experimental facilities ...................................................................... 137. Table 4-1. Properties of ZnO nanowires at different concentrations ................. 154. Table 4-2. Properties of ZnO nanowires for different growing time ................. 159. Table 4-3. EDS analysis of ZnO nanowires at different growing time .............. 160. Table 4-4. PL result of ZnO nanowries .............................................................. 168. Table 4-5. RF sputtering parameters of spacing-resistance experiment ............ 176. Table 4-6. RF sputtering parameters of Ar flow-resistance experiment ............ 177. Table 4-7. RF sputtering parameters of power-resistance experiment ............... 178. Table 4-8. Experimental parameters of RTA process for AZO film ................... 179. Table 4-9. RF sputtering parameters of spacing-transmittance experiment ....... 181. Table 4-10. RF sputtering parameters of Ar flow-transmittance experiment ....... 182. Table 4-11. RF sputtering parameters of power-transmittance experiment ......... 183. Table 4-12. Brief results of p-ZnO experiment .................................................... 186. - XVIII 國立臺灣師範大學機電科技學系.

(22) 第一章緒論. 第一章 1.1. 緒論. 前言光是人類文明賴以進步的原動力。1876 年 Thomas Edison 發明全世界第一. 個白熾燈泡(incandescent lamp)，並將此發明商業化，進行大規模的生產製造，開啟了電照明技術的發展。現今各式各樣照明光源與設備的開發，都顯示出人類對於各種照明設備的需求，而這些發明，不只帶動了經濟的發展，更大大的增進了人們生活上的舒適度。依光源的技術分類，大致上可將目前的照明燈源分為：第一類：燈絲結構的白熾燈與鹵素燈；第二類：螢光燈管結構的螢光燈與省電燈泡；第三類：氣體放電結構的水銀燈與複金屬燈；第四類：固態照明技術的發光二極體(light emitting diode, LED)。圖 1-1 為 LED 發展歷程圖【1】，LED 研究始於 1960 年代，當時紅光 LED 的誕生，促使許多學者投入相關研究工作。1970 年代 LED 開始商業化後，便朝向高亮度、多色化及高發光效率方向發展。1980 年代展發出 GaAsP 高亮度紅光 LED 及 AlGaAs 綠光 LED。1990 年代則開發出 AlGaInP 高亮度紅光、橙色 LED 與 GaInN 藍光、綠光 LED。目前日光燈是室內照明的主要光源，在追求節約能源、減少環境污染的科技發展趨勢下，LED 有機會成為下一世代的照明技術主流。一般認為，若 LED 的發光效率達 120 流明/瓦(lm/W)，極有可能取代目前節能效果最好的日光燈。圖 1-2 為各種照明光源之總光通量與發光效率分布圖【2】，縱軸是各種光源之總光通量(total luminescence)，單位是流明(lm)；橫軸為發光效率，單位是流. -1國立臺灣師範大學機電科技學系.

(23) 第一章緒論. 明/瓦(lm/W)。傳統之照明技術如螢光燈、高壓氣體放電燈、白熾燈等，其分布圖形均呈現出左下右上的趨勢，亦即總光通量越高其發光效率也越高。燈絲結構的白熾燈與鹵素燈之效率較差，總光通量為 140−2000 lm，效率從個位數至 20 lm/W。螢光燈源落在白熾燈源的右邊，其節能效果較好，因此只要光學特性及光通量足夠，此區塊均以螢光燈源為主，只有少數聚光用途的鹵素燈與 200 lm 以下的白熾燈未被取代。LED 的分布則為左上右下趨勢，亦即總光通量越高則發光效率越低。因此，即使達成節能之目的(發光效率高)，實際運用時卻無法滿足單顆光源之需求。表 1-1 為 2008 年 12 月市售高功率 LED 之效能統計表【2】，顯示出目前高功率 LED 總光通量越高，則發光效率越低之特性。以 Philips 公司的 K2 與 Rebel 兩種 LED 為例，其發光效率已達 100 lm/W，幾乎已超越傳統白熾燈與螢光燈源，但總光通量卻未達 10 W 鹵素燈之 140 lm。以 K2 5W 之 LED 來看，其總光通量已可比擬 25 W 鹵素燈，但發光效率卻大幅降低至 55 lm/W。若再考慮散熱問題、交直流供電及定電壓的電路耗損等問題，其發光效率更會大幅降低。圖 1-3 為 Cree 公司之 LED 光通量與接面溫度關係圖，若設定接面溫度 25 °C 之相對光通量為 100 %，當接面溫度上升至 85 °C，則光通量為原來的 85 %；若接面溫度上升至 150 °C，則光通量只剩下 65 %。由此可知，若 LED 的散熱機制設計不良，將大幅降低其發光效率，這也是目前 LED 技術發展所面臨的重要課題。綜上所述，LED 具有省電、元件固體性強、壽命長、耐震動、無汞污染與反應時間快等優點，被譽為未來的綠色照明技術。但目前 LED 仍有亮度(總光通量)不足、每單位流明之照明成本偏高、散熱不良造成發光效率降低等問題，限制其應用發展，有待產官學研各界研究克服。. -2國立臺灣師範大學機電科技學系.

(24) 第一章緒論. Figure 1-1. History of all kinds of LED【1】.. lm 200000 lm. 100000 lm. 10000 lm. LED. 1000 lm. LED LE D. 200 lm. lm/W 50 lm/W. 100 lm/W. 150 lm/W. 200 lm/W. Figure 1-2. Total luminescence versus light efficiency plot of lighting technology, modified from reference【2】.. -3國立臺灣師範大學機電科技學系.

(25) 第一章緒論. Table 1-1. High-efficiency LED (up to 2008/12), modified from reference【2】. Philips Lumileds. K2. Rebel. OSRAM. Golden. Cree. Ostar. Dragon Plus. AVAGO. Xlamp 7090. ASMT-. ASMT-. MC-E. MW. Mx. 1W. 5W. 1W. 1.8 W. 15 W. 1W. 4W. 1W. 3W. 95. 55. 100. 63. 50. 107. 92. 63. 52. lm/W. lm/W. lm/W. lm/W. lm/W. lm/W. lm/W. lm/W. lm/W. 95 lm. 275 lm. 100 lm. 117 lm. 750 lm. 107 lm. 370 lm. 63 lm. 161 lm. Figure 1-3. CREE relative luminous flux versus LED junction temperature.. -4國立臺灣師範大學機電科技學系.

(26) 第一章緒論. 1.2. 發光二極體之簡介. 1.2.1 發光二極體之原理發光二極體(light emitting diode, LED)的本質為 PN 接面二極體，通常由直接能隙之半導體材料構成 PN 接面，藉由外加電壓激發電子，而後透過電子、電洞對(electron hole pair, EHP)之輻射複合(radiative recombination)過程，產生光子而放射出光線。電子電洞對的複合速率，正比於電子與電洞濃度之乘積。圖 1-4 為最基本之 LED 結構，結構的最下方是磊晶 PN 層時使用的承載基板，作為 PN 層加工過程中的機械支撐，因此可以是不同於 PN 層的材料，但在材料選擇上必須考慮到晶格匹配的問題。若基板材料與 PN 材料的晶格常數(lattice constant)差異太大，則會在 LED 結構造成晶格應力，因而產生晶體缺陷，導致非輻射複合 (nonradiative recombination)增加，產生之能量最終造成晶格振動而非光子發射，如圖 1-5 所示，將造成 LED 發光效率的降低。PN 層通常為直接能隙材料，若要確定主要的輻射複合位於上方之 P 型材料層，則需在 N 型材料層進行重摻雜 (heavy doping)，以 N+為代表符號。由 P 型材料層產生之光子，將往四面八方放射，而往 N 側放射的光子，將被材料所吸收或從基板界面反射回 LED 之表面，這取決於基板厚度與 LED 的結構組成。圖 1-6 為 LED 之 PN 接面能帶圖【3】，延續前述之結構，N 側較 P 側為重摻雜。Ec 表示導帶(conductive band)能量，Ev 為價帶(valance band)能量，Eg 為材料之能隙(energy gap)，V0 為內建電位(build-in potential)。在無外加偏壓的情況下，內建電位 V0 將阻止電子從 N+層擴散至 P 層。當外加一順向偏壓 V，使內建電位降為 V0-V，讓 N+側之電子能夠擴散或注入 P 側，注入之電子在空乏區和 P 側複合導致光子自發發射。此複合主要發生在空乏區內，以及電子擴散長度可. -5國立臺灣師範大學機電科技學系.

(27) 第一章緒論. 達之 P 側中，稱之為主動區(active region)。光由電子電洞對複合而發光的現象，源自於少數載子的注入，稱為注入電致發光(injection electroluminescence)。由量子力學可知，當電子位於一個寬度為 L 之無限位能井(infinite potential well)中，如圖 1-19 所示，其能量將不再是連續狀態而呈現量子化，可表示為： En =. =2k 2 2me. (1-1). me 為電子之有效質量(effective mass)， = 為約化蒲朗克常數(Planck constant)，k 為波向量(wave vector)。波向量 k 為一量子數，可由下式表示： k=. nπ , n = 1, 2 ,3, ... L. (1-2). 直接能隙材料其導帶之最小處，與價帶之最大處，位於波向量軸之同處，氧化鋅、氮化鎵等都屬於直接能隙材料。圖 1-7 為簡化之直接能隙發光二極體能帶圖，假設位於導帶的電子與價帶的電洞具有拋物線的分佈關係： =2 k 2 En = Ec + (electron) 2me =2k 2 E p = Ev − (hole) 2m p. (1-3). 上式中，En 為電子能量，Ec 為導帶，Ep 為電洞能量，Ev 為價帶，mp 為電洞之質量。電子電洞對複合的過程中，受到激發而躍升於導帶之電子，與價帶之電洞複合產生光子，故光子的能量等於電子與電洞的能量差，約等於能隙： hν = En − E p ≅ Ec − Ev = Eg. 因此，選擇不同直接能隙的半導體材料，可以改變 LED 的發光波長。. -6國立臺灣師範大學機電科技學系. (1-4).

(28) 第一章緒論. 理想二極體的電壓電流關係可表示為： I = I s ⋅ (e. qV nkbT. − 1). (1-5). Is 為飽和電流(saturation current)，q 為基本電荷(elementary charge)，kb 為波茲曼常數(Boltzmann constant)，T 為接面之絕對溫度，n 為二極體之理想因子(ideal factor)，和二極體的材料及品質有關，通常介於 1−2 之間。但因為發光二極體的結構通常存在多層之堆疊，以增加載子的複合機率或光子射出機率，故往往會造成串接電阻 (series resistance)之增加。串接電阻 Rs 可能是由金屬與半導體界面的接觸電阻、LED 內部堆疊層間的接觸電阻或是材料本身的電阻所構成。若考慮到串接電阻的影響，則 LED 之電壓電流特性將變成： I = I s ⋅ (e. q (V − IRs ) nkT. − 1). (1-6). 在順向偏壓的操作條件下，上式之指數項將遠大於 1，故可改寫為： V = IRs +. nkT I ln( ) q Is. (1-7). 綜上所述，理想與考慮電阻效應後之實際 LED 電壓電流特性如圖 1-8 所示，且其臨界電壓(threshold voltage, Vth)約略等於擴散電壓(diffusion voltage, Vd)。 Vth Vd Eg / q. (1-8). 1.2.2 發光二極體之發光效率半導體發光效率通常由兩種方式來表達，第一種是量子效率 (quantum efficiency, QE)，第二種為功率轉換效率(wall-plug efficiency)；業界評估 LED 發光效率的單位−流明/瓦(lm/W)，即屬於功率轉換效率。. -7國立臺灣師範大學機電科技學系.

(29) 第一章緒論. 量子效率分為內部量子效率(internal quantum efficiency, IQE)、光萃取效率 (extraction efficiency)與外部量子效率(external quantum efficiency, EQE)。內部量子效率為光子數與電子電洞複合數量之比值；光萃取效率為電子電洞複合產生之光子，與順利脫離 LED 晶體之光子數量比；而內部量子效率與光萃取效率相乘積，即為外部量子效率。 ηint =. Pint /( hv) I/e. C ex =. P /( hv ) Pint /( hv ). ηext = ηint × Cex =. (1-9) p /( hv) I /e. 其中ηint 為內部量子效率、Cex 為光萃取效率、ηext 為外部量子效率、Pint 為光輸入功率、I 是電流、P 是光輸出功率、hv 是光子能量。影響 LED 內部量子效率的因素主要為非輻射複合速率，其正比於位於能隙中的非輻射缺陷密度(defect density)。當缺陷密度降低時，輻射效率就會增加。此外，由於電子的移動率高於電洞，所以輻射複合速率主要取決於 P 型摻雜。當 P 型摻雜增加時，複合速率將會增加，但同時又會降低注入速率，故需進行摻雜優化才能增進內部量子效率。功率轉換效率又可稱為輻射發光效率(radiant efficiency)，為光輸出功率與輸入功率之比。功率轉換效率較量子效率考量更多實際條件，所以較常用於評估 LED 的效率。功率轉換率可以用下式表達： η wp = ηint × Cex ×ηv ηv =. (hv) eV. (1-10). 其中ηwp 為功率效率、ηv 是電壓效率、V 是電壓、hν 是光子能量、e 是電荷。要提高ηv 主要就是藉由減少電阻與電壓以及臨界電流，綜合(1-9)、(1-10)之公式，可. -8國立臺灣師範大學機電科技學系.

(30) 第一章緒論. 以得到以下之結果： ηwp =. Pint /( hv) P /( hv ) ( hv ) P × × = I/e Pint /( hv ) eV IV. (1-11). 由上面的式子可以得知，在固定的 IV 功率之下，要提升功率轉換效率有幾種方法，增加內部量子效率、增加光取出效率或是改善電壓效率，這幾種方式即為得到高亮度與高效率 LED 最主要的方向。. 1.2.3 同質與異質結構發光二極體半導體元件之接面若為相同材料(相同能隙)，則可稱為同質接面 (homojunction) ；若兩接面為不同能隙之半導體材料，則稱為異質接面 (heterojunction)。對 LED 而言，若要選擇適當的半導體材料堆疊組成期望之元件，則必須考量到材料之間的晶格匹配與能隙。當堆疊的兩層材料不同，其晶格常數(lattice constant)差異太大，則會造成晶格不匹配(lattice mismatch)。晶格不匹配易造成磊晶製程的困難，但在磊晶薄膜未達臨界厚度 (critical layer thickness)前，可藉由晶格彈性應變(strain)的調整來達到異質磊晶效果。圖 1-9 為晶格不匹配之異質結構示意圖，假設兩材料皆為方形結構，其晶格常數分別為 a1 與 a2。當成長晶格匹配的磊晶層時，磊晶層將沿著晶面面邊界 (plane boundary)接合，以保持共同的二維單胞結構。但是在晶格不匹配的情況下，上方或下方之磊晶層產生彈性形變，才可以達到晶體結構的匹配。晶格不匹配可定義為【5】： Δa2 a2 − a1 ≡ a2 a2. (1-12). 若下方之磊晶層厚度遠大於上方之磊晶層，則上方磊晶層之原子將產生位移，在平行界面方向的晶格常數因應力而等同於下方磊晶層之晶格常數，而垂直方. -9國立臺灣師範大學機電科技學系.

(31) 第一章緒論. 向的晶格常數將因波伊松效應(Poisson effect)而改變。若平行的晶格常數收縮或產生壓縮形變(compressive strain)，則垂直方向的晶格常數會變大；反之，若平行的晶格常數膨脹或產生拉伸形變(tensile strain)，則垂直方向的晶格常數會變小。但是當上方之磊晶層達到臨界厚度，由於彈性應變能太高，晶體將藉由錯位差排(misfit dislocation)或貫穿式差排(threading dislocation)的產生來降低總自由能。差排基本理論指出，差排必須形成差排環、分支到其他差排處，或延伸到晶界處與晶體表面，不會終止於晶體內部【6】。當上方磊晶層的厚度持續增加，將會使位於兩磊晶層間的錯位差排，延伸至晶體之頂端或表面，稱之為貫穿式差排。差排缺陷通常產生於異質界面附近，此乃由於界面處屬於較不穩定且具有高應變能之區域，這些存在於晶體內部的差排，容易形成載子陷阱(trap) 及複合中心(recombination center)，使電子電洞之複合不以光子的型態發散能量，而轉為熱能，大幅降低 LED 之發光效率。此外，差排也提供載子快速擴散的路徑，對具備特定載子分佈的元件來說，載子沿著差排快速擴散或偏析 (segregation)，將影響元件之載子分佈設計，影響元件之性能。以圖 1-4 為例【3】，假設此為同質接面發光二極體，則其 PN 層為相同之材料，具有晶格常數匹配的優點，因此擁有較佳之接面品質。但此發光二極體之 P 側，在設計上必須較狹窄(厚度較薄)，以容許光子脫離而沒有過多的再吸收；當 P 側較為狹窄時，部分在 P 側注入之電子會擴散至結構表面，並經由缺陷在表面附近複合，此種非輻射複合會減少光的輸出。此外，若電子之擴散長度較長，則已發射之光子再度複合之機會增加，且光子被吸收的數量隨材料體積而增加，亦會減少光的輸出。異質接面的設計，可以提升發光二極體的光輸出強度。圖 1-10【4】為雙異質接面發光二極體之結構示意圖，由兩層能隙較大之材料構成侷限層 (confinement layer)，兩侷限層中間為能隙較小之材料，構成所謂的主動區(active. - 10 國立臺灣師範大學機電科技學系.

(32) 第一章緒論. region)，電子與電洞於此層複合產生光子放射，如圖 1-11【4】所示。光子自主動區脫離時，因侷限層之能隙大於主動區，而侷限層之折射率較小，故光子不會被吸收或反射而能穿透至元件表面，目前高亮度發光二極體均採用此種設計。 2006 年 Lim 等人提出 p-ZnO film/n-ZnO film 之氧化鋅同質結構 (homostructure)發光二極體【7】，其結構如圖 1-12 所示。此研究使用高溫濺鍍製程製作 P 型與 N 型之氧化鋅薄膜，P 型氧化鋅使用摻雜磷之氧化鋅靶材沉積，並以高溫熱退火製程活化 P 型氧化鋅之摻雜元素，也能提升其發光與電性表現。圖 1-13 為 PN 接面之 TEM 檢測結果，顯示 P 型氧化鋅薄膜在 N 型氧化鋅上磊晶成長，且無任何界面層。此發光二極體之臨界電壓為 3.2 V，接近理論值；EL 顯示於 380 nm 存在發光峰值。本篇文獻也提出氧化鋅雙異質接面發光二極體之研究，元件結構由上而下依序為 p-ZnO/Mg0.1Zn0.9O/ZnO:Ga/Mg0.1Zn0.9O /n-ZnO，侷限層(Mg0.1Zn0.9O)與主動區(ZnO:Ga)之厚度均為 40 nm，如圖 1-14 所示。兩種發光二極體之電激發光頻譜比較如圖 1-15，具量子井之發光二極體，其 EL 強度為同質接面之 1.55 倍。. - 11 國立臺灣師範大學機電科技學系.

(33) 第一章緒論. Figure 1-4. Typical LED structure【3】.. Figure 1-5. Radiative and nonradiative recombination of electron hole pair【4】.. - 12 國立臺灣師範大學機電科技學系.

(34) 第一章緒論. Electron energy n+. p. Ec. eVo (a). EF. Ec EF. Eg. n+. p. Eg hυ - Eg. Ev eVo. Ev. Distance into device. V. Electron in CB Hole in VB. Figure 1-6. Energy band diagram of unbiased and biased P−N+ LED【3】.. Figure 1-7. Parabolic electron and hole dispersion relations【4】.. - 13 國立臺灣師範大學機電科技學系.

(35) Injection current. 第一章緒論. Figure 1-8. Ideal and real I-V characteristic curve of LED.. Figure 1-9. Schematic diagram of lattice mismatch.. - 14 國立臺灣師範大學機電科技學系.

(36) 第一章緒論. Figure 1-10. Double heterojunction LED consisting of one quantum well【4】.. (a). (b) Figure 1-11. Carrier distribution of (a) homojunction; (b) heterojunction LED【4】.. - 15 國立臺灣師範大學機電科技學系.

(37) 第一章緒論. Figure 1-12. Schematic diagram of a p-n homojunction ZnO LED【7】.. Figure 1-13. Cross-sectional TEM image of the region around the p-type ZnO/n-type ZnO interface【7】.. - 16 國立臺灣師範大學機電科技學系.

(38) 第一章緒論. Figure 1-14. Schematic diagram and band diagram of ZnO heterojunction LED【7】.. Figure 1-15. Comparison of EL spectra of the p–n homojunction ZnO LED and the ZnO LED with Mg0.1Zn0.9O layers【7】.. - 17 國立臺灣師範大學機電科技學系.

(39) 第一章緒論. 氧化鋅材料. 1.3. 1.3.1 氧化鋅之材料特性氧化鋅 (ZnO) 是一種金屬氧化物材料，晶格結構為六方晶系 (hexagon close-packing )之纖維鋅礦(wurtzite)結構，晶格常數 a = 3.2539 Å，c = 5.2098 Å，結構如圖 1-16 所示【8】。其分子量為 81.39 g/mol，密度為 5.61 g/cm3，熔點約 2250 K，能隙(energy gap)約為 3.37 eV，激子束縛能(excition binding energy)為 60 meV，其他基本性質如表 1-2 所示。當鋅在氧氣的氣氛下形成氧化鋅單晶時，此單晶結構具有金屬多餘或氧缺乏的現象，伴隨間隙型鋅原子(zinc interstitials) 或氧空缺(oxygen vacancies)而產生 N 型半導體特性【9】，此因傳統半導體製程製備之氧化鋅薄膜，大多屬於鋅含量偏多之成份組成所致，其載子濃度約為 1017 cm-3，電阻率約 106 Ω⋅cm【10】。纖維鋅礦結構為六方對稱，不具有對稱中心，故氧化鋅具有顯著之壓電特性，適合用於發展表面聲波元件(surface acoustic wave, SAW)。氧化鋅為直接能隙材料，且具有相當大之激子束縛能，相對於氮化鎵之激子束縛能 25 meV，其室溫下的發光效率較一般材料好，可應用於短波長發光元件及雷射二極體【11】。氧化鋅的高能隙、高折射率、高透光性及非線性光學係數等特性，可應用於透明導電薄膜。用於一氧化碳、二氧化碳與氨氣等氣體感測器之製作，其操作溫度低且靈敏度高。此外，尚有變阻器、太陽能電池、薄膜電晶體液晶顯示器(TFT-LCD)等相關應用方向之研究。. 1.3.2. 氧化鋅之發光機制. 氧化鋅為直接能隙之半導體材料，激子束縛能高，適合應用於各種光電元件及短波長雷射。目前文獻中有各種關於氧化鋅發光機制的探討，包括紫外光. - 18 國立臺灣師範大學機電科技學系.

(40) 第一章緒論. (UV emission, 3.3 eV)、綠光(green emission, 2.2 eV)、黃光(yellow emission, 2.1 eV)、橘光(orange emission, 1.9 eV)、紅光(red emission, 1.8 eV)、近紅外光(near IR mission, 1.6 eV)等。在這些發光波段中，紫外光與綠光放射較為常見，且除了 UV emission 為本質發光(intrinsic emission)之外，其餘都與電子在雜質能階間躍遷有關。氧化鋅之紫外光放射機制主要有兩種，一種為帶間放射 (band-to-band emission)，另一種是激子放射(exciton emission)，如圖 1-17 所示。帶間放射是以外加能量，如電子束、雷射、X 射線等，加諸於氧化鋅材料上，使位於價帶(valance band)中較穩定的電子激發至導帶(conduction band)。處於激發態之電子相當不穩定，易由導帶跳回價帶，此時其能量釋放將以光子的形式產生。 Exciton 稱為激子，指的是透過庫倫作用力束縛在一起的電子電洞對。氧化鋅具有相當高的激子束縛能，約為 60 meV，常溫下的熱擾動不能將激子分離成自由電子與自由電洞，亦即氧化鋅內部之激子於常溫環境中穩定存在。氧化鋅的激子能階(exciton level)較靠近導帶，約為 3.2 eV，因此激發至導帶的電子跳回價帶時，可能會先落在激子能階，再落回價帶而放射出光子。氧化鋅的能隙約為 3.37 eV，光子的能量約為能隙之大小，且與波長成反比關係，如下所示： λ≅. 1240 Eg. (1-13). 激子放射形成的紫外光能量較小，且波長較長。綠光放射為氧化鋅另一種常見之發光波段，座落於可見光區域，且波長範圍較廣。對於綠光放射的機制眾說紛紜，常見的理論有氧空缺 (oxygen vacancies)、間隙型氧原子(interstitial oxygen)、鋅空缺(zinc vacancies)、間隙型鋅原子(zinc interstitial)、氧原子摻雜等結構缺陷，最常被引用的是 Vanheusden 等. - 19 國立臺灣師範大學機電科技學系.

(41) 第一章緒論. 人提出的 V 模型【12】，以單一氧化態的氧缺陷(single ionized oxygen defect)與能帶彎曲(band bending)來解釋氧化鋅的綠光放射，如圖 1-18 所示。氧化鋅晶體內部的氧空缺有三種不同形式：Vφ為捕捉兩個電子的氧空缺，相對於晶體屬於中性； Vo• 為捕捉一個電子的單一氧化態氧空缺，此缺陷具有順磁性 (paramagnetic)，相對於晶體為帶一價之正電；Vo••為沒有捕捉任何電子的氧空缺，相對於晶體為帶二價之正電。氧化鋅晶體內部主要的空缺以 Vo•為主，若以光激發氧化鋅使其價帶產生電洞，氧空缺捕捉之電子掉回價帶與電洞複合，將產生能量約為 2.42 eV 綠光，其波長約為 510−525 nm。文獻【13】研究單一氧化態的氧缺陷(Vo•)密度與綠光放射強度、自由載子密度之關係，如圖 1-19 所示。在較高溫度的環境下，PL 檢測顯示綠光發射強度與 Vo•密度均提高，顯示兩者具有正相關性。Dijken 等人以深陷阱能階(deep trap level)理論解釋綠光放射【14】，如圖 1-20 所示。深陷阱能階由 Vo•/ Vo••構成，圖左方為晶體表面 O2−/O− 之能量分佈，C 表示導帶，V 表示價帶，T 表示陷阱(trap)，S 表示晶體表面，此處假設電子電洞的非輻射複合發生於晶體表面。在晶體表面電子與電洞將產生帶間放射、激子放射或捕捉(trapping)，綠光放射源自於電子被激發後於價帶產生電洞，電洞被晶體表面之缺陷 Os2−捕捉，形成 Os−，而後經由穿隧(tunneling) 回到晶體內部，此時電洞處於 Vo•深陷阱能階，位於導帶之電子跳回深陷阱能階與電洞複合，產生光子放射。Lin 等人則計算出氧化鋅的各種缺陷能階【15】，如圖 1-21 所示，並提出綠光放射可能源自於氧原子佔據鋅空缺所形成的 OZn 缺陷(antisite defect)。. - 20 國立臺灣師範大學機電科技學系.

(42) 第一章緒論. Figure 1-16. Three-dimensional structure model of the unit cell of ZnO, and the corresponding projection along the a axis【8】.. Table 1-2. Basic material properties of Zinc oxide. Physical properties Molecular mass Density Melting point Boiling point Standard heat of formation Specific heat Thermal conductivity Direct bandgap energy Excition binding energy. Value 81.39 (g/mol) 5.61 (g/cm3) 2250 (K) 2073 (K) -83.24 (Kcal/mol) 0.125 (cal/g) 0.006 (cal/(cm⋅K)) 3.37 (eV) 60 (meV). - 21 國立臺灣師範大學機電科技學系.

(43) 第一章緒論. Figure 1-17. Energy band of ZnO. Eg is band gap, and Ex is exciton binding energy. (a) band-to-band emission; (b) exciton emission of UV light.. Figure 1-18. Schematic energy-band diagram of a ZnO grain in cross section【12】.. - 22 國立臺灣師範大學機電科技學系.

(44) 第一章緒論. Figure 1-19. Intensity of the (a) 510 nm green emission peak; (b) free-carrier electron density for powder ZnO as a function of reduction anneal temperature【13】.. - 23 國立臺灣師範大學機電科技學系.

(45) 第一章緒論. Figure 1-20. A schematic overview of the relaxation processes that take place upon photoexcitation of a ZnO particle【14】.. Figure 1-21. The draft of the calculated defect’s levels in ZnO film【15】.. - 24 國立臺灣師範大學機電科技學系.

(46) 第一章緒論. 量子侷限效應. 1.4. 1966 年研究人員從 10 nm 等級的超薄膜研究中發現量子侷限效應(quantum confinement effect)。在古典力學中，所有物質被明確地區分為純波動(wave)或純粒子(particle)。1905 年 Einstein 提出光電效應理論，顯示光波具備粒子之性質。 1924 年，de Broglie 提出物質波(matter wave)假說，認為一切物質均具有波動性質，如光子、電子等既是粒子也是波，同時具備波與粒子的特性，亦即所謂的波粒二象性(wavicle)。根據 de Broglie 學說，物質波波長 λ 與粒子動量 p 之關係為： λ = h/ p. (1-14). h 為蒲朗克常數(Planck constant)。物質之動量 p 可由下式求得： p = mv. (1-15). m 為物質之質量，v 為物質之速度。當質量 m 越小，波長 λ 越大，則其波動性質越明顯。以電子來說，其物質波長即如上式所述，與動量及速度有關。室溫下的半導體材料，其波長約為 10−30 nm。當材料尺寸縮小至奈米尺度時，其元件尺寸 L 接近其物質波波長 λ，此時可將材料內之電子視為機率波。當材料尺寸 L 與波長 λ 符合下列關係式，電子在材料內呈現駐波形式，而被侷限於此材料中。 L=. nλ , 2. n = 1, 2,3,.... (1-16). 奈米材料依維度可分為零維、一維和二維。零維奈米材料是指長、寬、高三維的尺度都在奈米尺寸(< 100 nm)內，形狀是點狀，例如奈米粒子、分子團 (cluster)、量子點(quantum dot)等。一維奈米材料則是指長、寬、高三維中，寬與高二維都是奈米尺度，形狀是長條狀，例如奈米柱 (nanorod) 、奈米線 (nanowire)、奈米管(nanotube)、奈米帶(nanobelt)等。二維奈米材料則是指長、寬、. - 25 國立臺灣師範大學機電科技學系.

(47) 第一章緒論. 高三維中，僅有高度是奈米尺度，形狀是平面，例如奈米薄膜、超晶格(superlattice) 等。在奈米材料內，因尺寸縮小導致的量子侷限效應，將影響光子與電子的運動。在零維奈米材料如量子點內，其長、寬、高三維均為奈米尺度，則其自由度為零。一維奈米材料如奈米線，其長與寬為奈米尺度，則其自由度為一維，意即電子將沿著奈米線的長軸方向運動。二維奈米材料如奈米薄膜，只有高度是奈米尺度，故其自由度為二維，亦即能在平面上移動。量子侷限效應影響電子在奈米材料的運動維度，可類比為光子在不同結構中被限制其運動方向，如圖 1-22 所示【16】。舉例而言，電子在奈米線中沿著長軸方向運動，可類比為光子在光纖中因折射率之差異產生全反射，而限制其運動方向為沿著光纖前進。電子在侷限效應下之行為，可用簡化之一維無限位能井(one dimension infinite potential well)探討【16】。考慮圖 1-23 之位能井，其邊界條件為： ⎧∞ , if x ≤ 0 and x ≥ L V ( x) = ⎨ ⎩ 0 , otherwise Ψ ( x) = 0 , if x = 0 and x = L. (1-17). 根據一維不隨時變之薛丁格方程式(Schrödinger equation)： =2 ∂ 2 − Ψ ( x) + V ( x ) ⋅ Ψ ( x ) = E ⋅ Ψ ( x) 2m ∂x 2. (1-18). 帶入位能井之邊界條件，可得電子能量 En 為： n2h2 , n = 1, 2, 3,... En = 8mL2. (1-19). 上式中，n 為量子數(quantum number)，h 為蒲朗克常數(Planck constant)，m 為電子質量，L 為位能井長度。由上式可知，電子能量呈現不連續分布。不同能階之電子能量差異ΔE 為： ΔE = (2n + 1). h2 8mL2. - 26 國立臺灣師範大學機電科技學系. (1-20).

(48) 第一章緒論. 波函數(wave function) Ψn(x)則為： Ψ n ( x) =. 2 nπ x ⋅ sin( ) L L. (1-21). 由能量差異公式可知，當位能井長度 L 減少，則 ΔE 增加，意即兩相繼能階 (successive level)之間距(spacing)越大。對此結論，1996 年 Alivisatos 有類似的探討【17】，文獻提出若以分子軌域與能帶的角度解釋，塊材(bulk)材料內部之原子數量較多，電子數量趨近於無限大，能階間距小能階密度大，故可視為連續性能帶。當材料尺寸下降至奈米等級，因原子數減少能階密度降低，能階間距增加，而呈現非連續式能階狀態，如圖 1-24 所示。前述一維無限位能井之結論，可推廣至二維、三維之無限位能井【16】。以二維無限位能井為例，其電子能量與波函數分別為： E n1,n 2 = (. n12 n2 2 h 2 + ) , n1 , n2 = 1, 2, 3,... L12 L2 2 8m. Ψ n1,n 2 ( x, y ) =. 4 nπx n πx ⋅ sin( 1 ) ⋅ sin( 2 ) L1 L2 L1 L2. (1-22). 但實際狀況中，位能並非無限大；以奈米線而言，假設電子於 x 與 z 軸方向受到侷限，沿著 y 方向前進，在位能有限且電子能量小於位能(E < V)的情況下，則其電子能量為：. En1,n 2,k y = Ec +. 2 2 n12 h 2 n2 2 h 2 = k y + + 8me Lx 2 8me Lz 2 2me. (1-23). n1 與 n2 分別為 x 與 z 方向之量子數，ky 為 y 方向的波向量(wavevector)，Lx 與 Lz 分別為 x 與 z 方向之長度，me 為電子有效質量(effective mass)。. - 27 國立臺灣師範大學機電科技學系.

(49) 第一章緒論. 以上針對一維奈米線結構進行量子侷限效應理論之探討，而在實際應用方面，目前最常見的應用為高亮度 LED 之多重量子井(multi quantum well, MQW) 結構。以砷化鎵(GaAs) LED 為例，其多重量子井結構由多層堆疊之 AlGaAs/GaAs 結構組成，每單一層 AlGaAs 或 GaAs 之厚度均為奈米等級，由兩層能隙較大之 AlGaAs 與中間層能隙較小之 GaAs，構成前述之有限位能井(量子井)，而對電子電洞形成侷限效應，使其只能於薄膜之平面方向移動，提高電子電洞之複合率，進而提昇 LED 之發光效率。在奈米線 / 奈米柱的部份， 2003 年 Park 等人在氧化鋅奈米柱上製作 ZnO/ZnMgO 多重量子井結構【18】，如圖 1-25 所示。此研究團隊使用有機金屬氣相磊晶(metal-organic vapor phase epitaxy, MOVPE)技術於氧化鋅奈米柱之頂端，製備 10 層週期性(period)的 ZnO/Zn(1-x)MgxO 多重量子井結構(x < 0.3)，單一層 ZnO 的厚度分別為 1.1 nm、1.7 nm 與 2.5 nm。奈米柱的整體長度為 0.5−2 μm，直徑為 20−70 nm。TEM 如圖 1-26 所示，顯示奈米柱確實存在多重量子井結構，但 ZnO 與 ZnMgO 之邊界不甚分明。2004 年 Kim 等人以 InGaN/GaN 多重量子井之奈米柱結構製作發光二極體【19】，其結構如圖 1-27 所示。此團隊以 MO-HVPE(metal-organic hydride vapor phase epitaxy)技術，製備 6 層週期性之 InGaN/GaN 多重量子井，每單一層之 InGaN 與 GaN 之厚度為 4.8 nm 與 12 nm，如圖 1-28 所示。光輸出功率檢測顯示，在 20 mA 之直流電源下，奈米柱發光二極體之光輸出功率，為不具備奈米柱結構之 4.3 倍，顯示奈米柱結構能大幅提升 LED 的發光效率。. - 28 國立臺灣師範大學機電科技學系.