以ALD沉積氧化鋅鎵透明導電薄膜 應用於正面發光近紫外發光二極體陣列

Fabrication of Surface-Emitting Near Ultraviolet Light-Emitting Diode Arrays with GZO Transparent Conductive Oxide Deposited by ALD

李偉豪(Lee, Wei Hao)

指導教授 ： 吳孟奇 ； 何充隆

國立清華大學/電機資訊學院/電子工程研究所/碩士(2015年)

摘要

近年來LED 產業特別是III-V族氮化鎵(GaN)之磊晶、製程等面上都發展的成熟與穩定，窄頻、指向性高、體積小、長壽命等都是LED的優點，而紫外光光源在近年來發展迅速，更有其相當廣泛的應用，其中近紫外光對核酸與蛋白質的吸收較低，對生物較無害處。紫外光發光二極體具備波長短、適於細微感測等特性，符合高演色性、發熱少、穩定性高之光源需求，因此在特殊應用領域仍被看好。然而，發光效率在紫外光 LED 上一直以來是個限制以及重要的課題，造成發光效率較差可能為電流擁擠效應帶來的熱、歐傑效應、磊晶造成的大量的缺陷以及缺陷造成的漏電流路徑，其中紫外光 LED 的製造原理，是藉著減少銦（In）在 InGaN/GaN LED 裡發光層的摻雜量，使得發光層能隙變大，進而造成輸出的光波長變短。隨著銦的摻雜減少，發光波段逐漸下降至400 nm以下，LED 主動層材料的能隙逐漸上升，使得量子井的效應消失，造成電子以及電洞的注入效率變差，最終造成光電轉換效率下降主要原因。所以在相同操作電流下，這也是主要為何紫光或紫外光 LED 的光輸出功率仍普遍比藍光LED差。
為了提升紫外光LED的發光效率，本論文設計8×8的紫外光LED陣列、pixel size為200 μm，配合原子層沉積設備成長具有高階梯覆蓋與均勻性的GZO作為連接各個pixel間的內導線，其厚度在100 nm可以有8 × 1020 cm-3高的載子濃度、約4 × 10-4 Ω-cm低電阻率以及在紫外波段仍有88%以上的高穿透率。在相同的磊晶結構下，同時製作傳統較大尺寸的LED元件，最後與8×8的LED陣列作比較是否在發光效率上有所提升。
製程完畢之元件在光譜儀上可量測到穩定的370 nm 發光波段，隨著注入電流增加有稍微紅移的趨勢；在光輸出功率上，8×8陣列元件在90 mA下可以有最大功率密度7.25 W/cm2，相較於傳統尺寸LED之70 mA下最大功率密度1.34 W/cm2有明顯的提升，並且可推論出元件可注入更高電流及延後衰減的原因為環繞式陰極電極設計可以有效分散電流擁擠效應所產生的熱；在發散角部分，陣列之設計可從128o提升之137o。以上實驗結果都指出8×8陣列的設計能有效提升 AlGaN/InGaN近紫外光的發光效率，較佳的光功率及發散角使得紫外光能更有效率且均勻的照射，在未來實驗設計配合尺寸微縮，可以設計出無光罩的微影技術。

關鍵字

發光二極體 ； 原子層級沉積系統 ； 陣列 ； 氧化鋅鎵 ； 透明導電膜 ； 紫外光

並列摘要

Ultraviolet light (200-400 nm) LEDs are not only developing rapidly but also used in many varied applications in recent years because of its short wavelength used for imperceptible biosensor, high color rendering, and excellent stability. Moreover, near ultraviolet light does less damage to human beings by comparison. However, efficiency droop of UV LED is a restriction and important issue. The reason of efficiency droop relates to current crowding effect, Auger recombination, and trap-assisted tunneling leakage by structural defects. Most of all, the main reason is that effect of quantum wells is gradually inefficient due to low In-doping in InGaN below 400 nm wavelength.
In order to increase the efficiency, surface-emitting 8×8 LED arrays with pixel size of 200 μm are designed and conventional broad-area LEDs (400μm×400μm) will also be fabricated from the same wafers for comparison studies. The atomic layer deposition (ALD) Ga-doped ZnO films with a thickness of 100 nm have an electron concentration of ~8 × 1020 cm-3, low resistivity of ~4 × 10-4 Ω-cm, and high transmittance (~88%) at the near ultraviolet wavelengths. ALD-GZO thin films are deposited as the internal wires connecting pixels because of excellent step coverage.
The peak wavelength is 370 nm and has a red-shift as the injection current increases. The single pixel of LED arrays reveals a maximum light output power density of 7.25 W/cm2 at 90 mA as compared to the broad-area LED of 1.34 W/cm2 at 70 mA. The LED arrays also exhibit an enhancement in divergence angle from 128o to 137o. These results reveal that LED arrays have significant improvement for the light output power in the UV region.

並列關鍵字

LED ； ALD ； Array ； Ga-doped ZnO ； Transparent Conductive Oxide ； Ultraviolet

參考文獻

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