Study of the GaN-based Alternating Current Light-emitting Diode
|關鍵字:||發光二極體;氮化鎵;交流電;Light emitting diode;GaN;Alternating current|
|摘要:||本篇論文詳細研究並討論各種不同的交流電發光二極體（ACLED）之操作模式、效率比較、壽命測試結果及失效機制等特性。當高壓（high voltage，HV）ACLED、橋式（Wheatstone bridge，WB）ACLED、反向並串（Anti parallel，AP）ACLED、及直流電發光二極體（DCLED）均操作於直流電 1 W時，其光輸出功率分別為353.8、299.3、269.4、及324.7mW。在直流電操作情況下，HV-ACLED的發光效率明顯優於其他LED，而由二維光強度分析結果可發現輸入HV-ACLED的電流密度明顯較其他LED低且均勻。然而，當ACLED改以交流電1瓦驅動時，HV-ACLED、WB-ACELD、及AP-ACLED的光輸出功率則降低為273.4、218.9、以及189.9 mW，週期性的高輸入電流密度是造成HV-ACLED以交流電操作時的光輸出功率低於DCLED的主因。此外，模擬與實驗結果均顯示當WB-ACLED中的輸出微晶粒面積為整流微晶粒的sqrt(2)倍時WB-ACLED將有最低的串聯電阻，且此結果與陣列中輸出及整流微晶粒的數量無關。而在研究過程中也發現WB-ACLED串聯電阻大小是影響其光電轉換效率最主要的因素，也因此有最低串聯電阻的WB-ACLED設計將同時具有最高的光輸出效率。
經壽命測試後，倍頻微晶粒（包括所有HV-ACLED中的微晶粒及WB-ACLED中的輸出微晶粒）光電特性的衰退情況都比整流微晶粒（包括WB-ACLED中的整流微晶粒及所有AP-ACLED中的微晶粒）嚴重，而經6,000小時的室溫壽命測試後，WB-ACLED的光輸出功率降低為初始值的70%，相較於DCLED仍可維持在初始值的90%，WB-ACLED特性衰退情況明顯較為嚴重。另外在進行失效分析時也發現當整流微晶粒處於逆向偏壓時，電洞會週期性的堆積在n型GaN材料並和氫氧離子產生反應而形成GaOx氧化物，這些GaOx生成物會破壞微晶粒結構與金屬導線而造成WB-ACLED元件的失效。此外， 整合蕭基二極體（Schottky barrier diode，SBD）作為整流元件的12 V SBD-ACLED也完成製作，而使用重複磊晶方式堆疊SBD與LED磊晶結構以製作SBD-ACLED之技術在本論文中亦進行研究與討論。|
Characteristics including operation conditions, efficiency comparisons, lifetime test result, and failure mechanisms of different ACLEDs were extensively researched in this study. Light output power of the high voltage (HV) ACLED, Wheatstone bridge (WB) ACLED, Anti parallel (AP) ACLED, and DCLED operating in DC 1W were 353.8, 299.3, 269.4, and 324.7 mW, respectively. The two-dimension light output intensity mapping result indicated that injection current density of the HV-ACLED was lower and more uniform than that of other LEDs. However, light output power of the HV-ACLED, WB-ACLED, and AP-ACLED operating in AC 1W were 273.4, 218.9, and 189.9 mW, respectively. The periodic excessive inject current of the ACLED operating in AC caused light output power of the HV-ACLED operating in AC to be lower than that of the DCLED operating in the same DC input power. Besides, both simulation and experiment results indicated that the WB-ACLED performed the minimum series resistance when area of the output micro-LED was sqrt(2) (~140%) times larger than that of the rectifying micro-LED, and the result was irrelevant to the number of micro-LEDs in each branch. The observation also showed that the electrical efficiency dominated the wall plug efficiency of the WB-ACLED, and therefore the WB-ACLED having the lowest series resistance performed the highest light output power than other designs did. The lifetime test result indicated that both electrical and optical degradation of the double frequency micro-LED (micro-LEDs in the HV-ACLED or output branch of the WB-ACLED) was more serious than that of the single frequency micro-LED (micro-LEDs in the AP-ACLED or rectifying branch of the WB-ACLED). After 6,000 hours room temperature lifetime test, light output power of the WB-ACLED was only 70% of its initial value, which was lower than the DCLED (about 90%) aged in the similar conditions. A particular failure mechanism was observed in this study. When the rectifying micro-LED was reverse biased, positive holes accumulated in the n-type GaN material periodically and combined with the GaN material and OH- ions to generate the GaOx oxidation, which resulted in the failure of the WB-ACLED eventually. Furthermore, a 12 V green SBD-ACLED was demonstrated in this study, and combining SBD and LED epitaxial structures by the re-grown method to fabricate the SBD-ACLED device was also studied and discussed.
|Appears in Collections:||Thesis|