透明指叉狀電極於氮化鎵發光二極體元件之光電特性研究

Abstract

本篇論文主要是研究將鎳/金薄膜應用於穿透式P型指叉狀電極在氮化鎵發光二極體元件上之光電特性差異，期望能以鎳/金薄膜的穿透特性，來改善傳統鉻/鉑/金電極的遮光效應，進而使其應用在氮化鎵發光二極體元件上時能具有高的光輸出功率，實驗設計以鎳/金薄膜做為穿透式P型指叉狀電極其應用在不同尺寸之氮化鎵發光二極體元件上，分析其光電特性。 首先進行鎳/金薄膜合金條件的優化，由合金前後的穿透率及電阻值差值分析，於合金溫度各為400℃、425℃、450℃、475℃、500℃、525℃、550℃時，穿透率在波段450nm，合金後減合金前的穿透率差值分別為23.2%、23.6%、27.7%、28.2%、29.6%、29.5%、30.8%，而合金後減合金前的電阻值差值則分別為2.00Ω、1.34Ω、3.22Ω、3.28Ω、3.71Ω、5.02Ω、8.33Ω，而取得最佳合金條件為475℃，可以獲得穿透率的提升並避免電阻值升幅過大。 接著以不同厚度之鎳/金薄膜分析穿透率及電阻值，由穿透率結果可知，鎳/金(5/5nm)、鎳/金(5/10nm)、鎳/金(5/20nm)與鎳/金(5/50nm)薄膜在波段450nm的穿透率值分別為71.3%、52.7%、34.3%與6.2%，相較於傳統鉻/鉑/金(50/25/2000nm)電極其穿透率為0%，皆具有較高之穿透率值；而穿透式電極在鎳/金(5/5nm)、鎳/金(5/10nm)、鎳/金(5/20nm)與鎳/金(5/50nm)之電阻值，分別為8.83Ω、1.64Ω、0.47Ω、0.13Ω，皆高於傳統鉻/鉑/金(50/25/2000nm)電極之電阻值2.7×10-3Ω，，在P型指叉狀電極的電流密度應以截面積為首要考量，所以我們進一步對於不同尺寸的氮化鎵二極體元件設計不同線寬的P型指叉狀電極，來進行後續實驗及結果分析。 最後探討不同厚度及不同線寬鎳/金薄膜穿透式P型指叉狀電極在09×36mil、20×20mil、22×46mil等不同尺寸氮化鎵發光二極體元件上之光電特性研究。於09×36mil操作電流為20mA時，傳統鉻/鉑/金(50nm/25nm/2000nm) P型指叉狀電極之操作電壓為2.88V，光輸出功率為17.0mW，功率轉換效率為29.52%，而在鎳/金薄膜穿透式P型指叉狀電極與傳統鉻/鉑/金(50nm/25nm/2000nm) P型指叉狀電極比較，操作電壓部份差異最低為0.1V，最高為0.17V，而在光輸出功率部份，差異最小為0.9mW，最大為3.0mW。而功率轉換效率(Wall Plug Efficiency)，差異最小為低9.5%，其條件為FW10μm@鎳/金(5/20nm)。於20×20mil操作電流為60mA時，傳統鉻/鉑/金(50nm/25nm/2000nm) P型指叉狀電極之操作電壓為3.04V，光輸出功率為43.4mW，功率轉換效率為23.80%，而在鎳/金薄膜穿透式P型指叉狀電極與傳統鉻/鉑/金(50nm/25nm/2000nm) P型指叉狀電極比較，操作電壓部份差異最低為0.03V，最高為0.25V，而在光輸出功率部份，差異最小為1.0mW，最大為5.6mW。而功率轉換效率(Wall Plug Efficiency)，差異最小為低6.7%，其條件為FW12μm@鎳/金(5/20nm)。於22×46mil操作電流為120mA時，傳統鉻/鉑/金(50nm/25nm/2000nm) P型指叉狀電極之操作電壓為3.11V，光輸出功率為126.1mW，功率轉換效率為34.80%，而在鎳/金薄膜穿透式P型指叉狀電極與傳統鉻/鉑/金(50nm/25nm/2000nm) P型指叉狀電極比較，操作電壓部份差異最低為0.4V，最高則為0.65V，而在光輸出功率部份，差異最小為9.9mW，最大為18.9mW。而功率轉換效率(Wall Plug Efficiency)，差異最小為低20.7%，其條件為FW8μm@鎳/金(5/50nm)。 綜合以上結論部分顯示出，鎳/金薄膜製作透明P-Finger與氮化鎵發光二極體，因無法有效的傳送電流，進而導致電流擁擠的現象，不僅導致其操作電壓高於傳統鉻/鉑/金P-Finger的氮化鎵發光二極體，更因元件的發光不均勻，使得光取出效率較傳統鉻/鉑/金P-Finger的氮化鎵發光二極體低。雖然從各種不同線寬及厚度條件鎳/金薄膜透明P-Finger的氮化鎵發光二極體間之比較及二維影像分佈均可以看出鎳/金薄膜透明P-Finger確實具備光穿透性並能增加光粹取效率，然而電流擁擠效應使得功率轉換效率不僅亦未見提昇，甚而遠低於傳統鉻/鉑/金P-Finger的氮化鎵發光二極體。

This thesis aims to study the photoelectric characteristic difference of the Ni/Au film applied to transparent P-finger electrode on gallium nitride light-emitting diode (GaN LED), particularly the transparent properties of the Ni/Au film to improve the traditional Cr/Pt/Au electrode blackout effect. The application of the said film on GaN LED can have a high optical output power. In the experiment, the Ni/Au film as transparent P-finger electrode is applied to different sizes of GaN LED, and its photovoltaic characteristics are analyzed. First, the optimization of the Ni/Au film was implemented based on the analysis of the resistance and transmittance differences before and after alloying. At temperatures of 400, 425, 450, 475, 500, 525, and 550 °C, the transmittance difference was 23.2%, 23.6%, 27.7%, 28.2%, 29.6%, 29.5%, and 30.8%, respectively, at a wavelength of 450 nm, and the resistance difference was 2.00, 1.34, 3.22, 3.28, 3.71, 5.02, and 8.33 Ω, respectively. We determined that 475 °C is the best alloying temperature that can enhance transmittance and significantly prevent an increase in resistance. Second, the resistance and transmittance differences of different thicknesses of Ni/Au were analyzed. The transmittance of Ni/Au (5/5 nm), Ni/Au (5/10 nm), Ni/Au (5/20 nm), and Ni/Au (5/50 nm) at a wavelength of 450 nm was 71.3%, 52.7%, 34.3%, and 6.2%, respectively, compared with that of the traditional Cr/Pt/Au electrode (50/25/2,000 nm) at 0%. The resistance of Ni/Au (5/5 nm), Ni/Au (5/10 nm), Ni/Au (5/20 nm), and Ni/Au (5/50 nm) was 8.83, 1.64, 0.47, and 0.13 Ω, respectively, compared with that of the traditional Cr/Pt/Au electrode (50/25/2,000 nm) at 2.7 × 10-3 Ω. However, given the current density of the P-finger electrode, the cross-sectional area of the electrode should be the primary consideration. Thus, we further designed and analyzed different finger widths of the P-finger electrode on GaN LED. Finally, we investigated the optical and electrical properties of different thicknesses and widths of the Ni/Au film as P-finger electrode on different sizes of GaN LED, that is, 09 × 36, 20 × 20, and 22 × 46 mil. At 09 × 36 mil with an operating current of 20 mA, the conventional Cr/Pt/Au P-finger electrode (50/25/2,000 nm) operating voltage is 2.88 V, the optical output power is 17.0 mW, and the wall plug efficiency (WPE) is 29.52%. Comparing the conventional Cr/Pt/Au P-finger electrode and Ni/Au P-finger electrode, the lowest operating voltage difference is 0.1 V up to 0.17 V and the lowest optical output power difference is 0.9 mW up to 3.0 mW. The minimum difference of the WPE is low at 9.5%, with the proviso that FW10µm@Ni/Au (5/20 nm). At 20 × 20 mil with an operating current of 60 mA, the conventional Cr/Pt/Au P-finger electrode (50/25/2,000 nm) operating voltage is 3.04 V, the optical output power is 43.4 mW, and the WPE is 23.80%. Comparing the conventional Cr/Pt/Au P-finger electrode and Ni/Au P-finger electrode, the lowest operating voltage difference is 0.03 V, the highest operating voltage difference is 0.25 V, and the lowest optical output power difference is 1.0 mW up to 5.6 mW. The minimum difference of the WPE is low at 6.7%, with the proviso that FW12µm@Ni/Au (5/20 nm). At 22 × 46 mil with an operating current of 120 mA, the conventional Cr/Pt/Au P-finger electrode (50/25/2,000 nm) operating voltage is 3.11 V, the optical output power is 126.1 mW, and the WPE is 34.80%. Comparing the conventional Cr/Pt/Au P-finger electrode with the Ni/Au P-finger electrode, the lowest and highest operating voltage differences are 0.4 and 0.65 V, respectively, and the lowest and highest optical output power differences are 9.9 and 18.9 mW, respectively. The minimum difference WPE is 20.7%, with the proviso that FW8µm@Ni/Au (5/50 nm). Based on these results, the inability of the Ni/Au film, as transparent P-finger electrode on GaN LEDs, to effectively transfer current leads not only to the current crowding phenomenon but also to a voltage higher than that of the conventional Cr/Pt/Au P-finger GaN LED. Moreover, the light extraction efficiency of the Ni/Au film is lower than that of the traditional Cr/Pt/Au P-finger GaN LED because of nonuniform light emission. Although the comparison of the distribution and two-dimensional images of different width and thickness conditions of Ni/Au film as transparent P-finger electrode on GaN LEDs shows an increase in the light extraction efficiency, the current crowding efficiency does not improve the WPE and is even lower than that of the traditional Cr/Pt/Au P-finger GaN LED.