開孔式電流阻障層結構於金屬反射式電極之氮化鎵發光二極體元件研究

Abstract

本論文為應用反射式電極(Cr/Al/Cr/Pt/Au)於氮化鎵發光二極體之研究，希望藉由開孔式結構應用於氮化鎵發光二極體的電流阻障層與透明導電層來增強反射式電極與氮化鎵發光二極體之間的附著力，並且分析不同開孔結構對於氮化鎵發光二極體之光電特性的影響。 首先以一般普通的氮化鎵發光二極體作為基礎，利用電流阻障層開孔結構來增加反射式電極與氮化鎵發光二極體的附著力。由實驗結果可知，電極脫落率由10%降低至5.5%；接著將這兩種結構應用於尺寸500×250µm2以及950×950µm2發光二極體元件，在20mA以及350mA的順向電流注入下，電流阻障層開孔結構的光輸出功率相較於普通結構衰減了1.76%以及1.97%，其主要的原因為電流阻障層開孔結構因移除了部分的電流阻障層，進而降低電流密度導致光輸出功率衰減。 再來利用透明導電層開孔結構來增加反射式電極與氮化鎵發光二極體的附著力，由實驗結果可知，相較於普通結構電極脫落率由10%降低至4%；接著將此結構應用於尺寸500×250µm2以及950×950µm2發光二極體元件，在20mA以及350mA的順向電流注入下，相較於普通結構光輸出功率衰減了1.71%以及0.5%，其主要的原因為透明導電層開孔結構因移除了部分的銦錫氧化物使得電流擴散不易，導致光輸出功率衰減。 最後利用電流阻障與透明導電層雙開孔結構來增加反射式電極與氮化鎵發光二極體的附著力，由實驗結果可知，相較於普通結構電極脫落的機率由10%降低至0%，成功的解決了反射式電極附著力不佳的問題；接著將此結構應用於尺寸500×250µm2發光二極體元件，在20mA的順向電流注入下，相較於普通結構光輸出功率增強了1.38%，造成光輸出功率增強的原因為雙開孔結構增加了反射式電極直接反射由P型氮化鎵所發出的光的面積，其面積相較於其他結構增加約36.58%，而此面積恰好占了整個發光區域的面積約2.25%；最後將此結構應用於尺寸950×950µm2發光二極體元件，在350mA的順向電流注入下，相較於普通結構光輸出功率衰減了4.51%，造成光輸出功率衰減的原因有：1.光罩圖形設計不良，2.隨著元件尺寸增大，光在元件中傳輸的路徑變長，導致光取出機率變小，3. 開孔結構改變了電流在元件中分布的狀況。4.磊晶片奇異點造成。 綜合上述，藉由電流阻障與透明導電層雙開孔結構可以成功的增加反射式電極(Cr/Al/Cr/Pt/Au) 與氮化鎵發光二極體的附著力，解決了反射式電極實際應用上最大的困擾。

In this study, the adhesion of the n and p reflective electrode (Cr/Al/Cr/Pt/Au) on GaN-based light-emitting diode (LED) was investigated. The application of a hole structure on the current blocking layer (CBL) and the transparent conductive layer (TCL) predictably enhanced the adhesion between the reflective electrode and the GaN-based LED. The effect of different hole structures on GaNLED optoelectric characteristics was also analyzed. We initially fabricated the normal structure GaN-based LED, which resulted in the CBL hole structure GaN-based LED. We used the CBL hole structure to enhance the adhesion between the reflective electrode and the GaN-based LED. According to the experimental results, the probability of pad peeling rate was reduced from 10% to 5.5%. Subsequently, the two structures were applied to GaN-based LEDs with chip sizes of 500×250 μm2 and 950×950 μm2. With injection currents of 20 and 350 mA, the light output power of the CBL hole structure was attenuated compared with the normal structure at 1.76% and 1.97% respectively. It’s attributed that the effect of the hole structure on the device’s current spreading status. Furthermore, we fabricated the TCL hole structure to enhance the adhesion between the reflective electrode and the GaN-based LED. In accordance with the experimental results, the probability of pad peeling rate was reduced from 10% to 4%. Then, we applied the TCL hole structure to GaN-based LEDs with chip sizes of 500×250 μm2 and 950×950 μm2. With injection currents of 20 and 350 mA, the light output power of the TCL hole structure was attenuated compared with the normal structure at 1.71% and 0.7% respectively. It’s attributed that the effect of the hole structure on the device’s current spreading status. Finally, we fabricated the CBL&TCL dual-layer hole structure to enhance the adhesion between the reflective electrode and the GaN-based LED. In accordance with the experimental results, the probability of pad peeling rate was reduced from 10% to 0%. These results reveal a successful solution to the poor adhesion of the reflective electrode. Subsequently, we applied the CBL&TCL dual-layer hole structure to GaN-based LEDs with a chip size of 500×250 μm2 and an injection current of 20 mA. The light output power of the CBL&TCL dual-layer hole structure was enhanced compared with that of the normal structure at 1.38%. It’s attributed that the 36.58% increase in direct reflection of the CBL&TCL dual-layer hole structure of the light area by the p-type reflective electrode. This entire light-emitting area accounted for 2.25%. This structure was then applied to GaN-based LEDs with a chip size of 950×950 μm2 and an injection current of 350 mA. The light output power of the CBL&TCL dual-layer hole structure was attenuated compared withthat of the normal structure at 4.51%. Analysis showed the reasons for the attenuation of light output power by the CBL&TCL dual-layer hole structure. The reasons are as follows: 1. poor design of mask pattern; 2. increase in device size results in longer light transmission path, which indicates the probability of smaller light escaping from the device; 3. the hole structure affects current spreading status in the device; and 4. poor epitaxy wafer. In summary, the CBL&TCL dual layer hole structure can successfully enhance the adhesion between the reflective electrode and the GaN-based LED to solve the practical application of reflective electrodes.