1 Low Operation Voltage of Nitride-Based LEDs with Al-Doped ZnO Transparent Contact Layer 授課老師： 李明倫 指導教授： 管鴻 學生：蘇奕昕 C. H. Kuo, a,z C. L. Yeh, a P. H. Chen, b W. C. Lai, b C. J. Tun, c J. K. Sheu, b and G. C. Chi a a.Department of Optics and Photonics, National Central University, Jhong-Li 32001, Taiwan b.Institute of Electro-Optical Science and Engineering, National Cheng Kung University,Tainan 70101, Taiwan c.National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan

2 目錄  序論  實驗步驟  實驗結果與討論  結論

3 序論  III-nitride-based materials have recently emerged as important semiconductor materials, leading to the realization of high-performance light emitters from the UV to blue and green spectralregions.  Lim et al. have shown a very low specific contact resistance of 3.4×10 −4 Ω cm 2 by using indium-oxide-doped ZnO (IZO) on p-GaN without an insert layer. Although the contact resistance was low, the operation voltage of the LED with an IZO transparent contact layer TCL was still high, 4.3 V at 20 mA, ~0.75 V higher than that of the LED with Ni/Au TCL.  Nakahara et al. have shown Ga-doped ZnO TCLs for nitride-based LED; by use of molecular beam epitaxy. It has been also shown that the LEDs with Ga-doped ZnO TCLs had the lowest forward voltage, 3.5 V at 20 mA.

4 實驗步驟 Sapphire MQW p-GaN n-GaN GaN-buffer TCL Step1 ：在藍寶石基板上沉積 30nm 的 GaN 緩衝 層，接著在 GaN 摻雜 4um 的 Si 長成 n- GaN 。 Step2 ：長 6 個循環的 InGaN(3nm)/GaN(15nm) 量子井。 Step3 ：摻雜 50nm 的 Mg 長成 p-GaN 。 Step4 ：沉積 TCL Step5 ：表面進行部份蝕刻直到 n-GaN 層顯露 出來 Step5 ：在 TCL 層與 n-GaN 層上沉積電極，材 料為 Cr/Pt/Au 。 30nm 4um 0.2um 50nm

5 實驗結果與討論 LEDI LEDII LEDIII (E-beam) (Sputter) Sapphire MQW p-GaN n-GaN GaN-buffer Sapphire MQW p-GaN n-GaN GaN-buffer AZO Sapphire MQW p-GaN n-GaN GaN-buffer AZO ITO

6 實驗結果與討論 在波長為 455nm 時個 別的穿透率為： ITO ： 90% AZO(E-beam) ： 93% AZO(sputter) ： 91%

7 實驗結果與討論

8 1. A lower E-beam evaporated AZO layer resistivity could result in a smaller contact resistance and a better current spreading. 2.it is well known that the electrical characteristics of p-GaN change its resistivity into semiconductor after the plasma damage. The worsened I-V characteristic of the sputtering-evaporated AZO contact that is attributed to the p-GaN layer was damaged by plasma in the sputter system.

9 實驗結果與討論 The 20 mA forward voltages measured from LEDI, LEDII, and LEDIII were 3.32, 3.33, and 4.91 V. The larger 20 mA forward voltage measured from the LEDIII could be attributed to the larger specific contact resistance between sputtering-evaporated AZO and the underneath n+-SPS structure,

10 實驗結果與討論 the output powers were 10.1, 11.8, and 6.0 mW for the LEDI, LEDII, and LEDIII, respectively. Compared with ITO LED, the 20 mA output power was enhanced by 16.8% for the E-beam evaporated AZO TCL LED. Sputtering-evaporated AZO contacts were deposited by dc sputtering result in a larger p- layer resistivity, a large AZO resistivity, a larger p-contact resistance, and a worse current spreading. Nonuniform current spreading leads to current crowding in localized areas of the device. Thus, LEDIII will have a severe heating effect. Severe self-heating can degrade the output power of the LED.

11 結論  E-beam evaporated AZO could provide an extremely high transparency(i.e.,93% at 455 nm) and a reasonably small 4.09 × 10 −2 Ωcm 2 specific contact resistance.  enhanced the 20 mA output power by 16.8%,as compared to the ITO LEDs, for the E-beam evaporated AZO TCL LED.  The 20 mA forward voltage of the LED with E-beam evaporated AZO TCL was almost the same as the LED with ITO TCL.  These results revealed that the E-beam evaporated AZO contact can serve as a suitable TCL for the fabrication of nitride-based lightemitting devices.