The application of GaN-based materials in optoelectronic devices has attracted more and more attention. Due to the recent improvement in the brightness of GaN-based light-emitting diodes, GaN-based light-emitting diodes have been used in many fields, such as traffic lights, mobile phone backlights, automotive taillights, short-range communications, and opto-electronic computer interconnects. However, due to the effect of non-radiative defects, the internal quantum efficiency of GaN-based light-emitting diodes is much less than 100% at room temperature. In addition, the reason why the external quantum efficiency of the GaN-based light-emitting diode is not high is largely due to the total reflection problem caused by the large difference in the reflection coefficient between the nitride epitaxial layer and the air. According to reports, the reflection coefficients of GaN and air are 2.5 and 1, respectively. Therefore, the critical angle at which light generated in the active region of InGaN-GaN can propagate out is about 23°. This greatly limits the external quantum efficiency of GaN-based LEDs [1]. Many people have done a lot of work in improving the light-emitting efficiency of GaN-based LEDs, and there are many methods. The following mainly introduces the use of surface roughening to improve the light extraction efficiency of the device. Huang et al. [2] used a laser irradiation method to form a nano-scale rough layer on the upper p-GaN surface of a conventional IaGaN/GaN light-emitting diode. The structure of the conventional GaN light-emitting diode mentioned here is: 560 °C, 30nm thick GaN low temperature buffer layer, a 2μm thick undoped GaN layer, a 1.5μm thick n-GaN layer grown at 1050 °C, and a 5 cycle In 0.21 Ga 0.79 N 2nm /GaN 5nm multi-quantum well layer , a 0.3 μm thick p-GaN layer. And the device using the surface roughening treatment and the conventional device are prepared by the same growth method and steps. After surface roughening, the root mean square roughness of p-GaN surface increased from 2.7nm to 13.2nm. The results show that the device with surface roughening increases the brightness by 25% with a current of 20 mA. However, the operating voltage has been reduced from 3.55 to 3.3V. The system resistance of the device using surface roughening was reduced by 29% because of the increased contact area after surface roughening and higher hole concentration after laser irradiation. Best Smartwatch Best Smartwatch everyone enjoys luck , https://www.eeluckwatch.com
Many people [3-7] have used surface roughening to improve light extraction efficiency. The main methods used include surface roughening, wafer bonding and laser substrate stripping. However, these studies have only focused on the roughening of a surface on top of a GaN-based LED. WC Peng et al. [8] studied the use of double-layer surface roughening to improve light extraction efficiency. Wei chih peng et al. prepared three LED devices. As shown in Figure 1. The CV-LED indicates an LED that has not been subjected to any surface roughening treatment. The PR-LED represents an LED that is roughened by p-GaN. The DR-LED represents an LED for roughening the p-GaN layer and the undoped-GaN layer.
Figure 1: Schematic diagram of the device structure The device structure of the LED here includes a buffer layer grown at 550 °C on a sapphire substrate, a 2μm thick undoped-GaN layer grown at 1050 °C, and a 2μm thick n- grown at 1050 °C. The GaN layer, a 6-cycle InGaN (3 nm) / GaN (9 nm) multi-quantum well grown at 800 °C, and a p-GaN layer grown at 950 °C. The roughened surface and the SEM image before untreated are shown in Figure 2.
Figure 2. Scanning photo results
(a) untreated p-GaN surface
(b) roughened p-GaN surface
(c) The surface of the undoped-GaN roughened before the roughening treatment, the root mean square roughness of the p-GaN surface was 11.8 nm. The root mean square roughness of the roughened p-GaN surface reached 71.6 nm. The roughened undoped-GaN surface has many three-dimensional island structures. The root mean square roughness reached 91.9 nm.
After surface roughening, the performance of the device was not affected. At an injection current of 20 mA, the front side of the DR-LED has an intensity of 133 mcd, which is 2.77 times that of the un-roughened device. The backside light intensity is 178mcd, which is 2.37 times that of CV-LED devices. This is because after the surface is roughened, the photon can be given more opportunities to exit, and the light whose exit angle is outside the critical angle can also be refracted multiple times, and finally enter the adjacent angle, so that the device can obtain more light. .
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