We report a multiple laser stealth dicing (multi-LSD) method to improve the light extraction efficiency (LEE) of InGaN-based light-emitting diodes (LEDs) using a picosecond (Ps) laser. Compared with conventional LEDs scribed by a nanosecond (Ns) laser and single stealth-diced LEDs, the light output power (LOP) of the LEDs using multi-LSD method can be improved by 26.5% and 11.2%, respectively. The enhanced LOP is due to the increased side emission from the large-area roughened sidewalls of the sapphire substrates fabricated in the multi-LSD process. Numerical simulation results show that the multi-LSD process has little thermal damages to the multiple quantum wells (MQWs) of the LEDs.
©2012 Optical Society of America
InGaN-based LEDs have attracted extensive interests because of their great advantages of high brightness, low energy consumption, and long lifetime, and thus have been widely used in various applications including general lighting, traffic lights, automobile headlights, and backlight units for liquid crystal displays. Due to the large difference of refraction index between GaN epitaxial material (n = 2.5) and air (n = 1), most photons emitted from the InGaN multiple quantum wells (MQWs) are trapped in the epi-layer and absorbed by GaN, which greatly limits the light extraction efficiency (LEE) of LED devices . Various techniques have been used to increase the LEE of LEDs such as patterned sapphire substrates [2, 3], surface roughening [4, 5], and two dimensional photonic crystal (2D-PC) structures [6, 7]. Recently, laser processing was extensively studied to improve the LED LEE. So far, prior work mainly focused on the n-GaN roughening in vertical structure by KrF laser , sapphire substrate backside roughening in flip chip by laser drilling technique  and geometrical shaping of LED chips by laser micromachining [10–13]. However, roughened sidewalls of the sapphire substrates were barely mentioned because of the chemically and mechanically stable structure of sapphire. To solve this problem, laser stealth dicing method was introduced to improve the LEE of LEDs [14, 15]. The enhanced LEE is due to the increased light extraction of LED devices from the roughened and low-thermal-damaged sidewalls of sapphire substrates. However, single stealth dicing method can only provide a very small area of roughened surface, which improves the LED LEE at a very limited extent. Moreover, the impact of the laser-scribed sidewalls of the sapphire substrates on the light extraction of LEDs still needs to be further investigated. And to analyze the thermal effect of the laser-scribing process to the InGaN MQWs of LEDs is also very important.
Based on such considerations, in this study, we introduce the multiple laser stealth dicing (multi-LSD) method using a Ps laser to fabricate large-area roughened sidewalls of the sapphire substrates, which greatly improve the LEE of GaN-based LEDs. Then we investigate the impact of different laser-scribing surfaces on the light extraction of LED devices. The LEDs were scribed by using an Ns and a Ps laser, respectively. Furthermore, the thermal effect of the multi-LSD process on the InGaN MQWs is analyzed through numerical simulations using finite element method.
The LED samples were grown on c-plane sapphire substrates by metal-organic chemical vapor deposition (MOCVD). The GaN epitaxial structure consisted of a 50 nm thick low-temperature-grown GaN buffer layer, a 2 μm thick undoped GaN layer, a 2 μm thick heavily Si-doped n-type GaN layer, 5 pairs of InGaN(3 nm)/GaN(12 nm) MQWs and a 100 nm thick Mg-doped p-type GaN layer. Conventional mesa structure chips (585 × 255 μm2) were manufactured after indium tin oxide (ITO) transparent conductive layer and p-n metal electrodes fabrication. Then the wafers were grinded and thinned to a thickness of 250 μm. After that, the LED chips were scribed from the backside of the polished sapphire substrates using an Ns pulse laser and a Ps pulse laser, respectively. The Ns pulse laser used here has a wavelength of 355 nm, a repetition rate of 120 kHz, a pulse duration of 150 ns, and a output power of 1.5 W. And the Ps pulse laser has a wavelength of 532 nm, a repetition rate of 15 kHz, a pulse duration of 80 ps, and a output power of 0.15 W. In order to investigate the impact of laser-scribing surfaces on light extraction of LEDs, different sidewalls of the sapphire substrates were fabricated under different laser-cutting conditions. In Ns laser-scribing process, we changed the depth of the laser-dicing layer by modifying the laser-dicing speed. In Ps laser-scribing process, multiple laser-dicing layers were formed on the sidewalls of the sapphire substrates via the adjustment of Ps laser-beam focus into the sapphire substrates. After the laser-scribing processes, the wafers were separated into individual LED chips after the mechanically splitting process.
In this study, surface morphology of the laser-scribing layers was observed by scanning electronic microscopy (SEM) and atomic force microscopy (AFM). Then, optoelectronic properties of the LEDs with different laser-scribing surfaces on the sidewalls of the sapphire substrates were analyzed. In addition, far-field radiation patterns of the laser-scribed LEDs were measured to investigate side emission of the LED chips. In addition, the side emission of the LEDs was also observed by optical microscopy when LED chips were vertically mounted on supporting substrates. In order to investigate the thermal effect of the multi-LSD process on the MQWs of LED chips, we also made numerical simulations using finite element method.
3. Results and Discussion
Figure 1(a)-(f) shows the SEM images of the LEDs with different sapphire sidewalls scribed by Ns/Ps pulse lasers. As shown in Fig. 1(a)-(c), Ns laser-scribing layers with depths of 30 μm, 60 μm, and 160 μm were fabricated on the sidewalls of the sapphire substrates by reducing the laser-scribing speed. Then, by modifying the focus of the Ps laser beam into the sapphire substrates and repeatedly scribing, multiple laser stealth dicing layers were fabricated on the sidewalls of the sapphire substrates. In this study, the Ps laser stealth dicing process was implemented for 1, 2, 3, 4, and 5 times, and the corresponding LED samples were defined as single laser stealth dicing LED (Single-LSD-LED), double laser stealth dicing LED (Double-LSD-LED), triple laser stealth dicing LED (Triple-LSD-LED), quadruple laser stealth dicing LED (Quadruple-LSD-LED), and quintuple laser stealth dicing LED (Quintuple-LSD-LED). The LEDs scribed by the multi-LSD method are shown in Fig. 1(d)-(f).
Figure 2(a)-(c) shows a close view of the surface morphology of a single laser-scribing layer on the sidewall of the sapphire substrates using multi-LSD method. Figure 2(a) shows the periodic stripe-shaped laser-scribing marks on the sidewall of the sapphire substrates. The AFM images in Fig. 2(b)-(c) shows the surface morphology in the yellow box of Fig. 2(a). The periodic grooves fabricated by the laser pulses were with a length of 20 μm and a depth of 490nm. A space of 3 μm between the grooves was determined by the laser-scribing speed. The roughened surface consisted of these laser-scratches on the sidewalls of the sapphire substrates decreased the total reflection effect occurred inside of the sapphire, resulting in an increased possibility for photons escaping from the LED devices. This enhancement was confirmed by our optoelectronic measurement results.
Figure 3(a) shows LOP-I-V curves of the LEDs with different laser-scribing surfaces. Here, all LED chips for the LOP measurements were packaged with epoxy. Compared with LEDs scribed by an Ns laser, the LOP of the scribed LEDs by using multi-LSD method was improved by 26.5%. The LOP improvement is attributed to the reduction in both debris and thermal damage of the sapphire substrates and the increase in side emission from the roughened surface of the sapphire substrate sidewalls . In addition, according to the measured current-forward voltage characteristics, there were no significant changes to the electrical properties of all these LEDs, as shown in Fig. 3(a). And to fully understand the impact of multi-LSD method on the electrical properties of the laser-scribed LEDs, we further investigated the reverse current-voltage characteristics of the laser-scribed LEDs. The results are shown in Fig. 3(b). The leakage currents of LED samples scribed by using an Ns laser-dicing (Ns-LD) method and multi-LSD method at a reverse voltage of −8 V are 1.62 μA(Ns-LD), 1.18 μA(Single-LSD), 1.15 μA(Double-LSD), 1.25 μA(Triple-LSD), 1.13 μA(Quadruple-LSD), and 1.20 μA(Quintuple-LSD), respectively. Based on these results, the leakage currents of multi-LSD LED samples at −8 V show negligible differences compared with that of the Single-LSD LED sample. It is concluded that the electrical properties of the laser-scribed LEDs are not damaged or degraded in the laser-scribing process. And it has been found that leakage current of 1.62 μA for the Ns-LD LED sample is a little larger than that of multi-LSD LED samples. This is due to the thermal impact of Ns LD process on the GaN epi-layer.
Figure 3(c) shows LOP of the LEDs with different laser-scribing surfaces at an injection current of 20 mA. As the laser stealth dicing process was implemented from 1 time to 5 times, the LOP of the LEDs was gradually increased from 16.01 mW to 17.81 mW. Based on the Single-LSD-LEDs, this 11.2% enhancement in the LOP of the LEDs scribed by using Quintuple-LSD method is due to the fact that the total reflection effect occurred inside of the sapphire was further reduced by the enlarged roughened laser-scribing surface on the sidewalls of the sapphire substrates. However, considering that the light extraction from the roughened laser-scribing surface, especially for the LEDs with less LSD processing times, is partly related to the laser-scribing position, it might be unfair to do comparison for Single-LSD-LEDs with other multi-LSD LEDs. So we measured the LOP of Single-LSD-LEDs with roughened layers at different positions at an injection current of 20mA. The LOP of Single-LSD-LEDs with roughened layers at a depth of 30 μm, 90 μm, 135 μm, and 195 μm on the sapphire sidewall is 15.55 mW, 16.05 mW, 16.15 mW, and 15.75 mW, respectively. Therefore, the optimized position for Single-LSD-LEDs is at the middle and upper part of the sapphire sidewall. Furthermore, taking into account the thermal damages, the scribing position should not be too deep so as not to deteriorate the electrical properties of LEDs. The results are illustrated in Fig. 3(d). Based on these results, for Single-LSD-LEDs, an optimized LOP of 16.15 mW is still lower than that of multi-LSD LED samples.
For Ns laser-scribing method, the LED light extraction was limited at a very low level due to the dirty laser-scribing surface on the sidewalls of sapphire substrates . However, it was worth noting that the sapphire sidewall of the LEDs scribed by the Ns laser was consisted of a contaminated layer, a roughened layer, and a cleaved layer, as shown in Fig. 1(a)-(c). And this roughened layer can improve the light extraction from the LED devices. Therefore, for Ns laser-scribing process, light extraction of LEDs from the sapphire sidewall could be greatly affected together by the roughened layer in the middle part and the contaminated layer in the outermost part of sapphire sidewalls. As illustrated in Fig. 1(a)-(c), keeping the depth of the contaminated layer at a level of 15 μm, as the roughened layer was increased from 15 μm to 45 μm, the output power of the LEDs at an injection current of 20 mA was improved from 14.5 mW to 15.45 mW. But when the contaminated layer was continually increased to 145 μm, the LOP of the LEDs gradually decreased to 14.02 mW, as shown in Fig. 3(c). As the laser focuses on the backside of the sapphire substrate, the generated heat caused by the high power laser pulses melts the sapphire substrate, and further diffuses away into the sapphire substrate during the pulse duration. During the dicing process, the evaporated sapphire usually produces debris onto the surface, which forms a low-transmittance contaminated layer. Moreover, as heat diffuses into the sapphire substrate, the temperature becomes lower, resulting in a less thermal-damaged roughened layer on the sidewall of the sapphire substrate .
Figure 4(a)-(f) shows corresponding luminescence pictures of the LEDs with different laser-scribing surfaces as shown in Fig. 1(a)-(f). In order to get the side emission photos, the LED chips were vertically mounted on the supporting substrates. For Ns laser-scribed sidewalls of the sapphire substrates, we can clearly see the different impacts of the laser-scribing layers on the light extraction of the LED devices. Not surprisingly, the dark contaminated layer shows that photons were absorbed by the debris, leading to a decrease of the light extraction of the Ns laser-scribed LEDs. The brighter roughened layer in the middle part of the sapphire sidewall demonstrated that more photons had escaped from the rough laser-scribed surface. However, for multi-LSD LEDs, the laser-scribed layers exhibited a higher brightness than the vicinal cleaved layers, which realized a side emission enhancement of the LED devices. In this part, we have to mention that, in order to make a good contrast of the impact of the laser-scribed surfaces on the LED side emission, all LED chips are driven at different currents. Moreover, as a result of the bad thermal conductivity of the supporting substrates and large injection current, the multi-LSD LEDs suffered great thermal issues, leading to a great red shift in the LED peak wavelength, as illustrated in Fig. 4(d)-(f).
To further investigate the influence of different laser-scribed sidewalls on the LEE of the LEDs, we also made measurements of the LED far-field radiation patterns. In order to obtain an accurate result, the LED chips used here were not covered with epoxy. Figure 5 shows the far-field radiation patterns of the multi-LSD LEDs compared with conventional LEDs scribed by an Ns laser. The LED chips used here were not covered with epoxy. The light intensity of the multi-LSD LEDs is greatly improved at a view angle of 30°-75°, and 105°-150° on the other side, compared with the LEDs scribed by using an Ns laser. Particularly, the light intensity could be enhanced as the stealth dicing times increased from 1, 3, to 5 times. This light intensity enhancement is considered as a result of an increased light scattering from the enlarged laser-scribed rough surface.
Finally, we made numerical simulations using finite element method to analyze the thermal damages of the multi-LSD process on the InGaN MQWs of the LEDs. Figure 6(a) shows the simulation conditions and the two-dimensional (2D) structure of the multi-LSD LED in our simulations. In order to obtain an accurate result, as shown in Fig. 4(f), the Ps laser stealth dicing process occurred at the nearest place to InGaN MQWs in our experiments was simulated. We assumed that the focus plane of the Ps laser was 25 μm under the InGaN MQWs. Moreover, our simulations were conducted under the irradiation conditions that the pulse energy Ep of the Ps laser was 10 μJ. And the pulse width pτ of the Ps laser varied from 40, 80 to 120 ps. The laser pulse was assumed to occur at t = 0. And the intensity distribution (spatial distribution) of the laser beam was considered to be Gaussian. The initial temperature was 300K and the LED structure in the laser-scribing process was assumed to have a thermal isolation boundary in the simulations. The simulation results are shown in Fig. 6(b)-(c). Figure 6(b) shows the time variation of the simulated temperature of the InGaN MQWs. The temperature around the InGaN MQWs will be rapidly increased in the laser pulse duration. And as the laser pulse duration is with a width of 40, 80, and 120 ps, when the laser pulse disappears, the temperature reaches a peak at about 440, 565, and 710 K, respectively. Then the heat diffuses away, which makes the temperature of InGaN MQWs decrease gradually in the few nanoseconds after the laser pulse ends. Therefore, in our experiments, with laser pulse duration of 80 ps, the InGaN MQWs will not be affected by the heat generated in the laser-scribing process. The simulated 2D temperature distributions at 40 ps, 80 ps, 1.65 ns, 13.5ns, and 37.0 ns were shown in Fig. 6(c).
We reported multiple laser stealth dicing method (multi-LSD) to improve the light extraction efficiency of the InGaN-based light-emitting diodes. Compared with the LEDs scribed by using single laser stealth dicing method, the LOP of the LEDs using multi-LSD method was improved by 11.2%. Furthermore, impact of different laser-scribed sidewalls of the sapphire substrates on the light extraction of the LED devices was investigated. Numerical simulations using finite element method demonstrated that the thermal damage of the multi-LSD process on the LED InGaN MQWs was little.
This work was partly supported by the National High Technology Program of China under Grant Nos. 2011AA03A105 and 2011AA03A103, National Natural Sciences Foundation of China under Grant No. 60806001, and National Basic Research Program of China under Grant No. 2011CB301904.
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