1. Introduction

Laser processing techniques have been widely applied to improve the performance of InGaN-based light-emitting diodes (LEDs) in recent years for its flexibility, high reliability, high-speed and low damages [1]. Nowadays, with the development of high-performance lasers with ultra-short pulses, besides the traditional applications in the laser lift-off process to fabricate vertical/thin-film LEDs [2–4] and laser dicing [5, 6], laser processing techniques are playing more important role in introducing micro/nano structures in GaN-based LEDs to improve their performance. Up until now, different methods, including chip shaping by laser micromachining [7], mesa definition by laser direct writing [8], n-GaN roughening in vertical LEDs [9, 10], deep holes at backside of sapphire in flip-chip LEDs [11, 12], p-GaN and indium tin oxide (ITO) texturing in lateral LEDs [13–15], have been attempted and proposed in previous studies to realize the enhancement in the output power of LED devices. Furthermore, roughened sidewalls assisted with laser scribing are also fabricated to improve the side emission of LEDs [16–19]. Recently, laser stealth dicing technique has been widely used for LED die separation to improve the device optoelectrical performance. Taking advantage of the nonlinear multi-photon absorption effect under extremely high energy density of ultra-short laser pulses, the Al-O bond can be directly decomposed and air-voids will be introduced in the center of sapphire substrate [20]. Apart from separating LED dies, the laser stealth dicing method can only create a very small rough area on the sidewalls of LED chips, which improves the LOP of LEDs at a very limited extent [21–23]. However, this non-contact low-damaged method has enlightened us that it can provide great opportunities to achieve the fabrication of micro/nano structures within the sapphire substrate for further enhancing the performance of GaN-LEDs.

In this study, we report a method to fabricate an array of dumbbell-like air-voids inside the sapphire substrates in InGaN-based light-emitting diodes (LEDs) to improve the light output power (LOP) from LED device by a picosecond (Ps) pulse laser. Different from other methods to fabricate air-voids using complicated epi-design in GaN-based LEDs [24], inducing air-voids inside the sapphire substrate by a laser is fast and low-cost, and it will not damage GaN epilayers, so the electrical performance and the reliability of LED devices is supposed not to be influenced or impaired. By using this method, the LOP of packaged LEDs with laser-induced air-voids can be improved by 24.7% compared with conventional LEDs at an injection current of 100 mA. Far-field radiation pattern has demonstrated that this great improvement in LOP is due to the light scattering occurred at the interface of sapphire/air-voids. Current-Voltage curves have proved that the laser processing of air-voids did not degrade the LED electrical properties. Furthermore, reverse current at a level of ~5 nA at −10V has demonstrated an enhancement in the LED electrical performance with laser-induced air-voids. Two-step focus model, which is originated in the local heating effect around the laser focus, has been proposed to explain the formation of dumbbell-like air-voids by optical transfer matrix theory.

2. Experimental

The LED samples were grown on c-plane patterned sapphire substrates by metal-organic chemical vapor deposition (MOCVD). The GaN epilayers were consisted of a 30 nm thick low-temperature-grown GaN buffer layer, a 2 μm undoped GaN layer, a 2 μm heavily Si-doped n-type GaN layer, 10 pairs of InGaN(3 nm)/GaN(12 nm) multiple quantum wells (MQWs) and a 150 nm Mg-doped p-type GaN layer. Conventional lateral structure chips (585 × 255 μm2) were manufactured after deposition of indium tin oxide (ITO) transparent conductive layer (TCL) and p-n metal electrodes (Cr/Pt/Au) by an e-beam evaporator. Then the wafers were thinned and polished to a thickness of 200 μm. After that, the LED chips were settled on a moving stage with the sapphire face-up for the laser processing using a Ps pulse laser. The Ps pulse laser used here has a wavelength of 1064 nm, a repetition rate of 15 kHz, a pulse duration of 50 ps, and a total output power of ~1.0 W. First, Ps laser focus was adjusted at the depth of 100 μm right at the middle part inside of the sapphire substrate. The sketch map illustrating the laser setup for inducing air-voids has been shown in the Fig. 1(a). The stage movements were completely automatic by a preset recipe so that we could fabricate an array of air-voids inside the sapphire substrate via the laser scanning back and forth. Here, the spacing between laser scan-lines was set to be 50 μm, which had already reached the limit resolution of the moving stage. After laser scanning, the wafer was separated into individual chips for Au metal wires bonding and package. Another set of LEDs as reference samples were also fabricated conventionally diced by an Ns pulse laser. 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. All the LED dies were then packaged and covered with epoxy. In this study, surface morphology of the laser-induced air-voids was observed by scanning electronic microscopy (SEM). Then, optoelectronic properties of the LEDs with these laser-induced air-voids were analyzed in detail. In addition, far-field radiation patterns were measured to investigate light emission characteristic. Especially, light scattering effect at the air-voids/sapphire was also directly observed by optical microscopy from the sapphire backside when LED chips were driven. Finally, we also build a physical model to explain the formation of these dumbbell-like air-voids.

 

Fig. 1 (a) Laser setup for inducing air-voids. (b) Bird-view photo for a LED die. (c) Optical images from the backside of LEDs with laser-induced air-voids. (d) Close cross-sectional view on the laser-induced air-voids inside the sapphire substrate.

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3. Results and discussions

Figure 1(b) shows the bird-view microscopic photo of an LED die with laser-induced air-voids. And an optical image taken from the backside of the sapphire substrate is presented in Fig. 1(c). It’s clear to see that there are several laser scan lines inside the sapphire substrate which has assembled an array of 50 × 50 μm² squares. To reveal more detailed morphology information, the LED die has been split to let the laser-induced air-voids exposed, as illustrated in Fig. 1(d). It is worthy to note that, for each laser pulse, a pair of vertically stacked air-voids can be formed with different diameters of 1.2 μm and 0.5 μm for the bottom and the upper air-void, respectively. As it is well known, LEE of LEDs is limited to a low level because of the total reflection effect at the LED interfaces with the air. The key factor to improve the light extraction from the LEDs is to change the optical paths to enhance the escaping possibility for emitted photons [25]. However, in most LEDs, the propagation direction cannot be changed once the light come into the sapphire substrate because the sapphire is so hard for processing. In this case, via scanning laser direct-writing method, these laser-induced air-voids can reflect light back at the air-voids/sapphire interface and further enhance the light scattering inside of the LED devices [24]. Our analysis has been confirmed by the optoelectronic properties measurements. As shown in Fig. 2(a), the LOP of the LEDs with laser-induced air-voids has been greatly improved compared with conventional LEDs. In particular, at an injection current of 20 mA, the output power of the LEDs with laser-induced air-voids has reached 20.6 mW, which is 22.2% more than that of conventional LEDs (16.8 mW). When the current is further increased to 100mA, the enhancement will be increased to 24.7%, with an output of 69.3 mW for the LEDs with laser-induced air-voids over the conventional LEDs (55.6 mW). To further study the impact of laser-induced air-voids on the LED emission characteristic, far-field radiation patterns of LEDs w/o laser-induced air-voids has been tested, as presented in Fig. 2(b). In this step, LED dies are located on a rotate Al plate and the light intensities from different angles are recorded by a high-resolution spectrometer. The overall enhancement in the light intensities at different angles is consistent with the former LOP results. On the other hand, we can also discover that, for LEDs with laser-induced air-voids, much of the light intensities are centered from the angles (−80°, 80°), which is much larger than that of conventional LEDs, illustrating that the divergence angle has been increased due to the improved light scattering effect from the air-voids/sapphire interface.

 

Fig. 2 (a) LOP-I curves and (b) Normalized far-field light intensity angular distribution for LEDs w/o the laser-induced air-voids.

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In order to investigate the electrical performance of LEDs with laser-induced air-voids, current-voltage (I-V) curves have been tested for two type LED samples, as shown in Fig. 3. The almost overlapped I-V curves have demonstrated that the electrical properties of LED w/o laser-induced air-voids are rather similar. This is understandable because we don’t introduce any extra fabrication procedures except a noncontact and low damaged laser processing step. In addition, in our previous study, we have demonstrated that the thermal impact of the short-pulse laser stealth-dicing far away from the MQWs will not harm the LED emission [23]. Therefore, it implies that, the ohmic contact of the ITO TCL and p-GaN layer as well as the injection and the recombination process of the carriers in the MQWs will not be influenced by the laser processing. To further prove this, the leakage currents under different reverse bias voltages are presented in the inset of Fig. 3. Throughout the measured voltages, the leakage current for LEDs with laser-induced air-voids is lower than that of conventional LEDs diced by an Ns laser, demonstrating improved reverse I-V properties. In detail, leakage currents at −10V and −15V for LEDs with laser-induced air-voids are as low as ~5 nA and ~48 nA, overperforming the conventional LEDs with ~8nA and ~71nA, respectively. The improvement in the electrical performance is due to the lower mechanical and thermal damages to the GaN sidewalls [21].

 

Fig. 3 Current-Voltage curves for LEDs w/o the laser-induced air-voids. Inset shows the reverse I-V characteristic.

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Another important issue we cannot skip is the absorption of the debris produced in the laser processing step. Actually, the impact of the laser-induced air-voids on the LED light emission can be determined by two factors: the absorption and reflectance at the interface of the air-voids/sapphire. The total effect can be characterized in the final light output and angular distribution from the LED devices if one of the factors is dominant. As we discussed above, since it is very hard to evaluate the absorption of damaged part in the laser process, we are trying to figure it out through investigating the optoelectronic properties, from which we can see that both the LOP and the light intensity are indeed greatly improved by introducing air-voids. This has demonstrated from the side that the light reflected from the air-voids is far more over the light absorption. Although the debris is unavoidable in the laser processing procedure, it seems like the absorption is not strong enough to influence the final LOP of LED devices, particularly for the LEDs processed using the laser with ultra-short pulses. This is confirmed by the optical image of LEDs with laser-induced air-voids driven at 5 mA, which has been shown in Fig. 4(a). Compared with the optical picture from the un-driven LEDs, we can easily find that there are some bright lines instead of dark lines along the laser scan lines. These bright lines are actually from the boundaries of laser-induced air-voids when the light is reflected at the air-voids/sapphire interface, further verifying that the LOP enhancement is from the light scattering around the air-voids.

 

Fig. 4 (a)-(b) optical images of LEDs with laser-induced air-voids driven at 5 mA and 0 mA. (c)-(d) sketch map and SEM image for the theoretical analysis. The scale bar is 500 nm.

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Finally, we will explain the formation mechanism of dumbbell-like stacked air-voids by building up a two-step-focus model based on ray transfer matrix analysis. When the laser was first focused and absorbed by the Al₂O₃ lattice, the temperature of sapphire around the focus will be immediately increased to a very high level, giving rise to an increased refraction index n’ = 1.77 (@2000 K) around the first focus. This increase of refraction index will change the propagation of laser beam, leading to a second-time focusing process right upper the first focus. However, when the temperature further increased, Al₂O₃ will be decomposed and laser beam should be divergent because of the light scattering induced by the air-voids. Here, to analyze the second-time focus, together obtaining the values of parameters from the SEM pictures in Fig. 4(c), we assume the laser beam to be Gaussian with a waist size of w₀ = 1.4 μm and an axial distance of z₀ = −0.35μm. The parameters of w’ = 1.0 μm and z’ represent the waist size and the axial distance after the laser beam is focused in the second time. Moreover, by assuming the heating area of the first focus to be an ellipsoid lens with a thickness of D⁰ = 2.5 μm and an effective refraction index of n’, according to the optical transfer matrix theory, we can get matrixes as follows:

4. Conclusion

In summary, we have demonstrated a great improvement in the light output power (LOP) of GaN-based LEDs by introducing an array of dumbbell-like air-voids in the sapphire substrate using a Ps pulse laser scanning stealth dicing. At an injection current of 100mA, the LOP of packaged LEDs with these laser-induced air-voids can be enhanced by 24.7% compared with conventional LEDs. Far-field radiation pattern confirmed that this enhancement is due to the light scattering effect at the air-voids/sapphire interface which has transferred the guidewave modes into the escaping modes. Moreover, the reverse current-voltage curves have shown that these LED electrical performance can be also improved. This noncontact low-damage laser direct-writing method has implied a low-cost, effective and reliable way to improve the overall performance of LED devices which can be potentially applied in the mass production.