Transferable Substrateless GaN LED Chips Produced by Femtosecond Laser Lift-Off for Flexible Sensor Applications ^†

Abstract

Transferable substrate-less InGaN/GaN light-emitting diode (LED) chips have successfully been fabricated in a laser lift-off (LLO) process employing high power ultrashort laser pulses with a wavelength of 520 nm. The irradiation of the sample was conducted in two sequential steps involving high and low pulse energies from the backside of the sapphire substrate, which led to self-detachment of the GaN stack layer without any additional tape release procedure. To guarantee their optoelectrical function and surface quality, the lifted LED chips were assessed in scanning electron microscopy (SEM) and electroluminescence (EL) measurements. Moreover, surface characterizations were done using atomic force microscopy (AFM) and Auger Electron Spectroscopy (AES).

1. Introduction

Gallium nitride (GaN) is a versatile semiconductor material that can be used to create LED devices for many different applications, including solid-state lighting and sensors. By transferring the nanostructured GaN LEDs onto flexible substrates, they could even exhibit enhanced performance compared to flexible organic LEDs in terms of their flexibility, longevity, and external quantum efficiency (EQE) [1]. Consequently, flexible inorganic LED devices have nowadays attracted more attention and possess potential to obtain novel functionalities (e.g., in optogenetics and biosensing applications) [2]. For the transfer of GaN LEDs from their original sapphire (Al₂O₃) substrate to other desired substrates, LLO has been developed and regularly used in the production of power LEDs as a reliable and reproducible technology. In a conventional LLO process, photons with energies above the band gap of GaN are normally used, so that the energy is dissipated at the sapphire/GaN interface [3]. These LLO processes are based on the expensive excimer laser technology. Alternatively, using femtosecond laser pulses with an extremely high density of photons allows for an induced multiphoton absorption, although the photon energy is lower than the semiconductor band gap [4]. In this work, a femtosecond LLO technology is proposed and the realization of free-standing blue GaN LED chips is demonstrated, which can potentially be employed for flexible device applications.

2. Material and Methods

The employed laser for LLO process emits at a wavelength of 520 nm, a pulse length of 350 fs, and a repetition rate of 200 kHz for the optical system as seen in Figure 1. The micromachining system consists of a galvanometer scanner, with 100 mm focal length telecentric f-theta lens integrated with linear motorized XYZ positioning stage system, which can control the sample position and working distance between the objective and the workpiece. The laser beam polarization qualities were kept by a λ/4 wave plate in this optical system. In this experiment, InGaN/GaN LED epitaxial layers on 2″ sapphire substrates from epitaxy competence center (ec²) are used, consisting of 4.4 μm heavily doped n-GaN layer in the range of 10¹⁸–10¹⁹ cm⁻³, 4 pairs of InGaN/GaN multi quantum wells (MQWs) and p-GaN with a total thickness of approximately 300 nm. The LED epi-layers were grown by MOVPE on 420 μm thick, non-patterned, double-side polished sapphire substrates.

The main process of LLO is scanning of a laser beam across the backside of the sapphire substrate to separate the whole GaN LED layer without tape release. Although the impinging photons from the laser have lower energy (2.3 eV) than the GaN band gap (3.4 eV), the directed laser pulse has a high density of photons and is transmitted through the sapphire to reach the interface between the n-GaN layer and sapphire, where it is absorbed by a non-linear two-photon excitation process. In this approach, the LED selective area release was performed using a two-step LLO process with different laser energies as illustrated in Figure 2a–c.

At the laser focus position, the beam is too small and the intensity is too high for lift-off [3]. Thus, the size of the beam was adjusted by moving the sample towards the objective lens until reaching a defocused position at the sapphire/GaN interface. In this case, the LLO process comprises two-step laser irradiation. The first step defines the GaN LED chip by irradiating the perimeter or frame on a 2 × 2 mm² area having a width of 0.5 mm from the outer line with a laser fluence of about 30 J/cm² as shown in Figure 2b. To freely release an LED chip area of 1 × 1 mm² from the sapphire substrate, the GaN LED was scanned with a lower laser fluence of 4.4 J/cm² (Figure 2c). The final result from the processes is a free-standing GaN LED chip, whose n-GaN surface was afterwards investigated by SEM (Figure 3).

3. Results and Discussion

Concerning the experimental results, InGaN/GaN LED chips with a total area of 1 × 1 mm² were successfully separated from the sapphire substrate in a reproducible process. Results from an LLO process of a free-standing GaN chip without transfer substrate and tape release are depicted in Figure 3.

Figure 3a shows an SEM image of a GaN LED wafer after selective outer frame removal by high laser fluence irradiation and remaining intact GaN LED in the middle of the area. After irradiating the intact GaN LED area with lower laser fluence, the GaN LED chip has been lifted and exhibited a crack-free p-GaN surface of the LED chip as seen in Figure 3b. On the other side, the n-GaN surface was affected by laser ablation resulting in marks on the surface (Figure 3c). To investigate the condition of the epilayer quality after LLO, we cut through the GaN LED chips and measured the thickness. From the cross-sectional view, a thickness of ~4.6 μm is observed, showing that the GaN LED epilayer is still complete and less damaged (Figure 3d). However, the n-GaN surface after LLO is relatively rough. Therefore, it has been investigated by AFM and AES (Figure 4a–c).

Figure 4a shows typical AFM images of the LLO-based n-GaN chip surfaces. From roughness measurements, the average and root mean square (RMS) roughnesses of the n-GaN surfaces were found to be in the 100 nm to 150 nm, with maximum height of the profile up to 0.5 μm. The RMS values are still comparable with the value of LLO using nanosecond laser reported by Xiaoying et al. (i.e., 50 nm) [5]. However, it should be noted that when the lifted chip needs to be transferred onto another foreign substrate, the produced roughness on its surface can still affect the bonding quality. As the surface chemical composition of the detached n-GaN layer becomes important [6], AES measurements were also conducted. The AES spectrum (Figure 4b) can be explained by a gallium surface oxide, while Al₂O₃ residues on the surface could not be detected. Presumably, the air-exposed GaN surface has been oxidized and also contaminated with carbon- containing species after the lift-off. The results from AFM and AES measurements indicate that the surface conditions of the LED chips need to be improved for a subsequent transfer to flexible substrates. In this case, short-time wet chemical etching treatment could be a candidate for solution to smoothen the surfaces [7]. To ensure the functionality of the LED chip after laser ablation, EL measurement has been performed. Regardless of the poor contacts between the probing needles with the sample, it is obvious that the chip could still emit the blue light (Figure 4c). Further improvements of the LLO process and chip transfer are foreseen.

4. Conclusions

We successfully separated ~4.6 μm thin InGaN/GaN LED chips from their sapphire substrate by a selective LLO technique with 520 nm femtosecond laser pulse irradiation. In this technique, two- step laser scanning with high and lower laser fluence 30 J/cm² and 4.4 J/cm², respectively, has been applied to release 1 × 1 mm² GaN LED. The AFM measurement after LLO shows that the n-GaN surface was relatively rough with an RMS roughness of ~150 nm. Further optimization of the lift-off process is therefore necessary to obtain smoother GaN surfaces suitable for transfer onto flexible substrates for various sensor applications.