Selective-area micromachining of InGaN/GaN LED chips using ultrashort pulse laser

Gallium nitride (GaN) is a promising semiconductor material for creating versatile light-emitting diode (LED) devices in various applications, including solid-state lighting and sensors. GaN LED layer stack has been normally grown on sapphire substrate using MOVPE, although sapphire has poor electrical and thermal conductivities. In the recent development of flexible LEDs, the nanostructured GaN LEDs, which have been transferred into soft substrates, could demonstrate better performance compared to conventional flexible organic LEDs in terms of their flexibility, longevity, and efficiency [1]. Moreover, in production of power LEDs, laser lift-off (LLO) has been regularly utilized as a reliable and reproducible technology to transfer GaN LED chips from their original sapphire substrates to other foreign substrates. For the LLO process with nanosecond laser technology, photons with energies above the band gap of GaN are used, in which the energy is dissipated at the sapphire/GaN interface inducing decomposition of GaN into metallic Ga and gaseous N₂ [2]. Alternatively, using femtosecond laser pulses with an extremely high density of photons allows an induced multiphoton absorption to occur with non-thermal ablation process, regardless of the lower energy of the photon compared to the semiconductor bandgap [3]. Here, the employed laser for this LLO process has a wavelength of 520 nm and a pulse width of 350 fs. Even though the impinging photons have lower energy (2.3 eV) than the GaN bandgap (3.4 eV), the directed laser pulse is transmitted through the sapphire reaching its interface with GaN buffer layer, where it is absorbed by non-linear two-photon excitation. Consequently, the selected-area of GaN layer is decomposed resulting in its lift-off from the sapphire. The LED selective-area transfer is performed using a two-step LLO process with different laser energies. From the experiments, thin 4.6 μm InGaN/GaN LED chips with a total area of 1 × 1 mm² were successfully released from the sapphire substrate, which exhibited an emission spectrum with peak wavelengths of ~465 nm during cathodoluminescence characterization. Moreover, surface quality and opto-electrical property of the lifted LEDs were further investigated in AFM and electroluminescence measurements, respectively.

References

1. Guan, Nan, et al. Chemical Science Flexible Inorganic Light Emitting Diodes Based on Semiconductor Nanowires. Royal Society of Chemistry, 2017, pp. 7904–11, doi:10.1039/c7sc02573d.
2. Kim, Jaegu, et al. “Selective Lift-off of GaN Light-Emitting Diode from a Sapphire Substrate Using 266-Nm Diode-Pumped Solid-State Laser Irradiation.” Applied Physics A: Materials Science and Processing, vol. 122, no. 4, Springer Berlin Heidelberg, 2016, pp. 1–6, doi:10.1007/s00339-016-9928-7.
3. Rethfeld, B., et al. “Modelling Ultrafast Laser Ablation.” Journal of Physics D: Applied Physics, vol. 50, no. 19, 2017, doi:10.1088/1361-6463/50/19/193001.