Laser-Induced Periodic Nanoripples Generation on Gallium Nitride Using Near-infrared Ultrashort Pulsed Laser

Laser-induced periodic surface structure (LIPSS), or nanoripples, is a fascinating laser-induced surface morphology observed on a wide range of solid-state materials, with many potential applications in surface engineering, photonics, and optoelectronic devices. On the specific material of gallium nitride (GaN), the well-known formation mechanism and the potential applications of LIPSS are still being explored. Here, a near-infrared spectra of an ultrashort pulsed laser were used to generate periodic nanostructures with dimensions smaller than the laser wavelength on the surface of a GaN LED. From the result, the LIPSS maintained the direction equivalent to the GaN surface, with periodicity around 140–220 nm perpendicular to <1–100> substrate orientation. Finally, the advances in fabrication of LIPSS are presented as a potential nanograting for increasing the efficiency of LED-based GaN.


Introduction
In the last couple of decades, gallium nitride (GaN)-based light-emitting diodes (LEDs) have been rapidly developed into a versatile optoelectronic technology.The excellent properties of GaN-based LED such as long lifetime, broad emission wavelength, and low power, are utilized not only in conventional solid-state lighting but also in other applications, e.g., in display, communication, gas sensors, microscopes, opto-microbial, and biomedical sensing [1].Beside increasing the internal efficiency during epitaxial growth, current GaN-based visible LED technology is designed to possess high light extraction efficiency (LEE) that is achieved by using several methods such as transfer on an unconventional substrate, contact and packaging improvisation, and surface nano-patterning.Among those methods, surface nanopatterning is a promising technique to modify the critical angle of light generated from the GaN LED active region (multi-quantum well) to the outer side of GaN, and then to the air [2].Typical optimizations for such nanopatterning include wet (chemical) and dry (laser and mechanical) etching techniques.In this context, the femtosecond laser micromachining technique is a suitable tool to produce nanopatterning tailored to electrical, tribological, and optical surface modifications.To this end, however, controllable and homogenic surface nanopatterning is challenging both on metal and semiconductor materials.Nevertheless, the interaction between femtosecond laser and semiconductor material surfaces is capable to yield nanostructure morphology with regular patterns (ripples); a structure commonly referred to as laser-induced periodic surface structures (LIPSS) [3].LIPSSs have a sub-wavelength periodicity of the incident laser beam and are self-formed without the use of a mask lithography.Several LIPSS laser type generation shapes are actively investigated (e.g., with nanosecond and femtosecond lasers [4,5]), and yet still pose many open questions, such as control of periodicity, nanoripples direction, and the physical mechanism.To this end, several models and mechanisms of its generation have been underpinned, including the activation of surface plasmon polaritons, a parametric decay process, second harmonic generation, and surface plasma created by ultrashort pulsed laser irradiation [6].Furthermore, the polarization shape, laser power, scanning direction, and crystalline dependence of LIPSS formation on GaN has important implications for optimization of this laser processing techniques.By carefully controlling all those parameters, it is possible to create well-defined LIPSS patterns with a desired periodicity, which can be used for various applications, such as surface texturing, anti-reflection coatings, and light extraction enhancement in LEDs [6].As a response, this paper demonstrates the generation of LIPSS patterns on GaN using a femtosecond laser and predicts the suitable physical mechanism.The laser power-dependent ablation is being investigated in order to obtain precise periods and shapes.

Method
The LIPSS generation process was carried out using a simple setup based on a femtosecond laser micromachining experiment, as shown in Fig. 1.The sample is planar InGaN/GaN blue LED (0001) wafers with a center wavelength of 455 nm on transparent double-side polished sapphire substrates purchased from E-Wave Corporation, the United Kingdom (UK).From their provided datasheet, the total thickness of the whole LED layer stack is ~4 µm, as depicted in Fig. 1(a).The laser scanning process was performed in the air using a femtosecond laser oscillator Ti:sapphire-based laser from Spectra Physics.The laser beam was guided towards an x-y micro-stage by several mirrors during the scanning process.The laser source emits light at a center wavelength of 800 nm with a pulse width of 100 fs and a repetition rate of 1 kHz, as shown in Fig. 1(b).The laser patterning for the LIPSS generation process was carried out using the raster scanning method with a scanning speed of 0.5 ms -1 and a 10 mm lateral distance between two lines for a working area of 2×2 mm 2 [7].The laser beam was focused with an objective lens onto the GaN surface at its focal position, while the beam diameter was ~100 μm from the normal direction.

Results and Discussion
In nanosecond laser processing, the formation of LIPSS is primarily attributed to the interference of incident laser light with surface plasmon polaritons and specific wave modes.However, using femtosecond laser, the formation of LIPSS is less dependent on thermal effects and more on the nonlinear interaction of ultrafast laser pulses with matter.Although the exact mechanisms are still under investigation, multiphoton and tunneling processes are involved in this mechanism [8].Here the photon energy from the laser source (1.5 eV) is below the GaN band gap (3.4 eV).Consequently, photons can be absorbed in GaN (non-linear absorption) and promote the electron from the valence band to the conduction band with at least three photons via the virtual bandgap energy (sub-bandgap) transition [9].Accordingly, the blue emission (455 nm) can be captured while the laser source wavelength is near infrared (800 nm) during the laser scanning process, as shown in Fig. 2(a).Generally, the LIPSS on the surface produces two types of ripples, i.e., low spatial frequency LIPSS (LSFL) and high spatial frequency LIPSS (HSFL), of which both have periods smaller than the wavelength of laser irradiation.
In this experiment, the laser scanning and polarization were maintained perpendicular and parallel, respectively, to the <1-100> of the GaN wafer.The laser power was varied from 0.3 to 0.9 W while the other parameters remained constant to investigate the laser power dependence of the shape and direction of the nanoripple.LIPSS formation occurred when the laser fluence exceed the induced damage threshold of GaN (~0.5 J cm -2 ) [10].Laser power and fluence in this experiment were varied at values of 0.3, 0.7, and 0.9 W at 0.7, 1.7, 2.2 J cm -2 , respectively.Fig. 2(b) shows SEM images of a laser-irradiated GaN surface with various laser powers and constant GaN crystal orientation directions.SEM images reveal that the formation of distinct HSFL embedded inside LSFL occurs only at specific energies as these structures are distorted or destroyed at higher energies.These HSFLs are parallel to the incident polarization and orthogonally oriented to the primary LSFL structures.In other words, the HSFL formations were generated perpendicular to <1-100> and parallel with <11-20> GaN crystal.The randomization of LIPSS occurred due to the laser fluence being too high, which turns the ripples into disintegrated and irregular shapes [11].From the experiment, the periods of LSFL tend to slightly increase with the laser power increment, from 0.3 to 0.9 W at 140 to 220 nm, respectively, as shown in Fig. 3(a).Moreover, HSFL formation has a relatively stable period at ~60 nm on the GaN surface.Other research has reported that the width and periods of ripple depends on the in-plane crystal direction of material [6], which we will investigate in a future report.In dielectric materials and semiconductors, models of surface plasmon polaritons (SPPs) and a nano-plasmonic enhancement of laser field can be used to predict the size of LIPSS.The estimation periods (˄) using the SPPs model can be calculated using the following formula [4,12,13] : where λ is the laser wavelength, is angle of laser incidence, is the SPP wavelength, and ℇ ℇ is the dielectric constant of air and GaN, respectively.Using equation (1), The periodicity of LSFL can be determined at a width of 430 nm, however this value does not align with the actual measurement.Therefore, nanoripples can be recalculated using nano-plasmonic model with formula of λ/2n (n = refractive index of GaN = 2.3), which results in ~170 nm of ripple periodicity.From this calculation, we propose that the nanoripples were created due to asymmetrically localized laser-field enhancement of the nano-plasma from various laser powers on the GaN surface.Using the identical optical setup (800 nm of 100 fs laser source), previous research was reported that the LIPSS periods were found at 140-320 nm in ZnO [4].However, the laser-etched GaN epilayers extend up to 3.5 µm, resulting in the opening of the n-GaN surface, as shown in Fig. 3(b).The profilometer was utilized to identify the presence of nanoripples on the oscillating surface resulting from a laser micromachining technique that can be employed as a grating in optoelectronics technology.The future challenge is to control the periodicity and depth of the GaN epilayers accurately.

Conclusion
In conclusion, we have investigated the power-dependent ablation for LIPSS generation in a transparent planar InGaN/GaN blue LED (0001) wafer.From the result, the LIPSS generation maintained a crystallinity equivalent to that of the GaN surface on sapphire substrate, with the majority having periodicity around 140-220 nm perpendicular to <1-100> substrate orientation after varying the femtosecond laser power from 0.3-0.9W.Although the in-plane crystal direction of GaN or the scanning direction had no effect on the direction or periodicity of the LIPSSs, it seems this factor has an impact on their depth and aspect ratio.By controlling the depth, direction, and periodicity of LIPSS, we can use this technique to generate maskless nanograting for optoelectronics applications.

Figure 1 .
Figure 1.LIPSS generation on GaN using Femtosecond laser.(a).Schematic of a GaN-based blue LED epitaxial wafer.(b).optical setup and laser scanning procedure on sample.

Figure 3 .
Figure 3. Surface characterization.(a).The width of LIPSS depends on the laser power.(b).Contour of GaN on the surface.