Gallium nitride(GaN)is a very important direct wide bandgap semiconductor material that is widely used in optoelectronic and power electronic devices[
Laser & Optoelectronics Progress, Volume. 60, Issue 7, 0714005(2023)
[in Chinese]
Gallium nitride (GaN) has widespread applications in the semiconductor industry because of its desirable optoelectronic properties. The fabrication of surface structures on GaN thin films can effectively modify their optical and electrical properties, providing additional degrees of freedom for controlling GaN-based devices. Compared with lithography-based techniques, laser processing is maskless and much more efficient. This paper shows how surface micro-nano structures can be produced on GaN thin films using 355 nm nanosecond laser irradiation. The effects of the laser pulse energy, number of pulses, and polarization direction were studied. It was found that distinct micro-nano structures were formed under different irradiation conditions, and their geometries and elemental compositions were analyzed. The results indicate that different types of surface micro-nano structures can be produced on GaN thin films in a controllable manner using 355 nm nanosecond laser irradiation. The results of our study provide valuable guidance for the surface modification of GaN-based optoelectronic devices.
1 Introduction
Gallium nitride(GaN)is a very important direct wide bandgap semiconductor material that is widely used in optoelectronic and power electronic devices[
As an improvement over conventional,complex,and time-consuming lithography-based surface texturing techniques,laser processing is maskless and efficient,representing a promising alternative strategy[
In this study,surface micro-nano structures were produced on GaN thin films using 355 nm nanosecond laser irradiation. Under different laser irradiation conditions,distinct patterns could be formed,including craters containing hillocks,randomly distributed nanoparticles,LIPSS,and alternate concentric stripes. The geometries and elemental compositions of these surface structures were analyzed in detail. The results of this study demonstrate the possibility of controlling the formation of micro-nano structures on GaN thin films,which provides an important physical foundation for the development of processing techniques for GaN-based optoelectronic devices.
2 Experimental setup
2.1 Preparation of GaN films
The GaN thin films(3 µm thick)used in the experiment were grown on the c-planes of sapphire substrates(440 µm thick)using metal organic chemical vapor deposition(MOCVD). The MOCVD was performed in a multi-wafer rotating disk reactor at 1040 ℃,with GaN buffer layers(20 nm thick)grown at 530 ℃.
2.2 Laser processing setup
A schematic of the laser processing setup is given in
Figure 1.Laser processing setup. (a) Schematic of the laser processing setup; (b) laser intensity distribution at the focal plane; (c) laser intensity distribution at the sample surface
2.3 Characterization
After laser processing,the surface morphology of each sample was analyzed using an emission scanning electron microscope(SEM,MIRA 3 LMH)and atomic force microscope(AFM,Veeco Nanoscope IIIa). An elemental analysis was performed by SEM-electron energy disperse spectroscopy(SEM-EDS,Aztec Energy X-Max 20).
3 Results and discussions
3.1 Single-pulse irradiation
Figure 2.SEM and AFM images of the micro-nano structures formed on GaN thin films using single 355 nm nanosecond pulses. (a)-(c) Structures induced by a 2.44 mJ laser pulse; (d)(e) locations in (a) where the elemental analyses were performed; (f)-(h) structures induced by a 0.67 mJ laser pulse; (i)-(k) structures induced by a 0.25 mJ laser pulse
When the pulse energy was tuned down to 0.67 mJ,as shown in the SEM and AFM images in
Finally,when the pulse energy was further reduced to 0.25 mJ,LIPSS formed on the GaN surface[
The experimental results show different regimes after the single-pulse processing of GaN thin films. At a high laser pulse energy,most of the GaN material is ablated away,leaving a deep crater containing randomly distributed hillocks. Ga2O3 or GaN nanoparticles can be observed as a result of the thermal effects of the laser pulse irradiation. At a medium laser pulse energy,a shallow crater forms at the center,and LIPSS form at the periphery. At a low laser pulse energy,LIPSS form on the GaN surface with embedded nanoparticles. The spatial frequency of the LIPSS correspond to LSFL.
3.2 Multi-pulse irradiation
We used the low laser pulse energy corresponding to the LIPSS formation regime to study the effect of multi-pulse irradiation.
Figure 3.SEM images of the micro-nano structures formed on GaN thin films by 400 ns laser pulses. (a) Overall view of induced surface patterns; (b) enlarged views showing alternate concentric stripes on the thin film surface; (c) element weight percentages in adjacent stripes
The differences in the oxidation and particle morphology indicate different temperatures on adjacent stripes. The regions with more oxidation experience a higher temperature during multi-pulse laser irradiation. It is possible that these alternate stripes are formed as a result of the alternate distribution of laser field energy during irradiation caused by some interference effects. The detailed reason deserves further investigation.
We only studied the formation of surface structures under a single pulse and 400 pulses. We selected N = 400 as the pulse number because clear and high-quality surface structures composed of alternate stripes form under 400 laser irradiation pulses. Future studies will consider the evolution of surface micro-nano structures with a gradual increase in the number of pulses.
3.3 Effect of laser polarization
Laser polarization is known to have a large influence on the formation of LIPSS. Thus,the polarization angle of the linearly polarized laser pulses was rotated,and we studied its effect on the LIPSS.
Figure 4.Effect of laser polarization direction (laser polarization angle is rotated by 30° between adjacent sets of experiments, and yellow arrows indicate the incident laser polarization directions)
Finally,as illustrated by
Figure 5.Surface micro-nano structures induced by circularly polarized laser pulses
4 Conclusions
Surface micro-nano structures can be induced on GaN thin films by 355 nm nanosecond laser pulse irradiation. The effects of the laser pulse energy,number of pulses,and polarization direction on the surface structure formation were studied. For single-pulse irradiation,distinct patterns,including craters containing hillocks,randomly distributed nanoparticles,and LIPSS,can be formed under different laser pulse energies. For multi-pulse irradiation,alternate stripes appear with different extents of oxidation corresponding to distributed nanoparticles and LIPSS. The laser polarization was found to play an important role in LIPSS formation. The controllable formation of micro-nano structures by laser processing has potential applications in GaN-based optoelectronic devices.
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Yonggang Gu, Jian Niu, Jian Yang, Fang Dong, Hongxing Xu. [J]. Laser & Optoelectronics Progress, 2023, 60(7): 0714005
Category: Lasers and Laser Optics
Received: Feb. 6, 2023
Accepted: Mar. 2, 2023
Published Online: Mar. 14, 2023
The Author Email: Dong Fang (dongfang@whu.edu.cn)