Surface morphology and straight crack generation of ultrafast laser irradiated β-Ga₂O₃

Recently, beta gallium oxide (β-Ga₂O₃) has received much attention due to its potential to dramatically improve wide bandgap semiconductor device applications in power electronics, UV solar blind detectors, and gas sensors.¹ The availability of bulk substrates via the melt growth technique,² wide bandgap energy (∼4.9 eV), and high breakdown voltage (8 MV/cm) make β-Ga₂O₃ an outstanding candidate material for next generation power electronics.³ The thermal stability of β-Ga₂O₃ facilitates the development of gas sensors operable at high temperatures. Realizing the potential of β-Ga₂O₃ for electronic and optical devices relies on the ability to control material properties and the development of material synthesis and processing techniques. Ultrafast (femtosecond pulse) laser processing provides a unique means of material modification⁴ which may provide a new pathway for β-Ga₂O₃ material synthesis and processing. The feasibility of irradiation in air ambient and direct laser writing provide opportunities for flexibility in material processing and low-cost processing approaches. Moreover, the femtosecond time scale allows transparent materials to absorb irradiation at sub-bandgap energy through nonlinear processes,⁵ leading to a nonequilibrium state between free electrons and phonons.^6,7 This nonequilibrium state can eventually be stabilized by the formation of surface nanostructures such as laser-induced periodic surface structures (LIPSS) at a high spatial frequency (HSFL). Furthermore, enhanced atomic drift associated with the high density of excited electrons can result in the generation of permanent point defects.^8–10 

The unique material interactions associated with ultrafast laser irradiation contribute to many applications. Modified surface and optical properties resulting from nanostructuring may be used to realize colorization,^11,12 hydrophobic surfaces,¹³ and surface texturing.¹⁴ The intentional introduction of point defects via ultrafast laser irradiation may also provide an opportunity to overcome challenges in the controlled doping of wide bandgap materials such as β-Ga₂O₃ by incorporating impurities into these point defects under nonequilibrium conditions introduced via femtosecond laser irradiation. Ultrafast laser irradiation on 4H-SiC, one of the widely utilized wide bandgap semiconductor materials, demonstrated electrical modification in the form of increased lateral conductance over orders of magnitude.¹⁵ 

Nakanishi et al.¹⁶ reported that the formation of nanogrooves under multiple femtosecond double-pulse irradiation focused on the laser beam at the depth of 100 μm from sample surface where the pulse energy (7.5 μJ per single pulse) is lower than the surface damage threshold (22 μJ for single pulse from this work). In contrast, our work focuses on a higher fluence regime near the damage threshold, and directly on surface modifications, where morphology differs dramatically from Nakanishi et al.¹⁶ Several damage thresholds of β-Ga₂O₃ associated with the generation of highly aligned straight thermal cracks and ablation are reported which can be interpreted as single exposure femtosecond laser-induced damage threshold (LIDT). The cracking behavior is observed for single pulse irradiation and without observable structural phase change of the initial β-Ga₂O₃ material. In addition, HSFL are observed without phase transition under multiple pulse irradiation below the ablation threshold.

A Clark MXR CPA-2001 Ti:sapphire laser (λ = 780 nm, 150 fs pulse width, 1 kHz repetition rate) was used for laser irradiation. The radius of the laser pulse is extracted by assuming a Gaussian beam with a DataRay WinCamD beam profiler. The laser power is measured by a thermal volume absorber (Ophir Optics). The laser energy was precisely controlled by optical components. The linearly polarized laser pulse propagates through a computer controlled halfwave plate, a polarizing beam splitter, and a neutral density filter in series. Fluence can be extracted by the relation $F=2P/fπw02$⁠, where f is repetition rate, $w0$ is the Gaussian beam radius extracted, and P is the laser power. Single pulse laser exposure (96 μm Gaussian beam radius) was investigated to extract the threshold fluence of the material and general morphological properties of β-Ga₂O₃ following ultrafast laser irradiation. Multiple pulse irradiation (19 μm Gaussian beam radius) and rastering (96 μm Gaussian beam radius) were conducted to explore the possibility of ultrafast laser processing of β-Ga₂O₃ over larger areas. The multiple pulse irradiation experiments were performed after high fluence single pulse (4 J/cm²) irradiation to scatter intentional debris throughout a defined grid area (100 μm between adjacent points on the grid). The intentional debris serves as “seed” sites that promote laser-induced damage and contribute to HSFL formation.¹⁷ The subsequent multiple pulse irradiation was performed between the two adjacent predamaged areas by high fluence exposure. Bulk β-Ga₂O₃ (010) substrates from the Tamura Corporation were used for experiments, consisting of both Sn-doped substrates with a nominal doping concentration of 10¹⁸ cm⁻³ and unintentionally doped (UID) substrates with a background donor concentration of 5 × 10¹⁷ cm⁻³. The Sn-doped material was irradiated for the single pulse irradiation and the rastering experiments, while the UID substrate material was used for the multiple pulse irradiation experiments. Single pulse irradiation of the substrate yields concentric regions with differing surface morphology, as evidenced by the contrast observed in an optical microscope. Threshold fluence for each modified region was extracted by a diameter regression method,¹⁸ plotting the effective radius of these boundaries vs laser fluence. Details of the experimental setup and the extraction method for threshold fluence using regression analysis are described in previous work.¹⁹ Various characterization techniques were used to examine the morphological and structure characteristics of laser irradiated β-Ga₂O₃ in greater detail. Scanning electron microscopy (SEM, Hitachi SU8000 FE-SEM), laser-confocal optical microscopy (Olympus OLS 4000 LEXT), and atomic force microscopy (AFM, Bruker ICON AFM) were implemented to study the surface morphology. Electron backscattered diffraction (EBSD, EDAX Hikari EBSD camera mounted on Tescan MIRA 3 XMU SEM) and Raman spectroscopy (Renishaw Invia confocal micro-Raman microscopy system) were performed to examine possible phase transitions of β-Ga₂O₃ during ultrafast laser irradiation.

Bulk (010) β-Ga₂O₃ was irradiated in air ambient with varying fluences (1.10–1.50 J/cm² for single pulse irradiation, 0.50–1.10 J/cm² for multiple pulse irradiation) and a number of pulses (single pulse and 10–50 000 shots for multiple pulse irradiation). Morphological changes are observed above particular laser fluence, as shown in the example SEM image for single-pulse-irradiated β-Ga₂O₃ at 1.40 J/cm² (Fig. 1). Concentric circles are observed in the contrast of the SEM images for the laser modified regions [Fig. 1(a)], implying the dependence of surface modification on local laser fluence. As can be seen in Figs. 1(b) and 1(c), the full area of the laser modified regions contains straight cracks. The characteristics of the outermost circular area forming the border with the unmodified β-Ga₂O₃ surface are attributed to a region covered by aligned straight cracks. The region inside the circular area was determined to be concave with a depth of approximately 50–80 nm (depending on the laser fluence) based on laser-confocal microscopy. The concave feature is attributed to laser ablation of β-Ga₂O₃. The inner circular area from the border of the ablation shows a darker area in the SEM contrast. The outer ring with darker SEM contrast is a result of the appearance of secondary cracks, which will be described further in Sec. IV. The threshold fluences for these three circular areas are extracted by the diameter regression method, applying a two-parameter nonlinear least squares regression analysis with effective radius and laser fluence (Fig. 2). The damaged area from five different batches with the same laser fluence was averaged and applied to the regression analysis in order to increase confidence in data fitting. The coefficient of determination (R²) of the data fit was larger than 0.99 for all thresholds indicating goodness of fit. The resulting threshold fluence for β-Ga₂O₃ irradiated by the Ti:sapphire laser for straight cracks, ablation, and the disappearance of secondary cracks are 1.11, 1.17, and 1.23 J/cm², respectively. For comparison, the ablation threshold under ultrafast laser irradiation (150 fs pulse width) have been reported as 0.32 J/cm² for Si.¹⁹ We suggest 1.11 J/cm² as the laser-induced damage threshold (LIDT) of β-Ga₂O₃. Here, we define “damage” of β-Ga₂O₃ as the straight crack formation (observable by optical microscopy), which indicates the mechanical breakdown of the material.

A closer examination of β-Ga₂O₃ following ultrafast laser irradiation above the ablation threshold reveals the nature of surface morphology changes, as shown in Fig. 1(b). A defining characteristic is the appearance of straight cracks that are aligned to a particular crystalline direction. Similar crack features have been reported in various single crystals: CaF₂,²⁰ NaCl,²¹ and MgO²² under femtosecond pulse irradiation and MgO,²³ SrTiO₃,²⁴ Mo,²⁵ sapphire,²⁶ and Si²⁷ under nanosecond or longer pulse irradiation. As shown in Figs. 1(a) and 1(c), the cracking covers the full laser modified region and crosses the ablation crater without discontinuity. This observation implies that the crack is formed after initial laser irradiation induced material modification. Moreover, the alignment of the crack direction is independent of the incident laser polarization direction [depicted in Fig. 1(a)]. Polarization independence was further verified by irradiation experiments where laser polarization was rotated by 45°, where the crack alignment was along the same crystalline direction and independent of the shift in the laser polarization direction. Two types of cracking characteristics were identified, represented by the thick and thin lines shown in Fig. 1(b). The thick and thin cracks appear to be nearly perpendicular, where the thick crack generally appears at a higher spatial frequency than the thin crack. A closer examination of the cracking characteristics is shown in the AFM image of Fig. 3, where the widths of the crack are around 60 nm and 30 nm and the depths are around 5 nm and 3 nm for the thick and thin cases, respectively. The depth of the crack may be an underestimate due to the physical limitation of the AFM tip size.

The response of β-Ga₂O₃ to multiple pulse irradiation below the single pulse LIDT was also examined. Significantly different surface morphology results for multiple pulse irradiation just below the single pulse ablation threshold, where HSFL features are observed (Fig. 4). Furthermore, along with HSFL formation, the irradiated Ga₂O₃ surface demonstrated significant ablation in the form of a crater as suggested by the contrast in SEM imaging. HSFL formation is attributed to optical coupling in combination with stress relaxation via accumulation of ultrafast laser-induced point defects in the case of semiconductors.⁸ However, in the case of wide bandgap materials such as β-Ga₂O₃, the HSFL formation mechanism would be different. The spatial period of the HSFL was determined using a 2-dimensional fast Fourier transform (FFT) analysis of SEM images. The period was determined to be approximately 227 nm, which is approximately a factor of 0.29 times the laser wavelength. For a small number of pulses, “dumbbell” shaped features are observed [see Fig. 4(b) for 10 shots at 0.90 J/cm²] indicating the early onset of HSFL formation, similar to prior reports on 4H-SiC.¹⁷ Moreover, the appearance of the debris near the dumbbell structure implies the contribution of Coulomb explosion during HSFL formation. The threshold fluence for HSFL formation due to multiple pulse irradiation can be defined by identifying the fluence where these small surface features begin to appear. The threshold fluence for β-Ga₂O₃ under a range of laser fluence and number of pulses is presented by Fig. 5 using this criterion to define the threshold. The fitting curve of Fig. 5 is the exponential decay formula representing the convergence of the damage threshold at a high number of shots.²⁸ In these experiments, no HSFL features were observed for fluences below 0.60 J/cm² for up to 50 000 shots.

The observed cracking characteristics for irradiation above the ablation threshold is not commonly observed for laser irradiated materials. To investigate the origin of the straight crack, the alignment relative to the crystallographic direction of β-Ga₂O₃ was examined. As shown in Fig. 1(b), the angle of the thick crack from the [102] direction is 50°, which compares closely to [001] and [102] directions with an angle difference of 53.8°. The thin crack which is approximately perpendicular to the thick crack does not appear to have a close match to the [100] direction, implying that the thin crack may not be attributed to the intrinsic well known as (001) and (100) natural cleavages plane of β-Ga₂O₃, which have been reported experimentally² and theoretically.²⁹ The thick, primary, crack crosses the ablation crater and continues to the border of laser modified region. Figure 1(c) which shows the area near the border including the ablation crater associated with low local fluence exhibits noticeable secondary cracks located between the primary cracks with an angle of 53° from the [102] direction. The angle matches with the angle of the (100) plane, implying that this secondary cracking behavior is related to the natural cleavage of β-Ga₂O₃. Surprisingly, the secondary crack is thin and less frequent in the central area having higher local fluence with certain threshold fluence, as indicated in Fig. 2. The decrease of the secondary crack in the central area results in the observed contrast difference in the SEM image. This implies that the cracking behavior is likely to have a strong correlation with local fluence but the reduction of secondary crack at the beam center is counterintuitive. In view of mechanics, the spacing of the periodic crack under thermal stress is in inverse proportion to applied stress.³⁰ The angular deviation from the (100) cleavage planes of the thick, primary, crack at the central area may be attributed to modified physical properties due to elevated temperature from the laser pulse. A possible origin of the crack formation is thermal stress after laser pulse irradiation. Nanosecond pulsed laser irradiation of Si showed very similar cracking characteristics, as reported by Lv et al.²⁷ In that work, the appearance of the cracking characteristics was attributed to laser-induced thermal stress, where cracks were observed when thermally induced stress overcomes the mechanical strength of Si. The two directions of Si cracks were similarly perpendicular to each other, with alignment along the [100] and [001] crystal directions. The cracking may be more apparent in β-Ga₂O₃ due to the low thermal conductivity relative to other semiconductor materials (27 W/m K for β-Ga₂O₃ along the [010] crystal direction and 10.9 W/m K along the [100] crystal direction,³¹ 149 W/m K for Si,³² and 330 W/m K for 4H-SiC along [0001] crystal direction³³). Furthermore, the mechanical strength of β-Ga₂O₃ is not as high as semiconductors such as Si or 4H-SiC: mechanical hardness of 10.7 GPa³⁴ (11.4 GPa for Si³⁵ and 36.2 GPa for 4H-SiC³⁶) and Young's modulus of 138 GPa³⁴ (150 GPa for Si³² and 413 GPa for 4H-SiC³⁶). While the short time scale associated with femtosecond laser irradiation is often characterized as nonthermal due to the short time scales relative to phonon interactions, some residual heating associated with laser irradiation can accumulate and will not be dissipated easily due to the low thermal conductivity of the material. The thermal stress induced from accumulated heat can reach the mechanical failure point where stress is released by the formation of the observed crack. The anisotropic nature of β-Ga₂O₃, including differences in thermal conductivity along varying crystalline direction, may affect the plane of maximum stress to be shifted from natural cleavage planes. Moreover, the frequency difference between the thin and the thick cracks of β-Ga₂O₃ may be directly attributed to anisotropy in mechanical properties. To summarize, the observed cracking characteristics following ultrafast laser irradiation may be attributed to heat accumulation and stress release via natural cleavage or mechanical failure, where the observed alignment is defined by the anisotropic nature of β-Ga₂O₃.

The observed morphological changes resulting from single pulse laser irradiation above the ablation threshold and multiple pulse laser irradiation below the ablation threshold raise the question of whether or not the structural properties of the material are altered. Ga₂O₃ has several crystalline polymorph (α, β, γ, ɛ, and κ) phases.³⁷ The β-phase has a monoclinic crystal structure with a 103.8° angle between [100] and [001] crystal directions in the (010) plane of the substrate, while the angle of α- (trigonal), γ- (cubic), ɛ- (hexagonal), and κ-phase (orthorhombic) are all perpendicular. Therefore, the fact that thin and thick cracks are perpendicular may imply a phase transition from β-phase to another phase containing natural cleavage planes with perpendicular orientation. To identify a possible phase transition occurring for the ultrafast laser irradiated surface, EBSD and Raman spectroscopy were performed. EBSD is a useful technique for microstructural characterization of crystalline phase and orientation. As shown in Fig. 6, the Kikuchi pattern of any single-pulse-irradiated β-Ga₂O₃ surface was identified as the β-phase, suggesting that the crystalline integrity of the material is preserved.

Samples with multiple pulse laser irradiation with raster scanning (2.5 mm/s scan speed, corresponding to 40 shots at one spatial location) provide a sizable optical area (100 × 100 μm² scale) to further investigate the structural properties with Raman spectroscopy. A probe beam with a wavelength of 532 nm was used in our Raman spectroscopy system where the penetration depth is 750 nm for β-Ga₂O₃ assuming an electron density of 5 × 10¹⁷ cm⁻³ and a mobility of 100 cm²/V s. Considering the HSFL period was 227 nm, the modification depth would be around 100–200 nm which is shorter than the penetration depth. The resulting Raman spectra for varying laser fluences is shown in Fig. 7, where the peaks are consistent with prior reports for β-Ga₂O₃.³⁸ There are no significant changes in peak location or relative peak intensity, even at fluences up to 0.75 J/cm² which is well above the multishot threshold of 0.60 J/cm² for HSFL formation. The Raman data suggest that there is no significant phase change or structural degradation. This result is in contrast with our prior Raman spectroscopic result of ultrafast laser irradiated 4H-SiC which showed significant evidence of amorphization from the irradiated surface measured by the same probe beam with 300 nm penetration depth.¹⁵ Although the penetration depth of the laser probe is four times longer than the modification depth for β-Ga₂O₃ case, the Raman data provide adequate support to suggest that there is no phase transition for ultrafast laser irradiated β-Ga₂O₃.

The combined EBSD and Raman spectroscopy data suggest that while ultrafast laser irradiation dramatically alters the surface morphology, the crystalline structural integrity of the materials is maintained. Further experiments based on X-ray diffraction (XRD) and transmission electron microscopy (TEM) are expected to provide further evidence that there is no phase transition for ultrafast laser irradiated β-Ga₂O₃.

In this paper, surface morphology and crystallinity of ultrafast laser irradiated β-Ga₂O₃ near the LIDT are reported. Threshold fluence for morphological change was evaluated to be 1.11 J/cm² for single pulse exposure and 0.6 J/cm² for multiple pulse exposure. Ultrafast laser irradiation above LIDT for single pulse irradiation introduces straight cracks that appear to be caused by the low thermal conductivity and mechanical brittleness of β-Ga₂O₃. Below LIDT, multiple pulse exposure results in the formation of HSFL with a period of approximately 227 nm. EBSD and Raman spectroscopy suggest that the ultrafast laser irradiation of β-Ga₂O₃ does not change the phase of the material. We believe that this work serves as a foundation to guide further ultrafast laser processing of β-Ga₂O₃.

We would like to thank Professor Marc De Graef of Carnegie Mellon University for assistance with the EBSD analysis. This work was performed in part at the University of Michigan Lurie Nanofabrication Facility (LNF) and Michigan Center for Materials Characterization (MC²). M.A. acknowledge support from the International Consortium of Nanotechnologies (ICON) funded by Lloyd’s Register Foundation, a charitable foundation which helps to protect life and property by supporting engineering-related education, public engagement, and the application of research. A.S., A.A., B.T., and S.Y. acknowledge that this material is based on work supported by the Air Force Office of Scientific Research under Award No. FA9550-16-1-0312.