Investigation of Radiation Effect on Structural and Optical Properties of GaAs under High-Energy Electron Irradiation

A systematic investigation of the changes in structural and optical properties of a semi-insulating GaAs (001) wafer under high-energy electron irradiation is presented in this study. GaAs wafers were exposed to high-energy electron beams under different energies of 10, 15, and 20 MeV for absorbed doses ranging from 0–2.0 MGy. The study showed high-energy electron bombardments caused roughening on the surface of the irradiated GaAs samples. At the maximum delivered energy of 20 MeV electrons, the observed root mean square (RMS) roughness increased from 5.993 (0.0 MGy) to 14.944 nm (2.0 MGy). The increased RMS roughness with radiation doses was consistent with an increased hole size of incident electrons on the GaAs surface from 0.015 (0.5 MGy) to 0.066 nm (2.0 MGy) at 20 MeV electrons. Interestingly, roughness on the surface of irradiated GaAs samples affected an increase in material wettability. The study also observed the changes in bandgap energy of GaAs samples after irradiation with 10, 15, and 20 MeV electrons. The band gap energy was found in the 1.364 to 1.397 eV range, and the observed intense UV-VIS spectra were higher than in non-irradiated samples. The results revealed an increase of light absorption in irradiated GaAs samples to be higher than in original-based samples.


Introduction
GaAs is a type of III-V semiconducting material composed of Ga element from column III and As element from column V of the periodic table of elements. This semiconducting material also includes InP, InAs, GaN, and InSb. GaAs is cubic crystals with a zincblende structure. It is the most studied and technologically utilized compound semiconductor material due to its several unique properties, such as its wider direct bandgap energy (1.42 eV at room temperature [1,2]), low exciton binding energy (4.2 meV) [3], and higher electron mobility (8800 cm 2 V −1 s −1 ) [4]) compared to crystalline silicon. GaAs also exhibits light emitting [5], electromagnetic [6], and photovoltaic [7] properties. It can be observed on the GaAs 1−x N x surface. The increase in N incorporation in both low-and high-N-content films after gamma-ray irradiation was attributed to the diffusion of N atoms at interstitial sites to either As lattice sites or vacancy sites during the irradiation process. The structural change in the irradiated GaAsN films came from atomic displacement caused by gamma-ray heating. To continue exploring the effect of a high-energy particle on the structural property of the GaAs-based material, we addressed the impact of high-energy electrons on GaAs' structural properties in this work. Moreover, we also studied the change of optical properties of GaAs samples after high-energy electron irradiation.
Thus, this work studied the effect of high-energy electron irradiation on the structural and optical properties of a semi-insulating GaAs (001) wafer. The GaAs wafers were irradiated with different electron energies of 10, 15, and 20 MeV and with various irradiation doses of 0.0, 0.5, 1.0, and 2.0 MGy. Structural change in microscopic and macroscopic scales of GaAs wafers after electron irradiation was clarified by high-resolution X-ray diffraction (HRXRD) measurement in three scan modes of the 2θ-ω scan, ω-rocking scan, and reciprocal space mapping (RSM), focus ion beam scanning electron microscopy (FIB-SEM), atomic force microscopy (AFM), contact angle goniometer and Raman spectroscopy. The results showed that electron irradiation could induce surface and crystalline changes. Furthermore, the change of optical property of GaAs sample after electron irradiation was investigated using UV-VIS spectroscopy. Our results found that electron irradiation treatment can improve the optical property of GaAs sample until electron energy increases to 20 MeV and electron dose of 1.0 MGy. In addition, the effect of structural change in microscopic and macroscopic scales on the optical property of GaAs was systematically discussed in detail.

GaAs Wafer
The commercial GaAs wafer was purchased from Wafer Technology Ltd., United Kingdom. The semi-insulating 625-thick (001) GaAs wafer was cut into 1 × 1 cm 2 and cleaned oxide layer with DI water and methanol several times. The cleaned GaAs samples were exposed to a high-energy electron beam in different potential energies and doses to investigate the GaAs wafer's radiation hardness against high energetic electrons. The GaAs wafer has a smooth surface with a small RMS roughness (R RMS ) of 5.993 nm and a hydrophobic surface with a contact angle of 91.34 • , as shown in Figure 1.

Electron-Beam Irradiation Facility
The study was carried out for irradiation at the Gems Irradiation Center (GIC), Thailand Institute of Nuclear Technology (TINT). High-energetic electron delivery was performed using the Mevex electron accelerator (Mevex Corporation Ltd. MB 20-16, Stitts-

Electron-Beam Irradiation Facility
The study was carried out for irradiation at the Gems Irradiation Center (GIC), Thailand Institute of Nuclear Technology (TINT). High-energetic electron delivery was performed using the Mevex electron accelerator (Mevex Corporation Ltd. MB 20-16, Stittsville, ON, Canada). The system consists of dual irradiation technology capabilities: X-ray and electron beam, which can produce high-energy electrons ranging from 8-25 MeV and accelerator power during 10-16 kW [6]. An aluminum collimator tailored the electron beam into a 1 × 1 cm 2 square, which was suitable for each GaAs sample's exact dimension. The GaAs samples were irradiated at room temperature under three different potential energies of 10, 15, and 20 MeV with irradiation dose rates ranging from 1.8-7.6 kGy/min to reach the accumulated doses of 0.5, 1.0, and 2.0 MGy, respectively.

Structural Features Analysis of GaAs Wafer
Changes in the crystal structure of the GaAs samples were studied with a highresolution X-ray diffractometer equipped with a graded parabolic mirror combined with a 2-bounce monochromator of a channel cut Ge (220) crystal monochromator and a secondary Ge (220) crystal monochromator, which was positioned in front of the beam source and the detector, respectively. The HRXRD measurements were carried out using the Rigaku TTRAX III equipped with a CuK α1 beam source at a wavelength of 1.5406 Å. Three HRXRD scan modes: 2θ-ω scan, ω-rocking curve scan, and RSM around GaAs (004) plane, were acquired to determine lattice distortion, mosaic, and strain-relaxation of the samples. Lattice distortion was also verified by Raman spectroscopy measured at room temperature using a 633 nm He-Ne laser as the excitation light source in backscattering geometry with a power laser of 25 mW. Surface morphology of the irradiated samples was observed by the third generation of FEI's Helios Nanolab Ultra High-resolution scanning electron microscope (SEM) equipped with focused ion beam (FIB) technology, FIB-SEM (FEI, Helios NanoLab G3 CX, Hillsboro, OR, USA) operating at an acceleration voltage of 15 kV. AFM was used with an XE-120 Park System AFM (Park System, Suwon, Korea) and non-contact silicon cantilever (PPP-NCHR, Nanosensors™, Neuchatel, Switzerland) with a spring constant of 0.16 N m −1 and a resonant frequency of 5 kHz were used for imaging. A scanning speed was set to 0.7 Hz. The microscope was equipped with a piezo scanner with a maximum scan range of 2 × 2 µm 2 . Analysis of the AFM images was performed with Park Systems XEI image analysis software. A contact angle goniometer (First Ten Angstroms FTA1000 drop Shape Instrument B Frame Analyzer System, Portsmouth, VA, USA) was carried out to measure the contact angle between the dropped water and the sample surface to verify surface wettability.

Optical Properties Analysis of GaAs Wafer
Optical absorption measurements were carried out at room temperature using a UV-1800 spectrophotometer (UV-1800, Shimadzu, Tokyo, Japan) located at the Faculty of Science, Khon Kaen University. The recorded spectra were measured on the GaAs samples oriented perpendicular to the c-axis before and after irradiation in wavelengths ranging from 200 to 2500 nm by setting a spectral resolution of 0.2 nm. GaAs samples' bandgap energy (E g ) was evaluated from the Tauc plot in Figure S1. The E g of the non-irradiated GaAs sample is approximately 1.373 eV at room temperature with low absorption intense UV-vis spectra of 18.59%.

Results and Discussion
The surface morphology of the irradiated GaAs samples is characterized by FIB-SEM and AFM, as shown in Figures 2 and 3, respectively. Figure  . The holes appearing on the surface may be due to a melt ability of the electrons' high energy, which caused some atoms (Ga or As) to escape from the lattice site. This occurrence of a hole on the irradiated surface was observed in our previous work, which was induced by gamma irradiation on the GaAsN surface [49].   [37] and our previous work [49]. Their works discovered the irradiated samples had a rougher surface with increasing gamma irradiation dose. The variation of the RRMS values with electron energy and electron dose is shown in Figure 4a. Based on the observation from the graph, the RRMS values of the irradiated surface tend to Moreover, the surface wettability of the irradiated samples was determined by the water droplet shapes on GaAs surfaces, as shown in Figure S2. The contact angles between the dropped water and the irradiated GaAs surface were plotted in Figure 4b. It is seen that the contact angle between the dropped water and the irradiation GaAs surface decreases with increasing irradiation energy and irradiation dose. The contact angle between the dropped water and the irradiated GaAs surface is in the range of 64.78°-87.17° less than that of the non-irradiated sample (91.34°). It implies that the irradiated samples have a higher hydrophilic surface than the non-irradiated sample. This may be because electron irradiation can cause an increase in the surface energy of the irradiated surface [50] due to its high surface roughness. Especially at the highest electron energy of 20 MeV and the highest irradiation dose of 2.0 MGy, the irradiated GaAs surface has the smallest contact angle of 64.78°, corresponding to the highest RRMS value of 14.944 nm.   [37] and our previous work [49]. Their works discovered the irradiated samples had a rougher surface with increasing gamma irradiation dose. The variation of the R RMS values with electron energy and electron dose is shown in Figure 4a.  To verify the crystalline quality of the non-irradiated and irradiated GaAs samples, three scan modes of HRXRD measurements of symmetric (004) peak, including 2θ-ω scan, ω-scan rocking curve, and RSM modes, were performed. Figure 5a-c shows HRXRD 2θω profiles of all samples. The diffraction peak of the GaAs (004) plane was adjusted to be constant at 66.049° for all samples. Especially at the highest electron energy of 20 MeV and the highest electron dose of 2.0 MGy, there is a shoulder peak at a high diffracted angle. The full width at half maximum (FWHM) of the GaAs (004) peak from the 2θ-ω scan curve is shown in Figure 5d. At electron energies of 10 and 15 MeV, the FWHM of the GaAs (004) peak tends to decrease with increasing irradiation doses, indicating improving crystal uniformity due to thermal annealing from the high-energy electron beam. This may be due to interstitial atoms moving to a lattice, or vacancy sites, improving lattice spacing uniformity. At electron energy of 20 MeV, FWHM of the GaAs (004) peak tends to decrease at the low irradiation doses of 0.5 and 1.0 MGy, resulting in improved crystallinity. However, at an irradiation dose of 2.0 MGy, the FWHM of the GaAs (004) peak is larger Moreover, the surface wettability of the irradiated samples was determined by the water droplet shapes on GaAs surfaces, as shown in Figure S2. The contact angles between the dropped water and the irradiated GaAs surface were plotted in Figure 4b. It is seen that the contact angle between the dropped water and the irradiation GaAs surface decreases with increasing irradiation energy and irradiation dose. The contact angle between the dropped water and the irradiated GaAs surface is in the range of 64.78 • -87.17 • less than that of the non-irradiated sample (91.34 • ). It implies that the irradiated samples have a higher hydrophilic surface than the non-irradiated sample. This may be because electron irradiation can cause an increase in the surface energy of the irradiated surface [50] due to its high surface roughness. Especially at the highest electron energy of 20 MeV and the highest irradiation dose of 2.0 MGy, the irradiated GaAs surface has the smallest contact angle of 64.78 • , corresponding to the highest R RMS value of 14.944 nm.
To verify the crystalline quality of the non-irradiated and irradiated GaAs samples, three scan modes of HRXRD measurements of symmetric (004) peak, including 2θ-ω scan, ω-scan rocking curve, and RSM modes, were performed. Figure 5a-c shows HRXRD 2θ-ω profiles of all samples. The diffraction peak of the GaAs (004) plane was adjusted to be constant at 66.049 • for all samples. Especially at the highest electron energy of 20 MeV and the highest electron dose of 2.0 Mgy, there is a shoulder peak at a high diffracted angle. The full width at half maximum (FWHM) of the GaAs (004) peak from the 2θ-ω scan curve is shown in Figure 5d. At electron energies of 10 and 15 MeV, the FWHM of the GaAs (004) peak tends to decrease with increasing irradiation doses, indicating improving crystal uniformity due to thermal annealing from the high-energy electron beam. This may be due to interstitial atoms moving to a lattice, or vacancy sites, improving lattice spacing uniformity. At electron energy of 20 MeV, FWHM of the GaAs (004) peak tends to decrease at the low irradiation doses of 0.5 and 1.0 Mgy, resulting in improved crystallinity. However, at an irradiation dose of 2.0 Mgy, the FWHM of the GaAs (004) peak is larger than that of the other samples. This indicates that a high irradiation dose can increase the non-uniformity of (004) plane lattice spacing. This result suggests that a longer exposure time at a higher electron energy of 20 MeV can break Ga-As bonds, induce Ga or As vacancies, induce interstitial atoms and induce amorphous formation structure, causing increase lattice spacing non-uniformity.
Materials 2022, 15, x FOR PEER REVIEW 9 o than that of the other samples. This indicates that a high irradiation dose can increase non-uniformity of (004) plane lattice spacing. This result suggests that a longer expos time at a higher electron energy of 20 MeV can break Ga-As bonds, induce Ga or As cancies, induce interstitial atoms and induce amorphous formation structure, causing crease lattice spacing non-uniformity.   Figure 6d. FWHM of (004) rocking curves for the irradiated samples is lar than that of the non-irradiated sample. It is known that an increase in FWHM of the (0 rocking curve indicates an increase in mosaic structures corresponding to distribution crystallographic orientations. This mosaic structure may be caused by amorphous mation. This high FWHM means a low crystal quality of the irradiated GaAs samples. further confirm the crystal quality of all GaAs samples, the RSM mode of the HRXRD s continuous around the (004) plane in both 2θ-ω and ω-rocking modes was perform   Figure 6d. FWHM of (004) rocking curves for the irradiated samples is larger than that of the non-irradiated sample. It is known that an increase in FWHM of the (004) rocking curve indicates an increase in mosaic structures corresponding to distributions of crystallographic orientations. This mosaic structure may be caused by amorphous formation. This high FWHM means a low crystal quality of the irradiated GaAs samples. To further confirm the crystal quality of all GaAs samples, the RSM mode of the HRXRD scan continuous around the (004) plane in both 2θ-ω and ω-rocking modes was performed, and the result is shown in Figure 7. The 2θ-ω axis (x-axis) is related to the parallel interplanar spacing. From the maps, the calculated lattice parameter of the GaAs wafer is 5.653 Å, coincident with the lattice parameter of a free-standing GaAs (5.653 Å), indicating that the irradiated GaAs wafer is relaxed in all samples. This corresponds to the circular-like shape of the contour plot of the RSM non-irradiated sample (Figure 7a). It is seen that the peak width in the 2θ-ω axis tends to decrease with increasing irradiation energy indicating improved uniformity of lattice spacing. This result corresponds to a single line of 2θ-ω scan mode in Figure 5. In ω-rocking (y-axis), ∆ω of the irradiated samples (Figure 7b-d) is larger with increasing irradiation energy, indicating an increase in mosaic structure in agreement with a single line of ω-rocking curve mode in Figure 6. This mosaic may be due to Ga or As atoms escaping from the lattice site and amorphous formation structure induced by thermal heating from high-energy electron bombardment. These phenomena affect microscopic scales inside the crystal structure, as evidenced by the HRXRD result and the macroscopic scales observed by surface roughness from FIB-SEM and AFM images.  Figure 7. The 2θ-ω axis (x-axis) is related to the parallel interplanar spacing. From the maps, the calculated lattice parameter of the GaAs wafer is 5.653 Å , coincident with the lattice parameter of a free-standing GaAs (5.653 Å ), indicating that the irradiated GaAs wafer is relaxed in all samples. This corresponds to the circular-like shape of the contour plot of the RSM non-irradiated sample (Figure 7a). It is seen that the peak width in the 2θ-ω axis tends to decrease with increasing irradiation energy indicating improved uniformity of lattice spacing. This result corresponds to a single line of 2θω scan mode in Figure 5. In ω-rocking (y-axis), ω of the irradiated samples (Figure 7bd) is larger with increasing irradiation energy, indicating an increase in mosaic structure in agreement with a single line of ω-rocking curve mode in Figure 6. This mosaic may be due to Ga or As atoms escaping from the lattice site and amorphous formation structure induced by thermal heating from high-energy electron bombardment. These phenomena affect microscopic scales inside the crystal structure, as evidenced by the HRXRD result and the macroscopic scales observed by surface roughness from FIB-SEM and AFM images.     [51] were observed for the non-irradiated sample. Based on the Raman selection rules, TO modes are not allowed for zincblende structures in backscattering geometry. This deviation from the selection rules is due to a breakdown in the long-range order of the zincblende crystal related to lattice distortion. However, the TO mode of all GaAs samples can be observed, and its intensity and peak width increase with increasing irradiation dose, indicating higher lattice distortion (Figure 8a) induced by loss of Ga or As atom after irradiation. For electron energies of 15 and 20 MeV, as shown in Figure 8b,c, it can be clearly seen that the Raman intensity obtained from the TO mode is stronger than that obtained from the LO mode with increasing irradiation dose. Moreover, a new peak appears at a wavenumber of around 200 cm −1 due to disorder-activated longitudinal acoustic (DALA) [52], as can be observed for high irradiation doses of 1.0 to 2.0 MGy (electron energy of 15 MeV) and 0.5 to 2.0 MGy (electron energy of 20 MeV). These results strongly imply that the irradiated GaAs samples at higher electron energies (15 and 20 MeV) have a larger lattice distortion than the irradiated sample at low irradiation energy   [51] were observed for the non-irradiated sample. Based on the Raman selection rules, TO modes are not allowed for zincblende structures in backscattering geometry. This deviation from the selection rules is due to a breakdown in the long-range order of the zincblende crystal related to lattice distortion. However, the TO mode of all GaAs samples can be observed, and its intensity and peak width increase with increasing irradiation dose, indicating higher lattice distortion (Figure 8a) induced by loss of Ga or As atom after irradiation. For electron energies of 15 and 20 MeV, as shown in Figure 8b,c, it can be clearly seen that the Raman intensity obtained from the TO mode is stronger than that obtained from the LO mode with increasing irradiation dose. Moreover, a new peak appears at a wavenumber of around 200 cm −1 due to disorder-activated longitudinal acoustic (DALA) [52], as can be observed for high irradiation doses of 1.0 to 2.0 Mgy (electron energy of 15 MeV) and 0.5 to 2.0 Mgy (electron energy of 20 MeV). These results strongly imply that the irradiated GaAs samples at higher electron energies (15 and 20 MeV) have a larger lattice distortion than the irradiated sample at low irradiation energy (10 MeV) induced by a larger amount of Ga or As vacancies, interstitial atoms, amorphous formation, and mosaic structure domain. These results coincide well with the HRXRD curve in three scan modes, as evidenced in Figures 5-7. (10 MeV) induced by a larger amount of Ga or As vacancies, interstitial atoms, amorphous formation, and mosaic structure domain. These results coincide well with the HRXRD curve in three scan modes, as evidenced in Figures 5-7.   Figure S1, was 1.364 to 1397 eV. As seen in Figure 9a, for electron energy of 10 MeV, the absorption band edge of the irradiated samples is sharper than that of the non-irradiated sample for all irradiation doses without a band tail. This may be due to the high crystal uniformity of the irradiated sample after electron irradiation treatment induced a loss of band tail of UV-Vis spectra. In addition, the absorption spectra of the irradiated samples have a higher intensity than that of the non-irradiated sample. This indicates that the irradiated samples can collect more light intensity than the nonirradiated sample. Significantly, the irradiated GaAs sample at 10 MeV electron for 2.0 MGy dose can absorb UV-Vis spectra of 122.66%, about 7 times the non-irradiated sample. Higher absorption spectra of the irradiated samples may be due to increased lattice  Figure 9 shows UV-VIS spectra of the GaAs samples irradiated with electron energies of (a) 10, (b) 15, and (c) 20 MeV. The E g of the irradiated GaAs samples, evaluated by Tauc plot, as shown in Figure S1, was 1.364 to 1397 eV. As seen in Figure 9a, for electron energy of 10 MeV, the absorption band edge of the irradiated samples is sharper than that of the non-irradiated sample for all irradiation doses without a band tail. This may be due to the high crystal uniformity of the irradiated sample after electron irradiation treatment induced a loss of band tail of UV-Vis spectra. In addition, the absorption spectra of the irradiated samples have a higher intensity than that of the non-irradiated sample. This indicates that the irradiated samples can collect more light intensity than the non-irradiated sample. Significantly, the irradiated GaAs sample at 10 MeV electron for 2.0 MGy dose can absorb UV-Vis spectra of 122.66%, about 7 times the non-irradiated sample. Higher absorption spectra of the irradiated samples may be due to increased lattice uniformity caused by reducing lattice defects (Ga or As vacancies, interstitial atoms) in the crystal structure agreeing with HRXRD and Raman scattering results or less reflected light on rough surfaces. The rough surface with a hole of the irradiated samples may act as nano-grating for protection against reflected light, as observed in Das et al.'s work [53,54]. In the case of electron energies of 15 and 20 MeV in Figure 9b,c, the irradiated samples' absorbance intensity also tends to be higher than that of the non-irradiated sample and increases with increasing irradiation dose. These high absorption spectra of the irradiated samples may help create large amounts of electron-hole pairs and carrier mobility in the irradiated sample. These could be improved the conversion efficiency of GaAs-based solar cells in space applications or environments of high-energy particle bombardment. However, comparing the absorption spectra of the irradiated sample at the 2.0 Mgy irradiation dose, the absorption spectra respective decrease with increasing irradiation energies from 10 to 20 MeV. This may be due to the higher crystal non-uniformity of the 20 MeV irradiated GaAs sample than that of the 10 and 15 MeV irradiated samples agreeing with HRXRD and Raman scattering results. However, the absorption spectra of the 20 MeV irradiated sample ( Figure 9c) increase with the irradiation dose. The highest absorption spectra can still be obtained from the 2.0 MeV irradiation dose sample, while the crystal structure has the highest non-uniformity. This may be because its largest roughening surface (R RMS = 14.944 nm) can reduce the reflection of light on the irradiated surface, increasing absorption spectra. Our results show that the irradiated GaAs samples still have high crystal quality and optical properties until irradiation energy and irradiation dose are as high as 20 MeV and 1.0 Mgy, respectively. Our results indicate that the optical property (absorption spectra) of the irradiated sample depends on its structural properties in microscopic (crystal uniformity involving lattice defects such as vacancies and interstitial) and macroscopic (surface roughness) properties. uniformity caused by reducing lattice defects (Ga or As vacancies, interstitial atoms) in the crystal structure agreeing with HRXRD and Raman scattering results or less reflected light on rough surfaces. The rough surface with a hole of the irradiated samples may act as nano-grating for protection against reflected light, as observed in Das et al.'s work [53,54]. In the case of electron energies of 15 and 20 MeV in Figure 9b,c, the irradiated samples' absorbance intensity also tends to be higher than that of the non-irradiated sample and increases with increasing irradiation dose. These high absorption spectra of the irradiated samples may help create large amounts of electron-hole pairs and carrier mobility in the irradiated sample. These could be improved the conversion efficiency of GaAs-based solar cells in space applications or environments of high-energy particle bombardment. However, comparing the absorption spectra of the irradiated sample at the 2.0 MGy irradiation dose, the absorption spectra respective decrease with increasing irradiation energies from 10 to 20 MeV. This may be due to the higher crystal non-uniformity of the 20 MeV irradiated GaAs sample than that of the 10 and 15 MeV irradiated samples agreeing with HRXRD and Raman scattering results. However, the absorption spectra of the 20 MeV irradiated sample ( Figure 9c) increase with the irradiation dose. The highest absorption spectra can still be obtained from the 2.0 MeV irradiation dose sample, while the crystal structure has the highest non-uniformity. This may be because its largest roughening surface (RRMS = 14.944 nm) can reduce the reflection of light on the irradiated surface, increasing absorption spectra. Our results show that the irradiated GaAs samples still have high crystal quality and optical properties until irradiation energy and irradiation dose are as high as 20 MeV and 1.0 MGy, respectively. Our results indicate that the optical property (absorption spectra) of the irradiated sample depends on its structural properties in microscopic (crystal uniformity involving lattice defects such as vacancies and interstitial) and macroscopic (surface roughness) properties.

Conclusions
This work systematically studied the effect of electron irradiat and optical properties under different electron energies of 10, 15, an doses ranging from 0-2.0 MGy. The surface morphology of the irr showed roughness surface, with many holes spreading on their GaAs sample at an electron energy of 20 MeV and electron dose o largest surface roughness of 14.944 nm and the largest hole size of of the irradiated GaAs samples became more hydrophilic compar ated surface, with the lowest contact angle of 64.78° observed in t irradiation energy of 20 MeV and irradiation dose of 2.0 MGy. HR tering results showed the crystallinity of the irradiated GaAs samp by low electron energies of 10 and 15 MeV. However, the crysta GaAs sample at the highest electron energy of 20 MeV tended to b formity with increasing irradiation strength to 2.0 MGy. After irr bandgap energy of GaAs can be tuned, ranging from 1.364 to 1 absorption spectra of the irradiated sample were higher than tha sample due to improving crystal uniformity and its roughening vealed that electron irradiation treatment could improve the struc erties of the irradiated GaAs samples when electron energy and el ceed 20 MeV and 1.0 MGy. Finally, outcome results can visualize th electron irradiation on microscopic defects in the crystal structur clarify the changes in the macroscopic properties of this material ular application.

Conclusions
This work systematically studied the effect of electron irradiation on GaAs' structural and optical properties under different electron energies of 10, 15, and 20 MeV and electron doses ranging from 0-2.0 MGy. The surface morphology of the irradiated GaAs samples showed roughness surface, with many holes spreading on their surface. The irradiated GaAs sample at an electron energy of 20 MeV and electron dose of 2.0 MGy obtained the largest surface roughness of 14.944 nm and the largest hole size of 0.066 nm. The surfaces of the irradiated GaAs samples became more hydrophilic compared with the non-irradiated surface, with the lowest contact angle of 64.78 • observed in the sample treated with irradiation energy of 20 MeV and irradiation dose of 2.0 MGy. HRXRD and Raman scattering results showed the crystallinity of the irradiated GaAs samples could be improved by low electron energies of 10 and 15 MeV. However, the crystallinity of the irradiated GaAs sample at the highest electron energy of 20 MeV tended to be damaged lattice uniformity with increasing irradiation strength to 2.0 MGy. After irradiation treatment, the bandgap energy of GaAs can be tuned, ranging from 1.364 to 1.397 eV. Moreover, the absorption spectra of the irradiated sample were higher than that of the non-irradiated sample due to improving crystal uniformity and its roughening surface. This result revealed that electron irradiation treatment could improve the structural and optical properties of the irradiated GaAs samples when electron energy and electron dose did not exceed 20 MeV and 1.0 MGy. Finally, outcome results can visualize the effect of high-energy electron irradiation on microscopic defects in the crystal structure of GaAs and further clarify the changes in the macroscopic properties of this material to utilize it for a particular application.