Influence of O2 Flow Rate on the Properties of Ga2O3 Growth by RF Magnetron Sputtering

The influence of the O2 flow rate on the properties of gallium oxide (Ga2O3) by RF magnetron sputtering was studied. X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), transmittance spectra, and photoluminescence (PL) spectra have been employed to study the Ga2O3 thin films. With the increase in oxygen flow rate, both the crystal quality and luminescence intensity of the Ga2O3 samples first decrease and then enhance. All these observations suggested that the reduction in the oxygen defect density is responsible for the improvement in the crystal quality and emission intensity of the material. Our results demonstrated that high-quality Ga2O3 materials could be obtained by adjusting the oxygen flow rate.


Introduction
Recently, gallium oxide (Ga 2 O 3 ) and related compounds (Al x Ga 2-X O 3 , (In x Ga 1-x ) 2 O 3 ) [1,2] have brought about the widespread attention, attributed to their outstanding electrical and photoelectric properties, such as wide fundamental bandgap (4.5-5 eV), a high off-state breakdown voltage of 755 V, high dielectric constant values from 10.2 to 14.2, and high mobility of 2790 cm 2 V −1 s −1 [3][4][5]. At present, Ga 2 O 3 has been widely used in solar blind ultraviolet detection [6], high power switching [7], metal oxide semiconductor field effect transistors (MOSFET) [8], high-temperature gas sensors [9], and many other fields. Ga 2 O 3 usually exists in six different polymorphic structures (α, β, γ, δ, ε, and k) [10]. Among these, β-Ga 2 O 3 is considered as the most stable phase and can be converted from other phases at high temperatures [11,12]. So far, many growth modes were used to develop β-Ga 2 O 3 , including RF Sputter [13], MBE [14], MOCVD [15], chemical vapor deposition [16], etc. RF magnetron sputtering is a comparatively economical deposition technique that has adequate control over stoichiometry and uniformity of the film compared to the above techniques. Until now, the effects of growth parameters, for instance, substrate temperature, oxygen/argon partial pressures, and sputtering power, on the properties of Ga 2 O 3 have been studied the most [17][18][19]. However, hardly any reports about the effect of the O 2 flow rate at fixed Ar flow rate on the structure and optical properties of β-Ga 2 O 3 thin films deposited by the reactive RF magnetron sputter. Since oxygen deficiency in the growth process can induce oxygen vacancies in Oxide semiconductor materials, oxygen vacancies affect the optical and electrical properties of oxide semiconductor films [20][21][22][23][24]. Therefore, it is of great significance to study the effect of the O 2 flow rate on the characteristics of Ga 2 O 3 thin films deposited by RF magnetron sputtering.
In this work, β-Ga 2 O 3 has been grown by RF magnetron sputtering, and the effect of O 2 flow rate on the structure and optical characteristic of β-Ga 2 O 3 have been studied in detail. The improvement of UV emission properties was observed in the β-Ga 2 O 3 samples with an increased O 2 flow rate. The mechanism of enhanced luminescence in the β-Ga 2 O 3 film was an in-depth study by careful inspection of the PL spectrum combined with XRD results. It is anticipated that this work will provide a meaningful step toward the fabrication of high-quality β-Ga 2 O 3 thin films.

Materials and Methods
Ga 2 O 3 samples were grown on single-polished c-plane (0006) sapphire substrates using an RF magnetron sputtering system. A sintered ceramic Ga 2 O 3 target of 99.99% purity was employed as the target. Before growth, the sapphire substrates were first cleaned with ultrasonic vibration in ethanol and then in high purity water. The argon gas flow rate was set at 30 sccm and pressure at 0.8 Pa. Sputtering power was adjusted to 80 w. The distance between the sample and target was 760 mm. The base pressure of vacuum chamber reached 5.6 × 10 −4 Pa. The Ar flow rate was kept constant at 30 sccm. In the experiments, the O 2 flow rate was set as 0 sccm, 1 sccm, 2 sccm, 4 sccm, respectively. The influence of other parameters was minimized.
The structure of Ga 2 O 3 was characterized by an XRD technique (XRD, X' Pert, Philips, Eindhoven, The Netherlands). The morphologies of samples were conducted on a fieldemission scanning electron microscopy (FE-SEM, ZEISS Merlin Compact, Oberkochen, Germany). Photoluminescence (PL) spectrums were investigated by Zolix responsivity measurement system (λ = 266 nm) as the excitation source (DSR600, Zolix, Beijing, China). Figure 1 shows the result of the X-ray diffraction of the Ga 2 O 3 films growth with various O 2 flow rates. The diffraction peaks located at 29.7 • , 37.6 • , and 58.4 • originate from the 400, 402, and 603 of the β-Ga 2 O 3 , respectively [1,25,26]. For the sample without the O 2 flow rate, 400, 402, and 603 of the β-Ga 2 O 3 diffraction peak coexisted; this suggests that the sample was polycrystalline. With the O 2 flow rate increased from 0 to 4 sccm, the diffraction peak intensity of the 400 β-Ga 2 O 3 decreased, while the intensity of both the 402 and 603 of β-Ga 2 O 3 diffraction peak increased. Both of these two diffractions belong to the 201 plane family of the monoclinic Ga 2 O 3 [27,28]. The above result illustrates that highly 201-textured β-Ga 2 O 3 samples have been prepared and the orientation of crystal is gradually enhanced when oxygen flow increased. Furthermore, the full width at half maximum (FWHM) values of the 402 β-Ga 2 O 3 peaks are 1.00 • , 1.10 • , 1.06 • , and 0.96 • for samples with the O 2 flow rate increased from 0 to 4 sccm, respectively. The FWHM value is dependent on the O 2 flow rate, and the results suggest a higher O 2 flow rate results in improved crystal quality. The minimal FWHM is obtained at 4 sccm of the O 2 flow rate, which means the grain size is the largest [29]. The combined results of the XRD peak intensity and the FWHM value of the samples show that higher O 2 flow rates lead to better quality.    The EDX spectroscopy analyses of the Ga2O3 films is shown in Figure 3. In the graph, O and Ga peaks can be observed. The composition of the Ga2O3 thin films are shown in Table 1. The atomic concentration of the O composition decreases from 39.88 to 10 at% with the rise in O2 flow rate. However, as the O2 flow rate continues to increase, the oxygen content starts to rise again.    The EDX spectroscopy analyses of the Ga2O3 films is shown in Figure 3. In the graph, O and Ga peaks can be observed. The composition of the Ga2O3 thin films are shown in Table 1. The atomic concentration of the O composition decreases from 39.88 to 10 at% with the rise in O2 flow rate. However, as the O2 flow rate continues to increase, the oxygen content starts to rise again. The EDX spectroscopy analyses of the Ga 2 O 3 films is shown in Figure 3. In the graph, O and Ga peaks can be observed. The composition of the Ga 2 O 3 thin films are shown in Table 1. The atomic concentration of the O composition decreases from 39.88 to 10 at% with the rise in O 2 flow rate. However, as the O 2 flow rate continues to increase, the oxygen content starts to rise again.  The thickness of samples measured by cross section scanning electron microscopy ( Figure 4) is also given in Table 1. According to the data in the table, the thickness of the sample decreases gradually as the O2 flow rate increases. Combined with XRD and AFM results, the Ar partial pressure in the cavity going down, the target atoms produced by bombardment decreasing, and both the growth rate and crystallinity of the Ga2O3 sample decreasing can be attributed to the O2 flow rate beginning to rise . As the O2 flow continues to rise, the oxygen vacancy defects decrease, and the crystallinity of gallium oxide films is improved.

Results
The transmission spectrum of the sample is shown in Figure 5. All the sample's transmissibility is over 75% and has interference fringes, indicating the existence of a smooth surface. As the O2 flow increases from 0 to 1 sccm, the transmittance of the sample decreases rapidly. With the increase in oxygen flow from 1 to 4 sccm, the transmittance of the sample increases gradually. The results of transmission spectrum and crystal mass can confirm each other; with the rise of O2 flow rates, the crystal mass of the sample decreases  The thickness of samples measured by cross section scanning electron microscopy ( Figure 4) is also given in Table 1. According to the data in the table, the thickness of the sample decreases gradually as the O 2 flow rate increases.  The thickness of samples measured by cross section scanning electron microscopy ( Figure 4) is also given in Table 1. According to the data in the table, the thickness of the sample decreases gradually as the O2 flow rate increases. Combined with XRD and AFM results, the Ar partial pressure in the cavity going down, the target atoms produced by bombardment decreasing, and both the growth rate and crystallinity of the Ga2O3 sample decreasing can be attributed to the O2 flow rate beginning to rise . As the O2 flow continues to rise, the oxygen vacancy defects decrease, and the crystallinity of gallium oxide films is improved.
The transmission spectrum of the sample is shown in Figure 5. All the sample's transmissibility is over 75% and has interference fringes, indicating the existence of a smooth surface. As the O2 flow increases from 0 to 1 sccm, the transmittance of the sample decreases rapidly. With the increase in oxygen flow from 1 to 4 sccm, the transmittance of the sample increases gradually. The results of transmission spectrum and crystal mass can confirm each other; with the rise of O2 flow rates, the crystal mass of the sample decreases Combined with XRD and AFM results, the Ar partial pressure in the cavity going down, the target atoms produced by bombardment decreasing, and both the growth rate and crystallinity of the Ga 2 O 3 sample decreasing can be attributed to the O 2 flow rate beginning to rise. As the O 2 flow continues to rise, the oxygen vacancy defects decrease, and the crystallinity of gallium oxide films is improved.
The transmission spectrum of the sample is shown in Figure 5. All the sample's transmissibility is over 75% and has interference fringes, indicating the existence of a smooth surface. As the O 2 flow increases from 0 to 1 sccm, the transmittance of the sample decreases rapidly. With the increase in oxygen flow from 1 to 4 sccm, the transmittance of the sample increases gradually. The results of transmission spectrum and crystal mass can confirm each other; with the rise of O 2 flow rates, the crystal mass of the sample decreases first and then increases, and the transmission spectrum also shows the same rule. In addition, the band gap of the sample becomes widened when the oxygen flow increases from 1 to 4 sccm. first and then increases, and the transmission spectrum also shows the same rule. In addition, the band gap of the sample becomes widened when the oxygen flow increases from 1 to 4 sccm. The PL spectra of Ga2O3 in the UV region at room temperature is shown in Figure 6a. The emission peak at 266 nm (4.66 eV) originated from the Ga2O3 samples [4,11]. When the O2 flow rate increased from 0 to 1 sccm, the intensity of Ga2O3 emission peaks decreased, and as the oxygen flow continued to rise, the intensity of Ga2O3 emission peaks increased. Combined with the above analysis of crystal quality, the PL result can be interpreted as when the O2 flow rate increased from 0 to 1 sccm, the crystalline quality of the sample deteriorated, which caused decreases in the luminescence intensity. As the oxygen flow continues to rise, the oxygen defect density decreases and the non-radiative composite center decreases, and this ultimately causes the luminescence intensity to increase. This is due to oxygen deficiency in the growth process producing oxygen defects in Ga2O3 and oxygen defects playing the role of non-radiation complex centers, and thus as oxygen flow continue to rise, the number of oxygen defects decreases and this increases the intensity of the Ga2O3 emission peaks. To confirm the discussion above, Figure 6b shows the PL spectra samples in the visible region. The emission band in the region of 450-600 nm in all PL spectra can be attributed to oxygen-defect-related deep-level emission [30][31][32][33]. It can be seen that the intensity of the oxygen-defect-related emission peak decreases gradually with the increase in the oxygen flow rate. Therefore, it is reasonable to conclude that increasing the oxygen The PL spectra of Ga 2 O 3 in the UV region at room temperature is shown in Figure 6a. The emission peak at 266 nm (4.66 eV) originated from the Ga 2 O 3 samples [4,11]. When the O 2 flow rate increased from 0 to 1 sccm, the intensity of Ga 2 O 3 emission peaks decreased, and as the oxygen flow continued to rise, the intensity of Ga 2 O 3 emission peaks increased. Combined with the above analysis of crystal quality, the PL result can be interpreted as when the O 2 flow rate increased from 0 to 1 sccm, the crystalline quality of the sample deteriorated, which caused decreases in the luminescence intensity. As the oxygen flow continues to rise, the oxygen defect density decreases and the non-radiative composite center decreases, and this ultimately causes the luminescence intensity to increase. This is due to oxygen deficiency in the growth process producing oxygen defects in Ga 2 O 3 and oxygen defects playing the role of non-radiation complex centers, and thus as oxygen flow continue to rise, the number of oxygen defects decreases and this increases the intensity of the Ga 2 O 3 emission peaks.
Micromachines 2021, 12, x FOR PEER REVIEW 5 of 7 first and then increases, and the transmission spectrum also shows the same rule. In addition, the band gap of the sample becomes widened when the oxygen flow increases from 1 to 4 sccm. The PL spectra of Ga2O3 in the UV region at room temperature is shown in Figure 6a. The emission peak at 266 nm (4.66 eV) originated from the Ga2O3 samples [4,11]. When the O2 flow rate increased from 0 to 1 sccm, the intensity of Ga2O3 emission peaks decreased, and as the oxygen flow continued to rise, the intensity of Ga2O3 emission peaks increased. Combined with the above analysis of crystal quality, the PL result can be interpreted as when the O2 flow rate increased from 0 to 1 sccm, the crystalline quality of the sample deteriorated, which caused decreases in the luminescence intensity. As the oxygen flow continues to rise, the oxygen defect density decreases and the non-radiative composite center decreases, and this ultimately causes the luminescence intensity to increase. This is due to oxygen deficiency in the growth process producing oxygen defects in Ga2O3 and oxygen defects playing the role of non-radiation complex centers, and thus as oxygen flow continue to rise, the number of oxygen defects decreases and this increases the intensity of the Ga2O3 emission peaks. To confirm the discussion above, Figure 6b shows the PL spectra samples in the visible region. The emission band in the region of 450-600 nm in all PL spectra can be attributed to oxygen-defect-related deep-level emission [30][31][32][33]. It can be seen that the intensity of the oxygen-defect-related emission peak decreases gradually with the increase in the oxygen flow rate. Therefore, it is reasonable to conclude that increasing the oxygen To confirm the discussion above, Figure 6b shows the PL spectra samples in the visible region. The emission band in the region of 450-600 nm in all PL spectra can be attributed to oxygen-defect-related deep-level emission [30][31][32][33]. It can be seen that the intensity of the oxygen-defect-related emission peak decreases gradually with the increase in the oxygen flow rate. Therefore, it is reasonable to conclude that increasing the oxygen flow rate leads to reductions in the oxygen defect density and improvements in the crystal quality and emission intensity of the material.

Conclusions
In summary, in terms of the effect of oxygen flow on the structure, optical l properties of the Ga 2 O 3 films have been investigated by XRD, EDX, AFM, transmission spectra, and PL spectra. With the increase in the oxygen flow rate, both the crystal quality and luminescence intensity of the sample first decreased and then enhanced. All these observations suggested that the reduction in the oxygen defect density is responsible for the improvement in the crystal quality and emission intensity of the material, however, there have been no reports about O 2 flow rate on the properties of the Ga 2 O 3 growth by RF magnetron sputtering. Our results were similar to those obtained by other techniques and the specific control of various experimental operating parameters. Vu found that the performance of β-Ga 2 O 3 -based photodetectors with a higher oxygen partial are better than those prepared at lower oxygen pressures [34]. Wang et al. studied the influence of oxygen flow ratio on the performance of Sn-doped Ga 2 O 3 films by RF magnetron sputtering; they found the sample with higher oxygen flow ratio displays an enhanced performance [35]. Shen's study revealed oxygen annealing will enhance the performance of β-Ga 2 O 3 solar-blind photodetectors grown by ion-cutting process [36]. Our results demonstrated that high-quality gallium oxide materials can be obtained by adjusting the oxygen flow rate.