**3. Results**

Figure 1 shows the XRD spectra of the samples with air, N2, and O2 as the carrier gases. The spectra are dominated by the di ffraction peaks at 40.26◦ and 41.66◦, which correspond to the (0006) planes of α-Ga2O3 epilayer and sapphire substrate, respectively. No other peaks are found, which shows that all of the Ga2O3 films show the obvious pure alpha phase. These XRD spectra show that the α-Ga2O3 films had a preferential c-axis orientation along the c-axis of the sapphire substrate and the calculated lattice constant along the c-axis is 1.34 nm. Paying attention to the sample grown with O2 as the carrier gas, the full-width at half maximum (FWHM) of the ω scan rocking curve is as small as 72 arcsec, indicating a high quality α-Ga2O3. However, for the samples grown with air and N2 as the carrier gases, FWHM of the ω scan rocking curves are 88.6 arcsec and 86.4 arcsec, respectively, indicating a relatively inferior crystal quality. Considering the di fferent oxygen content in the carrier gases, it is supposed that Ga2O3 film grown with O2 as the carrier gas has less defects, such as oxygen vacancies (V O), which may be the key to improving the quality of crystallization.

**Figure 1.** (**a**) X-ray diffraction 2θ/θ scan spectra for Ga2O3 films grown with different carrier gases. (**b**) The corresponding (0006) XRD diffraction rocking curves. The full-width at half maximum of Ga2O3 films grown with air, N2, and O2 as the carrier gases are 88.6 arcsec, 86.4 arcsec, and 72 arcsec, respectively.

*Materials* **2019**, *12*, 3670

Raman spectra of the epilayers grown with air, N2, and O2 as the carrier gases were measured between 100 cm<sup>−</sup><sup>1</sup> and 800 cm<sup>−</sup><sup>1</sup> at room temperature to confirm the crystalline quality of the deposited films, and the results are presented in Figure 2. A 514 nm laser was used as the excitation source and the laser beam was focused by a microscope lens system (×50 ulwd) yielding a spot size of 1 μm during the Raman measurement. The Raman peaks at 418 cm<sup>−</sup><sup>1</sup> and 749 cm<sup>−</sup><sup>1</sup> belong to the sapphire substrate [14]. The Raman peaks located at 431.3 cm<sup>−</sup>1, 577 cm<sup>−</sup>1, and 692 cm<sup>−</sup><sup>1</sup> are the Raman-allowed vibrational modes of Eg, A1g, and Eg for α-Ga2O3, respectively, and are consistent with the theoretical calculations [15]. The high-frequency A1g mode at 577 cm<sup>−</sup><sup>1</sup> mainly involves the vibration of oxygen atoms perpendicular to the c-axis. The linewidth of the peak A1g is as narrow as 3.9 cm<sup>−</sup><sup>1</sup> for the epilayer grown with O2 as the carrier gas, strongly suggesting the high crystallinity of the epilayer. By comparing the epilayers grown with di fferent carrier gases, the displacement and intensity of the Raman peaks do not change obviously, demonstrating that the stress in the films is mainly determined by other factors, such as lattice mismatch or growth temperature, instead of the carrier gases. It could also be observed that except the Raman peaks of α-Ga2O3, no other peaks are observed in the Raman spectra, which indicates that all of the epilayers are pure α-Ga2O3 without other phases and confirm the conclusion from the XRD measurements.

**Figure 2.** Raman spectra of the α-Ga2O3 epilayers grown with air, N2, and O2 as the carrier gases.

The variation of optical transmittance spectra (200–800 nm) were performed on α-Ga2O3 films as shown in Figure 3. All the samples exhibited a transmittance higher than 80% in the visible to near-UV regions. The relationship between the absorption coe fficient α and the optical bandgap (Eg) is αhν = A(h<sup>ν</sup>−Eg)<sup>1</sup>/2, where A is the material-dependent constant, h is the Planck's constant, and ν is the frequency of the incident light [16]. The optical band gap can be evaluated from the ( αhν) 2 versus photon energy (hν) graph by linear extrapolations to zero absorption coe fficient [17]. The inset shows the plot of ( αhν) 2 as function of photon-energy h<sup>ν</sup>. The bandgaps of the obtained materials remained at 5.1–5.3 eV, which is obviously larger than that of β-Ga2O3. The larger bandgap than that of β-Ga2O3 is obviously attributed to the crystal structure of α-Ga2O3 being di fferent from β-Ga2O3. We note that the α-Ga2O3 sample grown with O2 as the carrier gas is dropped more abruptly than those of air and N2 in ultraviolet region. This phenomenon may be caused by less defect in α-Ga2O3 samples carried by O2. During the film growth, extra oxygen will ensure the ideal ratio of O to Ga atomic, which results in higher crystalline quality and leads to its bandgap being close to the ideal value of 5.3 eV. The higher crystalline quality for the sample with O2 as the carrier gas is consistent with the XRD measurement results.

**Figure 3.** Optical transmission spectra and (αhv)2-hv plots of α-Ga2O3 films.

Figure 4a shows the electron di ffraction patterns of the Ga2O3/Al2O3 interface for the sample with O2 as the carrier gas. The di ffraction patterns of both the Ga2O3 film and the Al2O3 substrate are rectangular, corresponding to the corundum-structure. All of the di ffraction spots of α-Ga2O3 are situated almost in the α-Al2O3 spots. The cross-sectional HR-TEM image at the α-Ga2O3/α-Al2O3 interface is shown in Figure 4b. As a result of the in-plane strain, the α-Ga2O3/α-Al2O3 interface is unclearly identified. At the α-Ga2O3/α-Al2O3 interface, we can observe a dark area and this is induced by the in-plane compressive strain in the α-Ga2O3 layer because the lattice constant of α-Ga2O3 is larger than that of α-Al2O3 substrate. The TEM images for the samples, with air and N2 as the carrier gases in Figures S1 and S2 also demonstrate the similar electron di ffraction patterns and obvious HR-TEM lattice structures, indicating that all of the samples achieve a pure α-Ga2O3 phase and confirm the conclusion from the XRD measurements again. By comparing the HR-TEM images for the three di fferent samples, the samples with O2 and air as the carrier gases show a more complete lattice structure than the sample with N2 as the carrier gas, which shows that the adequate e fficient oxygen content is essential for the high quality film growth and this result is the same with that from the XRD measurement. The α-Ga2O3 films deposited on sapphire substrates obtain the thicknesses of 619 nm, 318 nm, and 146 nm for the samples with O2, air, and N2 as the carrier gases, respectively. These thicknesses correspond to the di fferent growth rates of 10.3 nm/min, 5.3 nm/min, and 2.4 nm/min as shown in Figure 4c. Variable-angle Spectral Ellipsometry (SE) measurements were performed at room temperature in ambient atmosphere with an electronically controlled rotating compensator and Glan Taylor polarizers (J. A. Woollam Co., Lincoln, NE, USA). The Cauchy model was used to fit the thickness of Ga2O3 thin film. Measurements were carried out at three di fferent incidence angles of 55◦, 65◦, and 75◦ over the 193–1000 nm wavelength range. Considering the di fferent oxygen contents in the carrier gases, it can be concluded that more oxygen in the carrier gas can promote the growth rate greatly, and at the same time it can also guarantee the crystal quality as demonstrated by the XRD, UV-VIS and TEM measurements.

**Figure 4.** TEM images. (**a**) Diffraction spots of α-Ga2O3/α-Al2O3 for the sample with O2 as the carrier gas, (**b**) cross-sectional α-Ga2O3/α-Al2O3 interface for the sample with O2 as the carrier gas. (**c**) Growth rate of α-Ga2O3 films grown by air, N2 and O2 carrier gas.

Figure 5 shows the AFM images for the α-Ga2O3 films prepared by different carrier gases of air (a), N2 (b), and O2(c). The films have root-mean-square (RMS) surface roughness values of 6.6 nm, 1.16 nm and 2.18 nm, respectively, measured over an area of 5 × 5 μm2. The surface morphology for the sample grown with air as the carrier gas is rough and composed of irregular stripes, while the surface morphologies of α-Ga2O3 thin films grown with N2 and O2 as the carrier gases are much smoother. It is inferred that the complex components in air induced this rough surface, and the reason behind that requires more research in the future. On the contrary, the pure carrier gas will lead to a much smoother surface. Comparing the sample grown with O2 as the carrier gas to the sample with N2 as the carrier gas, although the growth rate is improved by about five times, the difference between their RMS values is relatively small. This means that the rapid growth rate with O2 as the carrier gas will not degrade the surface morphology. To further investigate the microstructure, we measured SEM images of all α-Ga2O3 films as shown in Figure 5d–f. As demonstrated in the enlarged pictures, it can be seen that the film grown with air as the carrier gas has indeed a rough surface, and the other samples show the relatively smooth surface, which confirm the results from the AFM measurement.

**Figure 5.** *Cont.*

(**d**)Ga2O3 film grown with Air carrier gas

(**e**) Ga2O3 film grown with N2 carrier gas

(**f**) Ga2O3 film grown with O2 carrier gas

**Figure 5.** AFM surface images of Ga2O3 films grown with different carrier gases: (**a**) air (**b**) N2, and (**c**) O2. SEM pictures for the Ga2O3 films grown with different carrier gases: (**d**) air (**e**) N2, and (**f**) O2.

To assess the elemental composition of the α-Ga2O3 films, XPS measurements were conducted for the epitaxial films on sapphire. The result for the sample with O2 as the carrier gas is presented in Figure 6a and only three elements (C, Ga, and O) are observed in the films. The position of the Ga <sup>2</sup>*p*3/2 and Ga 3*d* binding energy peaks confirms the presence of Ga2O3 [18]. It can be observed from the spectra that the Ga2*p*3/2 and Ga2*p*1/2 signal peaks are located at 1118.5 eV and 1145.5 eV respectively, The energy difference between the two signal peaks is about 27 eV, which is consistent with the Ga2*p* signal peak energy difference reported in the literature [19]. The binding energy position of Ga3*d* signal is 21.05 eV, which is consistent with the results reported in the literature [20]. The result for the samples with air as the carrier gases is shown in Figures S3 and S4. The same peak positions appear in both samples and no significant peak shift and peak intensity changes are observed, indicating the same pure α-Ga2O3 and correspond to the XRD and Raman results. Figure 6b–d show the peaks of O 1*s* signal around 530.6 eV for the three samples. O 1s signal peaks do not show Gaussian symmetry so Gaussian fitting was carried out for the peaks. It is found that the strong O 1*s* signal peaks appear at 530.6 eV and 532.1 eV. The peak at 530.6 eV originated from the oxygen element in Ga–O bond, while the peak at 532.1 eV originated from the adsorption O on the sample surface [21]. Figure 5e–f show Ga 3*d* spectra. The chemical compositions of the surfaces were determined from the area of the Ga 3*d*, O 1*s*, and C 1*s* peaks taking into account the corresponding sensitivity factors [22]. The calculated atomic ratios of O to Ga for the samples grown with air, N2, and O2 as the carrier gases were 1.53, 1.51, and 1.56, respectively. It reveals that more oxygen present in the carrier gas can slightly increase the O content of the film, which confirms the guess in the XRD measurement.

**Figure 6.** *Cont.*

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**Figure 6.** (**a**) X-ray photoelectron wide spectra for the α-Ga2O3 sample grown with O2 and N2 as the carrier gas. X-ray photoelectron spectra of O 1*s* peaks for the samples grown with air (**b**), N2 (**c**), and O2 (**d**) as the carrier gases. X-ray photoelectron spectra of Ga 3*d* peaks for the samples grown with air (**e**), N2 (**f**), and O2 (**g**) as the carrier gases.

We also compared the quality of the same thickness and growth rate of the α-Ga2O3 films grown by different carrier gases. As shown in Figure S2 in the supplemental information, the growth rate of the α-Ga2O3 films was set to 10 nm/min for all samples. In Figure S3 in the supplemental information, the thicknesses of all α-Ga2O3 samples were 500 nm. The optical transmittance spectra show that when O2 is the carrier gas, the absorption edge is closer to 234 nm of α-Ga2O3 and the downward trend of the absorption edge is more obvious. From the AFM images, we assume that with the same growth rate and film thickness, the complex components in air induces rough surface and the pure carrier gas will lead to a much smoother surface. These results fully demonstrate that the partial pressure of oxygen in the carrier gas component can improve the quality of the film, consistent with the above results.
