Scanning Electron Microscope gave a morphological analysis on 3 sample substrates that had been used for Ga2O3 deposition. Figure 3 demonstrates the morphology of the Ga2O3 layer after annealing at a 5 µm scale with 3 samples, obtained using FESEM. Ga2O3 layer deposition is done for 3 samples at 550°C, 600°C, and 650°C temperatures respectively. According to the results from FESEM, Fig. 3(b) has a smooth-morphology compared to other samples. The thickness of the 3 samples has been estimated using cross-sectional imaging using FESEM. Figure 4 demonstrates the cross-sectional view of the Ga2O3 layer after annealing with 3 samples at 550°C,600°C, and 650°C temperatures, obtained using SEM. A smooth and efficient cross-sectional image in 3 samples is discovered in sample S2.
Figure (5) demonstrates XRD plots of sample S1, S2, and S3 substrates. These plots depict common diffraction peaks of 2θ at various angles for different phases of Ga2O3. Figure 5(a) shows common diffraction peaks of 2θ at 18.93°, 35.6°, 38.56° and 63.33° which correspond to (-201), (111), (-311), and (-710) of sample S1 substrate at temperature 550°C and other small peaks at 30.3°, 43.6°, 53.4°, 59.7° and 71.4° which correspond to (400), (113), (104), (313) and (444) [31], [32]. In sample S1, plane (113) at 43.6° 2θ angle belongs to the α phase of Ga2O3, plane (104) at 53.4° 2θ angle belongs to ε phase of Ga2O3, plane (444) at 71.4° 2θ angle belongs to δ phase of Ga2O3 and planes (-201), (400), (111), (-311), (313), (-710) at 18.93°, 30.3°, 35.6°, 38.56°, 59.7°, and 63.33° 2θ angles belongs to β phase of Ga2O3 [31], [32]. In sample S1, since most atomic planes shown in XRD belong to the β phase of Ga2O3, it can be said that the sample is β-Ga2O3.
Figure 4. (a) Shows SEM-cross-sectional image of deposited Ga2O3 on sample S1 sapphire substrate at a 1 µm scale, at 550°C. (b) Shows SEM-cross-sectional image of deposited Ga2O3 on sample S2 sapphire substrate at a 2 µm scale, at 600°C. (c) Shows SEM-cross-sectional image of deposited Ga2O3 on sample S3 sapphire substrate at a 500 nm scale, at 650°C.
Figure 5(b) shows common diffraction peaks of 2θ at 19.01°, 30.4°, 35.6° and 63.4° which correspond to (-201), (400), (111), and (-710) of sample S2 substrate at temperature 600°C and other small peaks at 30.3°, 43.6°, 53.4°, 59.7° and 71.4° which correspond to (110), (113), (112), (313) and (-710) [31]. In sample S2, planes (110), (113) at 30.3°, 43.6° 2θ angles belong to the α phase of Ga2O3, and planes (-201), (400), (111), (112), (313), (-710) at 19.01°, 30.4°, 35.6°, 53.4°, 59.7°, and 71.4° 2θ angles belong to β phase of Ga2O3 [31]. In sample S2, since most atomic planes shown in XRD belong to the β phase of Ga2O3, it can be said that the sample is β-Ga2O3.
Figure 5(c) shows common diffraction peaks of 2θ at 18.8°, 30.3°, 35.6° and 38.5° which correspond to (-201), (400), (111), and (-311) of sample S3 substrate at temperature 600°C and other small peaks at 43.6°, 53.4°, 59.5° and 63.4° which correspond to (112), (104), (313) and (020) [31], [32]. In sample S3, plane (104) at 53.4° 2θ angle belongs to ε phase of Ga2O3, and planes (-201), (400), (111), (-311), (112), (313), (020) at 18.8°, 30.3°, 35.6°, 38.5°, 43.6°, 59.5°, and 63.4° 2θ angles belong to β phase of Ga2O3 [31], [32].
The optical analysis has been done with UV-VIS (LAMBDA 750 UV/VIS/NIR, PerkinElmer, USA). The absorbance and tauc plot have been analyzed for 3 samples using UV-VIS spectroscopy and are shown in Fig. (6) and Fig. (7).
Ga2O3 shows high transparency in the spectral range except where the incident radiation is absorbed across the bandgap (Eg). Three plots had been acquired with three sample substrates through the Ga2O3 layer deposition process. Figure 6(a) demonstrates the band of absorbance ranging from 300 nm to 210 nm. Figure 6(b) demonstrates the band of absorbance ranging from 310 nm to 210nm. Figure 6(c) demonstrates the band of absorbance ranging from 320 nm to 230nm. After analyzing three UV plots, the sample S1 substrate had a smooth curve at the band of absorbance range. Sample S2 substrate had a smoother curve at the band of absorbance range. Sample S3 substrate had a sharper curve at the band of absorbance range. Now the resulting UV plot agrees with the sample substrate S3 morphology. For the direct bandgap, the absorption follows a power law of the form [33],
$$\left(\alpha hv\right)={B\left(hv-Eg\right)}^{\frac{1}{2}}$$
Here, these terms are elaborated \(hv\) as the energy of the incident photon, \(\alpha\) absorption coefficient, B is the absorption edge width parameter, and Eg is the bandgap. The optical absorption coefficient ‘\(\alpha\)’ of the Ga2O3 layer on sample substrates of different temperatures is evaluated using the relation [33],
$$a= \frac{1}{t}\text{ln}\left[\frac{T}{{(1-R)}^{2}}\right]$$
Here, these terms can be elaborated as T is the transmittance, R is the reflectance, and t is the thickness of the Ga2O3 layer on the sample sapphire substrate. The thickness of the Ga2O3 layer on sample substrates varies from one another as they are tested with different temperatures. The layer deposition is very rocky at 550°C, and at 600°C, the layer deposition is smooth compared to sample S1, and at 650°C, the layer deposition is very smooth compared to other substrates.
The UV-Vis Spectroscopy revealed that sample S1 substrate has a smooth slope cut at the absorbance band, sample S2 substrate has a smoother slope cut at the absorbance band, and sample S3 substrate has a sharper slope cut at the absorbance band. According to this analysis, the third sample plot now agrees more with the morphology of sample substrate S3.
The optical analysis of the tauc plot determines the optical gap of the deposited Ga2O3 layer on the sample substrate. The plot of (α\(hv\))2 vs (\(hv\)) now yields the energy gap value, which determines the material specifications. Extrapolating the plot line to the x-axis yields this energy gap. The band of absorbance of the plot in Fig. 7(a) at temperature 550°C is 4.5 eV and 5.5 eV. At 4.91 eV, the extrapolated line intersected the x-axis. The band of absorbance of the plot in Fig. 7(b) at a temperature of 600°C is 4.6 eV and 5.7 eV. At 4.85 eV, the extrapolated line intersected the x-axis. The band of absorbance of the plot in Fig. 7(c) at temperature 650°C is 4.2 eV and 5.6 eV. At 4.7 eV, the extrapolated line intersected the x-axis.
The tauc plot values of the three sample substrates will agree with the UV-Spectroscopy values for the energy band gap and the band of absorbance ranges. The UV plot of sample substrate S1 shows that the absorbance band ranges from 300nm to 210nm, and the tauc plot shows that the energy band gap is 4.91 eV. The absorbance band for sample substrate S2 ranges from 310nm to 210nm, and the energy band gap for the tauc plot of sample substrate S2 is 4.85 eV. The absorbance band for sample substrate S3 ranges from 320nm to 230nm, and the energy band gap for the tauc plot of sample substrate S3 is 4.7 eV. The sample substrate S3 tauc plot agrees more with the sample substrate C UV plot than the other two sample substrates' UV plots and tauc plots. These UV plots and tauc plots agree with the morphology of sample substrate S3.
The morphological analysis from Fig. 3 of three samples that have been deposited at three different temperatures, (550°C, 600°C and 650°C for S1, S2, and S3 samples correspondingly) have been shown that the S2 sample has the best morphology compared to S1 and S3 samples. At lower temperatures, α, δ, and ε phases have more chance to be in the material than the β phase. The XRD plot of sample S1 annealed at 550°C, demonstrated the α, δ, ε phases in the plot because this annealing is done at a lower temperature. But most atomic planes belong to the β phase of Ga2O3. The XRD plot of sample S2 annealed at 600°C, demonstrated the α phase at some planes because of annealing at low temperatures. But the majority of atomic planes belong to the β phase of Ga2O3. Sample S3 XRD analysis at 650°C, demonstrates the material belongs to the β phase of Ga2O3 and also the material shows one plane belongs to the ε phase. This XRD analysis shows that the most stable sample is S3, which annealed at 650°C and this elemental analysis agrees with morphological analysis. In optical analysis, UV-Vis Spectroscopy has been shown in Fig. 6 and when compared to samples S1 and S2, sample S3 has a sharper curve at the band of absorbance range. Tauc plots have been shown in Fig. 7, plots of tauc analysis and UV Vis spectroscopy agree with each other in sample S3 case, because of its absorbance band range and energy band gap (Eg ≈ 4.7–4.9 eV). From the comparative analysis of morphological, elemental, and optical analysis, sample S3 is the most stable β phase of Ga2O3. β-Ga2O3 is more stable in thermal and chemical conditions. α and δ phases are not stable compared to the β phase. So, the α and δ phase Ga2O3 can be used in Gas sensor applications. Applications of β-Ga2O3 are photodetectors.