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\b{eta}-phase (AlxGa1-x)2O3 thin film with Al composition more than 70%

In this work, we have demonstrated wide-composition-range \b{eta}-(AlxGa1-x)2O3 thin films with record-high Al compositions up to 77% for \b{eta}-(AlxGa1-x)2O3 covering bandgaps from 4.9 to 6.4 eV. With optimized thermal annealing conditions, the \b{eta}-Ga2O3 binary thin films on sapphire substrates transformed to the \b{eta}-(AlGa)2O3 ternary thin films with different compositions. The binary to ternary transformation resulted from the Al atom diffusion from sapphire into the oxide layers; meanwhile, the Ga atoms diffused into sapphire leading to thicker thin films than the original thicknesses. The interdiffusion processes were confirmed by transmission electron microscopy, which enhanced in proportion to the annealing temperature. The strain states of the \b{eta}-(AlGa)2O3 films have been analyzed showing reduced in-plane compressive strain with higher annealing temperature; and the film eventually became strain-free when the temperature was 1400 oC corresponding to the Al composition of 77%. The proposed method is promising for the preparation of the \b{eta}-(AlGa)2O3 thin films without employing sophisticated direct-growth techniques for alloys.

To form the (AlxGa1-x)2O3 alloys, researchers have employed RF sputtering, 27 PLD, 28,29 mist-CVD, 30 MOCVD, 31 and MBE, 32,33,34 as summarized in Table 1 which shows the highestreported Al compositions by different techniques for different phases. These 'direct-growth' techniques offer excellent tunability of material compositions and compositional homogeneity, albeit the highest reported Al composition for the β-(AlxGa1-x)2O3 alloys is 61% by MBE. 32 Also Zhang et al. have demonstrated β-(AlxGa1-x)2O3 thin films with the Al composition of 52%, above which the (-201) and higher-order peaks by X-ray diffraction (XRD) were weak or invisible. 29 However, the associated costs of the direct-growth techniques are not trivial because of various requirements: targets with different compositions, high-purity precursors/sources, or demanding environments such as an ultrahigh vacuum.
In the 1990s, Fleischer and Battiston et al. discovered that thermal annealing of Ga2O3 thin films on sapphire could lead to Al diffusion into the films. 35 37 They performed secondary ion mass spectrometry (SIMS) analysis which show Al diffusion into the films, though being largely inhomogeneous. Those studies hint a viable pathway of forming (AlxGa1-x)2O3 ternary alloys based on Ga2O3/sapphire templates by thermal diffusion, thereby being low cost and straightforward as opposed to the direct growth techniques. However, the Al compositions were still low and inhomogeneous with polycrystalline Ga2O3 templates in those studies, which greatly hindered the application prospects.
Besides, the thin film/sapphire interface has not been systematically examined, preventing an understanding of the annealing impact.  In this work, we have employed β-Ga2O3/sapphire templates to obtain ternary β-(AlxGa1x)2O3 alloys by thermal annealing at various elevated temperatures. We found that controlling annealing temperature from 1000 to 1400 o C can tune the diffusion rate, leading to Al compositions from 0 up to 77%, corresponding to a large bandgap from 4.9 to 6.4 eV for β-(AlxGa1-x)2O3. The corresponding material composition, film thickness, and strain state were characterized systematically. Besides, the thin film/sapphire interface was investigated by high-resolution transmission electron microscopy (HR-TEM).

Experiment
In this study, 50-nm thick β-Ga2O3 thin-film templates were deposited on c-plane sapphire substrates by the PLD. Although the PLD was utilized, it is important to note that the same studies can be performed for β-Ga2O3 thin films on sapphire grown by other techniques such as CVD, spectrophotometer. Afterward, the optical bandgap was deduced from the Tauc plot from transmission spectra. The TEM specimens were obtained by a Helios focused ion beam (FIB) system. The cross-sectional TEM images and Energy-dispersive X-ray spectroscopy (EDX) mapping were acquired by an FEI Titan ST and Gatan EDX system.   Figure 1 shows the XRD 2 scan spectra of the as-deposited and annealed Ga2O3 thin films on sapphire with different annealing temperatures for three hours, and a bare sapphire substrate.

Results and Discussion
For the as-deposited sample, there are three dominating peaks from the (-201) and higher-order diffraction planes of β-Ga2O3 thin films, demonstrating single crystallinity. The (0006) sapphire substrate diffraction plane peak can also be seen. As the annealing was introduced with temperatures increasing from 1000 to 1400 o C, the (-201) peak positions shifted toward higher angles as shown in Figure 1. Table 2      58, 67, 72, and 77%, respectively. The relationship between the temperature and the Al composition shows quasi-linear behavior exhibited in Figure 4(a), suggesting that the annealing temperature is an excellent knob to control the alloy composition. Figure 4 predicted that the monoclinic phase is the preferred structure for up to 71% Al incorporation into -(AlxGa1-x)2O3. 39 To understand this discrepancy, we performed the -(AlGa)2O3 lattice Al%: x = 77% Al%: x < 77% d (-201) d' (-201) d' (-201) d (-201) Compressive strain Strain-free parameters calculated from the Al content of these different temperatures annealed samples in Table 3. The -(AlxGa1-x)2O3 lattice parameters were obtained by follows: a = (12.21−0.42x), b = (3.04−0.13x), c = (95.81−0.17x), and  = (103.87+0.31x), which follow the Vegard's law and Kranert et al. 40 After that, from the d-spacing of the (hkl) plane of the monoclinic structure in Equation (1), we can calculate the d-spacing of (-201) diffraction planes of -(AlGa)2O3 for each Al content (annealing temperature). Such a d-spacing evaluation method from given Al compositions determined from the bandgap in the Tauc plot (Figure 3    and (d). The Al and Ga gradients in the EDX maps show that the Al atoms diffused from the sapphire substrate upward to the (AlGa)2O3 film layer, which was the opposite of the Ga diffusion.
Meanwhile, the O distribution was homogeneous. In Figure 6(b) and (d), the EDX signal overlap maps show a tiny orange-ish region comprising Al red and Ga green signals at the interface of -(AlGa)2O3/sapphire reaffirming the existence of the transition layer. For the EDX spectra in Figure   6(b) and (d), the Al signal intensity is weaker than that of Ga for the sample annealed at 1000 o C in (b); and the Al signal intensity is stronger than that of Ga for the sample annealed at 1400 o C in (d). The results reiterate that with higher temperature annealing, more Al atoms diffused into the AlGa2O3 layer. It is noted that because the film thickness increased amid the annealing due to the interdiffusion process, one could design experiments with different initial thicknesses to reach the desired thicknesses and compositions by utilizing the annealing temperature knob.

Conclusion
Based on the binary β-Ga2O3 templates on sapphire substrates, we have employed the thermal annealing method to demonstrate β-(AlxGa1-x)2O3 ternary thin films with Al compositions from 0 to record-high 77%, corresponding to optical bandgaps from 4.9 to 6.4 eV. The ternary alloy formation was caused by the interdiffusion of Ga and Al atoms in the template and substrate, respectively, which was found to be proportional to the annealing temperatures between 1000 and 1400 o C. Thus, the annealing temperatures were found to be an excellent knob to control the Al composition. The interdiffusion was confirmed by the TEM experiments. The Al compositions of the β-(AlxGa1-x)2O3 ternary thin films were largely homogeneous. With lower annealing temperatures, the β-(AlGa)2O3 thin films were under in-plane compressive strain, which decreased with higher temperatures and eventually reduced to zero at 1400 o C. The proposed method does not involve direct-growth techniques for alloys and therefore is promising for straightforward and low-cost production of the ternary β-(AlGa)2O3 alloys with desired Al compositions and thicknesses for deep UV and power devices.