*3.2. Part B: Discussion of Scandium Concentration in Al1*−*xScxN(0001)/Al2O3(0001) Thin Films*

In this section, the discussion is turned towards epitaxial *c*-axis columnar grown thin films of 1 µm Al1−xSc*x*N on Al2O<sup>3</sup> (sample set #2). The recorded XRD 2θ/θ-scans for 1 µm thick Al1−xSc*x*N (0 < *x* < 0.40) films before and after thermal annealing are shown in Figure 4a. All diffractograms of the as-grown sputtered Al1−xSc*x*N thin films demonstrate exclusive *c*-axis orientation as well as fixed in-plane orientation described by epitaxial relationships (0001)AlScN//(0001)Al2O<sup>3</sup> and (10–10)lScN//(11–20)Al2O3, respectively [17]. After annealing in situ (with oxygen atmosphere) to 1000 ◦C, only the 000` (` = 2, 4, 6)-reflections are observed, indicating decent temperature stability of the wurtzite-type phase for all examined Sc concentrations *x*.

**Figure 4.** Analysis of Al1−xScxN/Al2O3 (0 ≤ x ≤ 40.0) before and after in situ thermal annealing (sample set #2). (**a**) XRD 2θ/θ diffractograms. (**b**) XRD rocking curve measurements of the 0002-reflections. Summary of changes in lattice parameters *a* (**c**) and *c* (**d**) calculated from symmetric and asymmetric measurements, ω-FWHM (**e**) calculated from the XRD rocking curves. **Figure 4.** Analysis of Al1−xScxN/Al2O<sup>3</sup> (0 <sup>≤</sup> <sup>x</sup> <sup>≤</sup> 40.0) before and after in situ thermal annealing(sample set #2). (**a**) XRD 2θ/<sup>θ</sup> diffractograms. (**b**) XRD rocking curve measurements of the 0002 reflections. Summary of changes in lattice parameters *a* (**c**) and *c* (**d**) calculated from symmetric and asymmetric measurements, ω-FWHM (**e**) calculated from the XRD rocking curves.

**Table 2.** Lattice parameters and mosaicity analyses of Al1−xScxN (0 ≤ x ≤ 0.4) thin films by XRD before and after thermal annealing (sample set #2). **a [pm] c [pm] ω-FWHM [°] Sample As-GrownAnnealed Δa/a0 [×10−3] As-GrownAnnealed Δc/c0 [×10−3] As-GrownAnnealed Δω/ω0 [×10−2]** AlN 312.0 312.1 0.468 497.5 497.8 0.64 0.95 1.04 9.8 Al0.91Sc0.09N 313.6 316.0 7.64 498.3 498.5 0.39 1.07 1.10 2.2 Al0.77Sc0.23N 331.6 322.6 −27.13 498.8 499.6 1.66 1.53 1.59 3.7 However, a considerable XRD reflection broadening of the 000`-reflection profiles is observed after annealing for Sc concentrations x ≥ 0.23. This is paired with a shift in the reflection position to lower 2θ values. The broadening of the 000`-reflections features asymmetry, with the right tail of the Bragg reflection becoming extended relative to the left tail. No hump on the 0002-reflections is observed, which could indicate a better stability against oxidation for the epitaxial films with fewer grain boundaries. Indeed, rocking curve measurements of the 0002-reflection (Figure 4b) confirm this broadening and provide evidence for a structural degradation by the reduction in maximum diffracted intensity.

Al0.68Sc0.32N 327.4 326.7 −2.38 497.3 499.7 4.80 1.64 1.89 14.9 Al0.60Sc0.40N 332.7 330.5 −6.61 494.2 496.8 5.33 1.78 2.06 15.7 In further analysis of the in situ annealing experiments, the temperature-dependent changes in the reflection profiles are followed individually. In Figure 5, thin films of Al0.77Sc0.23N and Al0.60Sc0.40N are compared to demonstrate the difference between intermediate and high Sc contents. When comparing the reflection profiles, it is directly apparent that the Sc content influences the structural stability and the activation temperature of the observed degradation effects. The evolution of both the symmetric 2θ/θ 0002- and asymmetric ω/2θ 10–15-reflection profiles shows a strong degradation of the initial crystalline quality. The loss of structural coherence is most pronounced in the low-intensity asymmetric reflections, which limits the precise calculation of in-plane parameters at higher temperatures and after annealing. Concerning the XRD reflection broadening, for Sc *x* = The XRD reflection broadening is mainly attributed to reduced sizes of coherently scattering domains, the accumulation of defects and local lattice strain. A shift in the diffraction angle to lower values of 2θ in consequence of annealing is related to the expansion of the *c* lattice parameter, as discussed in part A. However, the exact origin of the reflection broadening remains speculative without a structure model and Rietveld refinement. Instrumental broadening of the reflections can be neglected due to the high mosaicity of the AlScN films. A detailed analysis of the in-plane and out-of-plane lattice parameterchanges using data from the symmetric 2θ/θ-scans and asymmetric <sup>ω</sup>/2<sup>θ</sup> 10–15(−) and 10–15(+)-reflection scans [30] is performed and the results are summarized in Figure 4c and Table 2. From the comparison, it is apparent that the high-temperature annealing induces irreversible changes to the lattice parameters of Al1−xSc*x*N. These changes manifest in the reduction in the *a* parameter and the increase in the *c* parameter in combination with the degradation of the overall crystal quality (broadening of FWHM). A clear trend is visible

in the magnitude of the effect which seems to scale with Sc concentration *x* indicating an increasing instability of high-Sc alloys at elevated temperatures. For instance, in the case of low-Sc content Al0.91Sc0.09N films, the irreversible change in the *c* lattice parameter is ∆*c/c*<sup>0</sup> 0.04% in contrast to high-Sc content Al0.60Sc0.40N films showing ∆*c/c*0~0.5%, associated with a relative broadening of FWHM of 2.2% and 15.7%, respectively. We note that the film with x = 0.23 shows a very large unexpected change in the *a* parameter, but follows the general trend regarding the other parameters.

**Table 2.** Lattice parameters and mosaicity analyses of Al1−xScxN (0 ≤ x ≤ 0.4) thin films by XRD before and after thermal annealing (sample set #2).


The structural origin of the observed reflection broadening in high-Sc content films could also be related to the competition between the hexagonal wurtzite-type phase and the cubic rocksalt-type structure when approaching 46% Sc [23–25,38]. High-temperature annealing could result in local phase destabilization and formation of nanosized cubic domains in Sc-enriched regions, e.g., at defect sites or grain boundaries [37]. Such nanosized domains, as well as the migration of defects, e.g., dislocations, will lead to reflection broadening and asymmetry by diffuse scattering and the formation of low-intensity shoulders in diffraction patterns [39,40]. Typically, we would expect a phase transition to be reversible, but here the strong and non-reversible increase in lattice parameters seems to indicate irreversibility.

In further analysis of the in situ annealing experiments, the temperature-dependent changes in the reflection profiles are followed individually. In Figure 5, thin films of Al0.77Sc0.23N and Al0.60Sc0.40N are compared to demonstrate the difference between intermediate and high Sc contents. When comparing the reflection profiles, it is directly apparent that the Sc content influences the structural stability and the activation temperature of the observed degradation effects. The evolution of both the symmetric 2θ/θ 0002 and asymmetric ω/2θ 10–15-reflection profiles shows a strong degradation of the initial crystalline quality. The loss of structural coherence is most pronounced in the low-intensity asymmetric reflections, which limits the precise calculation of in-plane parameters at higher temperatures and after annealing. Concerning the XRD reflection broadening, for Sc *x* = 0.23, the elevation of the background tails starts at a temperature of 850 ◦C, whereas for Sc *x* = 0.40 these features are already observed at 700 ◦C and with larger magnitude (Figure 5).

5).

**Figure 5.** Temperature-dependent changes in symmetric 2θ/θ 0002- and asymmetric ω/2θ 10–15 reflection profiles for (**a**) Al0.77Sc0.23N and (**b**) Al0.60Sc0.40N compositions (sample set #2). 10–15-reflections are measured with shallow (−) and steep (+) angles of incidence ω, see [30] for more information on the method. **Figure 5.** Temperature-dependent changes in symmetric 2θ/θ 0002- and asymmetric ω/2θ 10–15 reflection profiles for (**a**) Al0.77Sc0.23N and (**b**) Al0.60Sc0.40N compositions (sample set #2). 10–15 reflections are measured with shallow (−) and steep (+) angles of incidence ω, see [30] for more information on the method.

0.23, the elevation of the background tails starts at a temperature of 850 °C, whereas for Sc *x* = 0.40 these features are already observed at 700 °C and with larger magnitude (Figure

As before, the temperature-dependent changes in the reflection positions are used to calculate the relative thermal expansion of both *c* and *a* lattice parameters. The resulting thermal expansion is illustrated in Figure 6. Here, the relative changes in the *c* and *a* lattice parameters over the temperature range 25–1000 °C are presented for the investigated films in Figure 6a,b, respectively. In this analysis, the transition between the two expansion regimes is evidenced for all Al1-xSc*x*N compositions but not for AlN. Note that the films are not protected against available oxygen species, hence, intrinsic and extrinsic processes adding to the thermal expansion are superimposed. The transition from almost linear and low-value expansion at moderate temperatures into large values of expansion at high temperatures is observed for *a* and *c*. This expansion of the lattice is isotropic which is visible from the almost constant *c/a* ratio plotted in Figure 6c. The transition temperature *Ttr* seems to be related to the Sc content of the film, as well as the magnitude of the expansion. Roughly estimated values from the plots are *Ttr* ~ 850 °C for Al0.91Sc0.09N and *Ttr* ~ 550 °C for Al0.60Sc0.40N films as indicated by the vertical dotted lines in Figure 6b. In detail, the As before, the temperature-dependent changes in the reflection positions are used to calculate the relative thermal expansion of both *c* and *a* lattice parameters. The resulting thermal expansion is illustrated in Figure 6. Here, the relative changes in the *c* and *a* lattice parameters over the temperature range 25–1000 ◦C are presented for the investigated films in Figure 6a,b, respectively. In this analysis, the transition between the two expansion regimes is evidenced for all Al1−xSc*x*N compositions but not for AlN. Note that the films are not protected against available oxygen species, hence, intrinsic and extrinsic processes adding to the thermal expansion are superimposed. The transition from almost linear and low-value expansion at moderate temperatures into large values of expansion at high temperatures is observed for *a* and *c*. This expansion of the lattice is isotropic which is visible from the almost constant *c/a* ratio plotted in Figure 6c. The transition temperature *Ttr* seems to be related to the Sc content of the film, as well as the magnitude of the expansion. Roughly estimated values from the plots are *Ttr*∼850 ◦C for Al0.91Sc0.09N and *Ttr*∼550 ◦C for Al0.60Sc0.40N films as indicated by the vertical dotted lines in Figure 6b. In detail, the thermal expansion behavior in the low-temperature regime (<550 ◦C) is highly comparable for all Sc concentrations, in agreement with previous studies within this temperature range [17]. In strong contrast, the high-temperature regime is characterized by

a manifold increase in the expansion, depending on the Sc concentration (up to ∼8-fold for Al0.60Sc0.40N). After cooling back to room temperature, the lattice parameters show irreversible changes as discussed for Figure 4 and Table 2. increase in the expansion, depending on the Sc concentration (up to ~ 8-fold for Al0.60Sc0.40N). After cooling back to room temperature, the lattice parameters show irreversible changes as discussed for Figure 4 and Table 2. range [17]. In strong contrast, the high-temperature regime is characterized by a manifold increase in the expansion, depending on the Sc concentration (up to ~ 8-fold for Al0.60Sc0.40N). After cooling back to room temperature, the lattice parameters show irreversible changes as discussed for Figure 4 and Table 2.

thermal expansion behavior in the low-temperature regime (<550 °C) is highly comparable for all Sc concentrations, in agreement with previous studies within this temperature range [17]. In strong contrast, the high-temperature regime is characterized by a manifold

thermal expansion behavior in the low-temperature regime (<550 °C) is highly comparable for all Sc concentrations, in agreement with previous studies within this temperature

*Micromachines* **2022**, *13*, 1282 11 of 16

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**Figure 6.** XRD in situ (with oxygen atmosphere) characterization of thermal evolution of the lattice parameters (sample set #2). (**a**) Relative change in the lattice parameter *a* and (**b**) lattice parameter *c* indicating a high-temperature regime of isotropic thermal expansion and estimated values of the transition temperature *Ttr*. Dashed lines serve as a guide to the eye only. Residual values of lattice expansion after annealing are shown as square data points as given in Table 2. (**c**) Calculated *c*/*a* ratios showing approximately isotropic thermal expansion for the investigated thin films across the full temperature range. **Figure 6.** XRD in situ (with oxygen atmosphere) characterization of thermal evolution of the lattice parameters (sample set #2). (**a**) Relative change in the lattice parameter *a* and (**b**) lattice parameter *c* indicating a high-temperature regime of isotropic thermal expansion and estimated values of the transition temperature *Ttr*. Dashed lines serve as a guide to the eye only. Residual values of lattice expansion after annealing are shown as square data points as given in Table 2. (**c**) Calculated *c*/*a* ratios showing approximately isotropic thermal expansion for the investigated thin films across the full temperature range. **Figure 6.** XRD in situ (with oxygen atmosphere) characterization of thermal evolution of the lattice parameters (sample set #2). (**a**) Relative change in the lattice parameter *a* and (**b**) lattice parameter *c* indicating a high-temperature regime of isotropic thermal expansion and estimated values of the transition temperature *Ttr*. Dashed lines serve as a guide to the eye only. Residual values of lattice expansion after annealing are shown as square data points as given in Table 2. (**c**) Calculated *c*/*a* ratios showing approximately isotropic thermal expansion for the investigated thin films across the full temperature range.

For direct comparison with fiber textured samples from set #1, XRD measurements during two temperature cycles were conducted for the Al0.68Sc0.32N film, cf. Figure 7. The displayed temperature cycle and reflection profiles are highly congruent to the data recorded on the fiber textured thin films with similar composition. An irreversible change of Δ*c*/*c0*~3.5 × 10−3 remains after annealing at 1000 °C (Figure 7a) and no further changes are observed in the second temperature hysteresis, congruent with the measured reflection profiles shown in Figure 7b,c. For direct comparison with fiber textured samples from set #1, XRD measurements during two temperature cycles were conducted for the Al0.68Sc0.32N film, Figure 7. The displayed temperature cycle and reflection profiles are highly congruent to the data recorded on the fiber textured thin films with similar composition. An irreversible change of <sup>∆</sup>*c*/*c*0~3.5 <sup>×</sup> <sup>10</sup>−<sup>3</sup> remains after annealing at 1000 ◦C (Figure 7a) and no further changes are observed in the second temperature hysteresis, congruent with the measured reflection profiles shown in Figure 7b,c. For direct comparison with fiber textured samples from set #1, XRD measurements during two temperature cycles were conducted for the Al0.68Sc0.32N film, cf. Figure 7. The displayed temperature cycle and reflection profiles are highly congruent to the data recorded on the fiber textured thin films with similar composition. An irreversible change of Δ*c*/*c0*~3.5 × 10−3 remains after annealing at 1000 °C (Figure 7a) and no further changes are observed in the second temperature hysteresis, congruent with the measured reflection profiles shown in Figure 7b,c.

**Figure 7.** (**a**) XRD in situ (with oxygen atmosphere) temperature cycles in the example of Al0.68Sc0.32N (sample set #2). (**b**) Evolution of 0002-reflection profile in the first annealing cycle. (**c**) 2θ/θ-scan: Evolution of 0002-reflection profile in the second annealing cycle.
