*3.1. Part A: Intrinsic and Extrinsic Effects of Anomalous Thermal Expansion in AlScN Thin Films*

The exceptional robustness of the wurtzite-type structure and its ferroelectric polarization switching have been recently demonstrated for 400 nm Al0.73Sc0.27N/Mo(110)/AlN(0001)/ Si(001) thin films during and after high-temperature treatment up to 1100 ◦C [11]. However, the experimental data suggested a minor degradation of the crystalline quality during the process, which is illustrated by highly comparable types of data collected on Al0.73Sc0.27N thin films from the identical wafer as presented in Figure 1. The evolution with temperature of the Al0.73Sc0.27N 0002-reflection profile centered at 2θ~36◦ shown in Figure 1a displays the decrease in the maximum diffracted intensity of the Al0.73Sc0.27N component and the development of a hump as a shoulder at lower diffraction angles of the 0002-reflection. These observations indicate the reduction in the sizes of coherently scattering domains of the Al0.73Sc0.27N phase and the formation of a top oxide layer of (Al,Sc)(N,O)x, which is supported by energy-dispersive X-ray spectroscopy measurements on a scanning electron microscope before and after annealing (not shown). No further peaks indicating oxide formation were observed in the temperature-dependent 2θ/θ-scans (30–100 ◦C) (not shown). Further, the temperature dependent 0002-reflection profiles in Figure 1a reveal the 0002 reflection of the AlN seed layer at a temperature of 1000 ◦C (see arrows) at higher diffraction angle with respect to the intensity maximum. This could be explained by differences in thermal expansion coefficients of AlN and AlScN, which are known for temperatures up to 400 ◦C [17]. However, when treating AlScN films at high temperatures, a transition in thermal expansion is observed. The comparisons between the thermal shifts in the 0002-reflection maxima for Al0.73Sc0.27N and AlN thin films and the Mo(110)-reflection of the Mo layer underneath Al0.73Sc0.27N are presented in Figure 1b. Here, the relative change in the lattice spacings ∆*d[T]*/*d*<sup>0</sup> reveals strong non-linearity in the thermal lattice expansion of the Al0.73Sc0.27N film. The curve for the Al0.73Sc0.27N film (green curve in Figure 1b) shows a non-linearity in thermal expansion changing from a linear low-temperature branch to a high-temperature branch at a transition temperature of about *Ttr*~600 ◦C. The linear slope of the low-temperature branch is (∆*c*[25 ◦C–600 ◦C]/*c*0)/∆*T*~4.4 <sup>×</sup> <sup>10</sup>−6/ ◦C and increases by a factor of 3 to (∆*c*[700 ◦C–1000 ◦C]/*c*0)/∆*T*~13.7 <sup>×</sup> <sup>10</sup>−6/ ◦C in linear approximation to the high-temperature branch. Any influence of the underlayer is excluded by comparison to the relative change in the underlayer's Mo(110)-reflection which shows highly linear expansion. Further, an AlN film deposited under identical conditions was heated and shows a linear expansion of (∆*c*[25 ◦C–1100 ◦C]/*c*0)/∆*T*~3.5 <sup>×</sup> <sup>10</sup>−6/ ◦C. Due to the rather rough linear approximation over the entire temperature interval, the value

for AlN is somewhat smaller but still consistent with literature data of 4.2 [20–800 ◦C] − 4.65 [20–400 ◦C] <sup>×</sup> <sup>10</sup>−6/ ◦C [17,31]. tentially provide a Sc- and oxygen-enriched grain boundary structure [37], which could promote pathways for atmospheric oxygen species into the material.

and temperature [35,36].

erature data of 4.2 [20–800 °C] − 4.65 [20–400 °C] × 10−6/°C [17,31].

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changing from a linear low-temperature branch to a high-temperature branch at a transition temperature of about *Ttr*~600 °C. The linear slope of the low-temperature branch is (Δ*c*[25 °C–600 °C]/*c0*)/Δ*T*~4.4 × 10−6/°C and increases by a factor of 3 to (Δ*c*[700 °C–1000 °C]/*c0*)/Δ*T*~13.7 × 10−6/°C in linear approximation to the high-temperature branch. Any influence of the underlayer is excluded by comparison to the relative change in the underlayer's Mo(110)-reflection which shows highly linear expansion. Further, an AlN film deposited under identical conditions was heated and shows a linear expansion of (Δ*c*[25 °C– 1100 °C]/*c0*)/Δ*T*~3.5 × 10−6/°C. Due to the rather rough linear approximation over the entire temperature interval, the value for AlN is somewhat smaller but still consistent with lit-

To our knowledge, this strongly non-linear transition has not been observed to date for AlScN thin films. In the case of many oxide materials, non-linear thermal expansion behavior can be discussed in the context of oxygen- or oxygen vacancy-related effects termed 'chemical expansion' [32]. These chemical expansion phenomena can be non-reversible or reversible in nature. For instance, non-reversibility is observed in the case of cation–oxygen networks forming in metallic glasses [33] or small structural transitions in (Ba0.5Sr0.5)TiO3 induced by a change in oxygen site occupancy [34]. Reversible phenomena are commonly observed in non-stoichiometric perovskite-based ion-conducting ceramics which show pronounced chemical expansion depending on the oxygen partial pressure

Parallel to the strong oxidation of the AlScN film, the interaction with oxygen species and the intrinsic defect structure of the material could lead to the strong thermal expansion and irreversible changes. In this respect, the fiber textured columnar films could po-

**Figure 1.** XRD in situ (with oxygen atmosphere) annealing experiments on Al0.73Sc0.27N and AlN thin films grown on Mo(110)/AlN(0001)/Si(001) (sample set #1). (**a**) 2θ/θ-scan: Evolution of 0002-reflection profile of Al0.73Sc0.27N and (**b**) relative changes in lattice spacings *d* with temperature calculated from AlN(0002) and Al0.73Sc0.27N(0002)/Mo(110)-reflections. **Figure 1.** XRD in situ (with oxygen atmosphere) annealing experiments on Al0.73Sc0.27N and AlN thin films grown on Mo(110)/AlN(0001)/Si(001) (sample set #1). (**a**) 2θ/θ-scan: Evolution of 0002 reflection profile of Al0.73Sc0.27N and (**b**) relative changes in lattice spacings *d* with temperature calculated from AlN(0002) and Al0.73Sc0.27N(0002)/Mo(110)-reflections.

These preliminary experiments suggest the presence of residual oxygen contamination of the annealing atmosphere inside the graphitic dome placed in the diffractometer. Hence, the Al0.73Sc0.27N film was capped with a 100 nm thick SiNx layer to protect the film surface from oxidation-dependent effects, allowing us to investigate the purely intrinsic material contribution to the transition in thermal expansion behavior. With this experimental design, new in situ annealing experiments were conducted up to 1100 °C. The evolution of reflection profiles during the first and second annealing cycle and the corresponding Δ*c(T)/c0* plots are shown in Figure 2**.** The reflection profiles depicted in Figure To our knowledge, this strongly non-linear transition has not been observed to date for AlScN thin films. In the case of many oxide materials, non-linear thermal expansion behavior can be discussed in the context of oxygen- or oxygen vacancy-related effects termed 'chemical expansion' [32]. These chemical expansion phenomena can be nonreversible or reversible in nature. For instance, non-reversibility is observed in the case of cation–oxygen networks forming in metallic glasses [33] or small structural transitions in (Ba0.5Sr0.5)TiO<sup>3</sup> induced by a change in oxygen site occupancy [34]. Reversible phenomena are commonly observed in non-stoichiometric perovskite-based ion-conducting ceramics which show pronounced chemical expansion depending on the oxygen partial pressure and temperature [35,36].

Parallel to the strong oxidation of the AlScN film, the interaction with oxygen species and the intrinsic defect structure of the material could lead to the strong thermal expansion and irreversible changes. In this respect, the fiber textured columnar films could potentially provide a Sc- and oxygen-enriched grain boundary structure [37], which could promote pathways for atmospheric oxygen species into the material.

These preliminary experiments suggest the presence of residual oxygen contamination of the annealing atmosphere inside the graphitic dome placed in the diffractometer. Hence, the Al0.73Sc0.27N film was capped with a 100 nm thick SiN<sup>x</sup> layer to protect the film surface from oxidation-dependent effects, allowing us to investigate the purely intrinsic material contribution to the transition in thermal expansion behavior. With this experimental design, new in situ annealing experiments were conducted up to 1100 ◦C. The evolution of reflection profiles during the first and second annealing cycle and the corresponding ∆*c(T)/c*<sup>0</sup> plots are shown in Figure 2**.** The reflection profiles depicted in Figure 2a exhibit neither an oxide hump, nor a strong decrease in the reflection intensity, which is a sign of improved structural stability due to avoiding surface oxidation. Instead, a negligible XRD reflection broadening is observed by the increase in background intensity at higher diffraction angles at 550 ◦C, which could be due to changes in the average crystallite size, accumulation of defects and local lattice strains. Upon further annealing, no further changes in the reflection profile (Figure 2b) are observed for a second temperature cycle, indicating completed activation of any intrinsic processes until the applied temperature of 1100 ◦C.

ature of 1100 °C.

temperature.

sistent with the reflection profiles.

**Figure 2.** XRD in situ (with oxygen atmosphere) annealing experiments. Observation of anomalous intrinsic lattice expansion on oxygen-protected SiN passivated thin films of Al0.73Sc0.27N(0001)/Mo(110)/AlN(0001)/Si(100) (sample set #1). (**a**) 2θ/θ-scan: Evolution of the 0002 reflection profile during the first cycle. (**b**) 2θ/θ-scan: Evolution of the 0002-reflection profile during the second cycle. (**c**) Relative change Δ*c*/*c0* in lattice parameter with annealing temperature in two consecutive annealing cycles. **Figure 2.** XRD in situ (with oxygen atmosphere) annealing experiments. Observation of anomalous intrinsic lattice expansion on oxygen-protected SiN passivated thin films of Al0.73Sc0.27N(0001)/Mo(110)/AlN(0001)/Si(100) (sample set #1). (**a**) 2θ/θ-scan: Evolution of the 0002-reflection profile during the first cycle. (**b**) 2θ/θ-scan: Evolution of the 0002-reflection profile during the second cycle. (**c**) Relative change ∆*c*/*c*<sup>0</sup> in lattice parameter with annealing temperature in two consecutive annealing cycles.

2a exhibit neither an oxide hump, nor a strong decrease in the reflection intensity, which is a sign of improved structural stability due to avoiding surface oxidation. Instead, a negligible XRD reflection broadening is observed by the increase in background intensity at higher diffraction angles at 550 °C, which could be due to changes in the average crystallite size, accumulation of defects and local lattice strains. Upon further annealing, no further changes in the reflection profile (Figure 2b) are observed for a second temperature cycle, indicating completed activation of any intrinsic processes until the applied temper-

The corresponding Δ*c(T)/c0* plots for the first and second full temperature cycle are displayed in Figure 2c. In the first cycle (red curve), the thermal expansion in the lowtemperature regime is consistent with uncapped films with (Δ*c*[25 °C–400 °C]/*c0*)/Δ*T*~6.0 × 10−6/°C (compare Figure 1b) and literature reference data (6.38 × 10−6 K−1) for Al0.68Sc0.32N/Al2O3 [17]. However, a new and purely intrinsic regime at intermediate temperatures is observed with (Δ*c*[450 °C–650 °C]/*c0*)/Δ*T*~22.2 × 10−6/°C with much higher expansion and lower transition temperature *Ttr*~450 °C. This value of thermal expansion is almost double compared to the uncapped film at high temperatures with *Ttr* > 600 °C. In the high-temperature regime of >650 °C, the expansion slows down and reverses its sign (Δ*c*[750 °C–1100 °C])/*c0*)*/*Δ*T*~−1.5 × 10−6/°C. After cooling down to room temperature, linear thermal expansion over the entire temperature regime with (Δ*c*[1100 °C–30 °C]/*c0*)*/*Δ*T*~ −6.0 × 10−6/°C is observed and a small irreversible change in the *c* lattice parameter remains at Δ*c/c0*~0.5 × 10−3, which is much smaller than for the uncapped film. In a second temperature cycle (blue curve in Figure 2c), no anomalous thermal expansion is observed, con-

The comparison of both experiments suggests that oxidation effects or the interaction of material defects with oxygen species play a prominent role for high-temperature lattice expansion at >800 °C and the irreversible change in respective lattice parameters at room

This hypothesis is supported by the comparison of in situ XRD experiments with oxygen in the annealing atmosphere and ex situ XRD experiments without available oxygen in the atmosphere performed on uncapped films of Al0.73Sc0.27N/Mo(110)/AlN(0001)/Si(001). In this study, the first sample was introduced into the in situ XRD analysis when performing two consecutive heating cycles. The second sample was placed into a quartz tube furnace which was evacuated to 10−7 mbar. Both The corresponding ∆*c(T)/c*<sup>0</sup> plots for the first and second full temperature cycle are displayed in Figure 2c. In the first cycle (red curve), the thermal expansion in the low-temperature regime is consistent with uncapped films with (∆*c*[25 ◦C–400 ◦C]/*c*0)/ <sup>∆</sup>*T*~6.0 <sup>×</sup> <sup>10</sup>−6/ ◦C (compare Figure 1b) and literature reference data (6.38 <sup>×</sup> <sup>10</sup>−<sup>6</sup> <sup>K</sup> −1 ) for Al0.68Sc0.32N/Al2O<sup>3</sup> [17]. However, a new and purely intrinsic regime at intermediate temperatures is observed with (∆*c*[450 ◦C–650 ◦C]/*c*0)/∆*T*~22.2 <sup>×</sup> <sup>10</sup>−6/ ◦C with much higher expansion and lower transition temperature *Ttr*~450 ◦C. This value of thermal expansion is almost double compared to the uncapped film at high temperatures with *Ttr* > 600 ◦C. In the high-temperature regime of >650 ◦C, the expansion slows down and reverses its sign (∆*c*[750 ◦C–1100 ◦C])/*c*0)*/*∆*T*~−1.5 <sup>×</sup> <sup>10</sup>−6/ ◦C. After cooling down to room temperature, linear thermal expansion over the entire temperature regime with (∆*c*[1100 ◦C–30 ◦C]/*c*0)*/*∆*T*<sup>~</sup> <sup>−</sup>6.0 <sup>×</sup> <sup>10</sup>−6/ ◦C is observed and a small irreversible change in the *<sup>c</sup>* lattice parameter remains at <sup>∆</sup>*c/c*0~0.5 <sup>×</sup> <sup>10</sup>−<sup>3</sup> , which is much smaller than for the uncapped film. In a second temperature cycle (blue curve in Figure 2c), no anomalous thermal expansion is observed, consistent with the reflection profiles.

The comparison of both experiments suggests that oxidation effects or the interaction of material defects with oxygen species play a prominent role for high-temperature lattice expansion at >800 ◦C and the irreversible change in respective lattice parameters at room temperature.

This hypothesis is supported by the comparison of in situ XRD experiments with oxygen in the annealing atmosphere and ex situ XRD experiments without available oxygen in the atmosphere performed on uncapped films of Al0.73Sc0.27N/Mo(110)/AlN(0001)/Si(001). In this study, the first sample was introduced into the in situ XRD analysis when performing two consecutive heating cycles. The second sample was placed into a quartz tube furnace which was evacuated to 10−<sup>7</sup> mbar. Both samples were treated with identical temperature profiles. In the first heating cycles, a maximum temperature of 800 ◦C was applied, which is about the temperature at which the positive slope of the intrinsic expansion reverses, whereas a maximum of 1000 ◦C was used in the second in situ heating hysteresis. After the first ex situ cycle, a second temperature cycle was performed in situ to evaluate the 2θ shifts in the 0002-reflection at higher temperatures.

The reflection profiles and plots of the relative thermal expansion are summarized in Figure 3. Figure 3a shows the respective reflection profiles at selected stages during the experiment. As expected, no oxide hump is observed after the ex situ annealing (blue line), suggesting a purely intrinsic thermal expansion behavior. For the in situ annealed sample (red line), the oxide hump is observed. However, after performing the first cycles, both samples show an irreversible change in lattice parameter of about <sup>∆</sup>*c/c*0~1 <sup>×</sup> <sup>10</sup>−<sup>3</sup> in the plots of the relative thermal expansion shown in Figure 3b. This could indicate that the observed oxidation does not have a major effect on the expansion at intermediate temperatures and that the intrinsic contribution is dominating. By stopping the first cycle at 800 ◦C, it is assumed that the intrinsic effects on the anomalously high positive thermal expansion have all been activated and no further reaction would be observed. Indeed, this holds true for both samples in the second cycle up to 800 ◦C (golden and turquoise lines). By passing the 800 ◦C mark, comparable thermal expansion is observed when residual oxygen is supplied by the annealing atmosphere which results in a strong oxidation and large irreversible lattice changes <sup>∆</sup>*c/c*0~3 <sup>×</sup> <sup>10</sup>−<sup>3</sup> after annealing. suggesting a purely intrinsic thermal expansion behavior. For the in situ annealed sample (red line), the oxide hump is observed. However, after performing the first cycles, both samples show an irreversible change in lattice parameter of about *Δc/c0*~1 × 10−3 in the plots of the relative thermal expansion shown in Figure 3b. This could indicate that the observed oxidation does not have a major effect on the expansion at intermediate temperatures and that the intrinsic contribution is dominating. By stopping the first cycle at 800 °C, it is assumed that the intrinsic effects on the anomalously high positive thermal expansion have all been activated and no further reaction would be observed. Indeed, this holds true for both samples in the second cycle up to 800 °C (golden and turquoise lines). By passing the 800 °C mark, comparable thermal expansion is observed when residual oxygen is supplied by the annealing atmosphere which results in a strong oxidation and large irreversible lattice changes *Δc/c0*~3 × 10−3 after annealing.

samples were treated with identical temperature profiles. In the first heating cycles, a maximum temperature of 800 °C was applied, which is about the temperature at which the positive slope of the intrinsic expansion reverses, whereas a maximum of 1000 °C was used in the second in situ heating hysteresis. After the first ex situ cycle, a second temperature cycle was performed in situ to evaluate the 2θ shifts in the 0002-reflection at higher

The reflection profiles and plots of the relative thermal expansion are summarized in Figure 3. Figure 3a shows the respective reflection profiles at selected stages during the experiment. As expected, no oxide hump is observed after the ex situ annealing (blue line),

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temperatures.

**Figure 3.** XRD annealing experiments on Al0.73Sc0.27N thin films grown on Mo(110)/AlN(0001)/Si(001) (sample set #1). (**a**) 2θ/θ-scans: Evolution of 0002-reflection profile after ex situ (without oxygen atmosphere) and during in situ (with oxygen atmosphere) experiments. (**b**) Relative changes in lattice parameter *c* for Al0.73Sc0.27N thin films annealed ex situ and in situ. In summary, anomalous high thermal expansion and related irreversible lattice **Figure 3.** XRD annealing experiments on Al0.73Sc0.27N thin films grown on Mo(110)/AlN(0001)/Si(001) (sample set #1). (**a**) 2θ/θ-scans: Evolution of 0002-reflection profile after ex situ (without oxygen atmosphere) and during in situ (with oxygen atmosphere) experiments. (**b**) Relative changes in lattice parameter *c* for Al0.73Sc0.27N thin films annealed ex situ and in situ.

changes have been observed upon thermal activation by annealing of AlScN thin films. The experiments suggest an intrinsic material specific contribution activated at intermediate temperatures of >600–800 °C which is superimposed with extrinsic effects acting in parallel to oxidation of the films at temperatures exceeding 800 °C, if not protected by a surface covering layer. In summary, anomalous high thermal expansion and related irreversible lattice changes have been observed upon thermal activation by annealing of AlScN thin films. The experiments suggest an intrinsic material specific contribution activated at intermediate temperatures of >600–800 ◦C which is superimposed with extrinsic effects acting in parallel to oxidation of the films at temperatures exceeding 800 ◦C, if not protected by a surface covering layer.
