Nicolas Kurz

Wurtzite Al_1−xSc_xN thin films with scandium Sc concentrations up to x = 0.41 were prepared by reactive pulsed DC magnetron co-sputtering at heater temperatures between 300 °C and 400 °C on Al₂O₃ substrates. Spectroscopic ellipsometry was used to determine the dielectric functions of wurtzite Al_1−xSc_xN by modeling the spectra with a three-layer model involving parametric oscillator functions for the Al_1−xSc_xN layers. By combining ellipsometry with transmission spectroscopy, we determined the composition dependence of the optical band gap E_g. For x < 0.25, the experimentally determined band gap follows the theoretical prediction with an offset, which arises from residual sub-band gap absorption. For higher Sc concentrations, for which the band gap is expected to become indirect, the experimental data deviate from the theoretical values for the direct band gap. No absorption that can be attributed to cubic ScN crystallites was observed up to x = 0.41, indicating a high phase purity in line with X-ray diffraction data.

It has been found that introducing Sc into AIN to form Al _1-x Sc _x N can lead to a significant increase in piezoelectric response, making Al _1-x Sc _x N a promising material for electroacoustic devices, such as frequency filters for mobile communications. 1 μm thick highly c-axis oriented Al _1-x Sc _x N thin films were deposited on 100 mm Si(001)(x = 0, 0.14, 0.23, and 0.32)and on Al ₂ O ₃ (0001)(x = 0.14 and 0.23)substrates using reactive pulsed-DC magnetron sputtering and SAW resonators with wavelengths λ = 2-24 μm were fabricated. The effective electromechanical coupling k _eff ² of the resonators improved with increasing Sc concentration, for instance, the λ = 2 μm AIN (f _s = 2.08 GHz)and Al _0.68 Sc _0.32 N (f _s = 1.83 GHz)resonators exhibited coupling of 0.6 % and 2.6 %, respectively. Furthermore, 3 % increase in resonant frequency and 9.6 % increase in coupling were observed when comparing λ = 2 μm resonators based on Al ₂ O ₃ to those based on Si substrates. The temperature coefficient of frequency (TCF)was found to reach up to -27.4 ppm/K at x = 0.23 using Si substrates, while the equivalent resonators based on Al ₂ O ₃ substrates exhibited lower TCF of -26.1 ppm/K. Thermal hysteresis errors were calculated to be ~1%. Employing Al ₂ O ₃ substrates for λ = 2 μm Al _0.77 Sc _0.23 N-based resonators showed reduced TCF, higher resonant frequency, and higher coupling coefficient when compared to the same design resonators on Si substrates.

In this work, bar-and disk-shaped resonators were fabricated using Al _0.84 Sc _0.16 N thin film. Sensitivity analysis of the resonators' resonance frequencies with respect to the piezoelectric material parameters revealed that resonance analysis can be used for material parameter extraction. An optimization algorithm is employed to determine the piezoelectric material parameters s ₁₁ ^E , s ₁₂ ^E , and d ₃₁ of Al _0.84 Sc _0.16 N from the length-and radial-extensional mode. Using the piezoelectric constitutive equations and combining the results obtained in this work with the results obtained in a previous work, the stress-charge form of elastic and piezoelectric parameters could be derived.

The piezoelectric and pyroelectric properties of aluminum nitride (AlN) can be further enhanced by alloying AlN with scandium nitride (ScN). However, while the pyroelectric effect can strongly affect the performance of electroacoustic devices such as surface acoustic wave (SAW) resonators, the pyroelectric properties of aluminum scandium nitride (AlScN) and their thermal dependence are not well known. In this work, the crystallographic, microstructural, and pyroelectric properties of 900 nm thick pyroelectric Al_1−xSc_xN thin films (x = 0 to 0.3) are investigated. All films have wurtzite‐type crystal structure and are highly c‐axis oriented. The pyroelectric coefficient in the temperature range of 20–80 °C is determined by measuring the generated current as a function of sinusoidal temperature excitation. The effective pyroelectric coefficient of AlScN increases by approximately 85% in the investigated composition range. Moreover, no temperature dependence of the effective pyroelectric coefficient is observed for all samples. The increase of the effective pyroelectric coefficient is explained qualitatively by considering the contribution of primary and secondary pyroelectric effect as well as the substrate clamping.

It is more difficult to nucleate AlN-ScN alloy thin films (AlScN) in pure (0001)-texture than it is with pure AlN thin films. AlN thus can serve as seed layer for AlScN. Equipment limitations may lead to the problem of a vacuum break between AlN and AlScN deposition, as it leads to oxidation of the AlN surface. This issue was studied with high resolution TEM and electron diffraction. The formed oxide layer disturbed a lot the epitaxial growth, leading to additional grain orientations. A mild RF etching step introduced before AlScN deposition was able to remove the oxide layer, and allowed for growing Al _0.84 Sc _0.16 N in local epitaxy on AlN, as shown by Hyper map EDX images. The resulting AlScN films show a pure (0001) texture. Double beam laser interferometry and finite element modeling were used to determine d _{33, f} of both layers together as 6.6 pm/V, and of 6.85 pm/V for AlScN alone when using the standard value of 3.9 pm/V for pure AlN. At the same time, the relative dielectric constant of Al _0.84 Sc _0.16 N was determined as 14.1.

Partial substitution of Al by Sc in AlN wurtzite films leads to a strong enhancement of the piezoelectric properties as long as the wurtzite phase is maintained. This is very promising for improving piezoelectric MEMS devices and enlarging their application range. Nucleation of (0001)-AlScN works particularly well on Pt (111) thin films. However, in some applications, the growth on insulating substrates may be required. Even though AlN can be grown well oriented on smooth amorphous surfaces of SiO 2 , it is not the case for AlScN. AlN seed layers are an evident solution when there is no vacuum break between the AlN and AlScN growth. However, when a vacuum break is unavoidable, an oxide layer is formed on AlN, which makes the “regrowth” difficult. In this contribution we show that a mild RF etch can solve the problem.