Through its dependence on low symmetry crystal phases, ferroelectricity is inherently a property tied to the lower temperature ranges of the phase diagram for a given material. This paper presents conclusive evidence that in the case of ferroelectric Al1−xScxN, low temperature has to be seen as a purely relative term, since its ferroelectric-to-paraelectric transition temperature is confirmed to surpass 1100 °C and thus the transition temperature of virtually any other thin film ferroelectric. We arrived at this conclusion through investigating the structural stability of 0.4–2 μm thick Al0.73Sc0.27N films grown on Mo bottom electrodes via in situ high-temperature x-ray diffraction and permittivity measurements. Our studies reveal that the wurtzite-type structure of Al0.73Sc0.27N is conserved during the entire 1100 °C annealing cycle, apparent through a constant c/a lattice parameter ratio. In situ permittivity measurements performed up to 1000 °C strongly support this conclusion and include what could be the onset of a diverging permittivity only at the very upper end of the measurement interval. Our in situ measurements are well-supported by ex situ (scanning) transmission electron microscopy and polarization and capacity hysteresis measurements. These results confirm the structural stability on the sub-μm scale next to the stability of the inscribed polarization during the complete 1100 °C annealing treatment. Thus, Al1−xScxN, there is the first readily available thin film ferroelectric with a temperature stability that surpasses virtually all thermal budgets occurring in microtechnology, be it during fabrication or the lifetime of a device—even in harshest environments.

The piezoelectric and spontaneous polarization of wurtzite ScxAl1−xN, GaxAl1−xN, and InxAl1−xN ternary compounds dramatically affects the electrical properties of pseudomorphic MexAl1−xN/GaN, MexAl1−xN/AlN, and MexAl1−xN/InN heterostructures and devices (Me: = Sc, Ga, In), due to bound interface charges caused by gradients in polarization at surfaces and heterointerfaces. We have calculated the piezoelectric and spontaneous polarization of undoped, metal polar ScxAl1−xN barrier layers (0 ≤ x ≤ 0.5) pseudomorphically grown on InN, GaN, and AlN buffer layers, in order to compare the polarization induced surface and interface charges determined to the ones predicted and measured in heterostructures with GaxAl1−xN and InxAl1−xN barriers (0 ≤ x ≤ 1.0). To facilitate the inclusion of the predicted polarization in future simulations, we give explicit prescriptions to calculate polarization induced bound interface charges for arbitrary x and barrier thicknesses up to 50 nm in each of the ternary III-N alloy heterostructures. In addition, we predict the electron sheet charges confined in heterostructures with positive polarization induced interface charges taking limitations for the epitaxial growth by strain and critical barrier thicknesses into account. Based on these results, we provide a detailed comparison of the sheet resistances and current-carrying capabilities of the heterostructures investigated, pointing to a superior potential of ScAlN/GaN based heterostructures for processing improved high electron mobility transistors for high-frequency and power electronic applications.

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Electron charges and distribution profiles induced by polarization gradients at the interfaces of pseudomorphic, hexagonal Sc xAl1− xN/GaN- and Sc xAl1− xN/InN-heterostructures are simulated by using a Schrödinger–Poisson solver across the entire range of random and metal-face Sc xAl1− xN-alloys, considering the transition from wurtzite to hexagonal layered crystal structure. In contrast to previous calculations of polarization-induced sheet charges, we use Dryer’s modern theory of polarization, which allows for consideration of the spontaneous polarization measured on ferroelectric Sc xAl1− xN-layers. Because the sheet density of the electrons accumulating at the heterostructure interfaces can strongly depend both on the data set of the piezoelectric and structural coefficients and on the alloying region of the Sc xAl1− xN-layers in which the transition from the wurtzite to the hexagonal layered crystal structure occurs, we have calculated the charge carrier sheet densities and profiles for three representative data sets and evaluated their relevance for devices. We predict electron sheet densities of [Formula: see text] and [Formula: see text] for all three sets of data for Ni/AlN/InN- and Ni/ScN/InN-heterostructures, respectively. We demonstrate that the polarization-induced interface charges of Ni/Sc xAl1− xN/InN-heterostructures are always positive, tend to increase with increasing Sc-content, and can cause electron accumulations that lead to flooding of the triangular quantum wells at the semiconductor interface. We identify Ni/Sc xAl1− xN/GaN-heterostructures with [Formula: see text] as particularly promising candidates for the processing of energy-efficient high electron mobility transistors due to their missing or low mechanical strain and their large electron sheet densities between [Formula: see text] and [Formula: see text]. Furthermore, we present simulation results of highly strained Ni/Sc xAl1− xN/GaN-heterostructures for [Formula: see text], which point to electron accumulations of up to [Formula: see text]. These heterostructures are not suitable for transistor devices, but they may be of great interest for the implementation of low impedance contacts.

The structural, elastic, and basic thermodynamic properties of hexagonal Sc xAl1−xN crystals are calculated and discussed over the whole range of possible random alloys, including the transition from wurtzite to the layered hexagonal structure. Based on a review of lattice and internal parameters in combination with complete datasets of stiffness coefficients published in the literature, differing in the considered alloying intervals and the predicted structural transitions, changes in the crystal lattices caused by the substitution of aluminum by scandium atoms are discussed and illustrated. Crystal properties like the mass densities, average bond angles, and bond lengths are calculated, and the compliance coefficients, Young's modulus, shear modulus, Poisson's ratio, compressibility, and sound velocities are determined depending on the alloy composition and in relation to the orientation of crystal planes and axes. Particular attention is paid to the occurring directional anisotropies and the changes in structural and elastic properties in the alloy region of the structural transition between wurtzite and layered hexagonal Sc xAl1−xN crystals. The acoustic velocities determined are used to calculate basic thermodynamic properties such as the Debye temperature, heat capacity, and minimum heat conduction, as well as to evaluate both the influence of the alloying and the structural transition on these properties.