Spectroscopic ellipsometry of anisotropic AlScN pseudobinary alloys

Pseudobinary nitride alloys exhibit a great potential for applications in RF electronics, memory, and storage-based microelectronic hardware [1]. In particular, AlScN alloys are known for enhanced piezoelectric performance and accessible ferroelectric switching. Due to their inherently stochastic nature, pseudobinary nitride alloys manifest composition-sensitive mechanical and optical properties, which can hardly be exposed via DFT methods, aiming at single-crystal precision level of atomic structure. Moreover, provided that the only existing physical form of the alloys is thin films fabricated via physical vapor deposition methods, their structural integrity usually suffers from faults in microstructure and high impurity concentrations. It is thus necessary to provide a characterization method, such as spectroscopic ellipsometry, capable of entangling the physical properties of the alloys from the film quality effects. Recent works reporting the ellipsometry results of AlScN films showed that it is possible to extract the optical band gap, optical constants, absorption coefficients using a Cauchy function with an Urbach absorption term [2] or a set of Gaussian functions [3]. However, the effectiveness of such models originates from the fact that nitride films usually grow in c-axis texture enabling the use of isotropic models, which would inevitably fail when this texture is absent, as for a-plane AlScN films. Therefore, a reliable anisotropic model is strongly required. In this work, we propose the anisotropic optical model based on the Adachi model functions. The SE measurements of Al1-xScxN grown on sapphire substrates were performed in wide spectral range using Semilab SE-2000IR, while the spectral fitting has been performed via Semilab software suite. Based on the fitting results, a larger insight into the electronic properties of pseudobinary alloys is available enabling the information on the exciton strength and ground state energies.



[1] K.-H. Kim, I. Karpov, R.H. Olsson, and D. Jariwala, “Wurtzite and fluorite ferroelectric materials for electronic memory,” Nat. Nanotechnol. 18(5), 422–441 (2023).

[2] M. Baeumler, Y. Lu, N. Kurz, L. Kirste, M. Prescher, T. Christoph, J. Wagner, A. Žukauskaitė, and O. Ambacher, “Optical constants and band gap of wurtzite Al1−xScxN/Al2O3 prepared by magnetron sputter epitaxy for scandium concentrations up to x = 0.41,” Journal of Applied Physics 126(4), 045715 (2019).

[3] E.N. Jin, M.T. Hardy, A.L. Mock, J.L. Lyons, A.R. Kramer, M.J. Tadjer, N. Nepal, D.S. Katzer, and D.J. Meyer, “Band Alignment of ScxAl1– xN/GaN Heterojunctions,” ACS Appl. Mater. Interfaces 12(46), 52192–52200 (2020).