(Invited) Complex Relative Permittivity of UV-C Transparent AlN

Abstract

Development of high-power switching and radio frequency (RF) devices based on the ultrawide-bandgap (UWBG) AlGaN alloy system is generating interest in high Al composition alloys. The increase in bandgap as the alloy composition is varied from GaN to AlN is associated with an increase in the critical electric field, and thus the achievable breakdown voltage, of power devices based on these materials. In addition to the higher critical electric field, the relative insensitivity of saturation velocity to alloy composition offers prospects for RF applications. Also, high Al composition AlGaN transistors offer improved performance at high temperatures. Basic challenges, associated with controllable doping, electrical contacts, and passivation, remain for implementation of high Al composition alloys in these applications, but the expected improvement in device performance drives investigation of their properties. To date, both lateral and quasi-vertical power switching device geometries have been demonstrated (1), and the research on UWBG AlGaN transistors in power and RF electronics was recently reviewed (2).High-quality native substrates enable epitaxial growth of device heterostructures with low densities of threading dislocations, due to the low lattice and thermal mismatches to epitaxial active layers. Single crystal AlN substrates possess a high thermal conductivity and a close lattice match to high Al composition alloys, which make them an excellent choice for growth of AlGaN-based power switching and RF devices. Physical vapor transport growth (PVT) of 2-inch diameter AlN substrates free of low angle grain boundaries and with average threading dislocation densities below 103 cm-2 was recently demonstrated (3). However, high below-bandgap optical absorption in the ultraviolet-C (UV-C) region was observed in AlN grown by PVT in carbon-containing atmospheres, due to a deep-level absorption band related to the carbon impurity. This absorption band negatively impacts the efficiency of optoelectronic devices, which have driven adoption of AlN substrates to date, and typically require light extraction through the substrate. In this work, we studied the optical properties of double-side polished, 2-inch, c-plane AlN substrates by UV-Vis spectroscopy. Absorption coefficients were calculated by accurately accounting for reflection losses (4). Spatially uniform absorption coefficients below 30 cm-1 at 265 nm were demonstrated across 2-inch substrates. Furthermore, the calculated absorption and reflection coefficients were used to determine the complex refractive index and relative permittivity for the E⊥ c polarization in the 250-700 nm range, showing good agreement with published data for nominally unstrained, UV-C transparent bulk crystals.Finally, reliable values for the high-frequency and static permittivities, which are needed in the design of high-power electronic devices, were obtained.References R. J. Kaplar, A. A. Allerman, A. M. Armstrong, M. H. Crawford, J. R. Dickerson, A. J. Fischer, A. G. Baca, and E. A. Douglas, ECS J. Sol. State Sci. and Technol. 6(2), Q3061 (2017).A. G. Baca, A. M. Armstrong, B. A. Klein, A. A. Allerman, E. A. Douglas, and R. J. Kaplar, J. Vac. Sci. Technol. 38, 20803 (2020).R. Dalmau, J. Britt, H. Fang, B. Raghothamachar, M. Dudley, and R. Schlesser, Mater Sci. Forum 1004, 63 (2020).R. Dalmau, J. Britt, and R. Schlesser, ECS Trans. 98(6), 3 (2020).