High Dielectric Constant (111)-Oriented Sr_1-xCa_xTiO₃ Epitaxial Layers Integrated on AlGaN/GaN Heterostructures

Multifunctional perovskite oxides exhibit a wide array of exotic properties often not observed in conventional semiconductors, including multiferroicity, high sheet charge densities and dielectric constants (<i>κ</i>), superconductivity, and tunable electronic phases. Advances in epitaxial growth techniques in recent years have enabled the monolithic integration of these functional oxides with semiconductor materials including Si, Ge, GaAs, and more recently, GaN. Development of new electronics on wide and ultra-wide bandgap semiconductors such as GaN and AlN is especially attractive in the field of RF and power electronics, due to the increased saturation electron velocity, thermal and chemical stability, and larger breakdown electric field of these materials when compared to conventional semiconductors such as Si. One limitation to increasing output power density is the high peak electric field that occurs as operating voltage is increased, leading to premature breakdown. “Extreme-<i>κ</i>” (&gt;100) dielectric layers have been proposed as an approach to reduce peak electric fields and enhance breakdown voltage in a device. Perovskite oxides such as SrTiO₃ or CaTiO₃ can be considered “extreme-<i>κ</i>” materials in this sense, as their bulk <i>κ</i> values are &gt;100. However, <i>κ</i> values in thin film amorphous perovskite oxides are significantly lower (typically ~20).<br/>In this work, we demonstrate high permittivity thin films of Sr_1-xCa_xTiO₃ (SCTO), a solid solution of SrTiO₃ and CaTiO₃, epitaxially grown on AlGaN/GaN/SiC high-electron-mobility transistor heterostructures via RF-plasma-assisted oxide molecular beam epitaxy. The structural phase mismatch between the perovskite SCTO and wurtzite GaN is overcome by the deposition of a 1 nm rutile TiO₂ buffer layer on the GaN, which results in a (111), single-phase out-of-plane orientation of the SCTO layers. We investigate the structural and electrical properties of SCTO/AlGaN/GaN heterostructures with compositions ranging from <i>x</i> = 0 to <i>x</i> = 0.53 and oxide film thicknesses ranging from 7 nm to 126 nm. Current-voltage measurements show up to a 5 order of magnitude reduction in vertical leakage of the SCTO samples when compared to a Schottky-contacted control sample. Capacitance-voltage measurements show minimal hysteresis, an extracted<i> κ</i> value as high as 290, and a fixed positive interface charge density of 2.38 ×10¹³cm^-2 at the SCTO/AlGaN interface. RF characterization of interdigitated capacitors fabricated using the SCTO films grown on unintentionally doped GaN/SiC shows that the films maintain their high permittivity values at 2 GHz and only exhibit a slight reduction in <i>κ</i> with increased lateral electric fields. These results demonstrate the epitaxial integration of an “extreme<i> κ</i>” functional oxide with GaN that can potentially improve electric field management in RF high-electron-mobility transistors. Moreover, the epitaxial connection between a perovskite and wurtzite crystal structure enables the subsequent expansion of perovskite oxides onto ultra-wide bandgap semiconductors such as high-Al-content AlGaN and AlN to realize next-generation multifunctional oxide/nitride hybrid electronics.<br/>This work is supported by the Office of Naval Research.