Ivoyl P. Koutsaroff

This letter presents Sezawa mode surface acoustic wave (SAW) resonators based on 500 nm boron-doped aluminum scandium nitride (Al ${}_{\mathbf {1-x-y}}$ B ${}_{\mathbf {x}}$ Sc ${}_{\mathbf {y}}$ N), AlBScN thin films co-sputtered from AlB and Sc targets onto sapphire and silicon carbide (SiC) substrates. The devices exhibit series resonance frequencies ranging from 4.3 GHz to 7.6 GHz. Atomic Force Microscopy (AFM) revealed smooth film surfaces, with root mean square roughness values of 0.65 and 0.54 nm on sapphire and SiC, respectively. X-ray diffraction (XRD) $\omega $ -scans around the wurtzite (002) reflection confirmed a highly c-axis oriented film exhibiting a full width at half maximum of 0.96° on sapphire. Electrical measurements revealed a maximum Sezawa mode velocity of 6748 m/s on SiC, while the sapphire-based device achieved a peak parallel resonance quality factor ( $Q$ ${}_{\mathbf {p}}$ ) of 1400. These results demonstrate the suitability of AlBScN for high-frequency, high-performance resonator applications.[2025-0143]

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The impact of the residual strain induced by the thermal strain on the dielectric tunability was systematically studied for (100)-one-axis-oriented polycrystalline (Ba_0.5Sr_0.5)TiO₃films. These films were grown on various substrates with different thermal expansion coefficients [a_(sub.)] covered with a stack of (100)_cSrRuO₃/(100)_cLaNiO₃/(111)Pt layers. The residual strain was ascertained to linearly increase with the increase in a_(sub.)by enhancement of the surface-normal lattice spacing of (Ba_0.5Sr_0.5)TiO₃and Pt. Dielectric tunability of the films also linearly increased with the increase in a_(sub.). These results clearly demonstrate that dielectric tunability tailoring of the (Ba_0.5Sr_0.5)TiO₃films can be achieved by using residual thermal strain.

Thermal stability of epitaxial SrRuO₃ films grown on (100)SrTiO₃ substrates by RF magnetron sputter deposition as a function of the oxygen partial pressure (pO₂) was investigated in view of the crystal structure, surface roughness and electrical resistivity. The surface normal lattice parameter and diffraction peak intensity of SrRuO₃ films heat-treated at 700°C decreased with the decrease of pO₂ below 1.7 Pa, and finally SrRuO₃ peak completely disappeared at pO₂ below 1.7 × 10⁻⁴ Pa. Surface roughness also became high with decreasing the pO₂. X-ray diffraction and Non-Rutherford Elastic Resonance Scattering measurements suggest that SrRuO₃film most probably decomposes to metal Ru and amorphous SrO after the heat treatment at pO₂ below 1.7 × 10⁻⁴ Pa. Epitaxial (Ba_0.5Sr_0.5)TiO₃ films were grown on these pre-heat-treated SrRuO₃ bottom electrodes by RF magnetron sputter deposition. (Ba_0.5Sr_0.5)TiO₃ film with low crystallinity deposited on SrRuO₃ bottom electrode heat-treated under pO₂ below 1.7 × 10⁻⁴ Pa showed much higher leakage current than the others. This is considered to be due to the high surface roughness of (Ba_0.5Sr_0.5)TiO₃ film inducing the concentration of electric field and/or the diffusion of Ru into (Ba_0.5Sr_0.5)TiO₃ film. This result suggests that the characteristics of capacitors are significantly affected by the heat treatment conditions of SrRuO₃ bottom electrodes.

The dependences of electrical properties on (111) and (100) orientations were investigated for (Ba_0.5Sr_0.5)TiO₃ films with in-plane random (one-axis-oriented films) and aligned orientations (epitaxial films) prepared by RF magnetron sputtering. Epitaxial films with (100) and (111) orientations were grown on (100)_cSrRuO₃∥(100)SrTiO₃ and (111)_cSrRuO₃∥(111)SrTiO₃ substrates, while one-axis-oriented films were grown on (100) _c SrRuO₃/(100)_cLaNiO₃/(111)Pt/TiO₂/SiO₂/Al₂O₃ and (111) _c SrRuO₃/(111)Pt/TiO₂/SiO₂/Al₂O₃ substrates, respectively. Films with the (111) orientation had larger relative dielectric constants, ε _r, measured at an oscillation of 20 mV and 100 kHz under an applied dc bias electric field of 0 kV/cm and a larger tunability against the dc bias electric field than (100)-oriented films for both epitaxial and one-axis orientations. These data show the dependences of ε _r and tunability on orientation were existed, irrespective of the in-plane orientation and the thermal strain caused in the film by the substrates.

The power handling capability of tunable microwave devices employing planar and parallel-plate Ba/sub x/Sri/sub 1- x/TiO/sub 3/ film-based capacitors was experimentally measured and analyzed. A microstrip resonator, excited by either harmonic or two-tone microwave signals of elevated power, was selected as an example of tunable test fixture.