Featuring the low electroacoustic losses, that stem from an ultra-wide bandgap (≈6 eV) and hence low intrinsic electronic conduction, bulk single-crystalline aluminum nitride is potentially a key component for high-temperature piezoelectric resonators [1–3]. Despite the recent progress in growing of AlN bulk single crystals of high structural perfection, the control of incorporation of electrically active impurities and of formation of native point defects, which may adversely affect the resonator performance at high temperatures, is challenging . Therefore, we focused our efforts on the growth of AlN crystals maintaining high electrical resistivity at high temperatures and on comprehensive investigation of high-temperature electromechanical properties and atomistic transport in them.
Here we report the results of optical characterization and impedance spectroscopy of AlN wafers with certain amounts of typical growth-related impurities such as O, C, and Si. The optical properties, electrical resistivity and electromechanical losses in AlN depend significantly on the [O]/[C] concentration ratio. The photoluminescence bands at 2.9 eV and 2.3 eV, and a broad UV absorption band at 4.7 eV, inherent for carbon-dominated AlN, diminish with increasing oxygen content until complete suppression in samples with [O]/[C] > 3. The effect of increasing [O]/[C] ratio is also reflected in a conductivity rise while the activation energy of electrical conductivity remains around 2 eV. Smooth, only slightly parabolic temperature dependencies of resonance frequency fR in fundamental modes of thickness (fR.300K≈7.4 MHz), length-thickness (1.4 MHz) and thickness-shear (2.8 MHz) oscillations, and resonance quality factors in the range from T = 300 K to 900 K on the order of 1e4–1e5, imply figures-of-merit comparable to those of state-of-the-art quartz resonators at room temperature, confirming the suitability of AlN for high-temperature piezoelectric applications. However, the electroacoustic losses onset in the range from T = 873 K ([O]≈[C]) to 973 K ([C]>>[O]) and appear to correlate with increasing electrical conductivity in AlN. The temperature dependence of conductivity is discussed in the framework of charge carrier statistics in semiconductors assuming reasonable abundances of point defects, including the mentioned impurities. Factors contributing to electroacoustic losses in bulk AlN resonators are considered.