High-temperature electromechanical characterization of AlN single crystals

Hexagonal AlN is a non-ferroelectric material and does not have any phase transition up to its melting point (>2000�C), which indicates the potential use of AlN for hightemperature sensing. In this work, the elastic, dielectric, and piezoelectric constants of AlN single crystals were investigated at elevated temperatures up to 1000�C by the resonance method. We used resonators of five different modes to obtain a complete set of material constants of AlN single crystals. The electrical resistivity of AlN at elevated temperature (1000�C) was found to be greater than 5 � 10¹⁰ Ω� cm. The resonance frequency of the resonators, which was mainly determined by the elastic compliances, decreased linearly with increasing temperature, and was characterized by a relatively low temperature coefficient of frequency, in the range of -20 to -36 ppm/�C. For all the investigated resonator modes, the elastic constants and the electromechanical coupling factors exhibited excellent temperature stability, with small variations over the full temperature range, <11.2% and <17%, respectively. Of particular significance is that due to the pyroelectricity of AlN, both the dielectric and the piezoelectric constants had high thermal resistivity even at extreme high temperature (1000�C). Therefore, high electrical resistivity, temperature independence of electromechanical properties, as well as high thermal resistivity of the elastic, dielectric, and piezoelectric properties, suggest that AlN single crystals are a promising candidate for high-temperature piezoelectric sensing applications.