Temperature Dependence of the Electroacoustical Constants of Li-Doped ZnO Single Crystals

Abstract

A combination of ultrasonic velocity and piezoelectric resonator techniques has been used to determine the room-temperature electroacoustical constants and also their temperature derivatives for a hydrothermally grown, Li-doped single crystal of ZnO in the temperature range 10°–110°C. The crystal as initially grown had a bulk electrical resistivity of about 2×104Ω cm, which was low enough such that the ultrasonic velocity measurements were effectively made under zero electric-field conditions. The measured ultrasonic velocities and associated elastic moduli were in excellent agreement with Bateman's 25°C results for similar, low-resistivity material [(J. Appl. Phys. 33, 3309–12 (1962)]. The material was subsequently thermally annealed at several temperatures up to 800°C to increase the electrical resistivity (final value >1011Ω cm), and the electroacoustical constants and their temperature derivatives were remeasured. The electromechanical coupling coefficients calculated from the room-temperature data agree more closely with those of Jaffe and Berlincourt [Proc. IEEE 53, 1372–86 (1965)] than those of Chrisler et el. [Proc. IEEE 56, 225–6 (1968)]. The first-order temperature derivatives are negative for all five independent elastic and all three independent piezoelectric stress constants, but positive for the two independent dielectric constants. The relevance of these results to the design of piezoelectric resonator devices is briefly discussed.