High temperature impedance spectroscopy study of KTaO3 (001) single crystal

Abstract

Dielectric, impedance, electric modulus, and ac conductivity properties of KTaO3 (001) (KTO) single crystal have been investigated for temperature and frequency range of 573–973 K and 100 Hz - 1 MHz, respectively. Our results suggest two separate conduction processes attributed to bulk and the sample surface-electrode interface. The analysis of impedance and electrical modulus data shows a temperature-independent relaxation process dominated by delocalized charge carriers in bulk. For temperatures ≥ 773, Intriguing tailing behaviour at lower frequencies observed in Nyquist plots for impedance, and it has been attributed to the sample surface-electrode interface effect. The ac conductivity obeys Jonscher’s Power Law for temperatures<773 K. However, for temperatures ≥773, the ac conductivity exhibits an anomalous behaviour in the low-frequency region as ac conductivity decreases with an increase in frequency up to a critical point in the mid-frequency region. A conduction mechanism has been proposed to explain the electrical response related to bulk and sample surface-electrode interface. Thermally activated vacancies are responsible for bulk conduction. However, migration of K+ ions from bulk to surface at higher temperatures and consequent formation of KO islands is accountable for forming an additional conduction channel at the sample surface-electrode interface. Our topographical AFM images support the formation of KO islands at the KTO surface at higher temperatures. X-ray diffraction results confirm a single crystalline bulk phase of the sample even after high temperature annealing. Our study reveals that the cation (K+) role should be taken into account along with conventional anion vacancies hopping in bulk to gain an overall understanding of the high temperature impedance spectroscopy measurements of KTO.