Enhanced electrical properties of ferroelectric thin films by ultraviolet radiation

Abstract

Ferroelectric films in the paraelectric phase exhibit two undesirable properties: hysteresis in the voltage-capacitance characteristics and a significant relaxation time of the capacitance. Our experiments show that suppression of both of these is achieved by using UV radiation with wavelengths corresponding to the material forbidden gap. Experimentally we also observed UV radiation induced modulation of thin film permittivity without an applied electric field. The observed phenomena are believed to have the same origin: UV light generates nonequilibrium charge carriers that screen out local electric field induced by defects and interfaces inside ferroelectric thin films and change films effective dielectric properties. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.2137466͔ Ferroelectric materials have been the subject of extensive research for over 50 years. However, recent investigations of thin films of SrTiO 3 , BaTiO 3 , and their solid solution Ba x Sr 1−x TiO 3 ͑BSTO͒, motivated by the prospect of new applications such as electrically controllable microwave devices, 1,2 have uncovered many complexities not previously recognized. The most effort has been aimed at the optimization of thin film fabrication processes, i.e., to the microwave loss reduction ͑microwave loss tangent of ferroelectric thin films is much higher than that of corresponding bulk crystals͒, 3,4 increasing the tunability, 5 and improving the temperature stability. 6,7 These points are now better understood, but the issue of the residual polarization and the hysteresis phenomenon observed under varying bias voltage, resulting in slow relaxation of dielectric constant of ferroelectric films in paraelectric phase ͑above the Curie temperature͒, 8 has not been properly addressed.In principle, ferroelectric materials in the paraelectric phase should not exhibit hysteresis in the C͑V͒ behavior ͑voltage-capacitance characteristics͒. 9 They should demonstrate also very short response time comparable with the period of the lattice soft mode oscillations and the microwave 4 GHz intermodulation distortion measurements ͑subnano-second scale of response time͒ 10 and nanosecond pulse measurements 11 appeared to confirm this. However, a more careful analysis of the experimental results 11 revealed a difference between the variation of dielectric constant under ac and dc voltages, with a dc voltage producing a greater change in dielectric constant. The varactors' fast switching is at the expense of a decreased tunability. Furthermore, the difference between the variation of dielectric constant under ac and dc voltages varied with the magnitude of the applied voltage pulses, their duration and their repetition time making the control of devices problematic. Overcoming this problem will enable broader application of the ferroelectrics in microwave electronics.There exist a number of models explaining the residual polarization and dielectric relaxation in ferroelectric thin films. The charge injection 12 and space charge formation ͑migration of oxyg...

Λ-parameter of lattice QCD with Symanzik improved gluon actions

“…where a i are numerical constants (dependent on C 1 , C 2 ) evaluated via numerical integration over loop momenta. We consider ten sets of different values for the Symanzik coefficients, corresponding to the most commonly used actions, shown in Table I: The plaquette action, the tree-level Symanzik improved action, the Lüscher-Weisz tadpole improved actions (TILW), the Iwasaki action and the DBW2 action (see [12][13][14][15][16][17]). The quantities a i , for each one of the ten sets of parameters, are presented in Table II.…”

Effect of ultraviolet radiation on slow-relaxation processes in ferroelectric capacitance structures

“…Besides the fast response process ͑soft mode response, Ͻ1 ns͒, there are slowrelaxation processes, which cause up to 10% of the capacitance value to relax ͑i.e., return to initial state after the pulse duration͒ for a time ϳ 10 3 s. [5][6][7] This phenomenon is schematically presented in Fig. 9 In, 10 it was demonstrated that a way to suppress the slow-relaxation processes is irradiating the sample with UV light. This could cause a significant reduction in the efficiency of the capacitors tuning under unipolar pulse control voltages.…”

Correlations between microstructure and Q-factor of tunable thin film bulk acoustic wave resonators

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