Dielectric Relaxation of (Ba, Sr)TiO₃ Thin Films

Abstract

The dielectric relaxation of (Ba0.5Sr0.5)TiO3 thin films with high electric resistivity is investigated. The films are deposited by an rf-magnetron sputtering method in an atmosphere of argon and oxygen. The dielectric dispersion of the films is measured in the frequency range of 10-2-106 Hz. It is found that the dielectric constant ε slightly decreases with frequency, following the relationship of dε/d(log 10 f)∼-0.01ε, and the dielectric loss is almost constant at less than 1% in the measured frequency range. This type of dielectric relaxation causes absorption current which is inversely proportional to time, as the result of the dielectric aftereffect. In a dynamic random access memory (DRAM) operation, the dielectric relaxation would result in less than 10% loss of storage charge during the refresh cycle, and the film's DC leakage less affects the device operation.

(Ba,Sr)TiO3 Thin Films for Dram’s

“…This relationship has been measured over 9 decades (10-5 s to 10 4 s). The relaxation current has little temperature or field dependence [36,38]. As shown in Figure 11, the polarization current eventually decreases below that of the true leakage current labeled DC conduction in Figure 11.…”

“…Above the "knee" in Figure 10, the log of the current increases with the square root of voltage. Another measurement technique, shown in Figure 11, is used to measure the time dependent leakage current [25,32,[36][37][38][39][40][41]. By applying a voltage pulse to the sample and then measuring the current density with time a large current response is seen immediately after applying the voltage step and also after removing it.…”

“…Because the relaxation current is much larger than the DC conduction current for DRAM operating conditions (t < Is, V < IV), the relaxation current is responsible for almost all of the charge loss within refresh times, as shown in Figure 12 [32,36,39,42]. The dielectric relaxation has also been shown to control the dispersion in the capacitance with frequency and the loss tangent [31,32,36,37].…”

“…The dielectric relaxation has also been shown to control the dispersion in the capacitance with frequency and the loss tangent [31,32,36,37]. Measured loss tangents vary from as low as 0.006 in sputtered BST films [36] to 0.002 [37] for MOCVD BST films.…”

“…Dielectric relaxation current makes the greatest contribution to charge loss for DRAM conditions [32,36,39,42]. However, the true leakage current has also been studied and shown in SrTi03 to be controlled by the Schottky effect [38,43] by measuring DC conduction as function of voltage, film thickness and temperature for different top electrode materials (AI, Ti, Au, Pd and Pt).…”