Dielectric relaxation in gadolinium fluoride thin films

The preparation and studies of the electrical properties of thin insulating films has become of increasing interest in recent years. There is an increasing need for stable and reliable thin film capacitors for various applications in the field of microelectronics [1]. Because of their mechanical and chemical stability, the rare earth oxides and fluorides have attracted particular attention for device applications [2]. The dielectric behaviour of few rare earth fluoride thin films have been investigated [3-7]. This letter reports our work on the dielectric properties of thermally evaporated GdF3 thin films. Amorphous gadolinium fluoride thin films were deposited by thermal evaporation under a vacuum of 2 x 10 -5 torr on to precleaned glass substrates. Thin film capacitors of gadolinium fluoride (A1 GdF3-A1) were then fabricated using thick deposits of pure aluminium as lower and counter electrodes. The dielectric film thickness was measured using a multiple beam interferometer. The capacitors were stabilized by repeated annealing for several hours. The capacitance (C) and dielectric loss (tan 5) were measured in roughing vacuum in the frequency range 0.5 to 30 kHz at various temperatures (300 to 395 K) using a 0.1% Universal bridge (Eastern Electronics, India). X-ray diffraction studies were carried out on thermally evaporated films of GdF3 and the structure was found to be amorphous. Fig. 1 shows the variation of capacitance with frequency at five different temperatures. The capacitance decreases with frequency at all temperatures. It is observed that the capacitance becomes almost invariant with frequency at room temperature (300 K). The variation of tan 6 with frequency at different temperatures is shown in Fig. 2. It is observed that the value of tan 5 increases initially, attains a maximum and then decreases with frequency. It is also found that the loss peak shifts towards a highfrequency region with increasing temperature. Similar loss peaks were reported earlier in insulating films [8-12]. A Cole-Cole plot of e" against e' over the frequency range 0.5 to 30kHz at a temperature of 395 K is shown in Fig. 3. The activation energy for the relaxation process was calculated using the relation [131