Dielectric properties of Ba(Zn1/3Ta2/3)O3 thin films on Pt-coated Si substrates
Highlights
► Ba(Zn1/3Ta2/3)O3 thin films were grown by pulsed electron beam deposition method. ► Single-phase cubic perovskite structure after post-annealing ► Decrease of the temperature drift of the permittivity with an order of magnitude ► Ba(Zn1/3Ta2/3)O3 films with a dielectric constant around 21 were obtained.
Introduction
The complex perovskite ceramics have been extensively studied for their applications in microwave and millimeter wave domain. These compounds with the general formula Ba(B′1/3B″2/3)O3 (B′ = Mg, Zn; B″ = Nb or Ta) were prepared and widely studied [1]. A series of complex perovskite compounds were obtained by Galasso and coworkers [2], [3] and were found to have a 1:2 ordered hexagonal superstructure for B′ and B″ atoms [3]. When disordered (B′ and B″ ions arranged randomly), Ba(B′1/3B″2/3)O3 has a cubic perovskite structure [1]. In the case of Ba(Zn1/3Ta2/3)O3 ceramics, a reversible order–disorder phase transition occurs between 1600 and 1625 °C [4].
Despite the fact that the dielectric properties of the bulk Ba(Zn1/3Ta2/3)O3 (BZT) ceramics have been extensively studied and used typically in commercial communication devices as microwave dielectric resonators due to the very high quality factor at these frequencies [1], [5], [6], [7], [8], [9], [10], the formation of BZT thin films and their applications are at their beginning. A few papers reported the BZT thin films, obtained by sol–gel [11], aerosol deposition [12] methods, and pulsed laser deposition (PLD) [13], [14]. However, the high degree of miniaturization given by thin film is important in the integration of microelectronics systems.
In this work BZT thin films were grown by pulsed electron beam deposition (PED), which is a method for thin film growth similar to the PLD, but is based on the use of a pulsed electron beam instead of a laser beam to ablate a target material [15], [16]. Oxide films were grown by PED, with a good control of the surface morphology, cationic composition and crystalline structure of the films [16], [17], [18], [19], [20]. The composition of the various oxide thin films grown by PED is close to that of the target, as in the case of PLD, even in the case of complex oxide compounds (zirconium tin titanate, barium strontium titanate and biocompatible oxides, etc.) [16], [17], [18], [19], [20].
The Si/SiO2/TiO2/Pt substrates are one of the most currently used as metal–insulator–metal capacitive structure for the integration of complex dielectric oxides on silicon. One of the problems of dielectric measurements on perovskite thin films is that the behavior of platinum (on silicon wafers) as a metallic electrode at high growth temperatures leads to interface diffusion in silicon and formation of PtSi layers. Otherwise, high quality perovskite oxide films with good dielectric properties for applications require high growth temperatures and/or post-annealing treatments, regardless the thin film growth method used. Recently, we showed that PED method permits to obtain high quality, even epitaxial oxide thin films [18], [19], [20]. In this paper, we studied the dielectric properties of Ba(Zn1/3Ta2/3)O3 (BZT) thin films grown on Pt-coated Si substrates at substrate temperature of 500 °C and then post-annealed at 600 and 650 °C for 1 h. The influence of the degree of crystallization, surface morphology and film–substrate interface on the dielectric properties of the BZT film was investigated.
Section snippets
Experimental details
The BZT thin films were grown on commercial platinized silicon wafers, Si/SiO2 (450 nm)/TiO2 (15 nm)/Pt (100 nm), by the PED method in the experimental set-up at the National Institute for Lasers, Plasmas and Radiation Physics (NILPRP), which was described previously [16]. A pulsed electron beam (100 ns full width at half maximum) produced in a channel-spark discharge [15], [16] was used to ablate the BZT target [21] with a fluence of 2.5 J/cm2. An external capacitor of 16 nF was discharged at a high
Results and discussion
The X-ray diffraction patterns of BZT thin films grown on Pt-coated silicon substrates at a substrate temperature of 500 °C under different conditions are presented in Fig. 1. BZT films grown at substrate temperature less than 500 °C were found amorphous. The XRD pattern for the BZT films grown at 500 °C shows (110), (200), (211) and (100) reflection peaks, characteristic of the perovskite structure together with a large amount of amorphous phase, along with the (111) Pt layer reflections. The
Conclusions
BZT thin films were grown by PED method on Pt-coated Si substrate at 500 °C and post-annealed treatments at 600 and 650 °C for 1 h.
The XRD data showed that the BZT films grown on Pt-coated Si substrates are polycrystalline with a cubic (disorder) perovskite unit cell. The formation of cubic structure already occurs for partially crystallized BZT 1 films. With the post-annealing treatment the amorphous phase disappears and the degree of crystallization is improved. The unit cell constant decreases
Acknowledgments
This work was supported by a grant of the Romanian National Authority for Scientific Research, CNCS — UEFISCDI, project number PN-II-RU-PD-2011-3-0237. The authors thank Dr. Manuela Buda for performing the dielectric measurements.
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