Dielectric, impedance and ferroelectric characteristics of c-oriented bismuth vanadate films grown by pulsed laser deposition

Abstract

Ferroelectric bismuth vandante, Bi₂VO_5.5 (BVO) thin films with layered perovskite structure were deposited by pulsed excimer laser ablation technique on (1 1 1) Pt/TiO₂/SiO₂/Si substrates. The polarization hysteresis (P versus E) studies on the BVO thin films at 300 K confirmed the remnant polarization (P_r) and coercive field (E_c) to be 5.6 μC/cm² and 113 kV/cm, respectively. The same was corroborated via the capacitance–voltage measurements. The dielectric response and conduction mechanism of BVO thin films under small ac fields were analyzed using impendence spectroscopy. A strong low frequency dielectric dispersion (LFDD) was found to exist in these films, which was ascribed to the presence of the ionized space charge carriers such as oxygen ion vacancies and interfacial polarization. The room temperature dielectric constant and the loss (D) at 100 kHz were 233 and 0.07, respectively. The thermal activation energy for the relaxation process of the ionized space charge carriers was 0.85 eV. The frequency characteristics of BVO thin films under study showed universal dynamic response that was proposed by Jonscher for the systems associated with quasi-free charges.

Introduction

The studies concerning the ferroelectric thin films have drawn the attention of many researchers not only from the growth mechanism viewpoint but also from a variety of device applications that include non-volatile memory [1], infrared detectors [2], microelectromechanical systems [3], [4]; electro-optical switches [5], etc. Extensive work was done on lead zirconate titanate (PZT) [6], [7] for the development of non-volatile random access memories (NVRAMs) because of its non-volatility, large remnant polarization, and fast switching speed and radiation hardness. However, one of the most serious problems associated with Pt/PZT/Pt ferroelectric-based capacitor is the degradation of the polarization hysterises characteristics. In this context, bismuth-layered compounds were considered to be superior from their better fatigue properties point of view [8], [9]. These layered perovskites belong to the Aurivillius family [10], with the general formula (Bi₂O₂)²⁺(A_n−1B_nO_3n+1)²⁻. Among these materials, Bi₂Sr₂Ta₂O₉ and SrBi₂Nb₂O₉ and other similar compounds were widely studied [11], [12], [13]. The crystallization temperatures of these bismuth layer-structured ferroelectrics are relatively high and also possess higher dielectric constant at room temperature. In these Bi based layer-structured ferroelectrics, the dielectric constant decreases with decreasing the number of ‘n’ perovskite layers in the unit cell.

From the structural point of view, bismuth vanadate can be formulated as (Bi₂O₂)²⁺(VO_3.5□_0.5)²⁻, where □ represents oxide ion vacancies. Bi₂VO_5.5 (BVO) can hence be considered to be analogous to α-Bi₂WO₆, the n = 1 member of Aurivillius family of oxides with intrinsic oxygen vacancies in the perovskite layer [14], [15], [16], [17]. It crystallizes in a non-centrosymetric, polar orthorhombic class and is ferroelectric at room temperature. It exhibits three main polymorphs: a non-centrosymetric α-phase at room temperature, its transformation to a centrosymetric β-phase at 730 K and a centrosymmetric γ-phase is stable beyond 835 K; and BVO finally melts at 1153 K. Both the α-phase, which has a superstructure characterized by a tripling of the lattice parameter a and the β-phase, that has a superstructure with doubling of a, has an orthorhombic symmetry [19]. The γ-phase is described by a tetragonal symmetry. Because the distortions of crystal cell are small, these phases can be described as mean orthorhombic cell a_m = 5.53, b_m = 5.61, c_m = 15.26 Å. So the polymorphs of BVO can be briefly summarized as: Non-centrosymmetric orthorhombic (mm2) $\stackrel{\sim 730\text{K}}{⟶}$ centrosymmetric orthorhombic (mmm) $\stackrel{\sim 835}{⟶}$ centrosymmetric tetragonal (4/mmm). The lattice type of α-BVO is orthorhombic (space group B2cb) and the lattice parameters are a = 5.543 Å, b = 5.615 Å, and c = 15.321 Å (JCPDS 42-0135). Sooryanarayana et al. [20] reported a space group, Aba2 with the lattice parameters a = 5.602(2) Å, b = 15.269(3) Å, and c = 5.5250(8) Å using single crystal data. BVO thin films have been one of the promising for ferroelectric random access memory device applications because of their low dielectric constant. BVO in both bulk and thin film is known to exhibit the ferroelectric behavior along the c-axis apart from the a-axis (though a-axis was reported to be the polar axis) [17], [18], [21]. Similar observations have been made in the case of Bi₄Ti₃O₁₂ by Cummins and Cross [22], [23]. They have interpreted this kind of behavior to the electrical switching of P_s by 90^o under sufficiently high fields. BVO thin films have been fabricated using different deposition techniques including MOCVD [24], coating pyrolysis [25], chemical solution decomposition (CSD)[26], and pulsed laser ablation [21] on a variety of substrates. The pulsed laser technique is more suitable for depositing multicomponent oxides and has the advantage of maintaining excellent stoichiometry. Recently, we conducted detailed study on CSD deposited BVO thin films on Si substrates in a MOS configuration and found to exhibit excellent stability and lattice match on Si [26]. Since the number of pseudo-perovskite layers between the Bi₂O₂ layers is odd, i.e. remnant polarization P_r along the c-axis is expected to be preserved. Thus, fabrication of c-axis oriented BVO films is very important not only from the practical point of view, but also for elucidating the origin of ferroelectricity in bismuth-layered structure ferroelectrics.

To the best of the knowledge of the authors no reports exists in the literature on the growth and characterization of BVO thin films on platinum coated silicon substrates. Further, the understandings of the physical properties of BVO thin films are not conclusive. The high temperature γ-phase of BVO consists of (Bi₂O₂)²⁺ sheets interleaved with perovskite like layers of (VO_3.5□_0.5)²⁻. The high ionic conductivity associated with the γ-phase is attributed to the presence of oxide ion vacancies in the perovskite layer. The oxygen vacancies are disordered in α and β-phases of BVO and give rise to non-negligible ionic conductivity. It is known in the literature that the existence of electrical conductivity in ferroelectric materials is an undesirable feature as it may influence the ferroelectric characteristics: it is difficult to pole when leakage current is high; pyroelectric measurement may give a space charge induced current that overshadows the spontaneous polarization current; dielectric constant measurement no longer reflects the real contribution to the intrinsic polarization when space charge polarization becomes prevailing. In the present article we report the results and analyses of the growth and ac conduction properties of BVO thin films deposited on a Pt coated Si substrate in metal insulator metal (MIM) configuration.