Ferroelectric properties of Mn-Doped Bi_3.15Nd_0.85Ti₃O₁₂ thin films prepared under different annealing conditions

Abstract

Mn-doped Bi_3.15Nd_0.85Ti₃O₁₂ (BNTM) thin films were fabricated on Pt/Ti/SiO₂/Si(100) substrates by a chemical solution deposition technique and annealed at different temperatures from 650 to 800 °C. The structures of the films were analyzed using X-ray diffraction, which showed that the BNTM films exhibit polycrystalline structures and random orientations. The surface morphologies of the samples were investigated using scanning electron microscopy. The average grain size of the films increases with increasing annealing temperature. Electrical properties such as remanent polarization (2P_r) are quite dependent on the annealing temperature of BNTM films. It is found that the film annealed at 750 °C exhibits excellent ferroelectricity with a remanent polarization of 2P_r = 89.3 μC/cm² and a coercive field of E_c = 99.2 kV/cm respectively.

Introduction

Ferroelectric thin films have attracted considerable attention for their potential application in nonvolatile ferroelectric random access memory (NvFeRAM) [1], [2]. Pb(Zr,Ti)O₃ thin film has been widely investigated because of its large remnant polarization (P_r) and low processing temperature. However, it has some shortages such as imprint problem, lead toxicity, especially the fatigue when it is cycled beyond 10⁷ times, which limits its application [3]. In recent years, many researchers have paid their attentions to the bismuth-layered structure ferroelectrics (BLSFs) thin films due to their excellent fatigue-free nature and lead-free chemical composition. SrBi₂Ta₂O₉ (SBT) thin films, one type of BLSFs, show excellent endurance towards fatigue. However, the high processing temperature of SBT above 750 °C is an obstacle in integration with silicon devices [4]. Bi₄Ti₃O₁₂ (BiT) is another type of BLSFs and attracts much attention because of its lower processing temperature compared with that of SBT. However, the low P_r and poor fatigue property of BiT thin film limit its industrial application.

Site engineering is one of the best approaches to improve the ferroelectric properties of the parent BiT. In the case of A-site substitution in the BiT films, Park et al. have reported that La substituted BiT (Bi_3.25La_0.75Ti₃O₁₂, BLT) films have large P_r, low processing temperature and fatigue-free characteristics [5]. Some reports have also shown that other rare earth substituted BiT thin films have the similar results. It is noted that Nd-modified BiT (Bi_3.15Nd_0.85Ti₃O₁₂, BNT) films have the higher P_r than those of other A-site modified BiT films with the similar orientation [6]. In the case of B-site substitution in BiT, some ions such as V⁵⁺, Nb⁵⁺, W⁶⁺ and Mn³⁺ could improve the ferroelectric properties [7], [8], [9], [10]. Moreover, A- and B-sites cosubstituted BiT ferroelectric films, such as BLTV, BLTW, BLTM, and etc, were studied, and the results showed that A- and B-sites cosubstitution is an effective way to improve the ferroelectric properties [11], [12], [13], [14]. Considering that the B-site substitution of Mn in BiT thin films has been demonstrated to be an effective way to suppress leakage current, lower coercive field, and improve P_r of the films, Mn-doped Bi_3.15Nd_0.85Ti₃O₁₂ (Bi_3.15Nd_0.85Ti_2.99Mn_0.01O₁₂, BNTM) films were prepared on Pt/Ti/SiO₂/Si(100) substrates in the present study. To investigate ferroelectric properties dependence on the annealing temperature of BNTM films, the BNTM films were crystallized at different annealing temperatures in a moderate range, respectively.