Strain dependent microstructural modifications of BiCrO3 epitaxial thin films
Introduction
Multiferroic oxides possessing more than one ferroic order parameter have promising properties, both for fundamental and technological aspects [1]. These materials may be applied in integrated microelectronic devices, spintronic based magnetic field sensors, piezoelectric transducers, etc. [2], [3]. Among the known multiferroic materials, Bi-based perovskite oxides, such as BiMeO3 (Me = Fe, Cr, Mn, etc.), have been extensively studied due to their high magnetic and ferroelectric (FE) ordering temperatures [4], [5]. One of the abovementioned materials that possesses intrinsic multiferroic properties is BiCrO3 (BCO) [6], [7], [8]. BCO has not been explored much yet, unlike BiFeO3 (BFO), which is quite appealing due to its large FE polarisation at room temperature (RT) [9], [10], [11], [12]. In contrast to BFO, the existing literature on the synthesis and FE studies of BCO is very limited. Reports on the presence or absence of FE ordering in BCO at RT have led to controversial and ambiguous results [13], [14]. Furthermore, enhanced FE and magnetic moments for the theoretically predicted B-site ordered double perovskite, Bi2(FeCr)O6 [15] have led to a great deal of experimental efforts to synthesize bulk and thin films of these compounds. In general, the synthesis of B-site ordered perovskites has been tackled by fabricating artificial superlattices or heterostructures, consisting of BFO and BCO thin films [16]. This has proven challenging, both in bulk and thin film forms, mainly due to the difficulty in stabilising a phase pure BCO without predominating growth defects and coexistence of different crystallographic structures as reported in the literature. Phase pure BCO ceramics can be synthesized only by using high-pressure techniques, requiring about 6 GPa [6]. Some of the few reports on BCO are summarized here: (i) Belik et al. [17] have reported that bulk BCO ceramics exhibit a monoclinic (C2/c) crystal symmetry at RT, which transforms into an orthorhombic symmetry (Pnma) at around 420 K. However, later, the same authors [18] noted that the monoclinic BCO phase retains about 10 to 15 volume percentage of an orthorhombic phase, suggesting multiple phase coexistence in BCO, (ii) Goujon et al. [19] have pointed out the presence of ~ 10 nm sized twinned domains within the monoclinic BCO phase, indicating its defective growth behaviour, (iii) Niitaka et al. [20], in contrast, have suggested a distorted non-centrosymmetric crystal structure with C2 symmetry for BCO that could plausibly induce a FE structural distortion and (iv) David et al. [21] have recently shown that in the case of BCO epitaxial thin films, an inhomogeneous strain distribution and oxygen-related defects within the BCO film can stabilise the coexistence of several distinct phases such as: monoclinic (C2/c) and orthorhombic (Pbnm), similar to the crystal structures already reported for bulk BCO, and an “unknown” third phase giving rise to superlattice diffraction spots and whose crystal structure has not been identified so far. However, the nature and the origin of such multiple structural coexistences in BCO remain ambiguous.
Detailed knowledge in the crystal structure of BCO is fundamental to understand its FE properties [13], [14], as well as studying B-site ordered multiferroic heterostructures based on BCO and BFO [15]. The present work focuses on the fabrication and microstructural analysis of BCO epitaxial thin films fabricated under different in-plane strain conditions, i.e., tensile and compressive strains. All the fabricated BCO films were investigated using a variety of techniques: X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM). Our results show that the BCO films grown under in-plane tensile and compressive strained conditions exhibit different crystallographic structures at RT. In addition, we found that in-plane tensile strained BCO thin films exhibit local structural modulations, directly correlated to the BCO layer thickness.
Section snippets
Experimental details
Epitaxial BCO thin films were fabricated by pulsed laser deposition (PLD) with a KrF excimer laser (λ ~ 248 nm) using a laser fluence of ≈ 2 J/cm2. The BCO films were grown under optimised conditions as follows: (i) substrate temperature (Ts) ~ 650 °C to 700 °C, (ii) oxygen partial pressure (Po2) ~ 1.3 to 2.6 Pa and (iii) laser frequency of 3 to 5 Hz. Metallic SRO bottom electrodes (15–25 nm thick) were first deposited onto the substrates as a buffer layer, so that the BCO films could be subjected to
Results and discussion
The representative XRD data for the BCOSTO (d ~ 85 nm) and BCONGO (d ~ 160 nm) samples is displayed in Fig. 2(a) and (b), respectively. Phase pure and highly oriented epitaxial growth could be envisaged for both the films, without any minor secondary phases of Bi2O3 or Cr2O3, etc. The reflections around STO (003) and NGO (330) are enlarged as inset in Fig. 2(a) and (b), respectively, to clearly reveal the presence of close peak splitting corresponding to the BCO and SRO phases, which indicates an
Conclusions
Growing non-defective and relatively thick BCO epitaxial films that possess homogeneous crystal structure is a prerequisite to study their electrical properties, eliminating the large contributions from leakage current. The present study showed that it is possible to stabilise an epitaxial BiCrO3, one of the metastable phases of Bi-based multiferroic oxides by PLD utilizing the substrate induced strain. Furthermore, an appropriate choice of the substrate to fabricate an in-plane strained
Acknowledgement
This work was supported by the CNRS–MPG joint fellowship programme on nanoscience. V. K. would like to thank Dr. R. Ranjith and Dr. Wei Peng for the scientific discussions. The authors also acknowledge Mr. Schammelt for PLD lab maintenance, Mareike Herrmann for TEM sample preparations and Dr. Eckhard Pippel for STEM–EDX analysis.
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