Bismuth iron oxide thin films using atomic layer deposition of alternating bismuth oxide and iron oxide layers

Highlights

• Bismuth iron oxide thin films were grown by atomic layer deposition at 140 °C.

• The major phase formed in the films upon annealing at 500 °C was BiFeO₃.

• BiFeO₃ films and films containing excess Bi favored electrical charge polarization.

• Slight excess of iron oxide enhanced saturative magnetization behavior.

Abstract

Bismuth iron oxide films with varying contributions from Fe₂O₃ or Bi₂O₃ were prepared using atomic layer deposition. Bismuth (III) 2,3-dimethyl-2-butoxide, was used as the bismuth source, iron(III) tert-butoxide as the iron source and water vapor as the oxygen source. The films were deposited as stacks of alternate Bi₂O₃ and Fe₂O₃ layers. Films grown at 140 °C to the thickness of 200–220 nm were amorphous, but crystallized upon post-deposition annealing at 500 °C in nitrogen. Annealing of films with intermittent bismuth and iron oxide layers grown to different thicknesses influenced their surface morphology, crystal structure, composition, electrical and magnetic properties. Implications of multiferroic performance were recognized in the films with the remanent charge polarization varying from 1 to 5 μC/cm² and magnetic coercivity varying from a few up to 8000 A/m.

Introduction

Atomic layer deposition (ALD) is a low-temperature chemical growth method particularly suited to the build-up of very thin solid conformal films of arbitrary composition for a variety of applications, including those in electronics [1], [2], [3], [4], [5], [6], [7]. For nanoelectronics, spintronics and sensing applications, multiferroic materials are promising and have gained significant interest over the past few years due to their simultaneous ferromagnetic and ferroelectric ordering. Among the multiferroic compounds, bismuth ferrite, BiFeO₃, has been one of the most widely studied materials due to its high Curie and Néel temperatures [8], and recently in THz spectroscopy [9] observed optical diode effect at spin-wave excitations [10].

BiFeO₃ films have earlier been produced by pulsed laser deposition (PLD), sputtering and sol–gel techniques. At the early stages, BiFeO₃ films produced by PLD technique directly on Pt/TiO₂/SiO₂/Si substrates yielded a polarization saturation, P_s, and remanent polarization, P_r, of 2.2 and 0.83 μC/cm², respectively [11] which are at levels comparable to those achieved in the films produced by sol–gel technique [12], as well as in bulk BiFeO₃[13]. The polarized domains in the bulk BiFeO₃ have naturally been larger than those achievable in thin films [14]. Wang et al. [15] have later demonstrated high P_s, P_r, and noticeable magnetic coercivity, H_c, in epitaxial BiFeO₃ films deposited by the PLD technique on single crystal SrTiO₃ substrates. However, the use of single crystal SrTiO₃ substrates inevitably leads to high cost, which would hamper the commercialization process. Regarding the optimization of the process directly on Pt-coated silicon substrate electrodes, BiFeO₃ films recently produced by PLD and sputtering technique showed appreciably high P_r[16], [17], with also magnetization apparent in the same films [17]. A nanocomposite of BiFeO₃/Fe₂O₃ produced using PLD, possessing better electrical properties than BiFeO₃ alone, has been reported [18]. However, in the case of PLD and sputtering techniques, when BiFeO₃ thin films of different Bi/Fe ratio or heterostructures (multilayers) of BiFeO₃/Fe₂O₃ are to be produced, high purity ablation and sputtering targets of different compositions are required [19].

Often films, powders and crystals of bismuth iron oxides consist of mixed crystallographic phases instead of being phase-pure BiFeO₃. Herewith, phases with stoichiometry different from that of BiFeO₃ may also exhibit useful physical and physicochemical properties. Studies on the effects of nonstoichiometry on BiFeO₃ base material have revealed that excess of iron leads to the separation of pyrochlore Bi₂Fe₄O₉ and γ-Fe₂O₃ phases, causing an increase in conductivity in macroscopic scale [20]. However, such films with stoichiometry deviating from that of BiFeO₃ can still exhibit both magnetic and electric polarization loops [20]. Dominantly rhombohedral crystalline phase was formed in sol–gel BiFeO₃ films after annealing at 50 °C, but with Bi₂Fe₄O₉ and Bi₂₄Fe₂O₃₉ as impurity phases [21]. Bi₂Fe₄O₉ and Bi₂₅FeO₄₀ secondary phases have also been recognized in BiFeO₃ films chemical vapor deposited from metal tert-butoxide precursors [22]. Bi₂Fe₄O₉[22], [23], [24], [25] and Bi₂₅FeO₄₀[26], [27] have been of interest as photocatalytic films and powders. Dielectric behavior and conduction mechanisms in Bi₂Fe₄O₉ have been studied as well [28], and Bi₂Fe₄O₉ has also exhibited multiferroic performance [29]. Furthermore, sputtered high-permittivity tetragonal Bi₂₄Fe₂O₃₉ thin films have shown appreciable insulating and dielectric properties [30].

ALD has shown certain potential to overcome the limitations related to thickness non-uniformity, poor step coverage and pinholes often faced during thin film processing, allowing, in addition, uniform coating over large substrates. Hence, ALD appears to be a commercially viable technique and the corresponding process chemistry offers an attractive field of research. Regarding possible ALD routes to BiFeO₃, 5 nm thick continuous films have been grown at 250 °C on Nb-doped SrTiO₃ (STO:Nb) using equal amounts of alternate Bi₂O₃ and Fe₂O₃ ALD cycles from β-diketonates Bi(thd)₃ and Fe(thd)₃ with H₂O, where thd denotes 2,2,6,6-tetramethyl-3,5-heptanedionato ligands [31]. These films were crystallized upon annealing at 650 °C. In another ALD process, also carried out at 250 °C on STO:Nb the BiFeO₃ films were grown from ferrocene (FeCp₂, Cp = C₅H₅), tris(1-methoxy-2-methyl-2-propoxy)bismuth, Bi(mmp)₃, and ozone, O₃[32]. In the latter two studies, ferroelectric polarization domains in BiFeO₃ were observed using piezoresponse force microscopy (PFM). More recently, ferroelectric characterization by PFM has also been carried out on films grown again at 250 °C on STO:Nb substrates by alternate pulsing of Bi(thd)₃ and FeCp₂ combined with O₃[33]. Furthermore, bismuth ferrite films with Bi/Fe ratio of 1.00 ± 0.02 were grown by ALD to the thickness of 60 nm from tris(2,3-dimethyl-2-butoxy)bismuth(III), Bi(dmb)₃, iron(III) tert-butoxide, Fe(O^tBu)₃, and H₂O on Pt/SiO₂/Si substrates at a temperature as low as 150 °C [34]. The latter films were crystallized at 500 °C and characterized by superconducting quantum interference device (SQUID) magnetometry.

In this work, BiFeO₃ films with varying contributions from secondary phases (Bi₂Fe₄O₉, Bi₂O₃ or Fe₂O₃) were grown by ALD on Pt/SiO₂/Si substrates from Bi(dmb)₃, Fe(O^tBu)₃, and H₂O. The structure and bismuth to iron atomic ratio in the films was modified and studied after changing the relative amounts of Bi₂O₃ or Fe₂O₃. The films were initially grown as sequentially stacked layers of binary iron and bismuth oxides, with variable numbers of deposition cycles for either constituent oxide. For some samples regarded as uniform heterostructures or even-layered stacks, the period of the stacks, i.e. the thickness of the Bi₂O₃ or Fe₂O₃ double layers, was kept constant. These films will further be denoted as uniform stacks. For the rest of the samples with intentionally more nonuniformly distributed composition, the thickness of the double layers was changed during the growth, creating films consisting of two halves with different periods. These stacks will further be denoted as nonuniform ones. The crystallization and formation of different phases became evident after annealing of the films upon solid state reactions between constituent oxides and intermixing of the layers. Thicknesses and periods of stacked layers were changed in order to get possible implications of structural and magnetoelectric behavior to the variations in the eventual stoichiometry of the films. The double layers were deposited to somewhat higher thicknesses closer to the substrate to better complete the formation of chemically defined component oxides, starting with the deposition of Fe₂O₃ layer due to its higher chemical stability on the substrate. The ferroelectric and ferromagnetic characteristics of the films were examined.