Ferroelectric properties of ion-irradiated bismuth ferrite layers grown via molecular-beam epitaxy

We systematically investigate the role of defects, introduced by varying synthesis conditions and by carrying out ion irradiation treatments, on the structural and ferroelectric properties of commensurately strained bismuth ferrite Bi x Fe 2 − x O 3 layers grown on SrRuO 3 -coated DyScO 3 (110) o substrates using adsorption-controlled ozone molecular-beam epitaxy. Our findings highlight ion irradiation as an effective approach for reducing through-layer electrical leakage, a necessary condition for the development of reliable ferroelectrics-based electronics. is a nonlocal dipole-dipole interaction which acts, through G ( r − r ′ ), the Green function of the Laplacian, to suppress stray u ( r ) fields, and U [ u ( r )] is the poten-tial energy function of the order parameter, describing crystalline anisotropy. The interplay between the order parameter stiffness, which penalizes the formation of domain walls, and the nonlocal interactions, which encourages the formation of domains, determines the equilibrium width of domains and, consequentially, the periodicity of ferroelectric domain patterns.


I. INTRODUCTION
BiFeO 3 crystalizes in a rhombohedrally distorted perovskite structure (space group #161, R3c) and exhibits the combination of ferroelectricity and spin-canted weak ferromagnetism. [1][2][3] At room temperature, the polar and magnetic order parameters are coupled. As a result, when ferroelectric domains are poled, magnetic moments reorient deterministically. 2,4,5 Voltage-controlled magnetism is important for enabling low-power spintronic devices that operate efficiently and independently of current-based switching mechanisms, including Oersted induction and spin-transfer torque. 6 Utilizing BiFeO 3 for practical voltage-controlled spintronics, however, requires overcoming reliability challenges currently limiting ferroelectric-based devices. 7 Key among these is reducing throughfilm leakage. [8][9][10] In this letter, we investigate the influence of growth conditions and postdeposition ion-irradiation treatments on the composition, structure, and ferroelectric properties of epitaxial BixFe 2−x O 3 layers grown via adsorption-controlled ozone molecular-beam epitaxy. Structural characterization reveals that stoichiometric films, for which bismuth and iron concentrations are equal (i.e., x ≈ 1.00), exhibit the highest crystalline and ferroelectric domain perfection. The leakage characteristics of these highquality layers are found to be similar to defective layers grown near the single-phase field boundaries (i.e., x ≠ 1.00). Through film leakage is dramatically reduced by irradiating both stoichiometric and nonstoichiometric samples with He + ions.

II. FILM GROWTH
BixFe 2−x O 3 layers are grown to a thickness of ≃200 nm on SrRuO 3 -coated (110)o-oriented DyScO 3 substrates (o subscripts denote orthorhombic indices in the nonstandard Pbnm setting) via adsorption-controlled molecular-beam epitaxy in a Veeco GEN10 system (base pressure P Base = 1 × 10 −8 Torr = 1.3 × 10 −6 Pa). SrRuO 3 is selected as an epitaxial bottom electrode 11 due to its relatively low electrical resistivity (170 μΩ-cm at room temperature) 12 and lattice match with the DyScO 3 substrate (lattice mismatch m = 0.6%). The SrRuO 3 electrodes are deposited as described in Refs. 12 and 13 to thicknesses of ≃20 nm. BixFe 2−x O 3 films are grown subsequently without breaking vacuum at growth temperatures Ts between 550 ○ C and 650 ○ C, estimated using a thermocouple in indirect contact with the growth surface.

A. Structure and composition
Initial film growth experiments focus on determining the effects of incident flux ratios 2 ≤ J Bi /J Fe ≤ 16 and deposition temperatures 550 ○ C ≤ Ts ≤ 650 ○ C on the composition and structure of the BixFe 2−x O 3 layers grown on SrRuO 3 /DyScO 3 (110)o. Figure 1(a) is a variability chart 21 showing film bismuth fractions x determined from BixFe 2−x O 3 layers using Rutherford backscattering spectrometry. [22][23][24] Measured x values span 0.90 (J Bi /J Fe = 4, Ts = 650 ○ C) through 1.05 (J Bi /J Fe = 16, Ts = 600 ○ C). Increasing Ts and reducing J Bi /J Fe result in lower film bismuth factions x; in particular, we find that a 50 ○ C increase in Ts has an effect on x equivalent to a two-fold reduction in J Bi /J Fe . The loss of bismuth at higher temperatures results from thermally activated BiOx desorption. [25][26][27] The phase composition and structural perfection of asdeposited films are investigated using x-ray diffraction (XRD) θ-2θ scans and reciprocal space maps (RSM). The findings are summarized as a function of incident metal flux ratios J Bi /J Fe and deposition temperatures Ts in Fig. 1(b). Three structurally distinct growth regions are observed, comprised of phase-pure BixFe 2−x O 3 layers [white region, Fig. 1 Fig. 1(b)]. The single-phase regime spans a wide temperature window in excess of 100 ○ C, which has been modeled kinetically and shown to narrow rapidly with decreasing oxidant pressures. 15 Together with film compositional measurements [ Fig. 1(a)], a single-phase-field width spanning x = 0.90-1.05 is established, in close agreement with prior reports for layers grown via pulsed-laser deposition. 28 Within the singlephase field, RSMs performed about symmetric film reflections reveal two categorically distinct peak shapes [ Fig. 1 deposition temperatures and bismuth fluxes, the fundamental film reflections display diffuse features; at higher Ts and J Bi /J Fe values, the peaks exhibit coherent profiles. In the remainder of this letter, we focus on the latter set of films, for which the structural quality is superior. Figure 1(c) shows diffracted θ-2θ x-ray intensities collected near 002p film and substrate peaks from BixFe 2−x O 3 layers (p subscripts denotes pseudocubic indices) grown along the isoflux J Bi /J Fe = 16 and isotherm Ts = 650 ○ C lines. As the film bismuth fraction x is increased, film reflections shift to slightly lower 2θ values, resulting in out-of-plane lattice parameter values a which grow linearly with composition x [ Fig. 1(d)]. 29 The variation in a values suggests a changing concentration of point defects within the films.

B. Surface and domain morphology
The combination of reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM) is employed to investigate the surface morphology of epitaxial BixFe 2−x O 3 layers deposited on SrRuO 3 -coated DyScO 3 (110)o substrates.  For higher bismuth fractions of x = 1.07, arrays of bulk diffraction spots are observed [squares, Figs. 2(e) and 2(j)]. Bulk diffraction is a hallmark of three-dimensional growth 34 and occurs when glancing electrons penetrate through surface protrusions. 35 Such features are visible in AFM height images, including Fig. 2(o), and are attributed to Bi 2 O 2.5 grains. 26 The protrusions also lead to rough surfaces for which ρrms = 11.0 nm, exceeding the roughness values obtained here for stoichiometric bismuth ferrite layers by over an order of magnitude.
Systematic changes in the morphologies of the layers are also observed for bismuth-deficient layers. RHEED and AFM images collected from BixFe 2−x O 3 films with x = 0.95 exhibit half-order spots [rectangles, Fig. 2 Collectively, the direct-and Fourier-space analyses establish a rich morphological phase diagram in which topographical features vary systematically and depend sensitively on film composition. This makes RHEED a sensitive in situ monitor for characterizing the growth of BixFe 2−x O 3 layers in real time.  Fig. 2(r)], in agreement with prior reports. 13 The domains assemble into onedimensional stripes with remarkable long-range order and an inplane periodicity along [ In the as-deposited state, the films display pronounced leakage which impede ferroelectric poling. Leakage in bismuth ferrite arises from a combination of factors, including domain-wall conductivity 42 as well as electron and hole donor defects such as oxygen vacancies 20 and Fe 2+ -based complexes. 43 Ion irradiation was recently demonstrated as a successful avenue for increasing the resistivity of leaky ferroelectrics. 44,45 We adopt a similar strategy and bombard our patterned structures with 3.0 MeV He 2+ ions. At this energy, the ions penetrate to a mean depth of ∼15 μm, damaging the film lattice but preserving the film chemistry. Figure 3(a) shows P(E) curves obtained as a function of irradiation doses D between 0.3 × 10 15 and 1 × 10 16 /cm 2 from a stoichiometric BixFe 2−x O 3 film grown with x = 0.99. In contrast to P(E) curves measured from as-deposited heterostructures [for reference, also shown in Fig. 3(a)], devices irradiated with D ≥ 0.3 × 10 15 /cm 2 exhibit clear signatures of ferroelectricity, manifested in the form of hysteresis loops. For 0.3 × 10 15 ≤ D ≤ 1 × 10 15 /cm 2 , small residual leakage causes the hysteresis loop to be open, but increasing D further causes the loops to close completely, reflecting progressively decreased leakage. The suppression of leakage in irradiated layers is attribute to the formation of carrier scattering and trapping defects. 45 Figure 3(a) also demonstrates that, as irradiation doses are increased, ferroelectric coercive fields grow from Ec = 0.15 MV/cm (D = 0.3 × 10 15 /cm 2 ) to 0.90 MV/cm (1 × 10 16 /cm 2 ), following the exponential relationship Ec = 0.14e 0.19D (here, the units for Ec and D are MV/cm and 10 15 /cm 2 , respectively). Measured Ec values are comparable to those reported for epitaxial BiFeO 3 films deposited on SrRuO 3 /DyScO 3 (001) 45 and SrTiO 3 :Nb(001) 28 but are larger than the 0.08 MV/cm value obtained for free-standing bismuth ferrite membranes, for which domain walls move unobstructed by epitaxial strain. 46 The larger coercive fields of irradiated devices are attributed to the formation of domain-wall-pinning defects.

C. Ferroelectric properties
Hysteresis loops measured from BixFe 2−x O 3 -based devices 47 bombarded with 3 × 10 15 ions/cm 2 are presented as a function of film bismuth fractions x in Fig. 3(c). Remanent polarizations are approximately constant at 62 ± 6 μC/cm 2 , independent of the bismuth fraction. This is consistent with a spontaneous polarization of Ps = 107 ± 10 μC/cm 2 along ⟨111⟩ p , the polarization direction in bismuth ferrite. As x is increased, however, coercive fields Ec When cyclically poled, bismuth ferrite layers exhibit fatigueinduced failure. The mechanisms responsible for fatigue are diverse: conducting filaments form causing electrical shorts, 46 charge injection at ferroelectric/electrode interfaces suppresses domain nucleation, 48,49 and pinned domains grow in size. 50 To characterize the endurance of our BixFe 2−x O 3 layers and determine n, the number of polarization cycles tolerated before breakdown, we employ a 10 kHz rectangular waveform with variable bias amplitudes between ±0.35-0.65 MV/cm to ensure complete poling during testing. Figure 3(d) shows n as a function of the film bismuth fraction x. For bismuth-rich films with x = 1.05, repeated poling leads to breakdown above 3 × 10 2 cycles. As x is decreased, n grows exponentially to 6 × 10 3 (x = 0.99) and 4 × 10 4 (x = 0.90). The observed n values are typical of ferroelectric capacitor structures in which at least one of the electrodes is a metal 51 and can be enhanced by exclusively employing epitaxial conducting oxide electrodes. 52 The combination of oxide electrodes, ion irradiation, and bismuth deficient films thus provides an avenue for prolonging the reliability of BixFe 2−x O 3 capacitors.

III. CONCLUSIONS
Commensurately strained BixFe 2−x O 3 layers grown on SrRuO 3coated DyScO(110)o substrates using adsorption-controlled ozone molecular-beam epitaxy are employed to investigate the role of ARTICLE scitation.org/journal/apm defects, introduced by varying synthesis conditions and by performing postgrowth ion bombardment, on the chemical composition, structural characteristics, domain morphology, and ferroelectric attributes of bismuth ferrite. Within the explored ranges of growth temperature 550 ○ C ≤ Ts ≤ 650 ○ C and incident bismuth-toiron flux ratios 2 ≤ J Bi /J Fe ≤ 16, a single-phase field with bismuth fractions x spanning 0.90 through 1.07 is established. The varying film compositions are accompanied by topographical features that include pits (x = 0.90), mounds (x = 0.95), terraces (x ≈ 1.00), fractals (x = 1.05), and protrusions (x = 1.07); each feature produces unique diffraction signatures in RHEED suitable for monitoring film growth in real time. Film polarization morphologies generally consist of two domain variants arranged in stripe patterns. Pattern perfection and geometry depend sensitively on point defect profiles, with the widest domain widths and most periodic structures occurring near stoichiometry (x ≈ 1.00). In the as-deposited state, all films display excessive leakage which impede ferroelectric poling when tested using fabricated 40-μm-diameter platinum-capped capacitor structures. By performing postgrowth ion irradiation treatments, leakage is suppressed, yielding closed polarizationvs-field hysteresis loops. Remanent polarizations are constant at ∼60 μC/cm 2 and independent of film composition; coercive fields are reduced near stoichiometry, where the bismuth and iron concentration are equal. Bismuth deficiency is demonstrated as an avenue for enhancing the endurance of BixFe 2−x O 3 -based ferroelectric devices.