Structure of the BFO/LNO superlattice

The BFO/LNO superlattice was grown on a conductive Nb-doped single-crystal STO (001) substrate at temperatures in the range of 560 – 810 °C with a UHV double-gun RF magnetron-sputtering system. Figure 4.2.1 shows the radial scan along direction (00L) of the BFO/LNO superlattice thin films at various temperatures. Values of H, K, and L stated in this work are expressed in reciprocal lattice units referred to the STO lattice parameter, 0.3905 nm at 25 °C.

The intense and sharp feature centered at L=2 is the STO Bragg reflection (002) from the substrate. Satellite peaks of several orders are clearly observed on both sides of the main peaks (marked with arrows in Figure 4.2.1) at all deposition temperatures. An expanded view of the radial scan of the BFO/LNO superlattice film deposited at 660 °C is shown in the inset of Figure 4.2.1. The main peaks and satellite peaks accompanied by clearly discernible Pendellösung fringes on both sides of the main peak indicate the high crystalline quality of the BFO/LNO artificial superlattice structure formed on the STO substrate with RF magnetron sputtering.

1.6 1.8 2.0 2.2 2.4 2.6

Figure 4.2.1 Intensity distribution of the (002) radial scan of the BFO/LNO superlattice film deposited at various substrate temperatures. The inset shows an expanded view of the superlattice film deposited at 660 °C.

The epitaxial nature of the BFO and LNO sublayers in the superlattices is demonstrated by the in-plane orientation (2 0 0) with respect to the major axes of the STO substrate. The distribution of the in-plane X-ray intensity of the radial scans from superlattice films with varied deposition temperature is shown in Figure 4.2.2; the scan was performed in the vicinity of the STO (2 0 0) Bragg peak. A broad feature coexists with a sharp Bragg peak, which originates from the substrate. This broad feature, indicated by arrows in Figure 4.2.2, is assigned to the Bragg peak of the deposited layer, which is confirmed by the variation of the relative intensity between the two signals as a function of angle of incidence.

The H-value of the in-plane main peak (marked with arrows in Figure 4.2.2) decreases and approaches the STO substrate with increasing temperature of deposition in the range of 560–660 °C.

1.8 1.9 2.0 2.1 2.2

Figure 4.2.2 X-ray intensity of the radial scans along the (200) in-plane Bragg peak of BFO/LNO superlattice films deposited at varied substrate temperatures.

An arrow marks the position of the superlattice main peak.

The azimuthal diffraction patterns of the BFO/LNO superlattice film, deposited at 660 °C, in the vicinity of a surface peak and the substrate Bragg peak clearly exhibiting a four-fold symmetry with the same orientation appear in Figure 4.2.3. These results constitute evidence for strong epitaxial growth of the deposited layer on the substrate. No other feature is observed in the intervals between the four peaks, indicating a perfect alignment of axes a and b of the BFO and LNO unit cells along those axes of the STO substrate. The measurement of the radial scan around the (111) Bragg peak provides further structural information about the off-normal orientation. The radial scan around the (111) Bragg peak for the BFO/LNO superlattice film deposited at 660 °C is shown in Figure 4.2.4; distinct satellite peaks on both sides of the main peak are clearly observed, indicating that not only a well defined epitaxial relation with the substrate has developed along the (111) plane but also a smooth surface and an interface were formed.

0 50 100 150 200 250 300 Figure 4.2.3 Azimuthally scan (Φ scan) of the surface peak and the substrate Bragg peak for a superlattice film deposited at 660 °C.

0.6 0.8 1.0 1.2

Intensity (arb. units) -2

L (r.l.u.)

Figure 4.2.4 Intensity distribution of the (111) radial scan of the BFO/LNO superlattice film deposited at 660 °C.

To confirm these X-ray diffraction results of the structure layer by layer and the crystalline quality, we examined the superlattice films with a high- resolution transmission electron microscope (HRTEM) at selected cross sections.

Figure 4.2.5 illustrates the HRTEM cross-sectional images of a superlattice film prepared at 660 °C. According to Figure 4.2.5(a), a cross-sectional HRTEM image on a large scale, the superlattice film clearly shows a well defined structure of BFO/LNO layer by layer with the STO substrate; the thickness of the sublayer agrees satisfactorily with the designed value. Figure 4.2.5(b), a HRTEM image, indicates a satisfactory epitaxial relation between the film, interface and substrate, consistent with the experimental results of X-ray scattering of the surface normal and the in-plane direction.

Figure 4.2.5 HRTEM cross-sectional images of the BFO/LNO superlattice on the (001) SrTiO₃ substrate deposited at 660 °C; (a) large scale image, and (b) image of the interface regions between individual layers and between film and substrate.

Figures 4.2.1 and 4.2.2 show an extra peak (marked *) located at L=1.872 and H=1.868 for superlattice thin films deposited with the substrate temperature greater than 710 °C, which we attribute to the formation of the NiO phase due to protracted deposition at high temperature. This NiO compound is readily formed during decomposition of the LNO film heated for a long time at a high temperature of deposition [128]. To confirm the formation of this NiO phase, we measured the DAFS pattern for film deposited at 710 °C. DAFS is the X-ray spectrometric technique that combines a sensitivity to long-range order of X-ray diffraction and a sensitivity to short-range order of X-ray-absorption fine structure [112]. Figure 4.2.6 shows the DAFS result of the Ni K-edge and La L1-edge for the (002) superlattice diffraction peak and the extra peak shown in Figure 4.2.1 (marked *) for film deposited at 710 °C. These results clearly show a strong Ni absorption signal at 8.345 keV with no La absorption signal on the extra diffraction peak, whereas both Ni and La absorption signals exist on the (002) superlattice diffraction peak. According to this evidence from DAFS, the extra peak indicates a NiO phase. The superlattice structure became gradually degraded through the formation of the NiO phase for a film deposited at temperature ≧710 °C, and was destroyed at 810 °C. The modulation length of the superlattice and the thickness of the LNO layer altered slightly because of the formation of the NiO phase. The main peak of the superlattice has therefore an unsystematic shift for film deposited at high temperature.

8.1 8.2 8.3 8.4 8.5 8.6 8.7

Figure 4.2.6 DAFS result of the (002) superlattice diffraction peak and the extra peak as shown in figure 4.2.1 (marked *) for film deposited at 710 °C (a) Ni K-edge, and (b) La L1-edge.

The BFO and LNO films are described as having a pseudo-cubic structure, with bulk lattice parameters 0.3962 nm for BFO and 0.3861 nm for LNO. In this superlattice system, the BFO sublayer is in a biaxially compressive state, whereas the LNO sublayer is in a biaxially tensile state. These heteroepitaxial sublayers are hence characterized by either being strongly strained or containing many misfitted dislocations with a modulation length larger than a critical value.

The critical thickness for the misfitted dislocations is estimated according to a model proposed by Matthews and Blakeslee [129] as



in which h_c is the critical thickness, b is the magnitude of the Burgers vector of the dislocations, ν is the Poisson ratio, and f is the misfit between the least strained BFO/LNO bilayer and the STO substrate. The calculated critical thickness of the BFO/LNO superlattice was ~15 nm. The designed modulation length of the prepared superlattices (t_BFO + t_LNO= 4 nm) is within the theoretically estimated length. The total thickness of films, about 120 nm, was much greater than the critical value of the thickness as BFO/LNO superlattice films; this condition is one factor that leds to strain release. Both the sublayer and the total thickness were factors that contribute to the results of the partial strain effect in the superlattice structure.

The in-plane lattice parameter of BFO is determined directly from a crystal-truncation-rod (200) radial scan [130, 131]. Although the out-of-plane lattice parameter of the BFO layer is directly indeterminated from the (0 0 2) radial scan spectra shown in Figure 4.2.1, we evaluated the parameter through the elastic relation involving the strain normal to the interface in the cubic structure [132, 133]. The in-plane compressive strain of the BFO layer is defined

as (aBFO−abulk-BFO)/abulk-BFO, in which aBFO is the lattice parameter of the strained BFO layer that is obtained from the measured inplane lattice parameter of the superlattice and abulk-BFO is the bulk lattice parameter of unstrained BFO. Based on this derived lattice parameter, the evaluated in-plane and out-of-plane strains of BFO layers are shown in Figure 4.2.7. Deposition at a higher temperature results also in a clearly larger in-plane compressive strain of the BFO sublayers in the superlattice for films deposited at 560 – 660 °C. An in-plane lattice parameter 0.3905 nm is necessary for the fully strained pseudomorphic growth of BFO/LNO superlattices on the STO substrate. The lattice mismatch between BFO-LNO bilayer is ~2.5 % of the in-plane lattice parameter. A fully strained pseudomorphic growth of each layer thus does not occur in the superlattice during deposition, even though the thickness of individual BFO and LNO layers is less than the critical thickness [130, 134]. The BFO/LNO superlattice films reveal a partial strain relaxation, and a not fully strained state for all deposition temperatures, as shown in Figure 4.2.7.

550 600 650 700 750 800 850

: out-of plane strain

Out-of plane strain (%)

In-plane strain (%)

Temperature (^oC)

Figure 4.2.7 In-plane compressive strain and out-of plane tensile strain of the BFO layer as a function of deposition temperature.

Figure 4.2.8 shows the full width at half maximum (FWHM) of these in-plane rocking curves of BFO/LNO superlattice films deposited at various temperatures. The FWHM of the films increased greatly for the deposition temperature above 710 °C. The narrower is the FWHM, the better is the crystalline quality. The best crystal quality of the superlattice was shown at deposition temperature 660 ^oC, which can be the most suitable condition for growth of a BFO/LNO superlattice.

-6 -4 -2 0 2 4 6 Figure 4.2.8 Rocking curves of the in-plane (200) main peak of BFO/LNO superlattice films deposited at various temperatures.

Figure 4.2.9 presents reflectivity curves of BFO/LNO superlattice films deposited at various substrate temperatures, and their best-fitted results. The presence of clear oscillations indicates that both the surface and the interface correlate well with each other and are smooth enough to produce the oscillations.

Both the superlattice peaks and the Kiessig oscillations, which are well pronounced, reveal the presence of a well ordered layer structure of a superlattice, providing evidence for the vertically periodic modulation of the composition. To evaluate the physical parameters of the superlattice, we used a simulation of the specular reflectivity on the recursive formalism of Parratt [103]. We fitted the reflectivity data according to Ref. 104 [104] to determine the physical parameters of the superlattice, including roughness, thickness, and density. This program calculates the reflectivity of the material using the dynamic Fresnel equations for multilayer reflectivity, taking into account the absorption, instrumental resolution, interface roughness and abruptness, and the sample curvature.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 10^-3

10² 10⁷ 10¹² 10¹⁷ 10²²

560^oC 610^oC 660^oC 710^oC 760^oC 810^oC : experimental data : fitted data

q

( nm

)

Intensity (arb. units)

  Figure 4.2.9 Reflectivity curves of BFO/LNO superlattice films deposited at various substrate temperatures and their best-fit results (solid line) as a function of momentum transfer.

Table 4.2.1 shows the best-fitted results of the physical parameters of each thin-film layer, which indicate that the density of the BFO sublayer was less than the bulk value and decreases with increasing temperature, whereas for the LNO sublayer the density was slightly less than the bulk value. A Bi atom escapes readily from the thin film at the high deposition temperature [135]. The density of BFO was much less than the bulk density; this condition likely reflects an increased density of defects that inevitably occur during the deposition at high temperature. We found also that the thickness of the sublayer decreased with increasing substrate temperature, except the film deposited at 810 °C, indicating a growth rate inversely proportional to temperature. The thickness of the LNO sublayer decreased with increasing deposition temperature;

as the kinetic energy increases with increasing temperature, the effect of re-sputtering was much more obvious, leading to the decreased thickness of the sublayer.

At a high deposition temperature many vacancies are produced through the escape of Bi; the density of BFO decreased with increasing temperature. The fitted result shows not only that the surface and interface roughness increased with increasing deposition temperature but also that the interface roughness increased much for deposition at substrate temperature greater than 760 °C. The reflectivity curve also that the lower is the deposition temperature, the more clearly present are the superlattice peaks and the Kiessig fringes. Hence, the lower is the deposition temperature, the smoother is the surface and the less rough is the interface, except film deposited at ≧710 °C.

Table 4.2.1 Parameters obtained from best-fit results of reflectivity curves of BFO/LNO superlattice films deposited on the Nb-doped STO substrate with varied deposition temperature. The surface roughness determined from AFM measurements is listed in the last column for comparison. The relative standard deviations of the fitted data are for thickness ≦2.5%, density ≦3% and roughness ≦6%. The thickness of the SrTiO₃ substrate is set as infinite and the bulk density is 5.118 g cm⁻³. The bulk density of BFO is 8.366 g cm⁻³, and that of LNO is 7.086 g cm⁻³.

fitted thickness / nm

fitted density / g-cm^-3

fitted roughness

/ nm AFM / nm

deposition temperature

/ ^oC tLNO tBFO ρLNO ρBFO σLNO/sub σinterface σsurface σsurface

560 1.57 2.00 6.59 7.69 0.40 0.41 0.61 1.86 610 1.48 1.99 6.73 7.61 0.40 0.46 0.70 2.15 660 1.43 1.98 6.80 7.53 0.40 0.49 0.82 2.16 710 1.15 1.99 6.80 7.28 0.40 0.60 0.75 0.50 760 1.23 1.80 6.67 7.11 0.39 1.18 0.62 0.26 810 1.60 1.89 6.45 7.19 0.39 1.21 1.20 3.95

To confirm the X-ray reflectivity results of the surface roughness, we examined the surface roughness of the superlattice films using AFM. From observations conducted in a contact mode on area 1 μm × 1 μm, we calculated the root-mean-square (RMS) magnitude of surface roughness. The surface morphology of BFO/LNO superlattice films deposited at various substrate temperatures, as examined with the AFM, is shown in Figure 4.2.10. The AFM images show that the film surface is free of cracks, but exhibits surface/film features of three types—island-like for film deposited at ≦660 °C, nearly structureless or featureless for 710 – 760 °C, and platelet-like for 810 °C. For comparison, the surface roughness evaluated from AFM images is listed in Table 4.2.1. The surface roughness of the superlattice films shows the same tendency of increasing surface roughness with increasing deposition temperature for films deposited up to 660 °C. The results show a tendency consistent with the XRR fitted results.

Figure 4.2.10 AFM images of BFO/LNO superlattice films deposited at substrate temperatures/°C: (a) 560, (b) 610, (c) 660, (d) 710, (e) 760, and (f) 810.

4.2.2 Electrical properties of the BFO/LNO superlattice

Figure 4.2.11 shows the polarization–electric field (P-E) hysteresis loop of the BFO/LNO superlattice deposited at various substrate temperatures.

Hysteresis loops with the largest remanent polarizations (2 P_r) near 160 μC cm⁻² were observed for superlattice films deposited at 660 °C, although the exact P_r value was difficult to determine because there still existed a large component of leakage current, even with an epitaxial thin film of high quality. In a ferroelectric or conductive superlattice system, i.e., BFO/LNO superlattice, the intervening metallic layer increases the leakage current. The remanent polarization increased with increasing deposition temperature for films deposited up to 660 °C, and then decreased with further increasing temperature.

As illustrated in Figure 4.2.7, the BFO sublayers in the superlattices exhibit an increased compressive strain, or a larger lattice distortion, that much elongates the unit cells along the electric field in the superlattice structure with increasing deposition temperature in the range of 560–660 °C. This result clearly reveals that an effective manipulation of strain in BFO sublayers in a superlattice structure with alternating insertion of LNO sublayers can greatly enhance the ferroelectric properties of the BFO sublayers. The fitted result of the X-ray reflectivity curve shows that the interfacial roughness of superlattice films deposited above 760 °C is much larger than at other temperatures. Although these superlattice films have greater tensile stress along the c-axis, they have also narrower effective BFO layers. The greatly decreased remanent polarization of films deposited above 760 °C might be attributed to the formation of the NiO phase and the poor interface structure in the superlattice system. The value of remanent polarization (2 P_r) of the effective BFO layer, 2 P_r ~160 μC cm⁻², obtained from Figure 4.2.8 is twice as large as that, 2 P ~70 μC cm⁻², for the

single layer BFO film of thickness 60 nm prepared under the same sputtering conditions at 660 °C.

-1000 -500 0 500 1000

-100 -50 0 50 100

560^oc 610^oc 660^oc 710^oc 760^oc 810^oc single layer BFO at 660^oc

Polarization(C/cm2 )

E (kV/cm)

Figure 4.2.11 P-E hysteresis loops of BFO/LNO superlattice deposited at various substrate temperatures.

Figure 4.2.12 shows the J-E curve of BFO/LNO superlattice films with various deposition temperature, which indicates large leakage current densities in this superlattice system for all deposition temperatures, relative to a single BFO layer film of thickness 180 nm [25] or another BFO/STO superlattice system with the BFO layers of total thickness 200 nm prepared with the PLD technique [136]. In the BFO/LNO superlattice system, a metallic layer between leads increases the leakage current. These leakage current densities greatly increased for film deposited at ≧710 °C, which seems consistent with the tendency of the crystalline quality of the films and the formation of NiO phase.

-1000 -500 0 500 1000

10^-7

Figure 4.2.12 J-E curve of BFO/LNO superlattice films deposited at 560 – 710

°C. 

4.3 Sublayer thickness and leakage effect of the BFO/LNO superlattice

The growth condition of the BFO/LNO superlattice with satisfactory crystal structure and ferroelectric properties was optimized in previous work.

The sublayer thickness is an important issue of the superlattice structure, and the leakage property is a dominant problem of the hysteresis loop; those issues were studied in this work.

4.3.1 Crystal structure of the BFO/LNO superlattice with varied sublayer thickness

Figure 4.3.1, obtained from an eight-circle X-ray diffractometer and a high-resolution synchrotron source with a radial scan along direction (00L) of the BFO/LNO superlattice films, shows the crystalline quality of the superlattice structure. The intense and sharp peak centered at L=2 is the STO (002) Bragg reflection from the substrate. The main peak (marked with arrows in Figure 4.3.1) and well defined satellite peaks on both sides of the main peaks indicate the great crystalline quality of the BFO/LNO artificial superlattice structure formed with RF-magnetron sputtering. Expanded views of the fitted curve (solid line), using both symmetric and exponential composition profiles and experimental data (open circles) of BFO/LNO superlattice films with sublayers of thicknesses 3.6 and 8.5 nm, in the inset of Figure 4.3.1 indicate the position of the reflection of the satellite peak (marked with an arrow). Overall, both assumptions yielded satisfactory agreement with the experimental data; some discrepancy of the background intensity near the substrate peak in the composition modulation shape is indicated by the satellite intensities. The superlattice exhibits satellite peaks that allowed us to deduce the super-period of the superlattice (ΛWAXRD) according to the formula [137]

_WAXRD 

2 sin





in which λ is the X-ray wavelength and θn and θn−1 are the angular positions of two successive satellite peaks.

1.6 1.8 2.0 2.2 2.4

Figure 4.3.1 Intensity distribution of the (002) radial scan for the BFO/LNO superlattice with varied thickness of sublayer and periods. The inset shows the best fits (solid line) for the experimental XRD patterns for BFO/LNO superlattices at thicknesses 3.6 and 8.5 nm.

The period of the superlattice films obtained from the oscillations of the (002) radial scan shown in Table 4.3.1 agrees with the measurement result of the X-ray reflectivity

Table 4.3.1 Thickness parameters of the BFO/LNO superlattice obtained from the results of X-ray diffraction, X-ray reflectivity and SIMS.

ΛWAXRD / nm  Fitted thickness by X­ray 

To verify the vertically periodic modulation obtained from the X-ray measurements, we examined the vertical composition profile of the BFO/LNO superlattices with SIMS. A SIMS depth profile of the (BFO_8.5/LNO_8.5)₆ superlattice is shown in Figure 4.3.2. The variations in the signals of Bi, Fe, La, Ni, Sr, Ti and O are consistent with the designed period of six cycles of a

To verify the vertically periodic modulation obtained from the X-ray measurements, we examined the vertical composition profile of the BFO/LNO superlattices with SIMS. A SIMS depth profile of the (BFO_8.5/LNO_8.5)₆ superlattice is shown in Figure 4.3.2. The variations in the signals of Bi, Fe, La, Ni, Sr, Ti and O are consistent with the designed period of six cycles of a