Abstract
We report on differential etching behavior of the different orientations of the polarization in BiFeO3 (BFO), similar to other ferroelectrics, such as LiNbO3. We show how this effect can be used to fabricate epitaxial BiFeO3 nanostructures. By means of piezoresponse force microscopy (PFM) domains of arbitrary shape and size can be poled in an epitaxial BiFeO3 film, which are then reproduced in the film morphology by differential etching. Structures with a lateral size smaller than 200 nm were fabricated and very good retention properties as well as a highly increased piezoelectric response were detected by PFM.
| Original language | English |
|---|---|
| Article number | 082904 |
| Journal | Applied Physics Letters |
| Volume | 99 |
| Issue number | 8 |
| DOIs | |
| Publication status | Published (in print/issue) - 22 Aug 2011 |
Funding
3 nanostructures fabricated by differential etching of BiFeO 3 films Johann Florian a) Morelli Alessio Vrejoiu Ionela Max Planck Institute of Microstructure Physics , Weinberg 2, Halle 06120, Germany a) Electronic mail: [email protected] . 22 08 2011 99 8 082904 21 06 2011 05 08 2011 24 08 2011 2011-08-24T11:16:53 2011 American Institute of Physics 0003-6951/2011/99(8)/082904/3/ $30.00 We report on differential etching behavior of the different orientations of the polarization in BiFeO 3 (BFO), similar to other ferroelectrics, such as LiNbO 3 . We show how this effect can be used to fabricate epitaxial BiFeO 3 nanostructures. By means of piezoresponse force microscopy (PFM) domains of arbitrary shape and size can be poled in an epitaxial BiFeO 3 film, which are then reproduced in the film morphology by differential etching. Structures with a lateral size smaller than 200 nm were fabricated and very good retention properties as well as a highly increased piezoelectric response were detected by PFM. One of the most promising multiferroic materials with respect to applications is BiFeO 3 (BFO), being one of the few multiferroics showing ferroelectric and antiferromagnetic order at room temperature. BFO is ferroelectric with a high spontaneous polarization of ≈100 μ C/cm 2 along the pseudocubic [111] direction, accompanied with a slight distortion from cubic to rhombohedral. 1 BFO has a G-tpye antiferromagnetic order perpendicular to the direction of ferroelectric polarization and it has been shown that a ferroelectric/ferroelastic switching results in a reoriention of the antiferromagnetic plane as well. 2 In relation to potential utilization of BFO in data storage technologies, as well as for fundamental investigations, BFO nanostructures have to be fabricated and the influence of size effects on the functional properties studied. In particular, the polarization switching behavior and ferroelectric/ferroelastic domain formation within BFO nanostructures are of interest. Previously reported studies about patterning of BFO applied focused ion beam (FIB) milling to obtain nanostructures from an epitaxial film. 3–5 The main drawback of FIB milling is the high density of defects and Ga-implantation in the nanostructures. This damage leads to great reduction of the functional properties. In an earlier report, we showed already that a proper annealing method has to be employed for recovery. 4 However, for our FIB structures with size below 300 nm, switching of the ferroelectric polarization was impossible, most probably because not all the defects could be healed by the recovery procedure, which may have led to pinning of the domains. The fabrication of BFO nanostructures by FIB is, therefore, limited. In this paper, we report on differential etching of epitaxial (001)-oriented BFO films and how this property can be used to achieve BFO nanostructures in a simple way, with less defects compared to FIB machined ones. Differential etching refers to a different etching rate of different ferroelectric polarization orientations and is already known for a long time in case of other ferroelectric materials, such as BaTiO 3 , YMnO 3 , and LiNbO 3 . 6–8 The main application of differential etching has been the visualization of the ferroelectric domains by optical microscopy, but for LiNbO 3 , because of its high differential etch ratio, 9 it has been used to fabricate waveguides, 10 micro-cantilevers, 11 or photonic micro structures 12 as well. Here, we show that differential etching can be used in case of BFO to fabricate epitaxial BFO nanostructures with arbitrary shapes and in various sizes. Figure 1(a) shows an atomic force microscope (AFM) topography image and Fig. 1(b) a scanline across several fabricated structures as an example of the versatility of this method. Structures with lateral sizes down to 170 nm were achieved, which show good switching behavior, good retention properties, and enhanced piezoelectric response. An epitaxial BFO film of about 200 nm thickness has been grown by pulsed-laser deposition (PLD) on orthorhombic DyScO 3 (110), with an about 50 nm thick SrRuO 3 bottom electrode. The substrate was prepared atomically flat by an annealing procedure at 1000 °C for 2 h in air. SrRuO 3 and BiFeO 3 were deposited at 650 °C in an O 2 pressure of 0.14 mbar. After deposition, the sample was cooled down to room temperature with a rate of 600 °C/h in 200 mbar O 2 . Piezoresponse force microscopy (PFM) was performed on an Asylum Research MFP-3D and a XE-100 Park System. Etching was done in an aqueous solution of 0.5% hydrofluoric acid (HF) at room temperature for 45 s. As depicted in Fig. 2(a) , epitaxial (001)-oriented BFO films can have eight different ferroelectric domains, four domains where the polarization is directed upwards ( P + = P 1 + , P 2 + , P 3 + , and P 4 + ) and four domains where the polarization is directed downwards ( P - = P 1 - , P 2 - , P 3 - , and P 4 - ) . 13 We observed that on (001)-oriented BFO films, the four domains pointing upwards (P + ) exhibit a much higher etching rate than the domains pointing downwards (P − ) when treated with diluted HF. Figures 2(b)–2(d) show the topography, vertical PFM (VPFM), and lateral PFM (LPFM) images of the BFO film, respectively, before etching. In the VPFM images a bright/dark contrast corresponds to polarization pointing downwards/upwards, an intermediate color corresponds to a reduced piezoresponse. In case of LPFM images, a bright/dark contrast corresponds to a polarization pointing to the right/left side of the image. By applying a DC voltage of ±8 V to the tip during scanning, an area of 600 × 600 nm 2 of the BFO film was switched to domains pointing downwards, P − , surrounded by domains pointing upwards, P + . Figures 2(e)–2(h) show topography, its corresponding scanline, VPFM and LPFM of the same area after etching in the HF solution, respectively. By comparing Figs. 2(c) and 2(f) , it can be seen that the domains P − , the negative end of the dipole, remain almost unetched, whereas the domains P + , the positive end of the dipole, are etched orders of magnitude faster. This high differential etching ratio is similar to LiNbO 3 and allows the utilization of this effect for structure fabrication. However, in case of LiNbO 3 , the negative side of the dipole and, therefore, the opposite side compared to BiFeO 3 is etched, although BiFeO 3 and LiNbO 3 have the same crystal symmetry and very similar ion displacements. 14 Therefore, the different cations, which lead to a different surface chemistry, seem to be a crucial factor. As can be seen in Fig. 2(g) , the complete film is switched to domains P − after the etching procedure, which terminates the fast etching rate. The lateral domains in the structure remain unchanged, whereas the lateral domains in the etched region switch partly together with the out-of-plane direction. Presumably, the switching from P + to P − is a result of the instability of domains P + when the BFO film becomes very thin and is still in contact with HF. In very thin BFO films (e.g., 50 nm), P + domains written by PFM switch back directly to P − under the influence of HF, which is the predominant domain state in the as-grown film as well. Hence, the fabrication of nanostructures in our very thin films by this method has not been possible. It has already been shown that the direction of polarization in ferroelectrics can be influenced by the chemical environment. 15 In particular, the out-of-plane polarization in BFO films can be switched by oxygen plasma. 16 A chemical switching of our thin BFO films under the influence of HF is, therefore, reasonable. To verify the ferroelectric nature of the nanostructures, piezoelectric hysteresis loops on top of the structures were measured. The smallest structures were achieved by poling an area of 200 × 200 nm 2 in the as-grown film. Figure 3(a) shows the topography of such a domain after subsequent etching in the HF solution and Fig. 3(b) the corresponding scanline across the structure. The full width at half maximum is about 170 nm and using this value the aspect ratio is approximately a ≈ 0.7. Remanent hysteresis loops were measured by applying a stepwise DC voltage to the bottom electrode and measuring the PFM response in between the voltage steps at zero DC voltage. Figure 3(e) shows a comparison of the remanent hysteresis measured on the nanostructure shown in Figure 3(a) and measured on the same film at an area of the film unexposed to the acid. The piezoelectric response on the structure is highly enhanced compared to the film, which is likely due to the removal of the substrate clamping effect. 17 An increased piezoelectric response was already reported on Pb(Zr,Ti)O 3 nanostructures and attributed to two effects: on one hand to the removal of the clamping effect and on the other hand to the enhanced mobility of the 90° domain walls in the nanostructures. 18,19 Poling of the BFO structure is possible and good retention properties were observed. Figures 3(c) and 3(d) show the VPFM images before and after the hysteresis measurements, respectively, which switched the complete structure in the opposite state. After one week, no retention loss could be detected. In conclusion, BiFeO 3 exhibits differential etching rates for the different out-of-plane orientations of the polarization, similar to other ferroelectric materials, like LiNbO 3 . When exposed to HF, the positive end of the dipole is etched, whereas the negative end of the dipole remains mainly unetched. We demonstrate the possibility to use this effect for fabrication of nanostructures down to 170 nm in lateral size. The nanostructures retain their ferroelectricity and show very good retention properties. The out-of-plane piezoelectric response on the 170 nm structure is highly enhanced compared to the unpatterned film. By further optimizing the etching parameters, like the etching time and acid composition, it may be possible to achieve smaller nanostructures. This is a promising route for fabrication of epitaxial BiFeO 3 nanostructures and study their size effects. Financial support by DFG through SFB 762 is gratefully acknowledged. FIG. 1. (Color online) AFM topography (a) and corresponding scanline (b) showing BFO nanostructures with various sizes fabricated by differential etching. Scale bar is 2 μ m and color scale 170 nm. FIG. 2. (Color online) (a) Schematics of possible domain variants in (001)-oriented BFO. Topography (b), VPFM image (c), and LPFM image (d) after writing a 500 × 500 nm 2 square with polarization downwards. Topography (f), its corresponding scanline (e), VPFM image (g), and LPFM image (h) after etching in the HF solution. Scale bar is 500 nm, color scale is 20 nm in (b) and 180 nm in (f). FIG. 3. (Color online) Topography (a) and its corresponding scanline (b) of a BiFeO 3 nanostructure. VPFM image before (c) and after (d) hysteresis measurement, leaving the structure in the switched state. Scale bar in (a) is 200 nm. (e) PFM hysteresis measured on top of the structure and on the film.
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