E. Soergel

Piezoresponse force microscopy (PFM)

Piezoresponse force microscopy (PFM) detects the local piezoelectric deformation of a sample caused by an applied electric field from the tip of a scanning force microscope. PFM is able to measure deformations in the sub-picometre regime and can map ferroelectric domain patterns with a lateral resolution of a few nanometres. These two properties have made PFM the preferred technique for recording and investigating ferroelectric domain patterns. In this review we shall describe the technical aspects of PFM for domain imaging. Particular attention will be paid to the quantitative analysis of PFM images.

Quantitative analysis of ferroelectric domain imaging with piezoresponse force microscopy

The contrast mechanism for ferroelectric domain imaging via piezoresponse force microscopy (PFM) is investigated. A vectorial description of PFM measurements is presented which takes into account the background caused by the experimental setup. This allows a quantitative, frequency independent analysis of the domain contrast which is in good agreement with the expected values for the piezoelectric deformation of the sample and satisfies the generally required features of PFM imaging.

Electrostatic topology of ferroelectric domains in YMnO3

Trimerization-polarization domains in ferroelectric hexagonal YMnO3 were resolved in all three spatial dimensions by piezoresponse force microscopy. Their topology is dominated by electrostatic effects with a range of 100 unit cells and reflects the unusual electrostatic origin of the spontaneous polarization. The response of the domains to locally applied electric fields explains difficulties in transferring YMnO3 into a single-domain state. Our results demonstrate that the wealth of nondisplacive mechanisms driving ferroelectricity that emerged from the research on multiferroics are a rich source of alternative types of domains and domain-switching phenomena.

Nanoscale surface domain formation on the +z face of lithium niobate by pulsed ultraviolet laser illumination

Single-crystal congruent lithium niobate samples have been illuminated on the +z crystal face by pulsed ultraviolet laser wavelengths below (248 nm) and around (298-329 nm) the absorption edge. Following exposure, etching with hydrofluoric acid reveals highly regular precise domain-like features of widths ~150-300 nm, exhibiting distinct three-fold symmetry. Examination of illuminated unetched areas by scanning force microscopy shows a corresponding contrast in piezoelectric response. These observations indicate the formation of nanoscale ferroelectric surface domains, whose depth has been measured via focused ion beam milling to be ~2 micron. We envisage this direct optical poling technique as a viable route to precision domain-engineered structures for waveguide and other surface applications.

Precision nanoscale domain engineering of lithium niobate via UV laser induced inhibition of poling

Continuous wave ultraviolet (UV) laser irradiation at lambda=244 nm on the +z face of undoped and MgO doped congruent lithium niobate single crystals has been observed to inhibit ferroelectric domain inversion. The inhibition occurs directly beneath the illuminated regions, in a depth greater than 100 nm during subsequent electric field poling of the crystal. Domain inhibition was confirmed by both differential domain etching and piezoresponse force microscopy. This effect allows the formation of arbitrarily shaped domains in lithium niobate and forms the basis of a high spatial resolution micro-structuring approach when followed by chemical etching.

Ultraviolet light-assisted domain inversion in magnesium-doped lithium niobate crystals

The influence of ultraviolet (UV) light (wavelengths λ=334 and 305nm) on the ferroelectric domain inversion of lithium niobate crystals doped with different amounts of magnesium ranging from 0to7.5mol% is investigated. Illumination at λ=334nm leads to a coercive field reduction of up to 50%, but only in samples doped with a magnesium concentration above the so-called optical damage threshold. For λ=305nm the effective coercive field is reduced significantly in all samples. Different behavior of the coercive field reduction at both wavelengths indicates the presence of two mechanisms. To explain the effect occurring for 305nm illumination a model is presented in which an UV-induced photoconductivity alters the electric-field distribution through the crystal thickness. Utilizing an UV-interference pattern and a suitable homogeneous external electrical field, periodically poled lithium niobate with a period length of 55μm is produced.

Impact of ultraviolet light on coercive field, poling dynamics and poling quality of various lithium niobate crystals from different sources

Ferroelectric domain reversal by electric field poling of lithium niobate crystals (LiNbO3) with varying stoichiometry and magnesium (MgO) doping level obtained from various commercial suppliers is investigated. Magnesium doping lowers the domain-wall velocity, increases the uniformity of the growth of the domains, and reduces the impact of crystal symmetry on the shape of the domains. Illumination with ultraviolet (UV) laser light (305nm) reduces the coercive field by up to 34% in MgO-doped crystals, but is accompanied by a degradation of poling quality. UV light of longer wavelengths (334nm) has no influence on the coercive field except for the MgO:LiNbO3 material of one supplier, where the field is reduced by 27%. In this case the poling quality is excellent. UV-induced reduction of stress-induced birefringence is observed in some samples. The results are of crucial relevance for light-induced domain engineering of LiNbO3 crystals.

Visualization of ferroelectric domains with coherent light.

Illumination of a lithium niobate or lithium tantalate crystal along the crystallographic c axis with coherent light while simultaneously applying an external electrical field through transparent electrodes allows real-time, in situ, nondestructive monitoring of ferroelectric domain patterns. Imaging of the optical near-field through a lens directly visualizes the domain walls, whereas the far-field yields averaged information about the spatial orientation of the domain boundaries.

Consequences of the background in piezoresponse force microscopy on the imaging of ferroelectric domain structures

The interpretation of ferroelectric domain images obtained with a piezoresponse force microscope (PFM) is discussed. The influence of an inherent experimental background on the domain contrast in PFM images (enhancement, nulling, inversion) as well as on the shape and the location of the domain boundaries are described. We present experimental results to evidence our analysis of the influence of the background on the domain contrast in PFM images.

Contrast mechanisms for the detection of ferroelectric domains with scanning force microscopy

We present a full analysis of the contrast mechanisms for the detection of ferroelectric domains on all (x, y and z) faces of bulk single crystals using scanning force microscopy. The experiments were carried out with hexagonally poled lithium niobate to ensure access to a well-defined domain structure on every crystal face. The domain contrast can be attributed to three different mechanisms: (i) the thickness change of the sample due to an out-of-plane piezoelectric response (standard piezoresponse force microscopy), (ii) the lateral displacement of the sample surface due to an in-plane piezoresponse and (iii) the electrostatic tip–sample interaction at the domain boundaries caused by surface charges on the crystallographic y- and z-faces. A careful analysis of the movement of the cantilever with respect to its orientation relative to the crystallographic axes of the sample allows clear attribution of the observed domain contrast to the driving forces.