Tobias Jungk

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.

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.

Cross-talk correction in atomic force microscopy

Commercial atomic force microscopes usually use a position-sensitive photodiode to detect the motion of the cantilever via laser beam deflection. This readout technique makes it possible to measure bending and torsion of the cantilever separately. A slight angle between the orientation of the photodiode and the plane of the readout laser beam, however, causes false signals in both readout channels. This cross-talk may lead to misinterpretation of the acquired data. We demonstrate this fault with images recorded in contact mode on periodically poled ferroelectric crystals and present a simple electronic circuit to compensate for it. This circuit can correct for cross-talk with a bandwidth of approximately 1 MHz suppressing the the false signal to <<1%.

Challenges for the determination of piezoelectric constants with piezoresponse force microscopy

At first sight, piezoresponse force microscopy (PFM) seems an ideal technique for the determination of piezoelectric coefficients, thus making use of its ultrahigh vertical resolution (<0.1pm∕V). In general, however, only qualitative PFM imaging is performed and in the few publications containing quantitative data, the obtained values vary considerably with respect to macroscopic measurements. In this contribution, we present a reliable calibration procedure for PFM followed by a careful analysis of the encounted difficulties using PFM for determining piezoelectric coefficients. We point out different approaches for their solution and expose why those difficulties cannot be circumvented without an extensive effort.

Impact of the tip radius on the lateral resolution in piezoresponse force microscopy

We present a quantitative investigation of the impact of tip radius as well as sample type and thickness on the lateral resolution in piezoresponse force microscopy (PFM) investigating bulk single crystals. The observed linear dependence of the apparent width of a ferroelectric domain wall on the tip radius as well as the independence of the lateral resolution on the specific crystal-type are validated by a simple model. Using a Ti-Pt coated tip with a nominal radius of 15 nm the so far highest lateral resolution in bulk crystals of only 17 nm was obtained.

Direct-writing of inverted domains in lithium niobate using a continuous wave ultra violet laser

The inversion of ferroelectric domains in lithium niobate by a scanning focused ultra-violet laser beam (lambda = 244 nm) is demonstrated. The resulting domain patterns are interrogated using piezoresponse force microscopy and by chemical etching in hydrofluoric acid. Direct ultra-violet laser poling was observed in un-doped congruent, iron doped congruent and titanium in-diffused congruent lithium niobate single crystals. A model is proposed to explain the mechanism of domain inversion.