Scanning Pyroelectric Detection of Changes in the Spontaneous Polarization of P(VDF-TrFE) Thin Films Forschungsgesellschaft mbH Institute of Nanostructured Materials and Photonics
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Setup and Measurement Principle The developed application (PyroFM) comprises a two step process, also offering the possibility to investigate unpoled pyroelectric samples. In the first step a potential larger than the coercive field of the ferroelectric polymer is applied to the tip thus orienting the dipoles vertically to the sample surface with the polarization vector pointing in or out of surface plane depending on the direction of the applied field. This is done in contact mode in a small area of the sample and is referred to as poling. In the second step the pyroelectric readout is performed by detecting the surface potential simultaneously with the temperature
switching. Depending on the polarization direction having been previously defined in the poling step, the surface potential changes with the temperature modulation due to the pyroelectric effect which is caused by dipole librations or dimensional changes in the amorphous and crystalline phases (dipole-density effect). In the second step a scanning surface potential measurement was done with a standard atomic force microscope (AFM) by exploiting a two pass technique in order to separate topographic and surface potential information. In the first scan topographic and phase information is recorded and in the second scan the potential at the sample surface is detected using a Kelvin probe method, where the potential of the tip is adjusted to nullify the electrostatic interaction between tip and sample. To introduce different temperature states in the sample, light from a laser diode with 808 nm wavelength is coupled into a glass fibre via standard optics (see Figure). After outcoupling, the infrared light is absorbed at the bottom side of the sample in a black graphite layer on an area corresponding to that one of the glass fibre (~0.8 mm2 ). As the laser intensity is modulated by a square function between on and off, the temperature of the sample and the pyroelectric layer changes between two equilibrium states. Setup of the PyroFM method including a detail of the sample indicating the region of heat fluctuations.
Poling and Pyroelectric Scanning The poling of the ferroelectric domains of P(VDFTrFE) copolymer thin films was done in contact mode by applying a voltage between tip and bottom electrode ranging from 20 to 100 V during a specific amount of time. It is generally believed, that depending on the strength and the sign of the applied field, a preferential orientation of the dipoles is induced. The alignment of the dipoles results in a macroscopic spontaneous polarization, that in a subsequent scan, is assumed to show up as an increased surface potential confined to the poled region. The section profiles below clearly reveal a step in the surface potential at the edge of the poled and non-poled regions both for positive and negative poling voltages. The pyroelectric readout is done with the AFM set to scan five lines trace and retrace of topography and surface potential in one second and the laser is driven with a square function of 0.1 Hz. If two separated and inversely poled regions have been defined in the poling step the pyroelectric response is inverse for oppositely poled regions with upward and downward pointing polarization direction.
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In Fig. b the exci tation frequency de pendence of the tem perature enhancement in the pyroe lect ric sample is calculated for varying thickness of the glass substrate assuming an excitation power of 8 mW over a spot of about 3 mm2 (see below). From this it is quite clear that the substrate acts as a heat sink. For a modulation frequency of 0.1 Hz between the on and the off state of the laser, which is often used in the PyroFM measurements, the calculation yields an induced temperature change in the pyroelectric layer of about 2–3 K. These results, the quality of the model and the correctness of the assumed input values can additionally be validated by modelling and simulating the pyroelectric response of homogeneously poled macroscopic capacitor samples by c using an excitation setup (via glass fibre) similar to the one used for the PyroFM measurement. In Fig. c the excellent correspondence bet ween calculation and backside response measure ment is obvious thus con firming the calculated tem perature lift values.
In order to estimate the potential distribution produced by the application of a voltage between the tip and the bottom electrode the method of mirror charges was applied. The calculation of the field penetration depth is done for a point-like potential source in front of a dielectric and a conductive plane. In Fig. a the calculated spatial distribution of the z-direction of the electric field (Ez) induced by an 80 V constant potential applied to the tip is plotted over the cross-section of the pyroelectric layer. The tip is located at 0 nm lateral position. The club-shaped equipotential plane corresponding to a field of EP = 10 MV / m (dark red) extends to the bottom electrode, whereas the planes for EP = 20 MV / m (light red), EP = 60 MV / m (orange) and for 100 MV / m (yellow) have decreased penetration depths.
Pyroelectric Coefficient
That means that a temperature enhancement induces a decrease or an increase of the absolute potential value, depending on the polarization direction (see Figures a–c). (a) Variations of the surface potential due to a laser excitation at 0.1 Hz for inversely poled regions (Vp = 100 V). The surface potential changes appear as blue stripes in the dark blue negatively poled regions and as dark yellow/grey stripes in the positively poled region. (b) Surface potential scan across the line indicated in (a) illustrating the laser induced fluctuations at the centre of the poled region. (c) Surface potential changes showing inverse response of regions when the preferential direction of the dipoles is orientated pointing up or down perpendicular to the surface plane.
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Pyroelectricity is the electrical re sponse of a material to a change in tem perature. In general the pyroelectricity of a region depends on the amount of dipoles orientated in the same direction and therefore leading to a spontaneous polarization Ps. If exact values of the temperature change are available the pyroelectric coefficient can be extracted from the PyroFM measurement. In this regard the temperature change induced by the laser modulation is calculated from a heat conduction model following the approach of D. Setiadi et.al. [1]. A multilayer thin film system composed of a 175 µm thick glass substrate a 400 nm thick P(VDF-TrFE) layer and electrodes of 50 nm was assumed to resolve the one-dimensional thermal diffusion equation :
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Top : (left) Surface Potential of a (PVDF-TrFE) sample with 24 % VDF content measured 10min after poling with +80 V (upper row) and -80 V (bottom row) for time periodes ranging from 1 to 20 s. (right) Surface potential profiles extracted along the lines indicatet in the 2D plot. Bottom : (left) Poled regions of above Figure during laser excitation at 0.1 Hz done directly after poling. (right) Section profiels according to the lines indicated on the left side. The curves are shifted upwards for better presentation. 1s
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In a next step we inscribed pyroelectric active regions into the copolymer layers by selective positive and negative poling. This was done via sequential positive poling of spots forming the shape of a “J” and sequential negative poling of spots forming the shape of a “R” thus summing up to a pattern of our company’s logo. This has been done for ferroelectric P(VDF-TrFE) copolymer samples (70 : 30). The poling was performed with a poling voltage of Vp = 80 V applied over a period of 60s for each spot. Then the pyroelectric effect was induced by switching on the laser modulation with an intensity of 6 mW / mm2 and a frequency of 0.1 Hz resulting in changes of the surface potential from which the pyroelectric coefficient can be derived after :
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[1] D. Setiadi and P.P.L. Regtien, Ferroelectrics 173, 309 (2002)
Modelling of the Measurement Principle
Results and Discussion
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coefficients around 30 µC / m2K. Another promising feature of PyroFM is the ability to visualize “screened” polarization thus enabling in-depth profiling of polarization distributions and domain formation and to study the composition dependence and the time and frequency behaviour of ferroelectric nano-domains.
Setup and Theoretical Background of PyroFM
Dr. Martin Zirkl Franz-Pichler-Strasse 30 8160 Weiz, AUSTRIA
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imultaneously, the surface potential variations are detected by scanning surface potential microscopy thus forming the base for the pyroelectric coefficient map. The potential of the method is demonstrated on the basis of ferroelectric semi-crystalline copolymer thin films yielding local maxima of the pyroelectric
resolution. In domains of previously aligned dipole moments small heat fluctuations are achieved by laser diode excitation from the bottom side thus inducing changes in the surface potential due to the pyroelectric effect.
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JOANNEUM RESEARCH
his work demonstrates a novel surface scanning method for the quantitative determination of the local pyroelectric coefficient in ferroelectric thin films. It is called Pyroelectric Force Microscopy (PyroFM) and allows generating a map of the pyroelectric response with very high spatial
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Contact
Martin Zirkl, Jonas Groten, Barbara Stadlober, Anja Haase, Georg Jakopic, Joachim Krenn
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Fabrication of a pyroelectric active JR pattern of positive (“J”) and negative (“R”) poled regions in a P(VDF-TrFE) layer. The time specifications are with respect to the time span elapsed after poling. 1st row : topography of (PVDF-TrFE) with 24 % TrFE. 2nd row : surface potential scan of copolymer after poling. 3rd row : surface potential scan of copolymer during laser excitation. 4th row : cross-sections through one point of the positively poled “J”. last row : map of the pyroelectric coefficient of the copolymer for DT = 3 K and cross-sections of a positive and negative poled point respectively.
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