Abstract
We have studied the properties of thin PbTiO{sub 3} films grown pseudomorphically on SrTiO{sub 3} substrates. It was found that the properties are unchanged for film thickness down to 20 nm. For thinner films the interfaces and polarization screening mechanism starts influencing the properties. The TEM study and XRD measurements showed that the PbTiO{sub 3} films were of a good structural quality with a low density of extended defects. The major extrinsic impact on the ferroelectric properties was assumed to come from point defects in the film and from the interfaces and electrodes. The effect of the point defects can be illustrated with the response of films grown at different temperatures. It was seen from XPS measurements that the Pb to Ti ratio at the surface decreased with increasing growth temperature. Concomitant with this, there was an increase in the leakage current and a larger negative offset voltage in the hysteresis loops. Although XPS is a surface technique, we expect the results to be indicative of the composition of the bulk of the sample as well. Hence the samples grown at optimal temperature are close to stoichiometric, with Pb and O vacancies as the dominating point defects. This is in
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Citation Formats
Dahl, Oeystein.
Measurements and modelling of effect of interface on ferroelectricity in lead titanate thin films.
Norway: N. p.,
2010.
Web.
Dahl, Oeystein.
Measurements and modelling of effect of interface on ferroelectricity in lead titanate thin films.
Norway.
Dahl, Oeystein.
2010.
"Measurements and modelling of effect of interface on ferroelectricity in lead titanate thin films."
Norway.
@misc{etde_1009190,
title = {Measurements and modelling of effect of interface on ferroelectricity in lead titanate thin films}
author = {Dahl, Oeystein}
abstractNote = {We have studied the properties of thin PbTiO{sub 3} films grown pseudomorphically on SrTiO{sub 3} substrates. It was found that the properties are unchanged for film thickness down to 20 nm. For thinner films the interfaces and polarization screening mechanism starts influencing the properties. The TEM study and XRD measurements showed that the PbTiO{sub 3} films were of a good structural quality with a low density of extended defects. The major extrinsic impact on the ferroelectric properties was assumed to come from point defects in the film and from the interfaces and electrodes. The effect of the point defects can be illustrated with the response of films grown at different temperatures. It was seen from XPS measurements that the Pb to Ti ratio at the surface decreased with increasing growth temperature. Concomitant with this, there was an increase in the leakage current and a larger negative offset voltage in the hysteresis loops. Although XPS is a surface technique, we expect the results to be indicative of the composition of the bulk of the sample as well. Hence the samples grown at optimal temperature are close to stoichiometric, with Pb and O vacancies as the dominating point defects. This is in agreement with the prevailing literature [160 -166]. There are indications that the point defects have a stronger influence on the ferroelectric properties in very thin films. For films grown at optimal temperature, it was found that the offset voltage in the polarization hysteresis loops became negative in very thin films, similar to that observed in films grown at high temperatures. The stability of the negative polarization was lost after switching in very thin films. The TEM study found a strain gradient, parallel with the polarization direction, in a layer near the Nb doped SrTiO{sub 3} and pure SrTiO{sub 3} substrate. In this layer, the c-axis of the unit cell was gradually reduced compared to the bulk of the film. The strain is not from the epitaxial growth, but can be related to a reduced polarization near the interface. A similar strain gradient has recently been observed for Pb(Zr,Ti)O{sub 3} thin film grown on SrRuO{sub 3} electrode [103, 104]. Based on the capacitance measurements for different film thickness, we found that the dielectric properties of this layer was not significantly changed from the bulk properties. This is in agreement with others who have found that the interface capacitance is dominated by the electrodes [88, 90, 167, 168]. The polarization can be reduced near the interface as a compromise between the depolarization energy from incompletely screened polarization and the strain and polarization gradient energy for an inhomogeneous polarization. The XRD measurements showed a decreasing out-of-plane lattice constant with decreasing film thickness below 20 nm. This behaviour has also been observed previously in PbTiO{sub 3} thin films [101]. The polarization profile, calculated from a vacuum/ferroelectric/metal electrode structure, showed a decreasing bulk polarization for films in the same thickness region. The polarization as extracted from the measured lattice constant was in agreement with the model calculations. The pyroelectric measurements showed that the polarization had the thick-film limit value down to a film thickness of 20 nm. For thinner films there was a reduced response. This is in agreement with the XRD results, and with other studies, which have found a reduction of the polarization for film thickness less than 15 nm [100, 103, 108, 114, 169]. For thick films, the initial polarization was found to be negative, consistent with the model calculations, showing the possibility of controlling the polarization by adjusting the contact potential of the interface between the ferroelectric and the electrodes. The decay of the net polarization after applying switching pulses was found to be faster in thinner films, consistent with an increased depolarization field with decreasing film thickness. The final state in very thin films was not zero net polarization, but a finite positive net polarization, which we interpret as an effect of the presence of interface dipoles and defects that influences the polarization. Returning to the question of the factors that affect the switchability and stability of the polarization, it is clear that both the interfaces and defects are important. To clarify the relative importance of the interface itself, and interface defects, it would be of interest to investigate surface and interface defects in detail. This can be accomplished by measuring the depth profile and the nature of the defects, using e.g. XPS, Auger electron spectroscopy, and electron energy loss spectroscopy, which all give valuable information. However, these methods have either a low sensitivity, or are very local probes, so this is a challenging task.}
place = {Norway}
year = {2010}
month = {Jul}
}
title = {Measurements and modelling of effect of interface on ferroelectricity in lead titanate thin films}
author = {Dahl, Oeystein}
abstractNote = {We have studied the properties of thin PbTiO{sub 3} films grown pseudomorphically on SrTiO{sub 3} substrates. It was found that the properties are unchanged for film thickness down to 20 nm. For thinner films the interfaces and polarization screening mechanism starts influencing the properties. The TEM study and XRD measurements showed that the PbTiO{sub 3} films were of a good structural quality with a low density of extended defects. The major extrinsic impact on the ferroelectric properties was assumed to come from point defects in the film and from the interfaces and electrodes. The effect of the point defects can be illustrated with the response of films grown at different temperatures. It was seen from XPS measurements that the Pb to Ti ratio at the surface decreased with increasing growth temperature. Concomitant with this, there was an increase in the leakage current and a larger negative offset voltage in the hysteresis loops. Although XPS is a surface technique, we expect the results to be indicative of the composition of the bulk of the sample as well. Hence the samples grown at optimal temperature are close to stoichiometric, with Pb and O vacancies as the dominating point defects. This is in agreement with the prevailing literature [160 -166]. There are indications that the point defects have a stronger influence on the ferroelectric properties in very thin films. For films grown at optimal temperature, it was found that the offset voltage in the polarization hysteresis loops became negative in very thin films, similar to that observed in films grown at high temperatures. The stability of the negative polarization was lost after switching in very thin films. The TEM study found a strain gradient, parallel with the polarization direction, in a layer near the Nb doped SrTiO{sub 3} and pure SrTiO{sub 3} substrate. In this layer, the c-axis of the unit cell was gradually reduced compared to the bulk of the film. The strain is not from the epitaxial growth, but can be related to a reduced polarization near the interface. A similar strain gradient has recently been observed for Pb(Zr,Ti)O{sub 3} thin film grown on SrRuO{sub 3} electrode [103, 104]. Based on the capacitance measurements for different film thickness, we found that the dielectric properties of this layer was not significantly changed from the bulk properties. This is in agreement with others who have found that the interface capacitance is dominated by the electrodes [88, 90, 167, 168]. The polarization can be reduced near the interface as a compromise between the depolarization energy from incompletely screened polarization and the strain and polarization gradient energy for an inhomogeneous polarization. The XRD measurements showed a decreasing out-of-plane lattice constant with decreasing film thickness below 20 nm. This behaviour has also been observed previously in PbTiO{sub 3} thin films [101]. The polarization profile, calculated from a vacuum/ferroelectric/metal electrode structure, showed a decreasing bulk polarization for films in the same thickness region. The polarization as extracted from the measured lattice constant was in agreement with the model calculations. The pyroelectric measurements showed that the polarization had the thick-film limit value down to a film thickness of 20 nm. For thinner films there was a reduced response. This is in agreement with the XRD results, and with other studies, which have found a reduction of the polarization for film thickness less than 15 nm [100, 103, 108, 114, 169]. For thick films, the initial polarization was found to be negative, consistent with the model calculations, showing the possibility of controlling the polarization by adjusting the contact potential of the interface between the ferroelectric and the electrodes. The decay of the net polarization after applying switching pulses was found to be faster in thinner films, consistent with an increased depolarization field with decreasing film thickness. The final state in very thin films was not zero net polarization, but a finite positive net polarization, which we interpret as an effect of the presence of interface dipoles and defects that influences the polarization. Returning to the question of the factors that affect the switchability and stability of the polarization, it is clear that both the interfaces and defects are important. To clarify the relative importance of the interface itself, and interface defects, it would be of interest to investigate surface and interface defects in detail. This can be accomplished by measuring the depth profile and the nature of the defects, using e.g. XPS, Auger electron spectroscopy, and electron energy loss spectroscopy, which all give valuable information. However, these methods have either a low sensitivity, or are very local probes, so this is a challenging task.}
place = {Norway}
year = {2010}
month = {Jul}
}