A. A. Sidorkin

Fatigue of Lead Titanate and Lead Zirconate Titanate Thin Films

The fatigue of lead titanate and lead zirconate titanate ferroelectric thin films, i.e., a change in the polarization as a function of the number of switching cycles in an external electric field, is investigated experimentally. The threshold numbers of switching cycles are determined to be 1010–1011 for the lead titanate films and 109–1010 for the lead zirconate titanate films. It is shown that a change in the temperature does not substantially affect the threshold number of switching cycles at which the switched polarization decreases drastically. However, an increase in the external field strength leads to a noticeable decrease in the threshold number of switching cycles. The process of fatigue is accompanied by an increase in the coercive field and the internal bias field. It is established that, as the number of switching cycles increases, the internal bias field changes more significantly as compared to the coercive field. The absence of a change in the phase composition in repeatedly switched samples indicates that the fatigue processes have a nonchemical nature. The anomaly observed in the frequency dependence of the permittivity in the frequency range 106–107 Hz due to the domain structure disappears after multiple switching cycles. This suggests that the observed fatigue phenomenon has a domain nature.

Internal field in ferroelectric films with different electrodes

It has been shown that a difference between the binding energies of electrons in different electrodes induces an electric field in the bulk of a ferroelectric film, which can be interpreted as a kind of internal bias field. The values of the internal bias field in films of different thicknesses have been found and compared with the theoretical calculations.

Frequency dependence of the coercive field and the internal bias field in ferroelectric thin films

The dependences of the coercive field and the internal bias field on the switching field frequency for lead zirconate titanate and lead titanate thin films are experimentally studied over a wide temperature range. It is shown that, as the frequency increases, the coercive field and the bias field in these films increase at the rate dependent on the temperature and the substrate used.

Thermally Stimulated Electron Emission in the Paraelectric Phase of Triglycine Sulfate Crystals Heated at a High Rate

Thermally stimulated electron emission is experimentally observed in ferroelectric triglycine sulfate (TGS) crystals in a temperature range whose upper limit is 10–15 K above the Curie point. Samples of a nominally pure and a chromium-doped TGS crystal, heated at different constant rates q=dT/dt, are investigated. It is shown that an increase in the heating rate results in increased emission current density over the entire temperature range investigated. The temperature at which emission arises depends only slightly on the rate q. At the same time, the temperature at which emission ceases increases monotonically with increasing q; if q is less than 1 K/min, this temperature is below the Curie point, while at q=4–5 K/min, this temperature becomes as large as 60–65°C, which is more than 15°C above the Curie point. In chromium-doped TGS crystal, the electron emission onset temperature is close to that of pure TGS, but the width of the temperature range over which emission is observed in the paraelectric phase is approximately two times less than in the case of pure TGS heated at the same rate. The emission disappearance below the Curie point (in the ferroelectric phase) at low q is explained as a result of full emptying of the electron traps under slow heating. The reason for the occurrence of emission above the Curie point is related to the charges that shield the spontaneous polarization and, because of their slow relaxation, persists in the paraelectric phase.

Influence of electric field on fatigue processes of thin ferroelectric films

The field and frequency dependences of the remanent polarization, coercive field, and internal bias field on the number of switching cycles have been studied experimentally in thin ferroelectric lead titanate and lead zirconate titanate films. A model is proposed to describe the features of fatigue effect in these films.

Dielectric Non-Linearity of TGS Crystals with an Admixture of Europium Ions

Introduction of europium ions into TGS crystal was shown to lead in appearance of minima in the dependence of effective dielectric permittivity ϵ eff on the strength of external electric field E. Near Curie point a decrease in the values of ϵ eff in the peaks of considered dependences is less than for TGS crystals with the impurity of chromium ions and is close to similar values for TGS crystals with L, α -alanine. These results can be probably due to the appearance of large elastic stresses in the process of TGS crystal growth with europium ions due to the large radius of these ions. It is confirmed by a high degree of unipolarity and large values of the emission current for all investigated samples.

Thermally stimulated electron emission from chromium-doped triglycine sulfate in the paraelectric phase

Thermally stimulated electron emission from a ferroelectric chromium-doped triglycine sulfate (TGS) crystal was experimentally observed to occur in a temperature range 6 K above the Curie point from samples heated at a relatively high rate. Increasing the heating rate q was shown to cause the emission current density to increase throughout the temperature range studied. The emission onset temperature in chromium-doped TGS depends only weakly on the rate q and is close to that for pure TGS, and the emission cutoff temperature grows monotonically with q at comparatively low heating rates and stabilizes at high q. At the same time, the interval of emission extension into the paraelectric phase here is about one half that for pure TGS heated at the same rate. The specific features of emission observed for this crystal can be assigned to relaxation of the charges screening the spontaneous polarization. The lower emission cutoff temperature for the chromium-doped TGS compared to that for pure TGS is accounted for by the shorter Maxwellian relaxation time in the doped crystal.

Kinetics of Electron Emission from the TGS Ferroelectric Crystal

A study is reported on the behavior in time of the electron emission current density from a triglycine sulfate ferroelectric crystal measured at fixed temperatures. This relation is shown to have an exponential nature. The characteristic emission relaxation time depends on temperature and decreases as one approaches the phase-transition point. The magnitude of the relaxation time and its temperature dependence can be accounted for both within a mechanism in which the emission decay is associated with the emptying of surface electron states, and in terms of the Maxwellian relaxation process.

Thermostimulated electron emission from polar cleavage of a triglycine sulfate crystal

This paper reports the results of experimental investigations into thermostimulated electron emission from the positive and negative cleavage surfaces of a ferroelectric triglycine sulfate crystal. It is shown that the current density j of emission from the negative surface is universally higher than that from the positive surface over the entire range from room temperature to the Curie point. The shape of the j(T) curves substantially depends on the degree of natural unipolarity of the studied samples. The results obtained are discussed in the framework of the mechanism of field electron emission from the surface electronic states.

Parameters of Domain Boundaries in Thin PbTiO3 Films

Features of polarization switching processes in lead titanate thin films were investigated. The films under discussion were synthesized by plasma-enhanced magnetron sputtering of titanium and lead on Al 2 O 3 substrates in an argon medium, and subsequent annealing in an oxygen atmosphere. In terms of theory of lateral motion of domain boundaries the main parameters of domain walls, characterizing kinetics of domain structure in thin polycrystalline lead titanate films, were determined.