Abstract
The compensation of the depolarization field in ferroelectric layers requires the presence of a suitable amount of charges able to follow any variation of the ferroelectric polarization. These can be free carriers or charged defects located in the ferroelectric material or free carriers coming from the electrodes. Here we show that a self-doping phenomenon occurs in epitaxial, tetragonal ferroelectric films of Pb(Zr0.2Ti0.8)O3, consisting in generation of point defects (vacancies) acting as donors/acceptors. These are introducing free carriers that partly compensate the depolarization field occurring in the film. It is found that the concentration of the free carriers introduced by self-doping increases with decreasing the thickness of the ferroelectric layer, reaching values of the order of 1026 m−3 for 10 nm thick films. One the other hand, microscopic investigations show that, for thicknesses higher than 50 nm, the 2O/(Ti+Zr+Pb) atomic ratio increases with the thickness of the layers. These results suggest that the ratio between the oxygen and cation vacancies varies with the thickness of the layer in such a way that the net free carrier density is sufficient to efficiently compensate the depolarization field and to preserve the outward direction of the polarization.
Highlights
Raw materials used to grow the epitaxial films (e.g. the targets used for pulse laser deposition (PLD), or radio-frequency (RF) sputtering), or through the structural defects that occur during the film growth; 3) by the environment existent in the deposition chamber, that may contain charged species which are adsorbed on the film surface[17]
We focus on the free carriers which are present in the ferroelectric PZT epitaxial films and we show that their concentration is large and increases with decreasing the film thickness, playing a significant role in the screening of the polarization charges in ultra thin films
Samples from these films were used for structural investigations by X-ray diffraction (XRD), atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM), as well as for chemical analysis by X-ray photoelectron spectroscopy (XPS) and scanning TEM (STEM) combined with electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDS)
Summary
The self-doping hypothesis presented above was checked by performing complex investigations on a set of epitaxial PZT films with different thicknesses: 5 nm, 10 nm, 20 nm, 50 nm, 100 nm, 150 nm, 200 nm, 250 nm and 300 nm. Considering that the deposition parameters were the same, one can explain the increased density of free carriers with decreasing the thickness of the PZT layer by self-doping mechanism introducing the required free charges to efficiently compensate the dominant outward direction of polarization evidenced by PFM studies (see Fig. 2). To reconcile the TEM and XPS results, showing a minimum of the 2O/(Ti+ Zr+ Pb) ratio at a thickness of 50 nm, with the results of the electric measurements showing a monotonic increase in the density of the free carriers with decreasing the thickness, in the light of the self-doping hypothesis, one has to remind that Sr diffusion over a thickness of about 10 nm was reported in the case of PZT films of the same composition as in the present study[53]. Where one has to keep in mind that it may happen that only a percentage of the oxygen vacancies (their density is nOv) may become ionized and produce free electrons, according to (3), depending on the temperature T and on the position of the donor levels density of electrons injected from the metal may be EexOpv rwesistehdreassp: ect to the
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