Fe-doped Pb Research Articles

Resistive-like Behavior of Ferroelectric p–n Bilayer Structures Based on Epitaxial Pb(Zr0.2Ti0.8)O3 Thin Films

The p–n junctions are the building blocks of nowadays electronic devices. The n- or p-type conductivity is obtained in classic semiconductors, like Si, by doping with atoms acting as donors or acceptors, respectively. Doping was used in ferroelectrics to influence the transition temperature, magnitude of some physical properties, but not necessarily conduction type. Therefore, comprehensive studies to obtain true ferroelectric p–n junctions by controlled doping are missing. Recently, it has been shown that Pb(Zr0.2Ti0.8)O3 films doped with ≈1% atomic Nb (n-type doping) or Fe (p-type doping) have different orientations of polarization in the as-grown state. Knowing that polarization orientation depends on doping type, the next step is to build ferroelectric p–n homojunctions and to study their properties in relation to ferroelectric polarization. p–n and n–p structures were grown for this purpose by successive deposition of Nb-doped and Fe-doped Pb(Zr,Ti)O3 layers with different thicknesses. We find that these p–n homojunctions are ferroelectric, but the magnitude of the polarization and coercive field, as well as the dominant polarization orientation in the as-grown state, depend on the conduction type of the first grown layer. The I–V characteristics are quasi-linear, although the interfaces with the electrodes behaves as Schottky contacts. The resistance extracted from the I–V characteristics displays an exponential dependence on temperature, with an activation energy in the range of 0.14–0.17 eV. These results are explained assuming that the total current in the junction is the total of electron and hole injections at the electrode interfaces. It is shown that for relatively low doping concentrations, the current density contains a dominant term with a linear voltage dependence and an exponential temperature dependence, as observed experimentally, and a secondary (correction) term that is dependent on the free carrier density and can induce non-linear voltage dependence when this density is significant.

Optical response and structural properties of Fe-doped Pb(Zr0.52Ti0.48)O3 nanopowders

In this paper, the Pb(Zr0.52Ti0.48)1−xFexO3 (FePZT, x = 0.05) nanopowders were prepared by dry and wet sol–gel methods in the morphotropic phase boundary (MPB) region. The effect of Fe concentration on the structural, morphological, and optical properties of PZT nanopowders was investigated using X-ray diffraction (XRD), Field emission scanning electron microscopy (FESEM), Fourier-transform infrared (FTIR), and Ultraviolet–Visible (UV–Vis) analysis. XRD results showed that the nanopowders have a perovskite structure with the tetragonal phase. The lattice parameters and average crystallite size of samples decreased from 33 to 21 nm with increasing Fe incorporation due to the substitution of Fe atoms instead of Ti and Zr atoms. FESEM images showed that all average diameters of the nanopowders decreased with the Fe concentration. The optical properties of the pure and FePZT nanopowders such as longitudinal optical (LO) and transverse optical (TO) phonon frequencies, refractive index, extinction coefficient, and the real-imaginary parts of dielectric function were examined by the Kramers–Kronig model. As a result, the TO and refractive index of nanopowders are increased by substituting Ti and Zr with Fe atoms due to their different ionic radius. Also, while the crystallite size increases from 20.95 to 33.52 nm, the LO–TO splitting increases too. The optical band-gap values of the pure and FePZT nanopowders were estimated using UV–Vis spectroscopy and Kubelka–Munk model. The band-gap increased from 3.52 to 3.60 eV with a decrease in the crystal size of nanopowders.

Domain wall contributions in Pb(Zr,Ti)O3 ceramics at morphotropic phase boundary: A study of dielectric dispersion

The dielectric properties of undoped, Nb-, and Fe-doped Pb(Zr,Ti)O3 ceramics with composition near morphotropic phase boundary were investigated in the frequency range from 1 MHz to 20.2 GHz at room temperature. Temperature dependences of dielectric permittivity ε′ and loss ε″ are measured at 100 kHz from 50 to 300 K and around 13.4 GHz from 100 to 300 K. These measurements permit estimation of the upper limit of the intrinsic permittivity and lower limit of the extrinsic contributions to the permittivity as a function of temperature. The extrinsic contributions account for more than 50% of the quasistatic dielectric permittivity in studied samples.

Hardening-softening transition in Fe-doped Pb(Zr,Ti)O3 ceramics and evolution of the third harmonic of the polarization response

It is shown that the phase angle of the third harmonic of polarization response (δ3) is very sensitive to the aging state of hard, Fe-doped lead zirconate titanate [Pb(Zr,Ti)O3 or PZT] ceramics and may thus provide rich information on processes responsible for aging in hard ferroelectrics. When hard PZT ceramics experience hardening (aged state)-softening (deaged state) transition, δ3 changes by about 90° at subswitching fields and by about 180° at switching conditions. Evolution of δ3 with time, temperature and electric field amplitude during deaging suggests that at least two mechanisms of charge migration, one short-range and one long-range, may participate in the deaging process.

Growth and pyroelectric property of 0.2 mol % Fe-doped Pb(Mg1/3Nb2/3)O3-0.38PbTiO3 single crystals measured by a dynamic technique

To develop a high-performance pyroelectric infrared detector, large-size and high-quality 0.2 mol % Fe-doped Pb(Mg1/3Nb2/3)O3-0.38PbTiO3 single crystals have been grown by a modified Bridgman technique with a seed crystal. At 50 Hz, the doped single crystal showed a lower dielectric constant of 310 compared with the pure one. A low dielectric loss of 0.0067 was also obtained by doping with small concentration of iron ions. Piezoelectric coefficient (d33) of 〈001〉 oriented sample was ∼260 pC/N. The pyroelectric properties and their temperature dependence were measured by the dynamic technique. At room temperature, the pyroelectric coefficient and calculated detectivity figure of merit FD are 568 μC/m2 K and 53 μPa−1/2, respectively. The values of current response figure of merit Fi and voltage response figure of merit Fv are 227.2 pm/V and 0.083 m2/C, respectively. These excellent pyroelectric properties as well as being able to produce large-size and high-quality single crystals make this kind of single crystal very promising for high-performance infrared detectors and other pyroelectric applications.

Growth and characterization of Fe-doped Pb(Zn1/3Nb2/3)O3−PbTiO3 single crystals

Fe-doped 0.955Pb(Zn1/3Nb2/3)O3−0.045PbTiO3 (PZNT4.5) single crystals were grown using a high temperature flux technique and the dielectric, piezoelectric, electromechanical, electrostrictive, and electro-optic properties were characterized. The room temperature dielectric permittivity along the <001> direction after poling was about ∼6000 (5000−7000), increased compared to that of pure PZNT4.5 crystals ∼3000−5000. The Curie temperature (TC) and the rhombohedral to tetragonal phase transition temperature (Tr−t) of doped crystals were about 143 and 92 °C, respectively, more than 20 °C lower than that of the pure crystals. The remnant polarization (Pr) and coercive field (Ec) of <001> oriented doped crystals measured at 1 Hz and 20 kV/cm field were ∼27.3 μC/cm2 and 4.7 kV/cm, respectively, the latter significantly greater than the Ec of the pure crystals ∼3 kV/cm. Piezoelectric coefficients (d33) of <001> oriented doped PZNT4.5 crystals were ∼2500−3000 pC/N, being much higher than that of the pure (1400–2000 pC/N). The electric field induced strain was up to 0.5% with minimal hysteresis at low electric field (E∼15 kV/cm). The longitudinal electromechanical coupling factor k33 was 92%–93%. The shear mode and lateral mode properties were also studied with piezoelectric coefficient d15 of ∼3500 pC/N, d31 of ∼−1800 pC/N, and electromechanical coupling of k15∼93%, k31∼87%, respectively. The electrostrictive coefficients of Fe-doped PZNT4.5 crystals, Q11∼0.099 m4/C2, Q12∼−0.048 m4/C2, Q44∼0.012 m4/C2, and Qh∼0.0033 m4/C2 were calculated and compared to those of PZNT4.5 crystals. The apparent electro-optic coefficients of doped crystals were measured to be r33*∼118 pm/V and r13*∼29 pm/V, respectively.

Schottky barrier effects in the electronic conduction of sol–gel derived lead zirconate titanate thin film capacitors

Pure and Fe-doped Pb(Zr0.53Ti0.47)O3 (PZT) thin film capacitors were fabricated by the sol–gel method and the leakage current versus voltage characteristics of these films were investigated at several temperatures and initial polarization states. After the initial poling of ferroelectric thin films, we measured two kinds of leakage current: (i) the full-switching current measured against an initial polarization direction and (ii) the nonswitching current measured toward an initial polarization direction. In the case of the full-switching current measurement, the anomaly of leakage current due to the switching of space charges was observed, which we suggest to be accumulated at a Schottky barrier region near an electrode or a gain boundary. In the case of the nonswitching current measurement, the Schottky diode rectifier current and the field-enhanced Schottky (ES) emission current came out without the switching current. The Poole–Frenkel emission process was dominant over ES emission for a Fe-doped PZT thin film having a large amount of space charge and traps. The height of the Schottky barrier in a Au/PZT film/Pt capacitor was measured to be 0.64 eV.