Jiaping Han

Defect chemistry and electrical characteristics of undoped and Mn-doped ZnO

Abstract Undoped and Mn-doped ZnO samples were sintered at 1373, 1473, and 1573 K, for 2 h, in air, and then quenched to room temperature. Defect concentrations in these samples at the sintering temperatures and at room temperature and below were calculated using a refined defect model. Using the calculated electron concentrations and assuming a constant electron mobility of 100 cm2/Vs, conductivities at room temperature were calculated and compared with experimental ones. The agreement between the experimental and calculated conductivities is very good for all the samples of undoped and Mn-doped ZnO. In the Mn-doped ZnO case, the ionisation energy of the Mn defect in ZnO was estimated to be ∼2.0 eV. Using the experimental conductivities and the calculated electron concentrations, the electron mobilities were calculated between 70 and 300 K. The results show that the temperature dependence of the mobility in undoped ZnO is similar to that in ZnO single crystals observed in other works, and heavy Mn doping significantly reduces the electron mobility below room temperature probably due to impurity scattering. The role of Mn on the electrical conductivity of ZnO could be understood.

Effect of Al and Mn doping on the electrical conductivity of ZnO

Abstract Samples of Al-doped and Mn-doped ZnO with a doping level up to 1.2 mol% were sintered at temperatures from 1100 to 1400°C in air. dc Electrical conductivities of these samples at room temperature and below were measured, and the effects of the doping type, the doping level, the sintering temperature and time on the electrical conductivity of ZnO were investigated. It was found that Al increased the electrical conductivity of ZnO resulting in a manifestation of a metallic electrical conduction behaviour, and a semiconductor-metal transition occured in the Al-doped ZnO samples. For Mn-doped ZnO samples quenched from the sintering temperatures, the electrical conductivity decreased with the increase in the Mn content, but the samples still showed a semiconductor electrical conduction behaviour. In this way, one could obtain a systematic variation of the ZnO electrical conductivity from the high conductivity, Al-doped case, to the high resistivity, Mn-doped one, spanning over eight orders of magnitude, which is explained in the present communication.

Hopping conduction in Mn-doped ZnO

The dc and ac conductivities of Mn-doped ZnO were investigated at temperatures from 10 to 100 K. The temperature dependence of the dc conductivity from 10 to 100 K shows an abrupt change at ∼18 K, manifesting a much lower activation energy for conduction below 18 K. From 10 to 18 K, the ac conductivity, σac(ω), varies as σac(ω)=Aωs in the frequency range from 102 to 106 Hz with s in the range of 0.6–1. The dc and ac conductivity observations suggest that the dominant conduction mechanism at temperatures between 10 to 18 K in these samples is a hopping conduction.

Deep donors in polycrystalline Mn-doped ZnO

In a previous work, significant varistor behaviour was found in polycrystalline ZnO with Mn as the only additive. In the present work, the admittance spectra of these Mn-doped ZnO samples were investigated in the temperature range from 70 to 300 K. From this study, the microscopic parameters of the deep bulk donors such as their energy levels and capture cross-sections were obtained in the samples with different Mn contents. The origin of the donors in ZnO is discussed by comparing their characteristics found in Mn-doped ZnO with those observed in other ZnO varistor systems.

Varistor behaviour of Mn-doped ZnO ceramics

Abstract Polycrystalline ZnO doped with MnO, from 0.1 to 0.6 mol%, was prepared by conventional ceramic processing. The effect of Mn doping on the electrical properties of ZnO was investigated. Samples quenched from the sintering temperature show ohmic behaviour, while pronounced varistor behaviour is found in Mn-doped ZnO obtained by slow cooling from the sintering temperature or by annealing at a lower temperature. The origin of the varistor effect in the samples of polycrystalline ZnO with MnO as the only additive is discussed. The defect equilibrium analysis suggests that the varistor behaviour in these samples is due to the oxidation of the double ionised zinc interstitial defects present at grain boundaries by ambient oxygen during cooling or annealing, and the presence of Mn in the ZnO grains induces this process.

Synthesis and dielectric properties of tungsten-based complex perovskites

Ba 2 MeWO 6 (Me = Mg, Ni, Zn) double perovskites were prepared by the conventional solid-state reaction in a wide temperature range. Single-phase ceramics were obtained only at low temperatures approximately 1200 °C, whereas a small amount of second phases existed in the samples sintered at higher temperatures. All the compounds are characterized by the cubic perovskite structure (space group Fm3m) with a complete NaCl type ordering between B-site ions. Anomalous temperature variation of the dielectric loss tangent found in the Ba 2 NiWO 6 perovskite is supposed to be connected with a dielectric relaxation due to electronic hopping within thermally activated Ni 3 + -6W ( 6 - 1 / 6 ) + /W 5 + -6Ni ( 2 + 1 / 6 ) + clusters. Dielectric measurements showed that the other two perovskites-Ba 2 ZnWO 6 and Ba 2 MgWO 6 -exhibit a positive value of the temperature coefficient of permittivity. Such temperature variation is assumed to be caused by a considerable influence of the second polar mode involving B-site ion vibrations on the low-frequency dielectric properties.

Interweaving domain configurations in [001]-poled rhombohedral phase 0.68Pb(Mg1/3Nb2/3)O3–0.32PbTiO3 single crystals

Domain structures in [001]-poled rhombohedral phase 0.68Pb(Mg1/3Nb2/3)O3–0.32PbTiO3 single crystals have been investigated using polarized light microcopy. It was found that the observed domain structures are quite different from those previously assumed four domain states with cross intersecting 71° charged domain walls. The pattern can be interpreted as interweaving of two types of twins in three dimensions. Each local twin pattern contains only two domains with a 109° charged domain wall in either [110] or [110]. The stacking of two 90° rotated twins produces a cross-hatching pattern for transmission optical microscope and an effective 4mm macroscopic symmetry. The pattern becomes simple twins when the imaging is focused at different depth. The domain size is 1–2 μm in all the samples observed and the twin layer thickness is about 50–100 μm.

Dielectric relaxation of shallow donor in polycrystalline Mn-doped ZnO

The dielectric properties of Mn-doped ZnO ceramics with electrically active grain boundaries at low temperatures of 10–70 K were investigated by admittance spectroscopy. It was observed that the dielectric relaxation of the main shallow donors, zinc interstitial, in these samples occurred in the temperature range of 20–50 K. Based on theoretical analysis, an expression for the admittance of an electrically active grain boundary at low temperatures was obtained. It was concluded that the relaxation of the shallow donors is Debye type with a thermally activated relaxation time and is related to the ionization process of these donors. The activation energy for the relaxation, i.e., the ionization energy of the shallow donors, was determined to be about 0.04 eV, the capture cross section of the shallow donors to be around 10−12 cm2, and the relaxation time at room temperature to be about 10−13 s.

Structural, dielectric and optical properties of barium strontium sodium niobate (Sr0.7Ba0.3)2NaNb5O15 single crystals

Barium strontium sodium niobate (Sr0.7Ba0.3)2NaNb5O15 (SBNN70) crystals were grown by the Czochralski method. X-ray energy dispersive spectrometry showed that SBNN70 crystals have a stoichiometric composition nearly identical to the starting materials. X-ray diffraction of crushed powder indicated that the SBNN70 crystals are orthorhombic with a = 1.2748 nm, b = 1.1402 nm and c = 0.3898 nm. The measured temperature dependence of dielectric constants showed strongly anisotropic relaxor behaviour. The Curie temperature of the SBNN70 crystals is 186.7uC, which is 116uC higher than that of the (SrxBa1−x)5Nb10O30 (SBN) crystal. The transmissivity and refractive index of [001] oriented SBNN70 single crystals were measured, and a deep energy level of 3.54 eV was found. The SBNN70 crystal has better than 70% optical transmittance for wavelengths above 500 nm. The refractive index of SBNN70 is 2.004 at 632.8 nm, which is lower than 2.3117 for SBN crystals.

Electric-field-induced dielectric anomalies in C-oriented 0.955Pb(Zn1/3Nb2/3)O3–0.045PbTiO3 single crystals

Electrical-field-induced dielectric anomalies in C-oriented 0.955Pb(Zn1/3Nb2/3)O3–0.045PbTiO3 single crystals were observed in both the dielectric constant and loss variations with temperature. Three distinct ordered phases are revealed, which are distinguished by their frequency dispersion behavior. We propose the existence of a medium-range ordered phase between the long-range and the short-range ordered phases, resulting from the competing features of the relaxor and ferroelectric constituents, Pb(Zn1/3Nb2/3)O3 and PbTiO3. Some dispersion is evident in this medium-range ordered phase but global polarization is still present. The assumption is rationalized by the measured relationship between the remnant polarization and temperature.