摘要:

We have measured ac complex impedance and thermal depolarization current for aged(K0.50Na0.50)2(Sr0.75Ba0.25)4Nb10O30ceramic samples to investigate the influence of mobile space charges on the ferroelectric properties. The impedance behavior was analyzed using an equivalent circuit composed of a capacitor connected in series with a parallel resistor-capacitor circuit. The dielectric relaxation is found to involve two mechanisms; one is attributed to the hopping process of the space charges bound inside grains and the other is associated both with the intrinsic polarization of the ferroelectric material and with the space charges at the grain boundary layer. The experimental results are also discussed in terms of the aging mechanism. It is suggested that the space charges bound inside grains play an important role in the aging process of the ferroelectric ceramics. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366063

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The cooling process encountered during fabrication of epitaxialPb1−xLaxTiO3(PLT,x=0.00–0.12) thin films prepared on MgO(001) single-crystal substrates is simulated using an x-ray high-temperature attachment, and x-ray diffraction measurements are performed at various temperatures, during which a ferroelectric 90° domain structure has been evolved. The lattice constants and the degree ofc-axis orientation (&agr;) of the epitaxial PLT films are evaluated as a function of temperature below the deposition temperature (650 °C). The 0.00 La-PLT film shows nucleation ofcdomains atTC,growth of them at the expense ofadomains belowTC,andc-domain dominant structure at room temperature. By contrast, the 0.12 La-PLT film shows an abrupt evolution of thec-domain dominant structure atTC,and a higher degree ofc-axis orientation at room temperature. The full width at half-maximum of the 003 perovskite peak is used to quantify the crystal quality of the films as a function of temperature. Considerable change in crystal quality occurs after the cubic-to-tetragonal phase transformation. Significant improvement of the crystal quality of the PLT films with high La concentration have been realized predominantly at the Curie temperature when the films transformed to the tetragonal phase. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366064

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

It is postulated that porous media is made up of fractal solid skeleton structure and fractal pore surface. The model thus developed satisfies measured anomalous dielectric behavior of three distinctly different porous media: kaolin, montmorillonite, and shaly sand rock. It is shown that the underlying mechanism behind dielectric dispersion in the kHz range to high MHz range is indeed Maxwell–Wagner mechanism but modified to take into account the multiphase nature of the porous media as opposed to the traditional two-phase Maxwell–Wagner charge accumulation effect. The conductivity of the surface water associated with the solid surface and charge accumulation across the surface irregularities, asperity, and bridging between particles at the micro-scale-level pores are shown to contribute to this modified Maxwell–Wagner mechanism. The latter is dominant at low frequencies. The surface water thickness is calculated to be about 2–6 nm for a variety of porous media. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366065

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The practicality of modeling the power law degradation observed in thin dielectrics after Fowler–Nordheim stress has been demonstrated on the basis of a generalized trapping approach with appropriate trap cross-section and density profiles. A detailed mathematical analysis of the negative bulk oxide charge kinetics has been established using incomplete Gamma and generalized hypergeometric functions, after assuming exponentially varying trap cross-section and density profiles throughout the oxide. These spatial distributions could be due to the structural nature of the oxide after growth. Moreover, the asymmetry of the charge distribution centroid for negative and positive gate bias stress has been satisfactorily interpreted by neglecting the trapping in the tunneling region near the cathode. Overall this generalized kinetic trapping model provides very good fitting of the variation of the trapped oxide charge with the injection dose for oxide thicknesses between 5.5 and 10 nm. The evolution of the charge centroid is also well predicted but with less accuracy, due to the presence of other concurrent charge generation processes associated with positive and/or negative charge buildup. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366539

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

SputteredBa1−xSrxTiO3(BST) andSrTiO3(STO) films and capacitors made with these dielectrics have been characterized with respect to physical and electrical properties. Specific capacitance values included a high of96 fF/&mgr;m2for BST films deposited of 600 °C and a high of26 fF/&mgr;m2for STO films deposited at 400 °C. Leakage current densities at 3.3 V for the most part varied from mid10−8to mid10−6 A/cm2.All of the dielectrics are polycrystalline, although the lowest temperature STO films have a nearly amorphous layer which impacts their capacitance. Grain size increases with deposition temperature, which correlates to higher dielectric constants. The lattice parameter of the BST films is larger than that of bulk samples. Capacitance, leakage, breakdown, and lifetime results are reported. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366066

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Three dimensional (3D) thermoluminescence (TL) spectra of two yttrium orthovanadate(YVO4)single crystals grown fromYVO4melt by the Czochralski (CZ) technique and one additionalYVO4crystal pulled from lithium-metavanadate(LiVO3)flux by the top seeded solution growth (TSSG) method were investigated after x-ray irradiation. The TL spectra were recorded in the 200–800 nm wavelength and 20–300 K temperature ranges before and after annealing of the crystals in an oxygen atmosphere. In spite of the different growth conditions the measured 3D TL spectra of the CZ samples show similar TL characteristics. The main TL emission appears around 450 nm, and between 200 and 250 K. Further weak emissions were also detected at 570, 600, 650, and 710 nm wavelengths, which might be attributed to unwanted impurities. The TL spectrum of the TSSG crystal markedly differs from that of the CZ crystals. On one hand the major TL appears at lower temperatures whereas on the other hand the emission spectrum exhibits a broader band around 500 nm, while the weak bands between 500 and 700 nm could not be detected. Based on these observations the different defect structures inYVO4crystals are discussed in the article emphasizing that 3D TL measurement is a useful practical tool for the characterization and identification ofYVO4crystals grown by different methods. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366067

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

This article demonstrates, from the theoretical point of view, that owing to the phonon-assisted relaxation rate of the excited electron-hole pair’s transiting to lower states decreases as the scale of Si particle is reduced, the optically excited electron-hole pair in the nanometer silicon particle with sufficient small scale generally tunnel into theSiOxlayers which enclose the Si particles and recombine radiatively through the luminescence centers there to emit visible light rather than recombine radiatively in the Si particles. It is proved also that when the density of the luminescence center is2.5×1022/m3,the upper limit of the average scale of the nanometer silicon particles in the ensemble capable of emitting red light in room temperature is around 8.7 nm, which is much bigger than the generally estimated value of 3 nm. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366068

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Photoluminescence (PL) spectra ofEr3+in an Er-doped silicon film were recorded at 2 and 77 K over the wavelength range from 1.52 to 1.68 &mgr;m, which includes optical transitions between theEr3+4I13/2and4I15/2manifolds. These spectra were then analyzed to determine the nature of the electrostatic field at the site of the Er ion that governs the ion’s optical behavior. The PL spectra were analyzed three different ways: by considering theEr3+ions to be occupying sites ofTd,C3,orD2dpoint group symmetry. Transition energies and magnetic dipole transition probabilities were calculated for each case by diagonalizing a Hamiltonian representing the free-ion and crystal-field interactions in a Russell–Saunders basis of states spanning the lowest 15Jmultiplets of the4f11electronic configuration ofEr3+.Crystal-field parameters were varied in each case to find the best agreement with the experiment. Our results showed that the site symmetry of theEr3+ions most consistent with the data isD2d.For this case, the rms deviation between the experimental and calculated energy levels is3.3 cm−1with the calculated magnetic dipole transition probabilities in qualitative agreement with the experiment. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366069

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The optical density change of the CuCl-doped borosilicate glasses(2 wt&percent; CuCl+52.5SiO2−40B2O3−7.5Na2O(wt&percent;))heat treated at 550 °C for 0–10 hrs was measured with temperature at spectral vicinity of the CuCl resonant absorption and the effect of size and distribution of CuCl precipitate particles on the optical spectra was investigated. The optical absorption spectrum obtained during heating and cooling cycle showed a typical hysteresis loop because of the melting and solidification of the CuCl particles embedded in the glass matrix. The shape of the optical absorption curve measured at 375 nm with temperature upon melting of CuCl particles was dependent on the size and the distribution of the CuCl particles. While the average size of the CuCl particles increased with heat treatment time, the size difference of the precipitated CuCl particles was found to be decreased with time before 1 h and then increased. The optical absorption spectra of the CuCl doped glass was also predicted theoretically and the predicted spectra curve was found to be in good agreement with experimental data. It is proposed that if the semiconducting particles embedded in an optically transparent matrix and the melting point of semiconductor is lower than that of the matrix material, the average size and size distribution of the particles can be estimated by the optical absorption measurements with temperature. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366070

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The normal emissivity of liquid germanium was determined from the direct measurement of the thermal radiation from the liquid surface and a blackbody. The monochromatic emissivity in the visible region and the average emissivity in the near infrared region were measured to investigate the wavelength dependence. The emissivity of the liquid has little dependence on the wavelength and is0.217±0.002for the visible region (600–900 nm) and is0.200±0.005for the near infrared region at the melting point. The emissivity has a weak dependence on the temperature from 910 to 1160 °C. The weak dependence agrees well with a dielectric response of the free electrons. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366071

出版商:AIP    

年代:1997

数据来源: AIP