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15 May 2000

Volume 87, Issue 10, pp. 7147-7597

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Mechanistic study of metalorganic chemical vapor deposition of (Ba,Sr)TiO3 thin films

Y. Gao, C. L. Perkins, S. He, P. Alluri, T. Tran, S. Thevuthasan, and M. A. Henderson

J. Appl. Phys. 87, 7430 (2000); http://dx.doi.org/10.1063/1.373005 (8 pages) | Cited 6 times

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The metalorganic precursor chemistry was studied on Pt(111) surfaces in a 16O2 and 18O2 backgrounds. Using temperature programmed desorption (TPD) and static secondary ion mass spectrometry (SIMS). The precursor chemistry of Sr(thd)2 was found to be different on oxide covered Pt(111) surface as compared to the clean Pt(111) surface. In an oxygen ambient, TPD showed at least four different reaction processes which involved the removal of carbon from the precursor ligands on oxide covered Pt(111). In two of these, gas phase oxygen was incorporated into the oxidative products. In contrast, one carbon removing reaction was observed on the clean Pt(111) surface. Isotopic labeling experiments have also been carried out to understand the film-formation reactions in the metalorganic chemical vapor deposition of (Ba,Sr)TiO3 (BST) films. Time-of-flight SIMS and nuclear reaction analysis reveal that the oxygen in the BST films originates from both the gas phase oxidants (18O) and the precursor ligands (16O). The ligand substitution by gas phase O2 plays a more prominent role in the film formation at lower temperatures. On the other hand, the reactive oxygen radicals produced by microwave plasma involved more in breaking the O–C bonds than substituting the precursor ligands for the film formation. Use of the 50% 18O2–50% N2 16O2 mixture results in a reduction of 18O incorporation in the BST films, indicative of the direct involvement of N2O in the film-formation reactions. The mechanistic studies are essential for understanding the new BST precursors used in this study, and provide useful information to correlate the film microstructure, step coverage, and dielectric properties with the precursor properties. © 2000 American Institute of Physics.
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81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
68.03.Fg Evaporation and condensation of liquids
68.43.Mn Adsorption kinetics
82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI)

Domain configurations in domain engineered 0.955Pb(Zn1/3Nb2/3)O3–0.045PbTiO3 single crystals

Jianhua Yin and Wenwu Cao

J. Appl. Phys. 87, 7438 (2000); http://dx.doi.org/10.1063/1.373006 (4 pages) | Cited 61 times

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We report an optical microscopy study on the domain structures of 0.955Pb(Zn1/3Nb2/3)O3–0.045PbTiO3 rhombohedral phase single crystals poled along [001] of the cubic coordinates. Both charged and uncharged domain walls are observed in domain engineered samples and they could join together to form “L” and “T” shaped domain walls, which are very unique. The observed domains are between 10 and 100 μm in size, which are much bigger than expected. Most of the samples have only two of the four possible degenerate domain states, making the macroscopic symmetry to be mm2 or lower rather than 4mm previously assumed. © 2000 American Institute of Physics.
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77.80.Dj Domain structure; hysteresis
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates

Thickness and oxygen pressure dependent structural characteristics of BaTiO3 thin films grown by laser molecular beam epitaxy

Tong Zhao, Fan Chen, Huibin Lu, Guozhen Yang, and Zhenghao Chen

J. Appl. Phys. 87, 7442 (2000); http://dx.doi.org/10.1063/1.373007 (6 pages) | Cited 47 times

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A series of BaTiO3 thin films with various thicknesses from 10 to 400 nm were epitaxially grown under various oxygen pressures from 2×10−4 to 12 Pa on SrTiO3 (001) substrates using laser molecular beam epitaxy. Being confirmed by reflection high energy electron diffraction, atomic force microscopy, x-ray diffraction, and high resolution transmission electron microscopy the epitaxial single crystal BaTiO3 thin films are highly c-axis or a-axis oriented with a root-mean-square surface roughness of 0.14 nm. The observed thickness and oxygen pressure dependent structural characteristics of the BaTiO3 thin films are discussed by taking into account both the misfits in thermal expansion and lattice constants between BaTiO3 films and SrTiO3 substrates, and the effect of the energy of the sputtered particles, which is consistent with the established strain relaxation mechanism. An abnormal expansion of lattice volume of a BaTiO3 unit cell is found and attributed to the effect of oxygen vacancies in the BaTiO3 films. © 2000 American Institute of Physics.
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68.55.-a Thin film structure and morphology
65.40.De Thermal expansion; thermomechanical effects
77.55.-g Dielectric thin films
77.80.-e Ferroelectricity and antiferroelectricity
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
61.72.J- Point defects and defect clusters
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

Stress-induced enhancement of second-order nonlinear optical susceptibilities of barium titanate films

Tong Zhao, Huibin Lu, Fan Chen, Guozhen Yang, and Zhenghao Chen

J. Appl. Phys. 87, 7448 (2000); http://dx.doi.org/10.1063/1.373008 (4 pages) | Cited 16 times

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A method introducing stress is suggested to enhance optical nonlinearity. Three stress induced samples of BaTiO3 thin film, BaTiO3/SrTiO3 superlattice, and cerium doped BaTiO3/SrTiO3 superlattice were epitaxially grown on SrTiO3 (100) substrates by laser molecular beam epitaxy. The measurement of the dependence of the effective second-harmonic generation (SHG) coefficients on the fundamental polarization angle and incident angle indicates that SHG coefficients d33 of the three samples are greatly enhanced, with the maximum value being one order of magnitude larger than that of the bulk BaTiO3 crystal. Our results clearly show that the introduction of stress is a very powerful means to enhance nonlinear optical susceptibility. © 2000 American Institute of Physics.
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78.66.Nk Insulators
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
42.65.An Optical susceptibility, hyperpolarizability
78.20.hb Piezo-optical, elasto-optical, acousto-optical, and photoelastic effects
77.22.Ej Polarization and depolarization
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Dielectric relaxation processes in Cd2Nb2O7 compound

Chen Ang, Ruyan Guo, A. S. Bhalla, and L. E. Cross

J. Appl. Phys. 87, 7452 (2000); http://dx.doi.org/10.1063/1.373009 (5 pages) | Cited 25 times

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This article reports a study on the dielectric relaxation processes of Cd2Nb2O7 compound. Three dielectric relaxation modes I, II, and III were reexamined. By using the Cole–Cole equation fitting of the frequency dependence of dielectric constant and loss, we obtained more precise relaxation time data, compared to the data reported in the literature. The results indicate that the relaxation time for mode I follows the Arrhenius law with one slope rather than two slopes as reported in the literature in the frequency range of 102–105 Hz. However, the parameters obtained from the Arrhenius law fit are not physically reasonable. More physically reasonable parameters can be obtained by fitting the relaxation time to the empirical Vogel [Z. Phys. 22, 645 (1921)]–Fulcher [J. Am. Ceram. Soc. 8, 339 (1925)] relation with essentially the same fitting quality. A comparison of Cd2Nb2O7 with well known triglycine sulphate (TGS) is made. The similarity between the dielectric response of Cd2Nb2O7 and TGS is emphasized. The physical mechanism of the relaxation modes is briefly discussed. With increasing dc bias, the three relaxation modes were suppressed and eventually eliminated. This indicates that the dielectric responses of Cd2Nb2O7 at zero dc bias are the sum effect of several dielectric modes with relaxation polarization superimposed on the ferroelectric–paraelectric phase transition. © 2000 American Institute of Physics.
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77.22.Gm Dielectric loss and relaxation
77.22.Ch Permittivity (dielectric function)
77.22.Ej Polarization and depolarization
77.80.B- Phase transitions and Curie point

Studies of ambient dependent electrical behavior of nanocrystalline SnO2 thin films using impedance spectroscopy

Oomman K. Varghese and L. K. Malhotra

J. Appl. Phys. 87, 7457 (2000); http://dx.doi.org/10.1063/1.373010 (9 pages) | Cited 30 times

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Impedance spectroscopy has been used to investigate the properties of sol-gel derived ultrafine grained tin oxide thin films subjected to various ambients and temperatures. The impedance measurements have been carried out in the frequency range 250 kHz–10 mHz and Cole–Cole plots drawn. Equivalent circuit models have been suggested and fitting of the experimentally obtained data done on the basis of the approach of “universal dielectric behavior” introduced by Jonscher. The films have shown strong interaction with water vapor in the atmosphere at room temperature (300 K) as reflected from the changes in the values of resistive and capacitive components in the equivalent circuits at different humidity levels. The conduction mechanism appears to be due to the transfer of protons through the physisorbed layer of water molecules. A low frequency spur has been observed in the impedance diagram at medium and high humidity levels and attributed to the migration of adsorbed ions towards the electrode sample contact region under the influence of the electric field. These ions accumulate in the electrode sample contact region, giving rise to a non-Debye capacitance. The spur in the impedance diagram disappears on heating the films to 373 K due to the desorption of water molecules from the surface. The films thereafter exhibit semiconducting behavior till 473 K. A reversal in behavior has been observed during further increase of temperature due to the conversion of O2− ions to highly active O− ions which abstract electrons from the grain bulk. At very high temperatures in excess of 573 K, the spur in the impedance diagram reappears, but this time due to accumulation of the adsorbed O− ions, thereby giving rise to another non-Debye capacitance in the electrode sample contact region. The spur disappears in the presence of ethanol due to removal of oxygen ions by ethanol molecules. The films on float glass have exhibited large impedance changes in presence of ethanol but negligible changes in presence of other reducing gases like H2 and liquefied petroleum gas. © 2000 American Institute of Physics.
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73.61.Le Other inorganic semiconductors
68.03.Fg Evaporation and condensation of liquids
68.43.Mn Adsorption kinetics
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