Zhenghao Chen

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

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.

Phase transitions and polarizations in epitaxial BaTiO3/SrTiO3 superlattices studied by second-harmonic generation

BaTiO3/SrTiO3 superlattices are found to have a critical periodicity of 30 SrTiO3 unit cells, below which the room-temperature symmetry is mm2 orthorhombic. The orthorhombic-tetragonal phase-transition temperature is shifted upward from the bulk value (<3 K and normally not observed at finite temperature) of the quantum paraelectric pure strontium titanate to temperatures above ambient (599 K for 30/30-unit-cell superlattices). Antidipole patterns appear in superlattices with unit cells below 10/10. A model is presented that accounts quantitatively for the change in second-harmonic generation intensity with superlattice periodicity from 4 unit cells to 50.

Structural behavior of thin BaTiO3 film grown at different conditions by pulsed laser deposition

Thin epitaxial BaTiO3 (BTO) films having a high crystallinity and uniformity of grains were deposited on the SrTiO3 (STO) substrates. When the oxygen pressure is from 0.04 to 70Pa the lattice constant decreases from 4.1100 to 3.9972 Angstrom and the orientation normal to the substrate changes from (001) to (100). The surface topography changes from Bat to hilly with a surface roughness (rms) changing between 1.2 nm at 0.7 Pa, 3.4 nm at 7 Pa and 6.4 nm at 70 Pa. At the substrate temperature of 550, 750 and 850 degrees C the surface topography of the films varies from corrugated structure to rectangular grain and the surface roughness increases from 1.2, 3.4 to 3.4 nm. The epitaxial BTO films were also deposited on the YBCO/LaAlO3 (YBCO/LAO) substrates at 750 degrees C and 7 Pa. The films have a rough surface (rms=29.1 nm), but a good uniformity of grains. The c-axis oriented BTO films with a poor crystallinity were deposited on the MgO substrates at 750 degrees C and 7 Pa. The films have a smooth surface (rms=1.0 nm), but a poor uniformity of grains. The interfaces between the BTO films and the substrates were determined by transmission electron microscopy (TEM).

Linear and nonlinear optical properties of Ag nanocluster/BaTiO3 composite films

To investigate the optical properties of metallic nanocluster/oxide composite films, Ag nanocluster/BaTiO3 composite films were synthesized on MgO (100) substrates by co-depositing the Ag and BaTiO3 targets using pulsed laser deposition. The x-ray diffraction results demonstrated that the Ag and BaTiO3 were well crystallized. The x-ray photoelectron spectroscopy analysis indicated that the samples were composed of nano-metal Ag embedded in the BaTiO3 matrices. The optical absorption properties were measured from 300 to 800 nm, and the absorption peaks due to the surface plasmon resonance of Ag particles were observed. With increasing the Ag concentration, the peak absorption increased and shifted to longer wavelength (redshift). Furthermore, the z-scan results showed that the films exhibited large optical nonlinearities and the signs of the nonlinear absorption (β) and nonlinear refractive index (n2) changed with increasing the Ag concentration.

Picosecond photoelectric characteristic in La0.7Sr0.3MnO3∕Si p-n junctions

Ultrafast photoelectric effects have been observed in La0.7Sr0.3MnO3∕Si p-n junctions fabricated by laser molecular-beam epitaxy. The rise time was ∼210ps and the full width at half-maximum was ∼650ps for the photovoltaic pulse when the junction was irradiated by a 1064nm laser pulse of 25ps duration. The photovoltaic sensitivity was as large as 435mV∕mJ for a 1064nm laser pulse. No such photovoltaic signal was observed with irradiation from a 10.6μm CO2 laser pulse. The results reveal that this phenomenon is an ultrafast photoelectric effect.

Characteristics of LaAlO3/Si(100) deposited under various oxygen pressures

High-k dielectric LaAlO3 (LAO) thin films were deposited by laser molecular-beam epitaxy under various oxygen pressures. X-ray diffraction showed that the LAO films were amorphous. The chemical compositions and thickness of the LAO thin films were measured using Rutherford backscattering spectrometry. Al/LAO/Si metal–oxide–semiconductor capacitor structures were fabricated and measured. The leakage current density versus voltage curves showed that the leakage current of the LAO thin films decreased by increasing the oxygen pressure of the preparation. Specifically, when prepared under the active oxygen (containing atom oxygen), the leakage current of the LAO sample was lower than that prepared under the general oxygen. All of the capacitance–voltage curves have a positive shift along the voltage axis and the flatband voltage decreased with increasing oxygen pressures during the LAO thin-film preparation.

Transient lateral photovoltaic effect in p-n heterojunctions of La0.7Sr0.3MnO3 and Si

A transient lateral photovoltaic effect (LPVE) has been observed in p-La0.7Sr0.3MnO3∕n-Si heterojunctions. Under the nonuniform irradiation of a pulsed laser, the LPVE shows high sensitivity to the spot position on the La0.7Sr0.3MnO3 surface. A mechanism based on the well established model for the LPVE in conventional semiconductors has been applied to explain the LPVE in the heteroepitaxial junctions of perovskite-type metal oxides. The large LPVE in the heteroepitaxial junctions is expected to make the perovskite-type metal oxide a new and faster candidate for position-sensitive photodetectors.

Z-scan determination of the large third-order optical nonlinearity of Rh : BaTiO3 thin films deposited on MgO substrates

Highly c-axis oriented Rh-doped BaTiO3 thin films were grown on MgO (100) substrates by pulsed laser deposition using a single-crystal Rh:BaTiO3 target. The 150-nm thick films were deposited at 800degreesC under the oxygen pressure of 7 x 10(-2) Pa. The structural properties of the samples were characterized by X-ray diffraction and atomic force microscopy. The full width at half maximum of the (002) Rh: BaTiO3 rocking curve and the root-mean-square surface roughness within the 5 x 5 mum(2) area were 0.520degrees and 0.85 nm, respectively. The nonlinear optical properties of the films were determined by a single beam Z-scan at a wavelength of 532 nm with laser duration of 10 ns. The result shows that Rh:BaTiO3 thin films exhibit a great nonlinear optical response with the real and imaginary parts of the third-order nonlinear optical susceptibility chi((3)) being 3.59 x 10(-7) esu and 4.01 x 10(-8) esu, respectively

Ultraviolet fast-response photoelectric effect in tilted orientation SrTiO3 single crystals

Ultraviolet photoelectricity based on the vicinal cut as-supplied SrTiO3 single crystals has been experimentally studied in the absence of an applied bias at room temperature. An open-circuit photovoltage of 130 ps rise time and 230 ps full width at half maximum was observed under the irradiation of a 355 nm pulsed laser of 25 ps in duration. The dependence of the photoelectric effect on the tilting angles was studied, and the optimum angle is 20.9 degrees. Seebeck effect is proposed to elucidate the tilting angle dependence of laser-induced photovoltage. This work demonstrates the potential of SrTiO3 single crystals in ultraviolet detection. (c) 2006 American Institute of Physics.

Ultraviolet photovoltage characteristics of SrTiO3−δ∕Si heterojunction

A photovoltaic effect is observed in the heterostructure of p-Si∕n-SrTiO3−δ (p: hole carrier type, n: electron carrier type). The current–voltage curve exhibits a good rectifying characteristic similar to that of the traditional diode. The junction shows the open circuit voltage of 126mV∕mJ, the short circuit current of 1.78mA∕mJ, and the response time faster than 10ns for ultraviolet pulsed laser of 25ns in duration at room temperature, suggesting the promising potential of this junction as a new type of ultrafast ultraviolet detectors with high sensitivity for application.