Chen Zheng-Hao

Unit-Cell by Unit-Cell Epitaxial Growth of SrTiO3 and BaTiO3 Thin Films by Laser Molecular Beam Epitaxy

Atomically regulated unit-cell by unit-cell homoepitaxial SrTiO3 and heteroepitaxial BaTiO3 films were fabricated on SrTiO3(100) substrates by laser molecular beam epitaxy. The fine streak patterns and more than 1000 cycles undamping intensity oscillation were obtained by in situ reflection high-energy electron diffraction with precisely controlled intermittence between each unit-cell layer. The surface morphology of the films was examined by atomic force microscopy. The root-mean-square surface roughness of the films is about 0.1 nm.

Large Third-Order Nonlinear Optical Susceptibility of Rh-Doped BaTiO3 Thin Films Prepared by Pulsed Laser Deposition

Rh-doped barium titanate (BaTiO3) epitaxial thin films have been fabricated on SrTiO3 (100) substrates by pulsed laser deposition. The nonlinear optical properties of the films were determined using the z-scan method at a wavelength of 532 nm with a laser duration of 10 ns. The real and imaginary parts of the third-order nonlinear susceptibility χ(3) were 5.71×10-7 esu and 9.59×10-8 esu, respectively. The real part value of χ(3) of the Rh:BaTiO3 films is about one order larger than that of Ce-doped BaTiO3 thin films. The results show that Rh:BaTiO3 thin films have great potential applications for nonlinear optical devices.

Superconducting MgB2 Thin Films with Tc≈39 K Grown by Pulsed Laser Deposition

Superconducting MgB2 thin films were fabricated on Al2O3(0001) substrates under ex situ processing conditions. Boron thin films were deposited by pulsed laser deposition followed by a post-annealing process. Resistance measurements of the deposited MgB2 films show Tc of ~39 K, while scanning electron microscopy and x-ray diffraction analysis indicate that the films consist of well-crystallized grains with a highly c-axis-oriented structure.Superconducting MgB2 thin films were fabricated on Al2O3(0001) substrates under an ex-situ processing conditions. Boron thin films were deposited by pulsed laser deposition followed by a post-annealing process. Resistance measurements of the deposited MgB2 films show Tc of about 39K, while scanning electron microscopy and x-ray diffraction analysis indicate that the films consist of well-crystallized grains with a highly c-axis-oriented structure.

Laser Molecular Beam Epitaxy Growth of BaTiO3 in Seven Thousands of Unit-Cell Layers

BaTiO3 thin films in seven thousands of unit-cell layers have been successfully fabricated on SrTiO3 (001) substrates by laser molecular beam epitaxy. The fine streak pattern and the undamping intensity oscillation of reflection high-energy electron diffraction indicate that the BaTiO3 film was layer-by-layer epitaxial growth. The measurements of scanning electron microscopy and atomic force microscopy show that surfaces of the BaTiO3 thin film are atomically smooth. The measurements of x-ray diffraction and transmission electron microscopy, as well as selected-area electron diffraction reveal that the BaTiO3 thin film is a c-oriented epitaxial crystalline structure.

Fe-doped BaTiO3 Thin Films with Large Third-Order Nonlinear Optical Susceptibility

Iron-doped BaTiO3 epitaxial thin films have been fabricated using the pulsed laser deposition technique. The target was an Fe/BaTiO3 two-material physical assembly. The films were grown on MgO (100) substrates at 650°C. The crystalline property of the films was studied by x-ray diffraction. The result indicated that the Fe-doped BaTiO3 films did not degrade their crystallinity at the dopant concentration of 4 mol%. The nonlinear optical properties of the films were determined using the z-scan method at the wavelength of 532 nm with a laser duration of 10 ns. The real and imaginary parts of the third-order nonlinear susceptibility χ(3) were determined to be 3.67×10-7 esu and 4.82×10-8 esu, respectively.

Structure and electrical characteristics of Nb-doped SrTiO3 substrates

The studies on structure and electrical characteristic of Nb-doped SrTiO3 (SNTO) substrates by XRD, Hall measurements, R-T and SEM, show that the SNTO substrates with the same Nb-doping concentration have different electrical properties because of the nonuniformity of Nb-doping. The uniformly doped high-quality substrates are extremely important for fabricating multilayer structure and devices. In addition, the experimental results also provide us with SEM method to judge the uniformity of conducting material.

Structure, Electrical, and Optical Properties of Nb-doped BaTiO3 Thin Films Grown by Laser Molecular Beam Epitaxy

Structure, electrical, and optical properties of Nb-doped BaTiO3 (Nb:BTO) thin films on MgO substrates grown by laser molecular beam epitaxy with increasing Nb content were investigated. The Nb:BTO thin films with high crystallinity are epitaxially grown on MgO substrates. With more Nb-doped content, the impurity phases are found in Nb:BTO thin films. Hall measurement at room temperature confirms that the charge carriers of the Nb:BTO thin films are n-type. When the Nb-doped content increases, the carrier concentration and carrier mobility increase. Meanwhile the optical transmittance decreases with the increase of the Nb-doping, and the width of the forbidden band in each group is not affected by the presence of Nb in the samples. Raman spectra show that the structural phase transition may occur with the increase of the Nb-doping content, in the meantime more defects and impurities exist in the Nb:BTO thin films.

Photoelectric Characteristic of La0.9Sr0.1MnO3/SrNb0.01Ti0.99O3 p–n Heterojunctions

Good rectifying current–voltage characteristics and nanosecond photoelectric effects are observed in the p–n heterojunctions of La0.9Sr0.1MnO3/SrNb0.01Ti0.99O3 fabricated by laser molecular beam epitaxy. The rise time is about 26 ns and the full width at half maximum is about 125 ns for the open-circuit photovoltaic pulses when the La0.9Sr0.1MnO3 film in the heterojunction is irradiated by a laser operated at wavelength 308 nm with pulse duration of about 25 ns. A qualitative explanation is presented, based on an analysis of the photoelectric effect of p–n heterojunction.

Simulation of Anisotropic Resistivity for Mixed-Phase Manganite La2/3Ca1/3MnO3 Thin Films

We utilize the random network model based on phase separation scenario to simulate the conductive behaviour and anisotropic characteristics of resistivity for La2/3Ca1/3MnO3 (LCMO) thin films. The simulated results agree well with our experimental data, showing a metal-to-insulator transition from a high-T paramagnetic (PM) insulating phase to a low-T ferromagnetic (FM) metallic phase in both the untilted film deposited on a (001) SrTiO3 (STO) substrate and the tilted film grown on a vicinal cut STO substrate. It is found that the resistivity of the tilted sample is higher than that of the untilted one, displaying prominent anisotropic characteristics. The studies reveal that the tilting not only decreases the conduction of the FM domains, but also increases the activation energy of the PM regions, inducing the enhancement of resistivity. All those results suggest that the intrinsic inhomogeneity in the phase separation system plays a significant role in the electrical conductivity and the resistive anisotropy is related to the structure of the crystal lattice.

N-Doped LaAlO3/Si(100) Films with High-k, Low-Leakage Current and Good Thermal Stability

High quality amorphous N-doped LaAlO3 (LaAlON) films have been deposited on Si (100) in nitrogen gas by laser molecular beam epitaxy. The chemical nature and physical distribution of N in LaAlON films has been performed by x-ray photoemission spectroscopy. Compared with LaAlO3, a significant improvement in the interfacial quality and leakage current density was obtained by nitrogen additive. At gate voltage +1 V, the leakage current density of the LaAlON film with an equivalent oxide thickness as thin as 2 nm is obtained to be 2.9×10−6 A/cm2, which decreases almost two orders of magnitude from that of LaAlO3 film with the same thickness. Moreover, high-resolution transmission electron microscope analysis show that the LaAlON sample is still amorphous with a very sharp interface between the LaAlON layer and the Si substrate after annealed at 900 degrees C for 60 s.