Bruce W. Wessels

Epitaxial growth of BaTiO3 thin films by organometallic chemical vapor deposition

Epitaxial BaTiO3 thin films were grown in situ on (100) LaAlO3 by low‐pressure organometallic chemical vapor deposition using the precursors Ba (hexafluoroacetylacetonate)2 (tetraglyme) and titanium tetraisopropoxide. The phase composition and epitaxial quality were sensitive to the reactant partial pressures and growth temperature. Deposition at 800 °C yielded [100]‐oriented BaTiO3 films. In‐plane epitaxy was confirmed for the BaTiO3 films by x‐ray diffraction.

Optical properties of the deep Mn acceptor in GaN:Mn

The optical and electrical properties of Mn-doped epitaxial GaN were studied. Low-temperature optical absorption measurements indicate the presence of a Mn-related band with a well-resolved fine structure. The zero-phonon line is at 1.418±0.002 eV with a full width at half maximum of 20±1 meV. Two pseudolocal vibrational modes associated with manganese were observed with energies of hv1=20 and hv2=73 meV. Deep-level optical spectroscopy measurements on lightly Mn-doped samples indicate that Mn forms a deep acceptor level at Ev+1.42 eV. Using the vacuum referred binding energy model for transition metals and the measured Mn energy level, the electron affinity of GaN is calculated to be 3.4 eV, which agrees well with experimental values.

Dielectric properties of epitaxial BaTiO3 thin films

The dielectric response of epitaxial BaTiO3 thin films deposited on MgO was measured through surface electrodes as a function of applied bias, frequency, and temperature. The room temperature value of the dielectric constant was ∼500 with a dissipation factor, tan(δ), of 0.05 at 100 kHz. Measurements varying the bias field showed hysteresis of the dielectric response and a tunability of 30% for a maximum applied field of ∼7 MV/m. The frequency response of the dielectric constant is well described by a Curie–von Schweidler power law with an exponent ∼0.04 in the range 1 kHz–13 MHz. The films undergo a diffuse phase transition at temperatures higher than the bulk Curie temperature. The behavior of the dielectric response is attributed to the presence of residual strain in the epitaxial thin films.

Thin-film channel waveguide electro-optic modulator in epitaxial BaTiO3

We report on a thin-film channel waveguide electro-optic modulator fabricated in epitaxial BaTiO3 on MgO. Films had an effective dc electro-optic coefficient of reff∼50±5 pm/V and reff∼18±2 pm/V at 5 MHz for λ∼1.55 μm light. Extinction ratios of 14 dB were obtained. The electro-optic effect decreases to ∼60% of the dc value at 1 Hz, 50% of the dc value at 20 kHz, and ∼37% of the dc value at 5 MHz.

Organometallic chemical vapor deposition of high Tc superconducting films using a volatile, fluorocarbon-based precursor

Uniform films of the high Tc superconductor YBa2Cu3O7−δ have been prepared by organometallic chemical vapor deposition using the volatile metalorganic precursors Cu(acetylacetonate)2, Y(dipivaloylmethanate)3, and Ba(heptafluorodimethyloctanedionate)2. With argon as a carrier gas and water vapor as a reactant, film growth rates of 10–30 nm/min are achieved. After annealing under oxygen, energy dispersive x‐ray analysis, profilometry, and x‐ray diffraction data reveal that such YBa2Cu3O7−δ films on [100] single‐crystal MgO have good compositional and dimensional uniformity as well as preferential orientation of crystallite c axes perpendicular to the substrate surface. Four‐probe resistivity measurements reveal the onset of superconductivity at ∼90 K and zero resistance by 66.2 K.

Second‐harmonic generation of poled BaTiO3 thin films

Second‐harmonic generation of 1.064 μm incident light was measured on BaTiO3 thin films prepared by metalorganic chemical vapor deposition. Upon corona poling of the film, the second‐harmonic signal was significantly enhanced. The second‐order nonlinear optical susceptibility, d, of the poled film reaches ∼7.5 times that of the quartz. The enhanced second‐harmonic generation shows a very slow decay at room temperature.

Photoluminescence band near 2.9 eV in undoped GaN epitaxial layers

The broad photoluminescence band with a maximum at about 2.9 eV widely observed in undoped epitaxial GaN is studied as a function of temperature and excitation intensity. We attribute the band to transitions from a shallow donor to a deep localized acceptor. The zero-phonon transition for this band is at 3.098 eV as determined from the fine structure at low temperatures. A local vibrational mode in the ground state with an energy of 36 meV is found.

Investigation of the formation of the 2.8 eV luminescence band in p-type GaN:Mg

The stability of defects present in GaN:Mg has been investigated using photoluminescence (PL) spectroscopy. The two dominant defect-related PL emission bands in p-type GaN were investigated, the blue band at 2.8 eV and the ultraviolet (UV) emission band at 3.27 eV. The intensity of the 3.27 eV PL band increases with increasing resistivity, whereas the 2.8 eV PL band intensity increases with a decrease in resistivity. The luminescence data is explained by a model whereby the concentration of luminescent centers depends on the Fermi level position. The shallow donor responsible for the UV band is attributed to hydrogen, whereas the deep donor defect responsible for the 2.8 eV band is attributed to a nitrogen vacancy complex.

THIN FILM CHANNEL WAVEGUIDES FABRICATED IN METALORGANIC CHEMICAL VAPOR DEPOSITION GROWN BATIO3 ON MGO

We report on the fabrication of channel waveguides in epitaxial grown BaTiO3 layers on MgO. Layers were prepared by metalorganic chemical vapor deposition. Ridge waveguides with ridge heights ranging from 15 to 200 nm were fabricated in a 0.2‐μm‐thick film. Single mode waveguide throughput, scattering loss, and mode profiles are reported. Coating waveguides with spin on glass significantly increase waveguide throughput. Throughputs of up to 10.4% were measured in 15 nm ridge waveguides which were 2.85 mm long and coated with spin on glass. Waveguide throughput is found to increase significantly with an increase in wavelength from 1.06 to 1.55 μm.

Dimensional Reduction: A Design Tool for New Radiation Detection Materials

John Androulakis , Sebastian C. Peter , Hao Li , Christos D. Malliakas , John A. Peters , Zhifu Liu , Bruce W W. essels , Jung-Hwan Song , Hosub Jin , Arthur J. reeman , F and Mercouri G. Kanatzidis *