We address the issue of decreasing band-gap with increasing atomic number, inherent in semiconducting materials, by introducing a concept we call dimensional reduction. The concept leads to semiconductor compounds containing high atomic number elements and simultaneously exhibiting a large band gap and high mass density suggesting that dimensional reduction can be successfully employed in developing new γ-ray detecting materials. As an example we discuss the compound Cs2Hg6S7 that exhibits a band-gap of 1.65eV and mobility-lifetime products comparable to those of optimized Cd0.9Zn0.1Te.

The defect and interfacial structure in a Fe3O4/BaTiO3 heteroepitaxial bilayer was investigated by scanning transmission electron microscopy. The results show that the Fe3O4 film grew epitaxially on BaTiO3. The orientation relationship between Fe3O4, BaTiO3 and MgO is [100]Fe3O4//[100]BaTi3O//[100]MgO and (010)Fe3O4//(010)BaTiO3//(010)MgO. An initial interfacial nucleation layer was formed that partially accommodated the lattice mismatch strain between BaTiO3 and MgO. This investigation indicates that the formation of this buffer layer provides a high-quality BaTiO3 surface for subsequent Fe3O4 growth, resulting in a semicoherent interface. The Fe3O4 surface is nearly atomically abrupt (roughness Rrms = 0.78 nm). The Fe3O4 film exhibits magnetic domains with a diameter in the range of 0.4–2 μm.

Two dimensional photonic crystal (PhC) microcavity structures were fabricated using epitaxial ferroelectric thin film as the optical media and their resonant optical properties were measured. The PhC structures are utilized in order to achieve strong light localization to enhance the interaction between the incident light and the nonlinear optical barium titanate (BTO). Fluorescence measurements were used to assess the resonant properties. Two types of resonant structures were investigated consisting of either dopant or vacancy PhC arrays. The nano patterning on BTO thin films was achieved using dual beam focused ion beam (FIB). For the dopant type PhC microcavity structure, a larger air hole is generated in every 5 x 5 unit cells forming a super cell. The spatial profiles of PhC microcavity structures are characterized by laser scanning confocal microscopy. Structures with a feature size approaching the optical diffraction limit are clearly resolved. Fluorescence measurements on PhCs coated with a fluorescent dye were carried out to determine the relationship between the degree of light localization and the photonic band structure. Enhanced fluorescence at wavelengths 550-600 nm is observed in the dye covered PhCs with lattice period a =200 nm and a =400 nm. The large fluorescence enhancement results from the presence of PhC stop bands that increase the emission extraction efficiency due to the strong light confinement. Since the only allowed propagation direction for the scattered fluorescence light is out-of-plane, this enhances the vertically fluorescent extraction efficiency. These BTO optical microcavity structures can potentially serve as active nano-photonic components in bio-sensors and integrated photonic circuits.

We report the nanofabrication of Bragg reflector waveguide structure based on barium titanate (BTO) epitaxial thin film grown on MgO substrate and characterized its optical transmission. A low pressure (~1 Torr) nano-lithography was used for the formation of Bragg reflector waveguide structure. We demonstrate that there is sufficient refractive index contrast to form a well-defined stop band by forming gratings in a top strip layer of Si3N4 that was grown on BTO thin film. Simulation of the optical transmission and stop band of TE mode, and electro-optic (EO) modulator tunability are discussed as well. The non-linear photonic crystal waveguides are potentially suitable as tunable optical filters and ultra-wide bandwidth modulators.

Photonic crystal (PhC) can potentially result in enhanced optical properties around critical points in the photonic band gap. Of interest here are enhanced nonlinear optical effects. In this study, one and two dimensional nonlinear PhC were designed and fabricated using barium titanium oxide (BTO) thin films as the active medium. Nonlinear PhC made with barium titanate thin films potentially provide integrated devices with the advantages of wide tunability and high stability. Films 500 nm thick deposited on MgO substrates were utilized. Two dimensional PhC structures were defined by focused ion beams (FIB). Before patterning, a thin metal layer was deposited on the barium titanate layers in order to improve the conductivity of the samples. After writing the patterns, cylindrical air holes were generated in the thin film layers. The PhC lattice constant and the hole radius were selected in sub-micron region in order to satisfy the requirement of wave resonance. The PhCs with sub-micron features were characterized by the atomic force microscopy (AFM), scanning electron microscopy (SEM), and near field optical microscopy (NSOM). The transmission spectra of the PhC waveguides were calculated with a continuous wide band source that covered 1 to 2 micron wavelength. Simulations of the transmission characteristics were performed using the two dimensional finite difference time domain method (FDTD).

The dielectric properties of high k dielectric thick films prepared by thermal spray were investigated. BaTiO3 and Ba0.68Sr0.32TiO3 thick films were deposited using plasma spray on Ag-Pd screen-printed alumina substrates. The sprayed films were predominantly polycrystalline but contained an amorphous second phase. The dielectric constants of the films decreased with decreasing film thickness in 10 ∼ 60 μm range. This was attributed to the presence of an interfacial layer between the film and the substrate as determined by capacitance measurements. The capacitance density of the interfacial layer was determined to be ∼ 2.7 nF/cm2. The capacitance density of the interfacial layer increased to 12 nF/cm2 after post heat treatment at 500 °C for 20 hours.

Deep level defects formed in p-type GaN:Mg codoped with shallow donors have been investigated by photoluminescence (PL) spectroscopy. A donor-acceptor pair (DAP) luminescence band peaked at 2.45 eV dominates the room temperature PL spectrum in heavily codoped epilayers. A superlinear dependence of PL intensity on excitation density is observed for this band, with an exponent of 1.4∼1.7. The intensity of this band increases with increasing temperature with a maximum at 264K. To explain the luminescent behavior a DAP model was developed whereby the recombination involves a deep donor and shallow Mg acceptor. The deep donor is tentatively attributed to a DX center.

The factors affecting optical gain were studied for Er-doped BaTiO3 thin film waveguides. Er-doped BaTiO3 with dopant concentrations of 0.3 – 9 at.% was deposited by metal-organic chemical vapor deposition. The luminescence efficiency was maximized by optimizing the growth temperature and erbium concentration as well as by post-deposition annealing. Stimulated emission was studied using the pump-probe technique over the spectral range of 1,520-1,550 nm. A maximum differential gain of 3 dB/cm was measured at 1,540 nm in an 8 mm long, 8 μm wide ridge waveguide.

The factors affecting optical gain were studied for Er-doped BaTiO3 thin film waveguides. Er-doped BaTiO3 with dopant concentrations of 0.3 – 9 at.% was deposited by metal-organic chemical vapor deposition. The luminescence efficiency was maximized by optimizing the growth temperatureand erbium concentration as well as by post-deposition annealing. Stimulated emission was studied using the pump-probe technique over the spectral range of 1,520-1,550 nm. A maximum differential gain of 3 dB/cm wasmeasured at 1,540 nm in an 8 mm long, 8 μm wide ridge waveguide.

The microstructure and dielectric properties of BaTiO3 and Ba0.68Sr0.32TiO3 thick films deposited by thermal spray were investigated. The as-deposited films were predominantly crystalline with a small amount of an amorphous second phase. The as-deposited BaTiO3 films had dielectric constants as high as 240 at room temperature. Upon annealing in air at 500°C, the dielectric constant increased to 480. This increase in dielectric constant was attributed, in part, to the crystallization of the amorphous second phase. The stabilized phase depended on the spray process used. The high velocity oxy fuel (HVOF) spray process resulted in the deposition of paraelectric BaTiO3, whereas the plasma spray process resulted in the ferroelectric phase.

The nonlinear optical properties of epitaxial KNbO3 were studied for films grown by metalorganic chemical vapor deposition on different substrates. X-ray diffraction revealed epitaxial KNbO3 with a (110) orientation for layers deposited on single crystal strontium titanate. Whereas, films grown on (100) spinel exhibited a multi-variant structure, consisting of the (110) and (001) orientations. The volume fraction of each crystallographic variant in the film was calculated. Second harmonic generation measurements at 532 nm were performed to determine the ferroelectric domain variants present. The Maker-fringe technique was employed to obtain the effective nonlinear optical susceptibility of the films, which ranged from 0.1 - 1.8 pm/V. The SHG intensity profiles reveal a strong dependence upon substrate type.

Deep level defects in Mg compensated GaN grown by metal-organic vapor phase epitaxy were investigated using photocapacitance spectroscopy measurements on Schottky barrier diodes. Addition of magnesium resulted in the formation of a series of deep centers with optical threshold energies of 1.0, 1.2, 1.8, and 3.1 eV. Upon annealing the epitaxial GCN in nitrogen at 850°C the ind-gap levels disappeared and only the trapping level at 3.1 eV remained. The mid-gap levels are ascribed to Mg dopant complexes which may in part be responsible for low doping efficiency of Mg in the as-grown, doped GaN. The deep level at 3.1 eV commonly observed from all Mgdoped GaN most likely involves the Mg acceptor. The photo-excited state of the 3.1 eV level had relaxation times of the order of 103 sec at 295 K.

Thin films of the Bi-2212 and T1–2212 compounds were prepared by MOCVD deposition techniques. Resistivity versus temperature and critical current density measurements were used to characterize the electrical properties. An analysis of the data based on a proposed model determined the influence of intragranular weak links. Thin film samples in both systems with near optimum oxygen doping showed a correlation between the slope and magnitude of the resistivity in the normal state. Samples with reduced oxygen content displayed a strong increase in the intragrain boundary resistance, consistent with weak link defects. The results agree with a similar analysis of YBCO samples and support a common mechanism for the development of weak links in cuprates.

The heteroepitaxial growth of gallium phosphide on silicon (GaP/Si) is a useful step towards integration of III-V based devices onto silicon. GaP layers grown on silicon substrates of different orientations using metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) were characterized using transmission electron microscopy (TEM) with an attempt to understand the epilayer growth characteristics. Despite the fact that the GaP/Si system has a low misfit (≈0.4 %), a high density of crystal stacking defects was commonly observed. Inversion domain boundaries (IDBs) were another common defect observed in regions where the fault density was reduced.

Bi2Sr2CaCu2O8 thin films have been prepared in situ by low pressure metalorganic chemical vapor deposition using fluorinated β–diketonate precursors. The influence of the growth conditions on the oxide phase stability and impurity phase formation was examined as well as the superconducting properties of the films. Thin films deposited on LaAIO3 substrates were epitaxial as confirmed by x-ray diffraction measurements, including θ-2θ and φ scans. Four probe resistivity measurements showed the films to be superconducting with a maximum Tc0 of 90 K without post annealing. This Tc0 is among the highest reported for thin films of the BSCCO (2212) phase, and approaches reported bulk values.

We report here a plasma‐enhanced organometallic chemical vapor deposition process for the preparation of YBa2Cu3O7‐x thin films using two rf plasma coupling configurations. For the films grown under a direct plasma glow, the YBa2Cu3O7‐x phase is not found in the as‐deposited state. However, by employing plasma‐activated nitrous oxide as the reactant gas, superconducting YBa2Cu3O7‐x films having a low carbon content and a mirror‐like surface have been prepared in‐situ at a substrate temperature of 610°C using an organometallic chemical vapor deposition process.

Films of the Tl‐Ba‐Ca‐Cu‐O high‐Tc superconductor can be prepared by several organometallic chemical vapor deposition routes. Two of these involve Ba‐Ca‐Cu‐0 films that are first prepared using the volatile metal‐organic precursors Ba(heptafluorodimethyloctanedionate)2, Ca(dipivaloylmethanate)2, and Cu(acetylacetonate)2‐ Deposition is carried out at a pressure of 5 Torr with argon as the carrier gas and water vapor as the reactant gas. Thallium is next incorporated into these films either by organometallic chemical vapor deposition using Tl(cyclopentadienide) as the source, or by vapor diffusion using bulk Tl‐Ba‐Ca‐Cu‐O as the source. Thallium deposition is carried out at atmospheric pressure with an argon carrier and water‐saturated oxygen reactant gas, followed by rapid thermal annealing. Both procedures yield films that consist primarily of the TlBa2Ca2Cu3Ox phase, have preferential orientation of the Cu‐O planes parallel to the substrate surface, and exhibit onset of superconductivity at ∼125 K with zero resistance by 100 K.