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A wide variety of brookite TiO2 synthesis methods have been published over the past several decades, but few studies discuss the underlying mechanism that stabilizes brookite over its stable counterparts, rutile and anatase. Here, we investigate of the effect of pulsed laser deposition parameters on the as-deposited amorphous precursor titania thin films, which subsequently crystallize into stable and metastable TiO2 polymorphs upon annealing. We find that oxygen pressure in the deposition chamber strongly influences the non-equilibrium state of the amorphous precursor, which ultimately allows for selective polymorph formation. Rutile forms as the dominant phase at low pO2 5 mTorr. Brookite forms primarily at intermediate pO2 (0.5–1.0 mTorr). Controlling the amorphous structure (i.e. Ti—O bonding and polyhedral arrangement) of the precursors via oxygen deficiency is therefore likely for the selective formation of crystalline TiO2 polymorphs from sub-stoichiometric amorphous precursors. Directing phase selectivity by manipulating the structure and internal energy of the precursor amorphous state may have tremendous potential for synthesis of metastable crystalline phases that exhibit more desirable properties in comparison to their stable counterparts.

The thermal conductivity, measured by the 3ω method, of amorphous films of Al 2 P 1.2 O 6 (AlPO) deposited on Si substrates by an all-aqueous spin-coating technique is 0.93(3) W m − 1 K − 1 . The thermal conductivity of a degenerately doped n -Si substrate is 85(5) W m − 1 K − 1 and of a more lightly doped p -Si substrate is 139(7) W m − 1 K − 1 . The AlPO thermal conductivity is independent of film thickness in the range 45–200 nm. The total thermal resistance is dominated by the film contribution for film thicknesses above 50 nm, and for smaller thicknesses, interface contributions become significant.

Crystals of BiCuOSe were grown from a salt flux, and the crystal structure was determined by single-crystal X-ray diffraction. BiCuOSe adopts the tetragonal layered structure of LnCuOCh (Ln=lanthanide; Ch=S, Se, and Te) with bond lengths and bond angles in good agreement with those published for powders. Powders comprising mixed chalcogenides across the series BiCuOCh (Ch=S, Se, and Te) were made by reacting Bi2O2Ch and Cu2Ch. Band gaps determined via infrared diffuse reflectance from powders are Eg=0.82 eV for BiCuOSe, 0.89 eV for BiCuOS0.5Se0.5, and 1.07 eV for BiCuOS. The band gap of BiCuOSe inferred from infrared transmission measurements on single crystals is in good agreement with the value obtained from diffuse reflectance from the powder.

BaCuSeF/ZnTe is a model system to investigate physical and chemical properties of the interfaces of non-oxide wide-bandgap p-type semiconductors with materials used in chalcogenide solar cells. The BaCuSeF/ZnTe interface was studied using electron microscopy and photoelectron spectroscopy. Both techniques indicate that Se and Cu from BaCuSeF diffuse into ZnTe creating an interdiffused layer between these two materials. The interdiffusion may be attributed to the differences in materials formation enthalpies and to Fermi level pinning in BaCuSeF.

Thin-film solid solutions of BaCu(Ch1 − xChx′)F (Ch, Ch′ = S, Se, or Te) wide-band gap p-type semiconductors are obtained by pulsed laser deposition at elevated substrate temperatures from alternating layers of BaCuChF and BaCuCh′F. Adjusting the thickness of the component layers varies the relative chalcogen content, which allows tunability of the film transparency and results in a conductivity change of more than three orders of magnitude. The tunability of the physical properties makes these chalcogen-based semiconductors potentially useful for optoelectronics applications. Lattice parameters of BaCuChF calculated using density functional theory agree with those previously reported for the powders. Deviations from Vegard's law are observed in BaCu(S1 − xSex)F thin films with large sulfur content.

High-quality TiO2 thin films have been deposited from aqueous titanium-peroxo solutions via spin coating. The effects of precursor solution pH on the crystallization behavior, morphology, density, and refractive index of the films are reported. From X-ray diffraction measurements, the amorphous as-deposited films are found to crystallize in the anatase phase at 250 °C. Surface and cross-section SEM images reveal that films deposited from an acidic precursor are more uniform and denser than those deposited from a basic precursor. X-ray reflectivity measurements show that films with smooth surfaces and high densities (up to 87% of single-crystal anatase) can be produced at temperatures as low as 300 °C. Measured densities are consistent with high refractive indices at 633 nm of 2.24 and 2.11 for films derived from acidic and basic precursors, respectively. The uniformity and dense nature of the films have allowed fabrication of multilayer dielectric optical elements with thermal processing at only 300 °C. The distributed Bragg reflector with four bilayers exhibits a reflectance of 92% and a stop band width of 150 nm. The optical microcavity has a quality factor of 20. The optical properties of all elements agree well with theoretical models, indicating good optical quality. Use of the precursor chemistry for direct photopatterning of TiO2 films without a polymer resist is also demonstrated.

The simplest realization of a pin double-heterojunction thin-film solar cell would consist of a lightly doped, moderate-bandgap absorber i-layer; a heavily doped, wide-bandgap n-layer window (cathode); and a heavily doped, wide-bandgap p-layer window (anode) in which the anode and cathode are electrically contacted by at least one transparent conductor. The focus herein is on p-layer interfacial assessment, which is accomplished using modern Schottky barrier and heterojunction theory and is directed to the analysis of p-windows for copper indium gallium diselenide (CIGS) and cadmium telluride (CdTe) thin-film solar cells. A p-type window layer serves as an electron reflector and also aids in the formation of an ohmic anode contact. Ohmic anode contacts are particularly difficult to form in CIGS and CdTe thin-film solar cells since these materials have very large ionization potentials, i.e., IP S = 5.65 (CIGS) and 5.78 V (CdTe) and significant interfacial screening, characterized by extremely small Schottky barrier interface parameters, i.e., S = 0.14 (CIGS) and 0.21 (CdTe). An ideal p-type window material would be heavily doped, p-type, and would have a wide bandgap, a large ionization potential, and a smaller charge neutrality level energy than that of the absorber layer.

The structural, optical, and electronic properties of thin films of a family of wide band gap (Eg > 2.3 eV) p-type semiconductors Cu3TaQ4 (Q = S or Se) are presented. Thin films prepared by pulsed laser deposition of ceramic Cu3TaQ4 targets and ex-situ annealing of the as-deposited films in chalcogenide vapor exhibit mixed polycrystalline/[100]-directed growth on amorphous SiO2 substrates and strong (100) preferential orientation on single-crystal yttria-stabilized zirconia substrates. Cu3TaS4 (Eg = 2.70 eV) thin films are transparent over the entire visible spectrum while Cu3TaSe4 (Eg = 2.35 eV) thin films show some absorption in the blue. Thin film solid solutions of Cu3TaSe4 − xSx and Cu3TaSe4 − xTex can be prepared by annealing Cu3TaSe4 films in a mixed chalcogenide vapor. Powders and thin films of Cu3TaS4 exhibit visible photoluminescence when illuminated by UV light.

Transparent p-type conductive BaCuTeF thin films are reported. Epitaxial BaCuTeF films are obtained in situ by pulsed laser deposition in ultra-high vacuum and in Ar ambient up to 1 × 10−3 Torr on single-crystal MgO (100) substrates above 500 °C. Nominally undoped films exhibit hole carrier mobilities of 4–8 cm2/V s with a maximum conductivity of 167 S/cm. Polycrystalline films deposited under similar conditions on amorphous SiO2 substrates have hole mobilities of 0.2 cm2/V s and conductivities of 0.4–7 S/cm. The largest change in optical absorption occurs near 3 eV, with a weak feature near 2.4 eV in epitaxial films.

Zinc oxide powders prepared by decomposition of zinc peroxide and zinc nitrate show evidence of acceptor states from iodometric titrations. Chemical analysis also shows the presence of nitrogen in the samples prepared by nitrate decomposition. The evidence for acceptor states disappears on heating samples, eliminating the possibility of transport measurements on sintered samples. Complex impedance spectra suggest room temperature conductivities as high as 5 × 10−4 and 2 × 10−3 S/cm for these peroxide and nitrate derived zinc oxide powders, respectively. As zinc oxide particles become small there is an increase in unit cell dimensions and a red shift of the absorption edge. An even stronger red shift is observed in the case of N doping.

The compound BaCuTeF has been prepared by using high-temperature reaction methods, and its structure has been established via Rietveld refinement of powder X-ray diffraction data. It crystallizes in the tetragonal space group P4/nmm (No. 129) with Z=2, a=4.4297 (1), c=9.3706 (1) A, and V=183.87 (1) A{sup 3}; refinement residuals include R {sub p}/wR {sub p}/R {sub Bragg} (%)=6.72/4.42/5.72. A band gap of 2.3 eV is estimated from wavelength-dependent diffuse reflectance measurements. Room-temperature conductivities of pressed pellets are 6-8 S/cm, and variable-temperature Seebeck and electrical-conductivity measurements reveal p-type degenerate semiconductor behavior. - Graphical abstract: The telluride fluoride BaCuTeF has been synthesized and structurally characterized through Rietveld refinement of X-ray diffraction data. It crystallizes in the tetragonal structure of LaCuOS. Optical and transport measurements have been used to establish the material as a degenerate p-type semiconductor with a band gap near 2.3 eV.

Thin films of CuSc 1− x Mg x O 2+ y have been rendered conducting by intercalation of oxygen. Hole conductivities up to of order 10 S/cm can be achieved at the expense of visible transparency, especially in the blue region of the spectrum. Hole conductivities up to 100 S/cm have been achieved in pressed pellets of K-doped BaCuSF, and a band-gap of 3.2 eV has been measured on undoped films.

Several p-type oxides of the delafossite structure have been investigated in the hope that the conductivity and transparency will be high enough to render them useful in the manufacture of transparent p–n junction diodes and other transparent devices. The highest conductivity achieved to date has been 220 S/cm in CuCr1−xMgxO2 thin films. Oxygen intercalation in CuSc1−xMgxO2+y films improves the conductivity at the expense of optical transparency. We have improved the conductivity of CuGaO2-based films from 0.02 to 1 S/cm by substitution of Fe for Ga. p-Type conductivity has been demonstrated in an Ag-based delafossite film. A sputter-deposited AgCoO2 film has a conductivity of 0.2 S/cm, a Seebeck coefficient of 230 μV/K and a band gap of 4.1 eV at room temperature. CuNi2/3Sb1/3O2 films have been produced that are p-type conductors when doped with Sn.

Thin films of CuY1−xCaxO2 that are p-type conductors have been prepared by thermal co-evaporation. The films are polycrystalline on amorphous substrates and c-axis oriented on crystalline substrates. Calcium doping was necessary to produce conductive films, and the highest conductivity recorded was 1.0 S cm−1. The average transparency in the visible region is approximately 40–50%, for conducting films approximately 250 nm thick, and in the infrared approximately 70% for a 350-nm conducting film. The optical spectra give evidence for a bandgap of 3.5 eV. We have demonstrated an all-oxide p–n diode using CuY1–xCaxO2/Zn1–xAlxO.

Compounds of the type CuMO2 and AgMO2 have been doped in various ways to induce p-type conductivity. Most of the compounds have the delafossite structure which can exist in 2H and 3R polytypes. Both the 2H and 3R forms of CuScO2 have been intercalated with oxygen up to a formula of CuScO2.5. p-Type conductivity can also be induced in CuScO2 by substitution of Mg for Sc. Thin films of CuScO2 prepared by rf sputtering are transparent and show a conductivity of 30 S cm−1. Thin films of Mg-doped CuCrO2 are transparent and show a room-temperature conductivity of 220 S cm−1. Doping of AgMO2 compounds with M=Sc, In, Cr, and Ga have produced p-type conductivity, but never higher than about 0.01 S cm−1 at room temperature.

ZnS:Mn, ZnGaS:Mn and ZnS:CuCl2 phosphor thin films for alternating-current thin-film electroluminescent (ACTFEL) devices were prepared by thermal evaporation from two and three sources, respectively. Films are polycrystalline, strongly oriented and stoichiometric or nearly stoichiometric with high optical transmission in the visible part of the spectrum. The band gap of ZnS:Mn, ZnGaS:Mn and ZnS:CuCl2 films was found to be 3.63, 3.86 and 3.56 eV, respectively. The sheet resistance of the ZnS:Mn and ZnGaS:Mn films was greater than 100 MΩ. The resistivity of ZnS:CuCl2 films was between 1.5–80 Ω cm. Photoluminescent (PL) and electroluminescent (EL) characteristics were also studied. The results indicate that after the optimization the investigated phosphor thin films will be suitable for ACTFEL device applications.

Undoped and 3% Na doped polycrystalline PbCu 2 O 2 pellets were prepared by solid state reaction and the electrical and optical properties were measured. Undoped pellets were electrical insulators ( σ ∼5×10 −11 S cm −1 ). The room temperature electrical conductivity increased by six orders of magnitude upon substitution of 3% Na + for Pb 2+ . The positive sign of the Seebeck coefficient demonstrated p-type conduction in the doped pellets. The diffuse reflectance spectrum revealed that optical absorption begins at ∼740 nm.

Thermal co-evaporation of neodymium, cerium and copper in an oxygen atmosphere produces thin films of the 20 K, n-type superconductor, Nd 2− x Ce x CuO 4− δ . The resistive transition is sharp (0.9 K, 90%-10%) and complete at 22 K in ambient magnetic field. The resistive transition broadens if a magnetic field is applied parallel to the superconducting planes, and shifts and broadens if the field is applied perpendicular to these planes. The transition can be well described by an Arrhenius-type behavior with a temperature dependent activation energy.

Current-voltage isotherms for YBa2Cu3O6+x thin-film superconductors for several magnetic fields in the low-field regime have been obtained. A crossover from ohmic behavior at low currents to exponentially decreasing behavior occurs for all fields down to ambient. The relation between the crossover temperature and field is predicted by the vortex glass theory. The isotherms within about 1 K of the crossover can be collapsed onto two curves for each field, according to the prescription of the vortex glass theory; however some parameters are field dependent. In particular, the parameter z increases with decreasing field, and shows a correlation with the number of vortices in the film.

We have prepared thin films of superconducting YBa 2 Cu 3 O 7 on bare silicon (100) by thermal co-evaporation of yttrium, barium and copper in an oxygen atmosphere. The temperature never exceeds the deposition temperature of about 650°C. No post-deposition annealing is necessary to achieve superconductivity at 85 K and critical current densities, at 4.2 K, of 9 × 10 4 A cm −2 . X-Ray diffraction studies show that the best films are oriented with the c axis perpendicular to the substrate, with no trace of reflections other than those from (00 l ) planes. We have also produced c axis oriented superconducting YBCO films on silicon substrates with buffer layers of amorphous SiO 2 and amorphous Si 3 N 4 . The best superconducting characteristics of these films were transition temperatures of 68–69 K and critical current densities at 4.2 K of 3 × 10 3 A cm −2 .

We report ellipsometric spectra of YBa2Cu3O7 measured on ceramic, thin film, and single crystal samples. Absorption bands centered at 2.8, 4.1, and 4.7 eV are observed in all threee types of samples. The spectra change only slightly upon cooling from room temperature to 7 K. We assign the spectral structures to interband transitions involving O(2p)-Cu(3d) and O(2p)-Ba(5d) bands.

We have prepared 800 nm YBa2Cu3Ox films on Al2O3, SrTiO3 and Si substrates by simultaneously evaporating the three metals in an oxygen atmosphere. Resistance measurements indicate the onset of superconductivity in the films on Al2O3 and SrTiO3 at about 96K. The transtion widths depend on the substrate and the details of the preparation method. The best films have onset temperatures of 96 K and transition widths (90 % to 10 %) 0f 2 K. The films were characterised by RBS, X-ray, electron microscope and Raman analyses. We have investigated the absorption of phonons (f > 100 GHz) and found that their mean free path is larger than 1 μm below about 300 GHz and decreases rapidly with increasing frequency.

Thermal coevaporation of Y, Ba, and Cu in an oxygen atmosphere produced c-axis oriented YBaCuO films with T c between 80 and 85 K on bare Si substrates. These films were laser patterned to produce microbridges which showed flux-flow resistance characteristics. We investigated the performance of a microbridge as a phonon detector, and measured its response to phonons of different frequency and polarization.