M. Rubin

Chemical Structure and Physical Properties of Diamond-Like Amorphous Carbon Films Prepared by Magnetron Sputtering

Thin films of amorphous carbon (a-C) and amorphous hydrogenated carbon (a-C:H) were prepared using magnetron sputtering of a graphite target. The chemical structure of the films were characterized using electron energy loss spectroscopy (EELS) and Raman spectroscopy. The mass density, hardness, residual stress, optical bandgap, and electrical resistivity were determined, and their relation to the films chemical structure are discussed. It was found that the graphitic component increases with increasing sputtering power density. This is accompanied by a decrease in the electrical resistivity, optical bandgap, mass density and hardness. Increasing the hydrogen content in the sputtering gas mixture results in decreasing hardness (14 GPa to 3 GPa) and mass density, and increasing optical band gap and electrical resistivity. The variation in the physical properties and chemical structures of these films can be explained in terms of the changes in the volume of sp{sup 2}-bonded clusters in the a-C films and changes in the termination of the graphitic clusters and sp{sup 3}-bonded networks by hydrogen in the a-C:H films.

Effects of substrate temperature on chemical structure of amorphous carbon films

Amorphous carbon thin films were prepared at 30, 200, and 450 °C by magnetron sputtering of a graphite target. The surface structure and chemical bonding (sp2/sp3) of the carbon films were characterized by scanning tunneling microscopy (STM) and Raman spectroscopy. STM images show that graphite microcrystallites of 20–40 A in size are present at the surfaces of all the films and the number of the microcrystallites increases with increasing substrate temperature. The microcrystallites often contain structural defects. Raman measurements show that increasing the substrate temperature results in an increase in the sp2‐bonded fraction of carbon atoms and a decrease in the microstructural defects. These results indicate that the microstructural changes are correlated with changes in the chemical bonding ratio (sp3/sp3) and no diamond microcrystallites are present in the amorphous carbon. A three‐dimensional atomic structure of the graphite microcrystallites is discussed in terms of turbostratic graphite.

Refractive index changes of Pd-coated magnesium lanthanide switchable mirrors upon hydrogen insertion

The optical effect upon insertion of hydrogen into Pd-coated magnesium lanthanide switchable mirrors is investigated in terms of the changes of their complex refractive indices. A significant change in the optical constants of LnMg layers is seen between the as-deposited state and the dehydrided state after one cycle. Furthermore, the optical effect of switching the Pd cap layer to a PdH cap layer was determined. It is shown that the Pd layer mainly limits the visible transmittance of the hydrided stack to about 35%–40%. Whereas the extinction coefficient of dehydrided LnMg layers at 550 nm is between 2.2 and 3.1, it is as low as 10−4 in the transparent state. This is of great promise to applications requiring large optical contrast (e.g., optical switches).

Thermodynamic and kinetic processes involved in the growth of epitaxial GaN thin films

Our experimental results using reactive magnetron sputtering, combined with earlier literature, are used to understand the thermodynamic and kinetic processes involved in GaN film growth and the limiting factors involved in the incorporation of nitrogen during the growth process. We show that GaN films fabricated with low pressure growth techniques (<0.1 Torr) such as sputtering and molecular beam epitaxy are formed under metastable conditions with a nonequilibrium kinetically limited reaction. For these methods, the growth process is controlled by a competition between the forward reaction, which depends on the arrival of activated nitrogen species at the growing surface, and the reverse reaction whose rate is limited by the unusually large kinetic barrier of decomposition of GaN. In practice, the thermally activated rate of decomposition sets an upper bound to the growth temperature.

Optical Indices of Pyrolytic Tin-Oxide Glass

SnO{sub 2}:F is a widely used transparent conductor and commercially available in a multilayer structure as Tech glass. Current applications include photovoltaics, electrochromics and displays. Optical design of these and other applications requires knowledge of the optical constants, in some cases, over the whole solar spectrum. Various optical property measurements were performed including variable angle spectrosopic ellipsometry, ad spectral transmittance and reflectance measurements. This material is deposited in several steps and has a fairly complex structure. The measured data were fit to models based on this structure to obtain the optical indices. Atomic force microscopy confirmed the optically modeled surface roughness.

Optical Indices of Electrochromic Tungsten Oxide

Abstract Tungsten trioxide (WO 3 ) is the most widely used material for the active layer of electrochromic devices. Knowledge of the complex refractive index over the range of coloration states is required for device design. Optical constants of WO 3 over the whole solar spectrum were determined as a function of injected charge. Films of WO 3 were prepared by electron-beam evaporation, then colored in several steps by reduction with lithium (Li) up to 68 mC cm −2 μm −1 injected charge. Measurements included variable-angle spectroscopic ellipsometry and spectroscopic transmittance and reflectance at normal incidence. Analysis was complicated by the fact that a transparent-conducting layer of indium tin oxide (ITO) was required to perform lithiation. Optical indices of the glass substrate and ITO transparent conductor were determined separately and then fixed in the model. The indices of WO 3 could then be extracted from measurements on the complete structure. A parametric dispersion model corresponding to Gaussian broadening of the oscillators was used to represent the dielectric response of WO 3 .

Calculation of thermodynamic, electronic, and optical properties of monoclinic Mg2NiH4

Ab initio total-energy density functional theory is used to investigate the low temperature (LT) monoclinic form of Mg2NiH4. The calculated minimum energy geometry of LT Mg2NiH4 is close to that determined from neutron diffraction data, and the NiH4 complex is close to a regular tetrahedron. The enthalpies of the phase change to high temperature (HT) pseudo-cubic Mg2NiH4 and of hydrogen absorption by Mg2Ni are calculated and compared with experimental values. LT Mg2NiH4 is found to be a semiconductor with an indirect band gap of 1.4 eV. The optical dielectric function of LT Mg2NiH4 differs somewhat from that of the HT phase. A calculated thin film transmittance spectrum is consistent with an experimental spectrum.

Visual quality assessment of electrochromic and conventional glazings

In this study, a parametric set of photo-realistic images was generated using the RADIANCE simulation program to study luminance contrast, daylight levels, reflections on a VDT screen, and privacy with both electrochromic glazings and conventional glazings. Quantitative and qualitative data are provided. Recommendations are given regarding material design and use within the commercial office environment

Evaluation of predictive models for the angle-dependent total solar energy transmittance of glazing materials

The predictions of angle-dependent optical properties of glazings are discussed. A categorisation of windows depending on the type of coating on the glazing is discussed as a way of improving the accuracy in the predictive models. Four approximate ways to predict the angle dependence of the total solar energy transmittance are compared. The impact on the energy performance of windows with different angle dependence is assessed in a heating and a cooling dominated climate, respectively. Results imply that by simply using the clear glass angular profile for all types of windows gives quite low errors in the angle dependence prediction, lower than some other previously proposed models. By using a model with window category as input, the errors in angle dependence prediction can be further reduced. The impact on the energy performance from incorrect angle dependence is considerable in some cases but not necessarily critical.

Electrochromic lithium nickel oxide thin films by RF-sputtering from a LiNiO2 target

Thin films of lithium nickel oxide were deposited by rf sputtering from a stoichiometric LiNiO 2 target. The films exhibit excellent reversibility in the potential range 1.1 to 3.8 V versus Li/Li + and could be cycled in a liquid electrolyte half cell with a switching range ΔT Photopic close to 70%. The photopic coloration efficiency of this anodically coloring material was typically -30 to -40 cm 2 C -1 . The switching performance of a device utilizing a lithium nickel oxide film as counter electrode and a tungsten oxide electrochromic film is reported. The device switched from 75% photopic transmittance to 2%, with color neutrality and a coloration efficiency of 65 cm 2 C -1 .