This paper presents the results of a systematic study of oxygen incorporation in a-Si:H alloys produced by the glow-discharge decomposition of SiH4, H2, and 02. We identify four oxygen-related absorption bands, at 2090, 980, 780, 500 cm ', and show that the absorption strength in each band scales linearly with the oxygen concentration. We demonstrate that oxygen can increase the solubility of hydrogen in a-Si in the monohydride bonding geometry. The features identified above are shown to be characteristic of a bonding site in which the oxygen and hydrogen atoms are bonded to the same silicon atom. We find no features in the infrared absorption that are associated with bonding configurations having OH groups. In films containing both oxygen and polysilane bonding, as evidenced by the doublet absorption at 845 and 890 cm ', we find no evidence for bonding sites in which a substantial fraction of the silicon atoms have one oxygen and two hydrogen neighbors.

The theory and practice of the Secchi disc experiment are discussed. It is shown, in theory, that the Secchi disc reading can be used to calculate the sum of the total and diffuse attenuation coefficients, (Y and K. To obtain independent values of (Y and K it is necessary to make some other measurement. Some recent measurements of the per cent of surface light reaching various depths, obtained in conjunction with Secchi disc readings, are discussed with respect to their usefulness in determining QI and K independently. It is suggested that the Secchi disc could be calibrated against modern instrumental measurements of a and K and a practical value for apparent contrast thus determined for the Secchi disc experiment. However if modern instruments are available for measuring Q: and K, it would probably be better not to undertake such a calibration. JOHN E. TYLER

We have studied the local bonding of nitrogen atoms in glow-dischargedeposited films of a-Si:H by using ir absorption spectroscopy. We find two different bonding environments for N, which are identified through different frequencies for the Si-N asymmetric bond-stretching vibration, 840 cm ' for the high-T, films, and 790 cm ' for the low-T, films. In films deposited on substrates held at temperatures in excess of 300'C, the N is incorporated in a planar site with three silicon nearest neighbors, and one hydrogen second-nearest neighbor. In films produced on substrates held below 200'C, the N atom is also in a threefold-coordinated planar site, but with all of the nearest and second-nearest atoms being Si atoms.

The optical reflectivity of a metal-dielectric-metal microcavity in which the upper layer is periodically perforated by narrow slits is explored. Complete characterization of the observed modes in terms of their resonant electromagnetic fields is achieved by comparison of the experimental data to the predictions of a finite-element model. In particular, we demonstrate that the slits provide efficient diffractive coupling to a surface plasmon mode buried within the microcavity whose propagation is strongly confined to the dielectric layer. DOI: 10.1103/PhysRevB.74.073408 PACS number͑s͒: 78.20.Ci, 42.25.Bs, 42.79.Dj, 73.20.Mf The texturing of metal surfaces on a subwavelength scale is known to strongly modify their electromagnetic ͑EM͒ response. Excitation of surface plasmons ͑SPs͒ at these interfaces plays a crucial role in light-matter interactions ͑see Refs. 1 and 2, and references therein͒. One of the reasons why the field of plasmonics has received so much attention in recent years is the localization of EM fields associated with these modes, together with their ability to guide energy along interfaces. In addition, metallic microcavity structures have also been explored to provide enhanced performance in a range of optoelectronic devices ͑e.g., light-emitting diodes 3 ͒. Furthermore, light-guiding structures that allow subwavelength confinement of the optical mode are critical for the achievement of compact photonic components.4,5 The design and fabrication of metallic nanostructures combining surface wave properties with microcavity resonant behavior opens up substantial new device potential and has already seen application as ultrathin electromagnetic attenuators in the microwave regime. 6 In this paper, we study the visible EM response of a metal-dielectric-metal microcavity where the illuminated metal film is perforated with periodically spaced identical slits ͓Fig. 1͑a͔͒. Both the width of these slits and the thickness of the dielectric core are ϳ /10 ͑where is the incident wavelength͒, the latter being small enough to permit coupling between SPs of the two metal surfaces. The excitation of these coupled-SP modes 7 using radiation polarized orthogonal to the slits results in resonant absorption of the incident power via Joule heating of the metal. In addition, it is found that single-surface SPs are excited at the illuminated face of the slit array. By recording the positions of the resonances in the reflectivity spectra as a function of the angle of incidence, the band structure of the modes supported is elaborated. The predictions of a finite-element method ͑FEM͒ model 8 are then fitted to the experimental data with good agreement being obtained. Each of the modes supported by the structure is identified by studying the numerically predicted EM field distributions.To fabricate the sample, a Ag/ SiO x / Ag trilayer structure was deposited by vacuum evaporation onto a clean glass slide. The first silver layer is optically thick ͑Ͼ100 nm͒, with the thickness of the SiO x dielectric core ͑t c ͒ and up...

We have grown thin films of a-Ge:H and a-Ge:(H, O) by the glow-discharge process, and have used infrared (ir) absorption spectroscopy to study the local bonding of hydrogen and oxygen as a function of the substrate temperature T, and the amount of 02 and/or H&O added to the germane gas mixture in the plasma. The temperature dependence of the ir features associated with monohydride and dihydride groups in a-Ge:H alloys is different in these films, grown at relatively high plasma power, from what has been reported in films of a-Si:H grown from silane plasmas at lower power levels. In particular, dihydride incorporation in these a-Ge:H films displays a distinct temperature threshold, rather than a competition with the monohydride bonding as in the a-Si:H films.The bonding of H and 0 atoms in the ternary alloys a-Ge:(H, O) is also different from what has been reported for the corresponding Si ternary alloys, both in the nature of the local atomic arrangements of the 0 and H atoms, and in the relationship between these bonding environments and the source of the oxygen atoms (02 or H20). These differences in bonding in a-Ge:H and a-Ge:{H,O) films with respect to the corresponding a-Si alloy films reflect differences in both the plasma-phase precursor chemistry and temperature-dependent reactions at the growth surface.