Constantinos Christofides

Solid‐state sensors for trace hydrogen gas detection

This paper reviews the development, history, theoretical basis, and experimental performance of solid‐state hydrogen detectors under flow‐through conditions available to date such as pyroelectric, piezoelectric, fiber optic, and electrochemical devices. Semiconductor hydrogen detectors will only be reviewed briefly, as excellent reviews on this subject already exist. In view of the fact that almost all the devices that will be discussed later in this paper use Pd as a hydrogen trap, we devote a subsection to examining the role of palladium as a catalyst as well as some of the characteristics of the Pd‐H2 system. Non‐solid‐state hydrogen sensors, such as the flame ionization detector are not the object of this review. A useful feature of this review is a comparison of operating characteristics of each device in a general table in Sec. VII. In that section a general discussion is presented, including a critical comparison of the capabilities and parameters of various solid‐state hydrogen sensors in the form...

Statistical analysis of wind speed and direction in Cyprus

The aim of this study is to establish the meteorological basis for the assessment of wind energy resources in Cyprus and to provide suitable data for evaluating the potential wind power. For this purpose the mean values, the systematic daily and annual variations, the frequency distributions and the estimation of the extreme values are determined. Predictions were also obtained from the WAsP model and, finally, the Wind Atlas of the island, in the form of contours of constant wind speed, was produced.

Photothermal reflectance investigation of processed silicon. I. Room‐temperature study of the induced damage and of the annealing kinetics of defects in ion‐implanted wafers

Silicon films damaged by ion implantation have been examined using the photothermal reflectance technique in the thermal wave regime (low‐modulation frequencies, large laser beam spot sizes). Data are presented on the sensitivity of this method to the implant dose and to the effects of thermal annealing. It has been shown that the technique provides information about the state of the implanted layer, and is a sensitive probe for monitoring the annihilation of the induced damage as a function of the annealing temperature. A model for the kinetics of damage annihilation has been presented to estimate the activation energy of the local annealing recovery mechanism, found to be 0.15 eV. The presence of negative annealing has been detected at about 500 °C, an advantage over the essentially insensitive Hall mobility method.

Thermoelectronic-wave coupling in laser photothermal theory of semiconductors at elevated temperatures

quantitative analysis of linear temperature-dependent coupled thermoelectronic diffusion waves in the generation of the laserinduced IR photothermal radiometric signal from a photoexcited, plasmawave- dominated, semi-infinite semiconductor Si wafer is presented. The strong coupling between the (usually assumed decoupled) carrier and thermal wave transport equations is accounted for explicitly and the range of violation of the Vasil’ev-Sandomirskii condition is studied. Thermoelectronic coupling is found to degrade the sensitivity of the plasmaoriginating IR radiometric signal in high-quality process Si substrates at elevated background temperatures and high modulation frequencies, with the exception of highly degenerate plasmas

NON-CONTACTING MEASUREMENTS OF PHOTOCARRIER LIFETIMES IN BULK- AND POLYCRYSTALLINE THIN-FILM SI PHOTOCONDUCTIVE DEVICES BY PHOTOTHERMAL RADIOMETRY

Laser‐induced and frequency‐scanned infrared photothermal radiometry was applied to a crystalline‐Si photoconductive device, and to polysilicon thin‐film photoconductors deposited on oxidized Si substrates by an LPCVD method. A detailed theoretical model for the radiometric signal was developed and used to measure the free photoexcited carrier plasma recombination lifetime, electronic diffusivity and surface recombination velocity of these devices, with the simultaneous measurement of the bulk thermal diffusivity. A trade‐off between detectivity/gain and frequency‐response bandwidth was found via the lifetime dependence on the wafer background temperature rise induced by Joule‐heating due to the applied bias. This effect was most serious with the bulk‐Si device, but was limited by the high resistivity of the LPCVD thin‐film devices. In the case of the bulk‐Si device, the results of photothermal radiometry were compared with, and corroborated by, frequency‐scanned photocurrent measurements. More sophistica...

The role of surface vibrations and quantum confinement effect to the optical properties of very thin nanocrystalline silicon films

We report on a spectroscopic study of very thin nanocrystalline silicon films varying between 5 and 30nm. The role of quantum confinement effect and surface passivation of nanograins in optical properties are examined in detail. The coupling between surface vibrations and fundamental gap Eg as well as the increase of interaction between them at the strong confinement regime (⩽2nm) are proposed for the observable pinning of Eg in luminescence measurements.

Characterization of reflectivity inversion, α- and β-phase transitions and nanostructure formation in hydrogen activated thin Pd films on silicon based substrates

Optically thin palladium metal films evaporated on different silicon based substrates are investigated following exposure to different concentrations of hydrogen gas in air. Laser modulated reflectance off the palladium surface of silicon oxide and silicon nitride substrates is used to recover information regarding the reflectivity inversion and α/β-phases of the palladium complex after both first and multiple gas cycling. Atomic force microscopy confirms the formation of metal nanostructures following exposure to hydrogen of the optically thin palladium films.

Raman spectroscopy and spreading resistance analysis of phosphorus implanted and annealed silicon

Raman and electrical characterization measurements are performed in order to study the effects of thermal annealing on phosphorus implanted silicon wafers. The silicon layers were implanted for various implantation energies and doses, below, and over the critical dose of amorphization. The post‐implanted period was followed by thermal isochronal annealing at various temperatures. Special attention has been given to the amorphous/crystal transition occurring at various annealing temperatures. A bi layer model [R. Loudon, J. Phys. (Paris) 26, 677 (1965)] has been used for a quantitative determination of the annealing temperature at which a complete annihilation of implantation defects takes place. For this analysis, Raman spectra, resistivity depth profiles, as well as 1D‐SUPREM III simulation were used.

Photothermal reflectance investigation of processed silicon. II. Signal generation and lattice temperature dependence in ion‐implanted and amorphous thin layers

Photothermal reflectance investigations as a function of temperature of various forms of silicon (crystalline, ion‐implanted, amorphous) are reported largely in the thermal wave regime (low‐modulation frequencies and large laser beam spot sizes). The observed results have been explained by a thermal wave model that has been extended to include the effect of lattice temperature. The influence of substrate on the observed signal has been examined in light of the temperature and frequency dependence of the thermal diffusion length. The possible application of the technique to depth‐profiling studies, and to the investigation of ion‐implanted semiconductors, is discussed.

Linear correlation between binding energy and Young’s modulus in graphene nanoribbons

Graphene nanoribbons (GNRs) have been suggested as a promising material for its use as nanoelectromechanical reasonators for highly sensitive force, mass, and charge detection. Therefore the accurate determination of the size-dependent elastic properties of GNRs is desirable for the design of graphene-based nanoelectromechanical devices. In this study we determine the size-dependent Young’s modulus and carbon-carbon binding energy in a homologous series of GNRs, C4n2+6n+2H6n+4 (n=2–12), with the use of all electron first principles computations. An unexpected linearity between the binding energy and Young’s modulus is observed, making possible the prediction of the size-dependent Young’s modulus of GNRs through a single point energy calculation of the GNR ground state. A quantitative-structure-property relationship is derived, which correlates Young’s modulus to the total energy and the number of carbon atoms within the ribbon. In the limit of extended graphene sheets we determine the value of Young’s mod...