Franco Gaspari | University of Ontario Institute of Technology (original) (raw)

Papers by Franco Gaspari

Diamond-like hydrogenated carbon films were deposited using saddle-field glow-discharge in pure m... more Diamond-like hydrogenated carbon films were deposited using saddle-field glow-discharge in pure methane. XPS, XAES, Raman and x-ray spectroscopy suggest that the percentage of sp{sup 3}/sp{sup 2} bonding was varied over a relatively wide range by adjusting pressure and substrate bias. It was found that a very high percentage of sp{sup 3} bonding could be achieved.

Thin Solid Films, 2014

ABSTRACT We propose a method to analyse Reflectance and Transmittance spectra of hydrogenated amo... more ABSTRACT We propose a method to analyse Reflectance and Transmittance spectra of hydrogenated amorphous silicon-rich carbide alloys, which allows the determination of n-k spectra with very good accuracy. The method is based on the Tauc-Lorentz formulation proposed by Jellison and Modine, in which we introduce an absorption band to take into account the tail states. The method is used to retrieve the n-k spectra of a set of silicon-rich carbide films fabricated in a variety of conditions. The results on oscillator parameters are discussed. We show that by introducing this absorption band we achieve a better accuracy in the determination of the oscillator parameters. The resulting oscillator frequency appears to be well correlated with compositional data.

We developed and applied physically transparent and efficient formalism of photoconversion in sin... more We developed and applied physically transparent and efficient formalism of photoconversion in single junction solar cells. The photoconversion efficiency of high-efficiency silicon based solar cells (SCs) is analyzed focusing on the role of combined surface recombinations at the illuminated and back surfaces with its combined velocity Ss. Both silicon based p-n junction SCs and –Si:H–Si or –SiC:H–Si heterojunctions (the so-called heterojunction with intrinsic thin layer (HIT) structures) based SCs are studied within the same general approach. It is shown that the open circuit voltage Voc increase due to an additional contribution from the back surface is the common and essential feature of the photoconversion and high efficiency in the above SCs. Equations that comprehensively describe the photoconversion processes were derived and solved. This includes the most basic mechanisms of recombination: Shockley-Read-Hall (SRH) recombination, radiative recombination, surface recombination, recombination in the space charge region (SCR) and band-to-band Auger recombination. Calculated photoconversion efficiency of the above systems were compared to the experimental values at AM1.5 condition for highly efficient Si based SCs with = 25% for p-n junction based systems, = 24.7% for –Si:H–Si HIT structures and = 20.3% for -SixC1-x:H-Si SCs. Comparison of theory and experiment allowed us to estimate the combined surface recombination velocity Ss, which was minimized in the above solar cells to achieve record high photoconversion. Our calculated maximum efficiency max for the silicon based SCs also fits very well the max calculated using more complex formalisms, available in the literature, thus confirming practicality of our theoretical results.

We offer a comprehensive semi-analytical formalism to model the photoconversion efficiency for mu... more We offer a comprehensive semi-analytical formalism to model the photoconversion efficiency for multijunction solar cells (MJSCs) and apply this under AM0 and AM1.5 conditions. We take into account dominating recombination mechanisms (such as radiative recombination, Shockley-Read-Hall (SRH) recombinations, and space charge region (SCR) recombination). In addition, heat dissipation, which depends on the number of p-n junctions (subcells) n, system emissivity, and losses related to low light absorption coefficient near the band edge of each subcell are included in the formalism. The derived equations for photocurrent and photovoltage, combined with heat balance equation, were solved selfconsistently. Increasing n reduces thermalization losses and cools the MJSC down. The optimal doping level of MJSC subcells is found to be ~1017 cm-3. This is particularly important under AM0 conditions. Using large area radiators to improve the heat dissipation noticeably increases the efficiency . Our approach can be used to optimize MJSC for both terrestrial and space applications.

Materials Chemistry and Physics, 2019

We introduce a novel method for the analysis of the dependence of the optical parameters of Hydro... more We introduce a novel method for the analysis of the dependence of the optical parameters of Hydrogenated Silicon-rich Carbide (SRC:H) on the alloy composition by using a ternary-3D diagram. The SRC:H samples were fabricated using the very high frequency (VHF) plasma enhanced chemical vapor deposition technique. The composition is given as Si % +C % +H % = 100, with the carbon fraction C f = C % /(C % +Si %) varying between 0.18 and 0.72. The actual composition was determined by Rutherford Back Scattering analysis. The optical constants are determined by means of Reflectance and Transmittance UV-visible spectroscopy, and are modeled by means of the Jellison-Tauc-Lorentz-with Gaussian Band Tail model. The unique properties of the ternary-3D diagram give unexpected insight on the origin of the dependence of optical properties on composition. It is shown that the wavelength position of the Lorentz oscillator is governed by C f with virtually no influence of H %. Conversely, the refractive index is strongly affected by H % , besides C f. We show that using the ternary-3D diagram approach it is possible to separate the contributions of the different components, and derive the analytical dependence between optical parameters and composition. In the second part of the paper such findings are used for a reversed analysis, that is, we employ the obtained analytical formulations in order to retrieve the SRC:H composition within better than ±4% absolute error for all components.

The basic physics of semiconductors is reviewed with a focus on the optical and electronic proper... more The basic physics of semiconductors is reviewed with a focus on the optical and electronic properties relevant for energy applications. The interactions among photons, electrons, and atoms in semiconductors will be presented in correlation with the photoelectric and photovoltaic (PV) effects. Energy device structures will be discussed and applications based on the properties of semiconductors materials and their combinations are presented. In particular, p–n junctions will be analysed as precursors to electronic and PV devices. Other applications will also be discussed, including, lasers, particle detectors, LEDs, super lattices, etc. Finally, a description of energy applications of semiconductors will be given, focusing on the different forms and structures of this class of materials. In particular, standard semiconductor materials, like silicon, will be compared with the new wave of semiconductors, which includes large band-gap semiconductor, nanostructured semiconductors, and org...

Fusion Science and Technology

Hydrogen is known to strongly affect the physical properties of amorphous semiconductors. Indeed ... more Hydrogen is known to strongly affect the physical properties of amorphous semiconductors. Indeed hydrogen is introduced during the growth of amorphous silicon films, used in active matrix displays and solar cells, to passivate silicon dangling bonds and to relax the lattice thereby reducing the density of states in the energy gap by several orders of magnitude and giving rise to device grade material. Ideally, hydrogenated amorphous silicon (a-Si:H) is a continuous covalently bonded random network of silicon-silicon and silicon-hydrogen atoms, with the predominant nearest neighbour environment similar to that of crystalline silicon. a-Si:H typically contains about 10 atomic percent hydrogen.Tritium can readily substitute for hydrogen in a-Si:H without altering the physicochemical properties of the material. Tritium decay leads to a change in the local bond structure of the material as helium detaches from bonds leaving behind dangling bonds. The decay rate of tritium and therefore the rate of dangling bond formation is determined by the half-life of tritium. Hence, tritium provides a unique avenue to dynamically study the effect of dangling bonds on the density of states in the energy gap and therefore on the optoelectronic properties of a-Si:H. Tritiated hydrogenated amorphous silicon (a-Si:H:T) was deposited using mixtures of tritium and silane gases in a dc saddle-field glow-discharge deposition system. The amount of tritium in the films was controlled by adjusting the relative flow of tritium and silane gases into the deposition chamber.Photoluminescence, isothermal capacitive transient spectroscopy and constant photocurrent spectroscopy were used to measure defect concentration as a function of time in the films. The defect concentration was found to increase between 1 and 2 orders of magnitude, in about 300 hours. Thermal annealing decreased the defect concentration. It was found that tritium permits a study of the change in the density of defect states due to dangling bond formation in a-Si:H without the uncertainties introduced by the use of multiple samples.

1998 International Conference on Applications of Photonic Technology III: Closing the Gap between Theory, Development, and Applications, 1998

ABSTRACT

Applied Physics Letters, 1993

Strained Sij-xGex quantum wells and multi-quantum wells, synthesized by solid source ebeam evapor... more Strained Sij-xGex quantum wells and multi-quantum wells, synthesized by solid source ebeam evaporated MBE on Si(100) substrates have been studied by low temperature photoluminescence (PL) spectroscopy. Phonon resolved transitions originating from excitons bound to shallow impurities in Sil.xGex layers were observed over the temperature range 2K to 100K and used to characterize SilxGex/Si heterostructures. Thin Sij-xGex quantum wells exhibited phonon-resolved PL spectra, similar to bulk material, but shifted in energy due to strain, quantum well width and Ge fraction. In single quantum wells confinement shifts up to-200 meV were observed (1.2 nm wells with x = 0.38) and NP linewidths down to 1.37 meV were obtained. The confinement shifts were modeled by hole confinement in Sit-xGex wells. An annealing study was performed to investigate the role of Si-Ge interdiffusion on luminescence. Both the geometrical shape and optical emission of the quantum well were found to significantly change through intermixing. In addition to near edge luminescence a broad band of intense luminescence was obtained from several Sil-xGex/Si heterostructures. Some layers exhibited both types of PL spectra. However, the broad PL band (peak energy-120meV below the strained bandgap) was predominant when the alloy layer thickness was greater than 2-10nm, depending on x, growth temperature, and substrate surface preparation. The strength of the broad PL band was correlated with the areal density of strain perturbations (-10 9 cm 2 per quantum well corresponding to a spacing of 300nm in the plane of the well; local lattice dilation-1.5 nm in diameter) observed in plan-view TEM. The first few wells of MQW exhibited only band edge luminescence as was revealed by etching off the upper MQW periods. In addition, post growth anneals at temperatures in the range 700°C to 1100°C were found to enhance band edge luminescence, while the broad luminescence band decayed to zero intensity. Interdiffusion at these these temperatures has been shown to dramatically change the QW shape and consequently interfacial asperities would be expected to disappear, consequently only shallow phonon resolved luminescence is observed in PL after annealing. The influence of PL measurement parameters such as excitation power density and PL sample temperature on the relative strengths of band edge versus broad band luminescence were also consistent with the presence of exciton traps at sites of reduced bandgap. SUMMARY We have deposited coherent Sil-xGex epilayer and multiple quantum well samples by MBE on Si(100) substrates and we have characterized these samples by low temperature PL spectroscopy and TEM. Phonon-resolved transitions originating from excitons bound to shallow impurities (boron) were observed by PL in addition to a broad band of intense luminescence. The shallow luminescence shifted reproducibly and systematically to higher energies with increased quantum confinement as observed for a wide range of well widths and Ge fractions. The blueshift was quantitatively consistent with hole confinement in SiGe wells whose effective thickness is reduced by interfacial roughening from the nominal dimensions. The broad PL band, typical of MBE SiGe, was predominant when the alloy layer thickness was greater than 40-100 A, depending on x (or the cumulative strain energy density in a multi layer structure). The strength of the broad peak was correlated with the areal density (up to-109 cm-2) of strain perturbations (local lattice dilation-15 A in diameter) observed in plan view TEM. The role of MBE growth parameters in determining optical properties was investigated by changing the quantum well thickness, composition and surface preparation. The transition from phonon-resolved, near-bandgap luminescence in thin layers to the broad PL band typical of thick layers was explained by a change in growth morphology. Band edge luminescence typical of coherent SiGe strained layers can be restored by annealing in the range 700 to 900C or by etching away the uppermost MQW layers which contain the highest density of growth asperities. The dominance of the broad luminescence band at relatively high PL measurement temperatures >-75K is consistent with shallow bound excitons diffusing to regions of lower bandgap(the strain perturbation) before self annihilation. The saturation effect, observed for the broad luminescence when exciton density and strain perturbation density are comparable, is further indirect evidence for saturable exciton sinks, characteristic of the MBE growth process. 14 19.

Japanese Journal of Applied Physics, 2007

Canadian Society of Forensic Science Journal

Journal of Vacuum Science and Technology, 1998

The dc saddle-field glow-discharge technique was employed to deposit undoped, phosphorus-doped an... more The dc saddle-field glow-discharge technique was employed to deposit undoped, phosphorus-doped and boron-doped hydrogenated diamond-like amorphous carbon films. The undoped films were grown using pure methane, while the doped films were grown using methane diluted with dopant gases (phosphine and diborane) in mole fraction ranging from 0.05 to 1×10−5. Secondary ion mass spectroscopy was used to determine the composition of these films. The results showed that various levels of doping in amorphous carbon films can be achieved predictably by using the appropriate mole fraction of dopant gases. The fractions of tetrahedral and trigonal bonds in the films were obtained using x-ray Auger electron spectroscopy. The optical energy gaps of the films were determined from optical absorption measurements. These data were correlated with the doping levels of the films. The electrical conductivities of the undoped and doped samples were determined at temperatures in the range from 225 to 380 °C....

Nanomaterials Design for Sensing Applications

Abstract Radio-frequency identification (RFID) tags have been proposed as a low-cost item-level t... more Abstract Radio-frequency identification (RFID) tags have been proposed as a low-cost item-level tracking system supplementing or replacing printed bar codes. Numerous applications are related to this technology. In this chapter, we present some of the novel solutions, based on nanostructured materials, used to improve the efficiency and flexibility of application of these devices.

Hydrogenated amorphous silicon (a-Si:H) specimens have been prepared by several deposition method... more Hydrogenated amorphous silicon (a-Si:H) specimens have been prepared by several deposition methods, including a novel technique (saddle field glow discharge), in order to investigate the influence of hydrogen and of inhomogeneities on the density of localized states deep in the energy gap of the material. The composition of the samples has been analyzed by infrared absorption spectroscopy, secondary ion mass spectroscopy and ^{15} N nuclear reaction profiling. The samples have been further characterized by electrical transport and optical measurements. The roles of polyhydrides and of monohydrides on the optoelectronic characteristics of these samples have been studied. Contrary to previous results in the literature, the present results demonstrate that it is the total amount of bonded hydrogen that primarily determines the above characteristics, rather than the proportion of monohydride and polyhydride bonding. The densities of states in the gap have been investigated using deep level transient spectroscopy (DLTS) and the results have been compared with the most recent theoretical calculations. The existence of a D^{ -} center located approximately at midgap and a preponderance of hole trap states over electron trap states are confirmed. Furthermore, a second peak has been observed in the DOS, in the upper part of the gap, for some of the more inhomogeneous samples. This peak is identified with a D^{+} center and the presence of this center is correlated with the total hydrogen content (C_{rm H}) and with the inhomogeneities introduced by the fluctuations in space of C_{rm H}. Furthermore, it is shown that DLTS is very sensitive to the variations in the DOS in the gap and remains one of the best techniques to date for probing the deep regions in the energy gap. Finally, a variation of the DLTS technique, namely voltage DLTS under constant illumination, has been introduced.

Photovoltaics (PVs) market is currently dominated by mono- and multi-crystalline silicon, which h... more Photovoltaics (PVs) market is currently dominated by mono- and multi-crystalline silicon, which has formed the basis of PV technology. However, breakthroughs in silicon solar cells photovoltaic energy conversion efficiency seem to be unlikely, and although further development in solar cells design might further increase their efficiency, cost projections are still not favorable. Hence, other materials and structures have been developed or are under development. In this chapter, we will examine different solutions, including thin film, heterojunction structures, organic materials, and hybrid (organic–inorganic) solar cells as possible alternatives to conventional crystalline PV devices. Furthermore, optoelectronic materials’ properties and solar cells configurations will be analyzed with respect to various PV applications.

An overview of thin-film energy related technologies is presented. Thin films technology includes... more An overview of thin-film energy related technologies is presented. Thin films technology includes both organic and inorganic materials. We will first focus on inorganic materials, starting with thin-film silicon, and will describe in detail other inorganic compound materials such as gallium arsenide and copper indium selenide. Organic thin films will also be discussed and the main differences between the two categories will be discussed. Novel applications linked to organic thin films will be introduced. Furthermore, growth and characterization techniques for thin films of both types will also be discussed, as they are crucial for the understanding of the field. Although applications of thin films can be mechanical, optical, and structural, a list of energy oriented applications will be presented at the end of the chapter. In particular, it will be shown that thin films are a major subject of investigation for uses in energy production, energy storage, and energy distribution and ma...

Materials Advances

Sequential embedding of metal complexes of 4′-(pyridin-4-yl)-2,2′:6′,2′′-terpyridine to a surface... more Sequential embedding of metal complexes of 4′-(pyridin-4-yl)-2,2′:6′,2′′-terpyridine to a surface-enhanced supports pre-functionalized with a templating layer results in hetero-bimetallic (Os–Fe, Co–Fe) and hetero-trimetallic (Co–Os–Fe) monolayer materials.