Papers by Stavroula Georga
Atomic-Layer-Deposited Aluminum and Zirconium Oxides for Surface Passivation of TiO2in High-Efficiency Organic Photovoltaics
Advanced Energy Materials, 2014
ABSTRACT The reduction in electronic recombination losses by the passivation of surfaces is a key... more ABSTRACT The reduction in electronic recombination losses by the passivation of surfaces is a key factor enabling high-efficiency solar cells. Here a strategy to passivate surface trap states of TiO2 films, used as cathode interlayers in organic photovoltaics (OPVs), through applying alumina (Al2O3) or zirconia (ZrO2) insulating nanolayers by thermal atomic layer deposition (ALD) is investigated. The results suggest that the surface traps in TiO2 are oxygen vacancies, which cause undesirable recombination and high electron extraction barrier, reducing the open-circuit voltage and the short-circuit current of the complete OPV device. It is found that the ALD metal oxides enable excellent passivation of the TiO2 surface followed by a downward shift of the conduction band minimum. OPV devices based on different photoactive layers and using the passivated TiO2 electron extraction layers exhibit a significant enhancement of more than 30% in their power conversion efficiencies compared to their reference devices without the insulating metal oxide nanolayers. This is a result of significant suppression of charge recombination and enhanced electron extraction rates at the TiO2/ALD metal oxide/organic interface.
Ultra high density three dimensional capacitors based on Si nanowires array grown on a metal laye... more Ultra high density three dimensional capacitors based on Si nanowires array grown on a metal layer Appl. Phys. Lett. 101, 083110 (2012); 10.1063/1.4746762 Dispersive capacitance and conductance across the phase transition boundary in metal-vanadium oxide-silicon devices J. Appl. Phys. 106, 034101 (2009); 10.1063/1.3186024 Molecular-beam epitaxial growth of III-V semiconductors on Ge ∕ Si for metal-oxide-semiconductor device fabrication Appl. Phys. Lett. 92, 203502 (2008); 10.1063/1.2929386
A study on the influence of the cations and anions on the electrical conductivity in thin films of potassium and rubidium halides
Journal of Physics D Applied Physics
ABSTRACT
Construction of a new multiple thin-film deposition system
Journal of Physics E Scientific Instruments
... S I979 The temperature dependence of Drude absorption in thin polycrystalline gold films Thin... more ... S I979 The temperature dependence of Drude absorption in thin polycrystalline gold films Thin Solid Films 57 83 Nocerino G and Singer KE 1979 The electrical and compositional structure of thin Ni-Cr films Thin Solid Films 57 343 Walton D, Rhodin TN and Rollins RW 1963 ...
Photoconductivity studies on evaporated thin films of rubidium halides
Journal of Physics D Applied Physics
ABSTRACT
A study on the Dielectric Behavior of Epoxy/Hafnium Oxide nanopowder composites
Advanced Composites Letters
ZrO2 and Al2O3 Thin Films on Ge(100) Grown by ALD: An XPS Investigation
Surface Science Spectra, 2011
Microelectronics Reliability, 2007
This work is an attempt to estimate the electrical properties of SiO 2 thin films by recording an... more This work is an attempt to estimate the electrical properties of SiO 2 thin films by recording and analyzing their infrared transmission spectra. In order to study a big variety of films having different infrared and electrical properties, we studied SiO 2 films prepared by low pressure chemical vapor deposition (LPCVD) from SiH 4 + O 2 mixtures at 425°C and annealed at 750°C and 950°C for 30 min. In addition thermally grown gate quality SiO 2 films of similar thickness were studied in order to compare their infrared and electrical properties with the LPCVD oxides. It was found that all studied SiO 2 films have two groups of Si-O-Si bridges. The first group corresponds to bridges located in the bulk of the film and far away from the interfaces, the grain boundaries and defects and the second group corresponds to all other bridges located near the interfaces, the grain boundaries and defects. The relative population of the bulk over the boundary bridges was found equal to 0.60 for the LPCVD film after deposition and increased to 4.0 for the LPCVD films after annealing at 950°C. Thermally grown SiO 2 films at 950°C were found to have a relative population of Si-O-Si bridges equal to 3.9. The interface trap density of the LPCVD film after deposition was found equal to 5.47 · 10 12 eV À1 cm À2 and decreases to 6.50 · 10 10 eV À1 cm À2 after annealing at 950°C for 30 min. The interface trap density of the thermally grown film was found equal to 1.27 · 10 11 eV À1 cm À2 showing that films with similar Si-O-Si bridge populations calculated from the FTIR analysis have similar interface trap densities.
Thin Solid Films, 2011
In this work we examine the influence of thermal oxidation on the electrical characteristics of u... more In this work we examine the influence of thermal oxidation on the electrical characteristics of ultra-thin strained silicon layers grown on relaxed Si 0.78 Ge 0.22 substrates under moderate to high thermal budget conditions in N 2 O ambient at 800°C. The results reveal the presence of a large density of interfacial traps which depends on the oxidation process. As long as the strained silicon layer remains between the growing oxide and the underlying Si 0.78 Ge 0.22 layer, the density of interface traps increases with increasing oxidation time. When the oxidation process consumes the s-Si layer the interface state density undergoes a significant reduction of the order of 40%. This experimental evidence signifies that the strained silicon-Si 0.78 Ge 0.22 interface is a major source of the measured interfacial defects. This situation can be detected only when the front SiO 2 -strained silicon interface and the rear strained silicon-Si 0.78 Ge 0.22 interface are in close proximity, i.e. within a distance of 5 nm or less. Finally, the influence of the material quality deterioration-as a result of the thermal treatment-to the interfacial properties of the structure is discussed.
Thin Solid Films, 2008
In this work we examine the influence of thermal oxidation on the electrical characteristics of u... more In this work we examine the influence of thermal oxidation on the electrical characteristics of ultra-thin strained silicon layers grown on relaxed Si 0.78 Ge 0.22 substrates under moderate to high thermal budget conditions in N 2 O ambient at 800°C. The results reveal the presence of a large density of interfacial traps which depends on the oxidation process. As long as the strained silicon layer remains between the growing oxide and the underlying Si 0.78 Ge 0.22 layer, the density of interface traps increases with increasing oxidation time. When the oxidation process consumes the s-Si layer the interface state density undergoes a significant reduction of the order of 40%. This experimental evidence signifies that the strained silicon-Si 0.78 Ge 0.22 interface is a major source of the measured interfacial defects. This situation can be detected only when the front SiO 2 -strained silicon interface and the rear strained silicon-Si 0.78 Ge 0.22 interface are in close proximity, i.e. within a distance of 5 nm or less. Finally, the influence of the material quality deterioration-as a result of the thermal treatment-to the interfacial properties of the structure is discussed.
Thin Solid Films, 1994
Titanium disilicide is formed by interdiffusion of multilayer structures, obtained by alternate a... more Titanium disilicide is formed by interdiffusion of multilayer structures, obtained by alternate amorphous silicon and titanium thin films, deposited by electron gun evaporation. The results presented here are complementary to previously published results on the influence of the bilayer Si-Ti thickness ratio on the thin film properties. Transmission electron microscopy, spectroscopic ellipsometry and electrical measurements in the temperature range 80-450 K are used. It is found that all samples with an excess of silicon in the deposited layers show lower sheet resistivity, compared with samples with a deficiency in the deposited silicon. This is in agreement with the grain sizes measured by transmission electron microscopy. The surface and interface of TiSi2 obtained from multilayers are very sharp compared with TiSi 2 films obtained from a single deposited titanium layer on top of the silicon substrate.
Polymer, 1995
POLYMER Volume 36 Number 25 1
Dielectric characterization of macroporous thick silicon films in the frequency range 1 Hz-1 MHz
physica status solidi (c), 2008
ABSTRACT
Porous anodic alumina thin films on Si: interface characterization
physica status solidi (c), 2008
... 1 IMEL/NCSR Demokritos, Terma Patriarchou Grigoriou, Aghia Paraskevi, 15310 Athens, Greece ... more ... 1 IMEL/NCSR Demokritos, Terma Patriarchou Grigoriou, Aghia Paraskevi, 15310 Athens, Greece 2 Department of Physics, University of Patras, 26500 Patras ... through PAA, used as masking layer with openings in the pores, resulted in the for-mation of SiO2 dots at each pore ...
Physica Scripta, 2012
The dielectric response of hafnium oxide nanopowder was studied in the frequency range of 10 −2 -... more The dielectric response of hafnium oxide nanopowder was studied in the frequency range of 10 −2 -10 6 MHz and in the temperature range of 20-180 • C. Broadband dielectric spectroscopy was applied and the experimental results were analyzed and discussed using the electric modulus (M * ) and alternating current (ac) conductivity formalisms. The analyses of the dc conductivity and electric modulus data revealed the presence of mechanisms which are thermally activated, both with almost the same activation energy of 1.01 eV. A fitting procedure involving the superposition of the thermally activated dc conductivity, the universal dielectric responce and the near constant loss terms has been used to describe the frequency evolution of the real part of the specific electrical conductivity. The conductivity master curve was obtained, suggesting that the time-temperature superposition principle applies for the studied system, thus implying that the conductivity mechanisms are temperature independent.
Organic Electronics, 2013
In this work, we demonstrate efficient polyfluorene-based light emitting diodes on which conforma... more In this work, we demonstrate efficient polyfluorene-based light emitting diodes on which conformal, thin ZrO 2 layers, formed by atomic layer deposition at a relatively low temperature (175°C), in order to avoid introducing any damage in the organic under layer, efficiently inject electrons from their high lying conduction band to the polymer's LUMO. An optimal thickness of 2 nm for ZrO 2 results in a threefold improvement in luminous current efficiency compared to the reference device. The relationship between the thickness of the ZrO 2 layer and the device operational characteristics is further investigated and the possible reasons for the improved device performance are discussed based on the experimental results obtained by a combination of photoemission spectroscopy and electrical/ optical measurements.
Nanotechnology, 2009
A versatile processing technique for fabricating epoxy nanocomposites with a high weight fraction... more A versatile processing technique for fabricating epoxy nanocomposites with a high weight fraction of oxidized multi-walled carbon nanotubes is presented. Thin carbon nanotube based preforms were prepared through an oxidation-filtration protocol and then immersed in a pre-polymerized epoxy/curing agent solution in acetone. By adjusting the conditions for the oxidation of carbon nanotubes and the epoxy concentration in the as-prepared solution, high loading of graphitic nanostructures was obtained. Tensile tests indicated that the elastic modulus and strength of certain composites prepared by in situ polymerization as above were improved by 100% and 60%, respectively, compared to neat epoxy. In addition, the composite sheets showed comparable electrical conductivity values to the neat carbon nanotube paper. These results suggest that targeted chemical modification of the carbon nanotube surface is an effective way to enhance the electrical and mechanical properties of carbon nanotube-polymer composites.
Macromolecular Symposia, 2013
Epoxy resin/barium titanate nano-and micro-composites were prepared and studied, varying the fill... more Epoxy resin/barium titanate nano-and micro-composites were prepared and studied, varying the filler content. Thermal properties of the composite systems were examined by means of Thermogravimetric/Differential Thermal Analysis, while dielectric response was assessed via Dielectric Relaxation Spectroscopy. Decomposition temperature found not to vary with filler size, while thermal stability of reinforced systems is enhanced compared to epoxy resin. Nanocomposites exhibit, improved thermal properties compared to microcomposites at the same filler content. Composite systems attain higher values of dielectric permittivity, and relaxation processes are present in all studied systems.
Probing the properties of atomic layer deposited ZrO2 films on p-Germanium substrates
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2013
ABSTRACT Zirconium oxide (ZrO2) thin films of 5 and 25 nm thickness were deposited by atomic laye... more ABSTRACT Zirconium oxide (ZrO2) thin films of 5 and 25 nm thickness were deposited by atomic layer deposition at 250 °C on p-type Ge substrates. The stoichiometry, thickness, and valence band electronic structure of the ZrO2 films were investigated by x-ray and ultraviolet photoelectron spectroscopies. For the electrical characterization, metal-oxide-semiconductor (MOS) capacitive structures (Pt/ZrO2/p-Ge) have been fabricated. Capacitance–voltage and conductance–voltage (C–V, G–V) measurements performed by ac impedance spectroscopy in the temperature range from 153 to 313 K reveal a typical MOS behaviour with moderate frequency dispersion at the accumulation region attributed to leakage currents. For the determination of the leakage currents conduction mechanisms, current density–voltage (J–V) measurements were carried out in the whole temperature range.
Journal of Physics: Condensed Matter, 1990
This is a report of measurements of the electrical resistivity and of the Hall coefficient of CoG... more This is a report of measurements of the electrical resistivity and of the Hall coefficient of CoGe and CoGe2 films between 80 and 520 K. Both germanides behave as metallic conductors with resistivities increasing linearly with increasing T over the whole temperature range. There is a slight deviation from linearity for CoGe,! which is attributed to the fact that the mean free path of the carriers becomes comparable with the lattice dimensions at high temperatures. The Hall coefficient is negative for CoGe, giving an apparent carrier concentration of (0.86 2 0.01) x lo2* m-3 and a Hall mobility of (11.0 f 0.1) x m2 V-' SKI at room temperature. On the other hand, p-type conduction is revealed for CoGe,! with apparent carrier concentration and Hall mobility equal to (1.93 f 0.01) x lo2* m-3 and (2.2 & 0.1) x m2 V-' SKI, respectively, at 300 K. For a quantitative explanation of the resistivity and Hall behaviour of germanides, detailed information about the energy band structure near the Fermi level is required. As this information is lacking, the rough approximation of the two-band model is the only way to obtain insight into the electrical properties of germanides.
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Papers by Stavroula Georga