C. Minarini

Amorphous silicon waveguides and light modulators for integrated photonics realized by low-temperature plasma-enhanced chemical-vapor deposition.

A new amorphous silicon waveguide is realized by use of amorphous silicon carbon as cladding material. The structure is characterized both experimentally and theoretically, and its application for optical interconnections in photonic integrated circuits on silicon motherboards is proposed. The fabrication process is based on low-temperature (220 degrees C) plasma-enhanced chemical-vapor deposition and is compatible with standard microelectronic processes. Propagation losses of 1.8 dB/cm have been measured at the fiber-optic wavelength of 1.3 microm. A strong thermo-optic coefficient has been measured in this material at this wavelength and exploited for the realization of a light-intensity modulator based on a Fabry-Perot interferometer that is tunable by temperature.

Investigations on the crystallisation properties of RF magnetron sputtered indium tin oxide thin films

X-Ray diffraction and X-ray texture analysis have been carried out on RF magnetron sputtered ITO (indium tin oxide) thin films on glass substrates in order to understand their crystal growth behaviour. Films deposited at various deposition rates and gas compositions were subjected to investigation. X-Ray diffraction analysis revealed the polycrystalline structure of the films with a difference in crystallographic orientation without any change in the BCC (Body Centred Cubic) structure. Both the increase in deposition rate and gas composition reflects the change in crystallographic orientation. X-Ray texture analysis on the grown films revealed also a difference in film texturization following the change in crystallographic orientation. Strong orientation of crystallites perpendicular to the substrate were observed for films grown with (400) preferential plane.

Structural and optical characterization of amorphous and crystalline evaporated WO3 layers

Abstract Amorphous and crystalline WO3 films show electrochromic properties by means of double injection of cations and electrons. At low substrate temperature thermal evaporation technique produces an amorphous material mixed to a nanocrystalline phase. When this material is annealed at temperatures higher than 400°C a fully crystalline structure is obtained. Different absorption mechanisms are responsible for colored amorphous and crystalline WO3 layers, small polaron transitions and Drude-like free electron scattering, respectively. Furthermore, both O/W ratio and crystalline structure affect electrochromic efficiency of a WO3 film. It has been confirmed that a hydrogen tungsten bronze structure appears when the material is colored. Finally, a self-bleaching process occurs when a colored WO3 film is exposed to an oxidative environment.

Lithium iron oxide as alternative anode for li-ion batteries

Lithium–iron oxide Li–Fe–O was synthesized by solid state reaction between Li2CO3 and Fe2O3. The sample was characterized by X-ray powder diffraction. The XRD patterns showed well defined reflections corresponding to α-LiFeO2 and the spinel LiFe5O8 in a molar ratio of 9:1. The material was tested as alternative anode for lithium-ion batteries. It exhibited good cyclability delivering about 120 mAh/g after 500 deep charge/discharge cycles. Unlikely, the use of the material as intercalation anode in practical cells is hindered by the irreversible uptake of lithium that takes place during the first lithium insertion. X-ray diffraction pattern showed that during this step a reduction of the lithium iron oxide occurs leading to the formation of lithium oxide and iron metal.

Influences of Sputtering Power and Substrate Temperature on the Properties of RF Magnetron Sputtered Indium Tin Oxide Thin Films

Indium tin oxide thin films have been deposited on glass substrates using rf magnetron sputtering at different power densities (0.27–0.80 W/cm2) and at different substrate temperatures (RT-250) °C. Film structure, crystallite size and orientation, optical absorption and bandgap have been studied to characterize the films. Carrier concentration and Hall mobility have been determined by Hall effect. X-ray diffraction (XRD) analysis of room temperature (RT) deposited samples reveals a structural change from amorphous to mixed amorphous/polycrystalline structure with preferred orientation with increasing rf power density. The increase in substrate temperature results in a similar structural evolution from amorphous to a mixed phase followed, at temperatures higher than 200°C, by a polycrystalline phase with orientation. The study clearly indicates that ITO films dominated by oxygen vacancies prefer to grow with oriented crystallites whereas the oriented films are characterized by a more effective doping by tin.

P-type strontium-copper mixed oxide deposited by e-beam evaporation

Abstract P-type thin films of strontium–copper mixed oxide have been deposited by e-beam evaporation technique on glass and quartz substrates starting from SrCu 2 O 2 powders. A study of optical, electrical and structural properties was performed on the thin films, varying deposition parameters such as the substrate temperature and the oxygen partial pressure. For the first time, polycrystalline films of SrCu 2 O 2 were obtained using e-beam evaporation in O 2 atmosphere at relatively low temperature (350 °C) with transparency of over 60% in the visible light range and a high optical transmittance in near-infrared region. The optical band gap of thin film was estimated to be ∼3.12 eV. Seebeck and Hall effects measurements confirmed the p-type nature of semiconductors and conductivity as high as 5.3×10 −2 S/cm for non-intentionally doped materials was measured. Hole concentration and mobility at room temperature were 1.5×10 17 cm −3 and 2.2 cm 2 /Vs, respectively. X-Ray diffraction measurements revealed a polycrystalline structure of the films.

Improvement in electrochromic response for an amorphous/crystalline WO3 double layer

Abstract Amorphous WO3 films have been deposited by E-beam evaporation technique at a substrate temperature of 80°C using different base pressure values. The best results have been obtained starting from a mild base pressure (1×10−5 Torr) probably due to the presence of water and oxidative atmosphere in the evaporation chamber. TEM investigations have shown that thermally evaporated WO3 films have an amorphous metastable structure, which subjected to energetic solicitations changes towards a nanocrystalline phase. The amorphous material subsequently annealed at a temperature of 500°C exhibits a fully crystalline structure. Also, in this case good electrochromic properties have been observed. Afterwards a WO3 double layer structure has been realised depositing an amorphous film upon a crystalline layer. The amorphous/crystalline structure properties have been compared with the amorphous and crystalline single layer ones. The colouration response and the transmittance asymptotic value during the bleaching phase have been better for the double layer.

Printing of polymer microlenses by a pyroelectrohydrodynamic dispensing approach.

The investigation of a method for fabricating microlenses by a nozzle-free inkjet printing approach is reported. The new method, based on a pyroelectrohydrodynamic mechanism, is also able to dispense viscous liquids and to draw liquid phase drops directly from the reservoir. Specifically, by dispensing optical grade polymer dissolved in different solvent mixtures, microlenses were printed with a pattern defined directly through this deposition method. The reliability of the microlenses and the tunability of their focal properties were demonstrated through profilometric and inteferometric analyses.

Optical properties of polystyrene-ZnO nanocomposite scattering layer to improve light extraction in organic light-emitting diode

In this work, experimental measurements on polystyrene-ZnO nanocomposite scattering films and on organic light-emitting device with and without the scattering layers are presented. The results are also compared with Henyey-Greenstein radiativetransfer model to narrow down the parameters that can be important in the identification of more suitable scattering layers. As a result, an increase of efficiency of about 30% has been obtained that it can be translated in 60% of outcoupled light in respect to the total generated amount.

Insights into thermal degradation of organic light emitting diodes induced by glass transition through impedance spectroscopy

Highly sensitive alternate current (ac) impedance measurements with variable temperature have been performed to investigate the optical and electrical failure mechanisms during the glass transition phenomena in the archetypal ITO/TPD/Alq3/Al organic light emitting diode (OLED) structure. Since the device degradation is mainly related to the lower glass transition temperature (Tg) of the N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD), this study is focused on the frequency response of thin TPD films approaching the glassy region. The related experimental data are discussed in the framework of the universal dielectric response model. By ac measurements, TPD glass transition temperature is located and temperature regions with different OLED behaviors are evidenced. The relation between the behaviors of TPD frequency response and of the OLED electro-optical response, while the temperature approaches the glass transition region, is discussed.