D. Della Sala

Lateral growth control in excimer laser crystallized polysilicon

Abstract The control of the structural properties of polysilicon obtained by excimer laser crystallization has become of great importance to further develop the polysilicon thin-film transistors technology. The most attractive crystallization regime is the so-called super lateral growth (SLG), characterized, however, by a very narrow energy density window and a strongly non-uniform grain size distribution. In this work we have investigated several approaches to achieve a control of the lateral growth mechanism through lateral thermal gradients, established by the opportune spatial modulation of the heating. To this purpose, three different patterned capping layers have been used: anti-reflective (SiO2), heat-sink (silicon nitride) and reflective (metal) overlayers. For all three types of overlayers, when the conditions to trigger the lateral growth mechanism are achieved, a band of oriented grains (1–2 μm wide) appears at the boundary between capped and uncapped region and extending in the more heated region. Among the different approach the use of reflective overlayers appears promising and further engineering of this process is in progress.

Gap density of states in amorphous silicon-germanium alloy: influence on photothermal deflection spectroscopy and steady-state conductivity measurements

A series of amorphous hydrogenated silicon‐germanium films has been produced by the glow‐discharge dissociation of a GeH4 +SiH4 +H2 gas mixture. The subgap optical absorption has been measured with the photothermal deflection spectroscopy (PDS) technique, and the results are discussed and referred to a model density of states. The density of defects has been found meaningfully correlated to the bulk material, despite a number of possible complications intrinsic of the PDS technique and/or of the material itself, which can potentially prevent the correct determination of the density of defects from the subgap optical absorption. Namely, the defect density ranges from 1016 cm−3 in a‐Si:H to 5×1017 cm−3 in a Ge:H, whereas the Urbach tail width is only slightly increased from 50 to 70 meV. The amount of disorder and the defect density of the material are compared to the predictions of an available thermodynamical model: the increasing number of defects brought about by germanium results in being an intrinsic ...

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.

Low-temperature laser-CVD thin film growth of SiC from Si2H6 and C2H2

Abstract SiC thin films have been synthesised by combining low pressure chemical vapour deposition from Si 2 H 6 and C 2 H 2 at 520°C and in situ KrF-excimer laser annealing. Glancing incidence X-ray analysis showed the quantity of polycrystalline SiC in the laser-irradiated area depends on the mass-flow ratio of the precursor gases. In the region outside the laser spot where the films are amorphous, X-ray photoelectron spectroscopy measurements showed an increased SiC character for low disilane:acetylene mass-flow ratios.

Morphological and structural effects of excimer laser treatment of amorphous silicon

The excimer laser irradiation of thin film amorphous silicon (a-Si) precursors on glass is a suitable method for obtaining high-performance polycrystalline silicon (p-Si) active layers for devices and circuits. By changing the experimental conditions, the recrystallization method generates a variety of microstructures that have direct impact on the material performance. An additional reason for microstructural characterization is introduced by the methods for spatially locating the recrystallization nuclei, used in more ergonomic concepts of device fabrication. Metal and SiO2 strip overlayers have been applied here, on a-Si to fix the position of the solidification seeds after laser melting. The control of many aspects of the thin film microstructure can be achieved with a collection of a few inspection techniques like AFM, SEM, EC contrast, TEM, X-ray diffraction (XRD), some of which require preliminary grain decoration treatment, and some do not. The results of different irradiation experiments, are herein illustrated, enlightened by the above characterization techniques, for providing information on surface morphology, grain arrangement, preferred orientation.

Electrooptical Analysis of Effects Induced by Floating Metallic Interlayers in Organic LEDs

The aim of this paper is to investigate the electrical and optical property modifications that can be related to the presence of a nanometric metallic layer at the interface between two organic emissive materials in a stacked structure. For purposes of comparison, reference devices have also been analyzed to emphasize the increase of electrical switching and hysteresis behaviors in current-voltage plots and spectral variations in electroluminescence. In this paper, we have tried to summarize the electrical effects of the floating nanometric thin metallic layer by extracting a small number of parameters which can represent the current state of the device.

Laser-assisted chemical vapor deposition of thick poly-Si layers for solar cells

Abstract The growth of polycrystalline silicon on glass by laser-assisted chemical vapor deposition has been studied with the aim of identifying a light absorber layer for solar cells, with superior material quality compared to other technologies available for low-temperature substrates. One-dimensional calculations of the thermal wave produced by laser irradiation have been used to elucidate the complex interaction of the molten silicon surface layer with the substrate during the growth. The experiments show the relevant role played by the seed layer used as the growth initiator. The morphology of the laser-crystallized films has been analysed by scanning electron microscopy and X-ray diffraction. Polysilicon films, 2 μm thick, with a compact structure consisting of 1–2-μm grains that are almost monocrystalline, have been obtained.

ENABLING DISTRIBUTED VOC SENSING APPLICATIONS: TOWARD TINYNOSE, A POLYMERIC WIRELESS E-NOSE

In this work, we present the development of a novel wireless e-nose platform designed for indoor distributed VOC detection and quantification. The proposed w-nose, called TinyNose, rely on a small polymeric sensor array that is connected to a commercial wireless mote by means of custom developed electronics. A custom developed software architecture allow for signal acquisition, processing and transmission to a data sink where data are stored and/or presented to the remote user. In this work a preliminary assessment of TinyNose capabilities to operate in open air configuration is conducted by using different source of indoor VOC pollution to be detected and classified by the developed architecture.

Characterisation of thin amorphous silicon films with multiple internal reflectance spectroscopy

Abstract Infrared multiple internal reflection spectroscopy has been applied to the characterisation (ex situ) of thin amorphous silicon layers on crystalline silicon substrates. The specimens are tightly clamped against a Ge-prism with a 45° bevel angle for the entrance and exit sides, allowing for 25 multiple internal reflections. The stretching bands of SiHn bonds in amorphous Si are detected over a range of film thickness from 3 to 600 nm. Very thin films (with thickness less than 50 nm) exhibit different hydrogen bonding compared to the thicker ones, due to sublayers with different bonding configurations.

Effect of the Layer Geometry on Ink-Jet Sensor Device Perfomances

In this work, we report on ink-jet manufactured chemical sensors based on polymer carbon black composites. In particular, in order to study the correlation between ink-jet pattern geometry and device response sensors characterized by sensing layers consisting of polystyrene/carbon black composites lines of various shape and surfaces were fabricated. The morphology of films, the electrical characteristics and time stability of sensors were investigated.