Rodrigo Martins

We report, for the first time, results on transparent ZnO:Al thin films deposited on polyester (Mylar type D, 100 μm thickness) substrates at room temperature by magnetron sputtering. The structural, optical and electrical properties of the deposited films have been studied. The samples are polycrystalline with a hexagonal wurtzite structure and a strong crystallographic c-axis orientation (0 0 2) perpendicular to the substrate surface. The ZnO:Al thin films with 83% transmittance in the visible region and a resistivity as low as 3.6 × 10-2 Ωcm have been obtained, as deposited. The obtained results are comparable to those obtained on glass substrates, opening a new field of low cost, light weight, small volume, flexible and unbreakable large area optoelectronic devices. © 2002 Elsevier Science Ltd. All rights reserved.

In this work, we study the electro-optical behaviour of cellulose/liquid crystal-based composite systems, in particular the influence of the flexible substrates and its conductive layers in the electro-optical behaviour of these kind of cells. Four cells were made using, respectively, two different substrates (a flexible polymer (poly(ethylene terephthalate) (PET)) and a soda lime glass) and two different conductive layers (indium tin oxide (ITO) and aluminium zinc oxide (AZO)). The conductive layer (AZO) was deposited in both, flexible and rigid substrates, for the same conditions, and the same substrates coated with ITO are commercially available. The cells were prepared from solid films of hydroxypropylcellulose (HPC) (30 μm thick) cross linked with 1,4-diisocyanatobutane (BDI) (7% w/w) and the nematic liquid crystal E7 (Merck, UK). The four different substrates were electrically and morphologically characterised. We have analysed all samples by light transmission and determined the maximum transmission, contrast and Von. We show a comparison of the results obtained for both flexible and rigid cells and discuss them in terms of the proposed working mechanism for these systems. © 2002 Elsevier Science B.V. All rights reserved.

In this work we present results concerning the composition and structure of intrinsic thin film silicon carbide alloys obtained by hot wire and hot wire plasma assisted techniques using ethylene as carbon gas source. The data show that by increasing the percentage of ethylene in the gas mixture from 14% to 60% the optical band gap is enhanced from 1.8 eV to 2.3 eV, for films produced by hot wire technique at a filament temperature of 2123K (1850°C). This is attributed to the increase of carbon incorporation, which was confirmed by the infrared spectra data where an increase is observed in the SiC stretching vibration mode ascribed to the peak located at around 750cm-1. On the other hand, the films produced by combining hot wire and rf plasma show a more efficient carbon incorporation. The SEM photographs of samples produced with hot wire technique reveal an amorphous structure, confirmed by micro-Raman spectroscopy data, while the samples produced with plasma assisting the process show a granular structure with grain sizes in the range of 100-200nm. © 2002 Elsevier Science Ltd. All rights reserved.

Aluminium doped zinc oxide thin films (ZnO:Al) have been deposited on polyester (Mylar type D, 100 μm thickness) substrates at room temperature by r.f. magnetron sputtering. The structural, morphological, optical and electrical properties of the deposited films have been studied. The samples are polycrystalline with a hexagonal wurtzite structure and a strong crystallographic c-axis orientation (002) perpendicular to the substrate surface. The ZnO:Al thin films with 85% transmittance in the visible and infra-red region and a resistivity as low as 3.6×10-2 Ωcm have been obtained, as deposited. The obtained results are comparable to those ones obtained on glass substrates, opening a new field of low cost, light weight, small volume, flexible and unbreakable large area optoelectronic devices.

In order to correlate the MIS devices performance with different surface oxidation methods, AFM, spectroscopic ellipsometry and infrared spectroscopy measurements were performed in a-Si:H films, before and after surface oxidation, using different oxidation techniques and oxides: thermal dry (in air), wet (in H2O2) and by oxygen plasma, while MIS (metal-insulator-semiconductor) devices were characterized by I-V curves, under dark and AM1.5 illumination conditions. The a-Si:H films were grown by the PECVD technique, in a modified triode configuration reactor to allow a precise control of the ion bombardment during the film deposition. We found that the growth of a thin layer of oxide by chemical processes on the top of the a-Si:H surface can cause changes on the surface morphology that are reflected in the electrical behaviour of the devices. The oxygen plasma treatment, cause the rearrangement of the surface atoms leading to a change of their morphology and to the improvement of the electrical properties of the surface for a MIS applications.

This paper deals with the determination of plasma impedance and ion density in r.f. plasmas using different mixtures of silane with methane or ethylene and r.f. powers. The aim is to correlate these parameters with carbon and hydrogen contents of the films produced. The data achieved show that the best carbon incorporation is achieved using ethylene gas mixtures, under low gas mixture concentration, where the substrate also sustains a low ion bombardment. The data also show that particulates in the plasma can be more easily formed in the ethylene-based processes. © 2001 Published by Elsevier Science B.V.

Hall effect measurements have been used to evaluate the conduction mechanism, exhibited by tin oxide thin film gas sensors deposited by spray pyrolysis. Two experiments have been carried out: (i) Hall measurements in air and (ii) in the presence of methane (first results reported), both as a function of temperature. From the measurements performed it was possible to infer the potential barrier and its dependence with the atmosphere used. The results obtained for the carrier concentration and mobility have been analysed in the light of the oxygen diffusion mechanism at grain boundaries by using the grain boundary-trapping model. In the presence of the methane gas the electrical resistivity decreases due to the lowering of the inter-grain boundary barrier height. © 2001 Published by Elsevier Science Ltd.

In order to understand the kinetics of formation of interface/surface states and its correlation on the final device performance, a preliminary study was performed on MIS structures, before and after surface oxidation/passivation, using different oxidation techniques and oxides: thermal (in air), chemical (in H2O2) and oxygen plasma. The devices used in this work are based on a glass/Cr/a-Si:H(n+)/a-Si:H(i)/SiOx/Pd structures, where the amorphous silicon intrinsic layer (i a-Si:H) with a photosensitivity of 107 was deposited by a modified plasma enhanced chemical vapour deposition (PECVD) triode system. The electrical properties of a-Si:H MIS structures were investigated by measuring their diode current-voltage characteristics in the dark and under illumination as well as the spectral response, as a function of the various oxidation techniques. Infrared spectroscopy and spectroscopic ellipsometry were used as a complementary tool to characterise the oxidised surface.

The increasing demand in automation processes in finishing techniques also calls for automatic measurement and inspection methods. These methods ought to be installed as close as possible to the production process and they ought to measure the values needed in a safe and fast way, without disturbing the process itself. Simultaneously they should be free of wear and insensitive against mechanical perturbations. This approach can be reached by proper combination of the laser triangulation technique with an array of linear position sensitive detectors, able to supply information about the surface finishing of an object. This is the aim of this paper that envisages to present experimental results of the performances exhibited by such an array constituting 128 elements. The analogue information supplied by this array is processed by an analogue/digital converter, directly coupled to the array and whose information is computer processed, concerning the recognition of patterns and the processing of information collected over the object to be inspected. © 1999 Elsevier Science S.A. All rights reserved.

This work's aim is to report for the first time the cubic to hexagonal phase transition in tin-doped In2O3 films with a Sn/In atomic ratio of 0.03, fabricated at low temperature and normal pressure from alcoholic solution of InCl3 and SnCl4. The performed X-ray diffraction measurements show a difference between crystallographic symmetry of thin (100 nm) and thick (400 nm) films prepared in the same conditions: the structure of thick films can be related to high pressure In2O3 hexagonal system with a preferred orientation of c-axis parallel to the substrate surface, while thin films present a cubic symmetry with columnar (400) grain orientation. Phase transition nature is connected with non-axial tensile deformation of indium oxide grid due to insertion of chlorine ions in the position of two diagonally opposite oxygen vacancies in In2O3 network.

Wide band gap microcrystalline silicon films have aroused considerable interest since they combine some electro-optical advantages of amorphous and crystalline materials highly important to produce electro-optical devices such as TFTs and solar cells. In this paper we present results concerning the electro-optical characteristics of highly transparent and conductive n-type μc-Si based films. Here, emphasis is given to the production of n-type μc-films with optical gaps of 2.3 eV and dark conductivity's of 6.5 Scm-1.

Using the Flying Spot Technique (FST) we have studied minority carrier transport parallel and perpendicular to the surface of amorphous silicon films (a-Si:H). To reduce slow transients due to charge redistribution in low resistivity regions during the measurement we have applied a strong homogeneously absorbed bias light. The defect density was estimated from CPM measurements. The steady-state photocarrier grating technique (SSPG) is a 1-dimensional approach. However, the modulation depth of the carrier profile is also dependent on film surface properties, like surface recombination velocity. Both methods yield comparable diffusion lengths when applied to a-Si:H.

The current transport in metal-amorphous semiconductor barriers is examined by solving the proper Poisson's equation and transport equations within the semiconductor's space charge region taking into account the role of trap shallow states distribution function. The effect of metal is also included through appropriate boundary conditions of the above solutions. Generalized transport equations will be derived either when thermionic drift-diffusion emission process dominates or when the conduction mechanism is mainly due to drift-diffusion emission. Both situations will be analysed with or without neglecting carriers losses during their collision free path, from which a tractable expression for the current-voltage characteristic will be determined.