Rodrigo Martins

This paper studies the dc and ac impedance behaviour of undoped ZnO thin films produced by spray pyrolysis and rf magnetron sputtering under UV light illumination, at room temperature, emphasising the role that the crystallite size, structure, surface morphology and the state of surface have on the electrical responsivities obtained. The results achieved show that the sputtered films with crystal sizes of about 4 nm exhibit dc electrical UV responsivities of 108. On the other hand, the spray pyrolysis films exhibit the lowest dc responsivities, due the high crystal sizes and state of surface contamination, to which very good capacitance responses were obtained, mainly due to the degree of porosity exhibit by these films when produced at low temperatures. Based on that, a two-phase electrical model is proposed to explain the set of behaviours observed. © 2005 Elsevier B.V. All rights reserved.

The aim of this work is to present an analytical model able to interpret the experimental data of the dependence of film's uniformity on the discharge pressure, gas flow and temperature used during the production of thin films by the plasma enhancement chemical vapour deposition technique, under optimised electrode's geometry and electric field distribution. To do so, the gas flow is considered to be quasi-incompressible and inviscous leading to the establishment of the electro-fluid-mechanics equations able to interpret the film's uniformity over the substrate area, when the discharge process takes place in the low power regime.

The aim of this work is to present an analytical model which can to interpret the role of the collecting resistive layer on the static performances exhibited by 2D amorphous silicon hydrogenated p-i-n thin film position sensitive detectors. In addition, experimental results concerning the device linearity and spatial resolution are presented and checked against the predicted values of the analytical model proposed. © 1999 Elsevier Science S.A. All rights reserved.

This paper deals with the study of the role of gas temperature and of the ratio of r.f. power to gas flow on the particle's formation in amorphous silicon films grown by the plasma enhanced chemical vapour deposition technique, by monitoring the plasma impedance behaviour under different process conditions. The results achieved show the existence of two main boundary regions separating the so-called α-regime (no powder formed) from the γ-regime (powder formed). Those regions are reached either by heating the gas, changing the gas pressure or using high power to gas flow ratios, corresponding to the establishment of a balance between the plasma resistance and the plasma reactance. In the β-region the probability to incorporate nanoparticles in the films is low and the films exhibit photosensitivity's of about 105 with density of states determined by the constant photocurrent method below 6×1015 cm-3 with Urbach energies below 50 meV. In the θ-region small nanoparticles can be incorporated leading to films with density of states below 3×1015 cm-3, with Urbach energies above 50 meV and photosensitivity's above 106, about two orders of magnitude larger than that of conventional amorphous silicon grown in the α-regime.

This paper aims to discuss the effect of order and disorder on the electrical performances of covalent silicon semiconductors and ZnO based ionic oxide semiconductors used as active channel layers in thin film transistors. The effect of disorder on covalent semiconductors directly affects their electrical transport properties due to the asymmetric behaviour of sp states, while in ionic oxide semiconductors it is found that this effect is small due to the fact that angular disorder has no effect on the spherical symmetry of s states. To this we must add that the mobility of carriers in both systems is quite different, being also affected by electron-phonon interactions (weak in silicon and strong in ionic oxides leading to formation of polarons). Besides, the impurity doping effect and the presence of vacancies in disordered silicon and in ionic oxides behave differently, which will influence the thin film properties and so, the performances of the devices produced. © 2007 Springer-Verlag.

The aim of this paper was to present results of the role of the d.c. grid bias on the silane plasma impedance and its I-V characteristics to grow undoped amorphous silicon (a-Si:H) films by plasma enhanced chemical vapour deposition (PECVD) in conditions where some nanoparticles can be formed in the growth region of the deposition process, under proper ion bombardment. The results achieved show that the performances of the films produced are dependent on the self bias voltage that can present photosensitivities of approximately 107 (two orders of magnitude larger than the one exhibited by films grown under conventional conditions) with density of states determined by the constant photocurrent method below 4×1015 cm-3. Apart from that, the films grown are less affected by light soaking than the conventional films.

The aim of this work is to interpret the role of the lateral leakage current on the a-Si:H solar cell performances (J-V characteristics, responsivity and the apparent device degradation behaviour), under low illumination conditions.

The objective of this work is to present an analytical model able to interpret the experimental dependence of the uniformity of films produced by the plasma-enhanced chemical vapour deposition technique on the deposition parameters (discharge pressure, gas flow temperature and rf power density). The model proposed is based on the Navier-Stokes equations applied to a gas flow considered to be quasi-incompressible and quasi-inviscous, whenever the Mach number is below 0·3. This condition leads to the establishment of the proper quasisteady-state gas flow equations, and the corresponding equations of energy and momentum balance ascribed to the mass profile of the species formed, under the presence of a low-rf-power plasma density, are able to predict the uniformity distribution of the film over the entire deposited substrate area.

The ability to process and dimensionally scale field-effect transistors with and on paper and to integrate them as a core component for low-power-consumption analog and digital circuits is demonstrated. Low-temperature-processed p- and n-channel integrated oxide thin-film transistors in the complementary metal oxide semiconductor (CMOS) inverter architecture are seamlessly layered on mechanically flexible, low-cost, recyclable paper substrates. The possibility of building these circuits using low-temperature processes opens the door to new applications ranging from smart labels and sensors on clothing and packaging to electronic displays printed on paper pages for use in newspapers, magazines, books, signs, and advertising billboards. Because the CMOS circuits reported constitute fundamental building blocks for analog and digital electronics, this development creates the potential to have flexible form factor computers seamlessly layered onto paper. The holistic approach of merging low-power circuitry with a recyclable substrate is an important step towards greener electronics. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

This paper deals with the study of the role of gas temperature and of the ratio of r.f. power to gas flow on the particle’s formation in amorphous silicon films grown by plasma enhanced chemical vapour deposition technique, by monitoring the plasma impedance behaviour under different process conditions. The aim is to determine in which conditions the particles formed do not deteriorate the performances of the films grown or even can lead to an improvement of the properties of the films deposited. The results achieved show the existence of two main boundary regions (β- and θ-regions) separating the so-called α-regime (no powder formed) from the γ-regime (powder formed). Those regions are reached either by heating the gas, changing the gas pressure or using high power to gas flow ratios. In the β-region the probability of incorporating nanoparticles in the films is low and the films exhibit properties similar to those of the ones grown in the α-regime, with a low density of voids incorporated. In the θ-region small nanoparticles can be incorporated leading to films with density of states below 5×1015 cm-3, widened Urbach energies and photosensitivities about two orders of magnitude larger than that of conventional amorphous silicon grown in the α-regime. © 2000 Elsevier Science Ltd.

In this work we present a study aiming to determine the role of Ga2O3-In2O3-ZnO (GIZO) channel layer composition on the electrical performance and stability exhibited by thin-film transistors (TFTs). The GIZO films were obtained by magnetron sputtering using ceramic targets of different compositions (Ga:In:Zn = 2:2:1, 2:2:2, 2:4:1 and 2:4:2 at.). Structural analysis corroborates the fully amorphous character of the GIZO deposited layers. For the target compositional range used we observe a Zn deficiency on the produced films, which affects the In/Ga atomic concentration ratios. Resistivity and mobility are found to show a general trend against the measured In/Ga ratio that reveals the role played by In and Ga cations on the transport mechanisms. Targets with increased In concentrations (2:4:1 and 2:4:2) allow to obtain the best TFT performances with field effect mobilities reaching values of 53.0 and 51.7cm2 V-1 s-1, respectively. In addition, the In-richer GIZO compositions result in considerably more stable TFTs, especially under positive gate bias stress conditions. Finally, it is verified that by using a target with a slightly lower In atomic composition (2:4:2 in comparison to 2:4:1), good stability and mobility can be achieved with potentially lower material costs.© 2011 Elsevier B.V. All rights reserved.

Hafnium oxide-aluminum oxide (HfAlO) dielectric films were cosputtered using HfO2 and Al2 O3 targets, and their properties are studied in comparison with pure HfO2 films. The X-ray diffraction studies confirmed that the HfO2 films are nanocrystalline with a monoclinic phase. The as-deposited HfAlO films with a chemical composition of (HfO2) 0.86 (Al2 O3) 0.14 are amorphous even after annealing at 500°C. Further, the cosputtered films show a slight reduction in leakage current. The leakage current density may be significantly reduced below 3× 10-10 A cm-2 at an electric field of 0.25 MV/cm when applying the proper radio-frequency bias to the substrate. © 2009 The Electrochemical Society.

This paper deals with the role the substrate on the structure of undoped and n-doped polycrystalline silicon (poly-Si) films produced by Low Pressure Chemical Vapour Deposition (LPCVD). The structural and electrical properties of the films deposited on glass, glass covered with molybdenum (Mo), oxidised crystalline silicon and oxidised crystalline silicon covered with Mo were analysed using X-ray diffraction and Spectroscopic Ellipsometry, dark conductivity and Hall effect measurements. Undoped poly-Si films deposited over Mo present modifications in the crystalline structure relatively to those deposited on the other substrates. The presence of Mo changes the preferential growth orientation, enhancing the Si {111} grains orientation, leading to more compact films. The electrical measurements also confirm that the films grown on Mo substrates present better characteristics. Some differences are also observed during the initial growth stages when using glass or oxidised silicon. Very thin n-doped films present a less effective doping effect when deposited on oxidised silicon than the ones deposited on glass substrates.

In this paper we present the effect of thickness on the electrical and optical properties of intrinsic/nondoped zinc oxide thin films deposited at room temperature by radio frequency magnetron sputtering, able to be used as a semiconductor material on electronic devices, like for example ozone gas sensors and ultraviolet detectors. These films are polycrystalline with a c-axis preferential orientation parallel to the substrate. The films present a resistivity that varies from 5.0 × 104 Ω cm to 1.0 × 109 Ω cm with an optical visible transmittance of 85%. The sensor response exceeds more than five orders of magnitude when exposed to UV light recovering to the initial state in the presence of ozone. © 2006 Elsevier B.V. All rights reserved.

In this work we studied the influence of the power density of hydrogen plasma on electrical and optical properties (Hall mobility, free carrier concentration, sheet resistance, optical transmittance and a.c. impedance) of indium zinc oxide films, aiming to determine their chemical stability. This is an important factor for the optimization of amorphous/nanocrystalline p-i-n hydrogenated silicon (a/nc-Si:H) solar cells, since they should remain chemically highly stable during the p layer deposition. To perform this work the transparent conductive oxide was exposed to hydrogen plasma at substrate temperature of 473 K, 87 Pa of pressure and 20 seem of hydrogen flow. The results achieved show that IZO films were reduced for all plasma conditions used, which leads mainly to a decrease on films transmittance. For the lowest power density used in the first minute of plasma exposition the transmittance of the IZO films decreases about 29%.

The aim of this paper is to present results concerning the role of the buffer layer on pin devices, deposited in a single chamber for plasma enhanced chemical vapor deposition, using high hydrogen dilution and pressures at 27.12 MHz. By doing so, we allow the incorporation of nanoparticles into the i-layer, during plasma process. The results show solar cells with 8.8% efficiency with a collection efficiency of 95% in the blue region of the spectra. Apart from that, the results from impedance spectroscopy, imaginary impedance vs. real impedance, show difference of a semicircle radius as function of sample temperatures, which could be explained by total device series resistance variation. © 2005 Elsevier B.V. All rights reserved.

In this paper we study the electro-optical behaviour and the structure of different TCOs, namely the ZGO, ITO and IZO films before and after being submitted to different hydrogen plasma power densities, for times up to 60 s, aiming their use in a/nc-Si:H solar cells. The results achieved show that ZGO films do not reduce for all plasma conditions used and so, the solar cells produced evidence high current density, about 17% larger that the one recorded in the other TCOs. Besides that, by combining the electrical and optical characteristics of the films through a figure of merit, the data reveal that for the ITO and IZO films even when exposed to very low hydrogen power plasma, the figure of merit is reduced up to 50%. © 2005 Elsevier B.V. All rights reserved.

PIN solar cells were light soaked up to 60 hours. The cell characteristics, the optoelectronic properties and the microstructure parameter (R = I2100/I2100+I2000) as well as the hydrogen content (CH) and density of states (g(Ef)) of the active i-layer were monitored throughout the entire light induced degradation process and compared with the correspondents μτ product (for both carriers) inferred through steady photoconductivity and FST measurements. Data show a strong correlation between the decrease of μτ product for electron and the increase of the fraction of hydrogen bonded on internal surfaces (R increases from 0.1 to 0.4) suggesting structural changes during the light induced defects' formation. For holes, the μτ product remains approximately constant and only dependent on the initial hydrogen content. As g(Ef) increases, μτ presents an asymmetrical decrease showing that electrons are more sensitive to defects' growth than holes. We also observe that the rate of degradation is faster for samples having the lowest defect densities, R and CH, showing that the amount of degradation is not a simple function of the photon exposure (Gt product) but also depends on the material microstructure.