Rodrigo Martins

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 process conditions of growing thin silicon films by plasma enhanced chemical vapour deposition (PECVD) were presented. The plasma impedance was found to monitor the powders in the PECVD systems and good quality silicon films were grown close to the plasma regime where the powders were formed. The silicon films exhibited properties which were interpreted based on a two-phase model where silicon nanostructures were embedded in a disordered network.

This paper deals with a new design method for the interfaces of a-Si:H pin solar cells that improves the stability and performances of devices deposited in a single batch chamber process. The method consists in removing a deposited sacrificial layer placed between the p/i and/or i/n interfaces by etching. This layer is an absorber of defects and impurities that are introduced in the interfaces, mainly from the chamber walls cross-contamination and the substrate surface. The results achieved increase the device fill factor and short circuit current density, respectively towards 75% and 16.3 mA cm-2, with a final efficiency of about 10%, before light soaking experiments. © 2000 Elsevier Science B.V. All rights reserved.

In this paper the influence of the DC grid bias on the plasma impedance and the I-V behaviour of silane plasmas used to grow undoped amorphous silicon films by plasma enhanced chemical vapour deposition technique using a triode configuration at or close to the powder regime is studied. The aim is to determine the correlation between the r.f. power and the DC grid voltage with the plasma parameters, under isothermal gas conditions. The results should lead to the production of nanostructured films, with the required optoelectronic characteristics for photovoltaic applications. The results achieved show the existence of a boundary region close to the γ-regime (powder formed) where nanoparticles can be formed by moderated ion bombardment of the growing surface. This is characterised by the plasma resistance of the same order of magnitude of the plasma reactance. Under this condition, it is possible to grow amorphous silicon films that can incorporate nanoparticles, exhibiting photosensitivities of about 107 (two orders of magnitude larger than the one exhibited by films grown under conventional conditions) with densities of states determined by the constant photocurrent method below 3 × 1015 cm3. Apart from that, the growth of the films is less affected by light soaking than the conventional films grown by standard techniques. © 2001 Elsevier Science Ltd. All rights reserved.

The effects of Sn thickness electrodeposited over Cu on the structural and morphological performance of the joints formed were investigated. The electrical stability of the joints formed was analyzed under extreme aggressive conditions. Results indicated that the proposed soldering technology greatly satisfied the demands concerning soldering specifications.

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.

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 deals with a new process to improve the stability of a-Si:H pin solar cells deposited in a single batch process by proper passivation of the interfaces. The process consists in removing partially a deposited sacrificial oxide layer grown between the p/i or i/n interfaces by SF6 etching. This layer is an absorber of defects and impurities that are introduced in the interfaces, mainly from the chamber walls and the substrate surface. The results achieved in laboratory samples lead to devices in which the fill factor and short circuit current density were improved respectively towards 75% and 16.5 mA cm-2, with a final working efficiency of about 9.5%. © 2001 Elsevier Science Ltd. All rights reserved.

This work reports on the performances of undoped and doped amorphous/nanocrystalline silicon films grown by hot wire plasma assisted technique. The structure (including the presence of several nanoparticles with sizes ranging from 5 nm to 50 nm), composition (oxygen and hydrogen content) and transport properties of the films are highly dependent on the temperature of the filament and on the hydrogen dilution. The undoped films grown under low r.f. power (≈4mWcm-2) and filament temperatures around 1850 K present dark conductivities below 10-10 Scm-1, optical gaps of about 1.6 eV and photosensitivities above 105, (under AM 1.5 light intensities), with almost no traces of oxygen content. For the n- and the p-doped silicon films also fabricated under the same conditions the conductivities obtained are of about 10-2Scm-1 and 10-5Scm-1, respectively. © 1998 Elsevier Science Ltd. All rights reserved.

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 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.

We have deposited by Plasma Enhanced Chemical Vapour Deposition phosphorus doped amorphous and microcrystalline silicon films, as a function of the RF power (10-300 W), using a PH3/(SiH4 + H2 + He)mixture. It was found that films microcrystallization occurs for powers above 130 W, where a clear phase transition occurs. The microcrystalline films produced present high dark conductivities and optical band gaps, where the crystalline volume fraction is above 25%, as revealed by micro Raman spectroscopy. The Hall mobility have been also determined for amorphous and microcrystalline films, as a function of temperature, in the range 280-340 K. The data show that for the microcrystalline films the conduction is mainly in the extended states of the microcrystals, confirming also the double sign anomaly. That is, for n-type films, the sign is positive for the amorphous case while it is negative for the microcrystalline case. © 1998 Elsevier Science S.A.

We present the modelling of a new two-terminal and low-voltage operating optoelectronic device based on MIS silicon structure with multichannel insulator and having as gate a transparent metallic tin-doped indium oxide (ITO) layer deposited by spray pyrolysis technique over the insulator layer. ITO layer has a multiple non-rectifier electrical contact with silicon substrate, in the SiO2 channel's region. Construction details of the process, together with its operating characteristics are given. The devices developed do not require external active electronic components (transistors, microschemes) to execute their functions and to transform analogue input optical signals to digital output form, highly important for a wide range of optoelectronic applications.

In this paper we report the success in fabricating FTO/Si surface-barrier photodiodes produced by spray pyrolysis deposition technique, under ambient conditions. Three types of photodetectors for low-voltage-bias operation were developed based on high-resistivity silicon: 1. X-Ray detectors with energy resolution of 16.5% at 661.5 keV (137Cs source), consisting of surface-barrier PIN photodiode with an active area of 50 mm2 operating at 5 V reverse bias, scintillator based on monocrystalline Bi4Ge3O12 and preamplifier (noise of 250 e- RMS.); 2. Fast-response surface-barrier FTO/n–n+ silicon epitaxial photodiodes, operating at 10 V bias with rise times of 2 ns at λ = 0.85 μm; 3. Radiation-resistant drift epitaxial surface-barrier PIN photodiodes for unbiased operating conditions, with an exponential impurity distribution in a 8 μm thick epitaxial layer. A built-in electrical field due to the carrier concentration distribution in the epitaxial layer provides a considerable improvement in the `critical fluence' value (3×1014 cm-2) for neutron irradiation.

We report the properties and optoelectronic applications of transparent and conductive indium and tin oxide films prepared by the spray pyrolysis method and doped with Sn or F, respectively. The film properties have been measured using X-ray diffraction, optical and electrical absorption. As examples of applications we produced a set of selective optical detectors for different spectral regions, covering the wavelength range from 0.25 to 1.1 μm, based on metal oxide-semiconductor heterostructures and using different substrates such as: GaP, GaSe, AlxGa1-xAs, GaAs and Si. The fabricated devices exhibit several features such as: production simplicity, high quantum efficiency, uniform sensitivity over the entire active area and a high response speed. Finally, we present a high quantum efficiency and solar blind monocrystalline zinc sulphide optical sensor fabricated by spray deposition as an alternative to the ultraviolet-enhanced SiC and GaN photodetectors and the performances of a solar cell. © 1998 Elsevier Science B.V. All rights reserved.

A new high quantum efficiency gallium phosphide Schottky photodiode has been developed by spray deposition of heavily doped tin oxide films on n-type epitaxial structures, as an alternative to the conventional Schottky photodiodes using a semitransparent gold electrode. It is shown that fluorine-doped tin oxide films are more effective as transparent electrodes than tin-doped indium oxide films. The proposed photodiodes have a typical responsivity near 0.33 A W-1 at 440 nm and an unbiased internal quantum efficiency close to 100%, in the range from 250 to 450 nm. The model used to calculate the internal quantum efficiency (based on the optical constants of tin oxide films and gallium phosphide epitaxial layers) is found to be in good agreement with the experimental results. The data show that the quantum efficiency is strongly dependent on the thickness of the transparent electrode, owing to optical interference effects. The noise equivalent power for 440 nm is 2.7 × 10-15 W Hz-1/2, which indicates that these photodiodes can be used for accurate measurements in the short-wavelength range, even in the presence of stronger infrared background radiation.

UV-enhanced monocrystalline zinc sulphide optical sensors with high quantum efficiency have been developed by spray deposition of heavy fluorine-doped tin oxide (FTO) thin films onto the surface of zinc sulphide monocrystals as an alternative to the UV-enhanced high-efficiency silicon photodetectors commonly used in precise radiometric and spectroscopic measurements as well as to new sensors based on SiC and GaN. The fabricated sensors have an unbiased internal quantum efficiency that is nearly 100% from 250 to 320 nm, and the typical sensitivity at 250 nm is 0.15 A W-1. The sensors are insensitive to solar radiation in conditions on the earth and can be used as solar blind photodetectors for precision UV measurements under direct solar illumination for both terrestrial and space applications. © 1998 Elsevier Science S.A. All rights reserved.

UV-enhanced monocrystalline zinc sulphide optical sensors with high quantum efficiency have been developed by spray deposition of heavy fluorine-doped tin oxide (FTO) thin films onto the surface of zinc sulphide monocrystals as an alternative to the UV-enhanced high-efficiency silicon photodetectors commonly used in precise radiometric and spectroscopic measurements as well as to new sensors based on SiC and GaN. The fabricated sensors have an unbiased internal quantum efficiency that is nearly 100% from 250 to 320 nm, and the typical sensitivity at 250 nm is 0.15 A W-1. The sensors are insensitive to solar radiation in conditions on the earth and can be used as solar blind photodetectors for precision UV measurements under direct solar illumination for both terrestrial and space applications.

The aim of this work is to study the role of hydrogen dilution and filament temperature on the properties of nanocrystalline silicon thin films (undoped and doped) produced by the hot wire technique. These deposition parameters are correlated to the film's structure, composition and electro-optical properties with special emphasis on boron doped nanocrystalline silicon carbide reported here. © 1998 Elsevier Science B.V. All rights reserved.

We present a set of high-efficiency optical sensors for the spectral range from 0.25 to 1.1 μm based on metal oxide-semiconductor heterostructures using different substrates: GaP, GaSe, AlxGa1 - xAs, GaAs and Si. A set of several transparent conductive metal oxide films such as indium, tin and zinc oxides fabricated by the spray pyrolysis method and its doping procedure has been investigated. The results show that heavily doped indium and tin oxide films are preferable as the active transparent conductive electrode in heterojunction surface-barrier structures. The fabricated sensors exhibit several features such as process simplicity, high quantum efficiency, uniformity of sensitivity over the active area and a high response speed. Such sensors can be used for precision measurements in different scientific and technical applications. © 1998 Elsevier Science S.A. All rights reserved.

We present a set of high-efficiency optical sensors for the spectral range from 0.25 to 1.1 μm based on metal oxide-semiconductor heterostructures using different substrates: GaP, GaSe, AlxGa1-xAs, GaAs and Si. A set of several transparent conductive metal oxide films such as indium, tin and zinc oxides fabricated by the spray pyrolysis method and its doping procedure has been investigated. The results show that heavily doped indium and tin oxide films are preferable as the active transparent conductive electrode in heterojunction surface-barrier structures. The fabricated sensors exhibit several features such as process simplicity, high quantum efficiency, uniformity of sensitivity over the active area and a high response speed. Such sensors can be used for precision measurements in different scientific and technical applications.

We have developed new types of functional and smart optical silicon sensors, based on ITO/multichannel insulator/silicon structures, which are able to execute electronic functions such as amplifying the photocurrent (without avalanche multiplication), transforming the input optical signal into a radio frequency output signal and transforming the analogue input optical signal to a digital output form, without external active electronic components. These new functional optical sensors allow a substantial simplification of the registration of optical signals as well as of the electronic scheme to be used. © 1998 Elsevier Science S.A. All rights reserved.

We have developed new types of functional and smart optical silicon sensors, based on ITO/multichannel insulator/silicon structures, which are able to execute electronic functions such as amplifying the photocurrent (without avalanche multiplication), transforming the input optical signal into a radio frequency output signal and transforming the analogue input optical signal to a digital output form, without external active electronic components. These new functional optical sensors allow as substantial simplification of the registration of optical signals as well as of the electronic scheme to be used.

The aim of this work is to present a model able to explain the role of the lateral drift current on the experimental behaviour exhibited by p-i-n amorphous silicon solar cells (J-V characteristics, responsivity and the apparent device degradation behaviour), when the ratio between the covered and uncovered metal collected areas of the device is higher than one or recrystallization occurs in the edges of the p-i-n junction.

The aim of this paper is to present data on the dependence of the electro-optical characteristics and structure of n-type microcrystalline silicon films on the r.f. power used during the deposition of films produced by the plasma-enhanced chemical vapour deposition technique. The interest of these films arise from the fact that they combine some electro-optical advantages of amorphous (wide optical gap) and crystalline materials (electronic behaviour), highly interesting in the production of a wide variety of optoelectronic devices such as solar cells and thin film transistors. In this paper, microcrystalline n-type films presenting simultaneously optical gaps of about 2.3 eV, dark conductivity of 6.5 S cm-1 and Hall mobility of about 0.86 cm2 V-1 s-1 will be reported, the highest combined values for n-type microcrystalline silicon films, as far as we know. © 1997 Elsevier Science S.A.