Rodrigo Martins

Nanocrystalline p-doped silicon films were deposited at low substrate temperatures (around 200°C) in a hot wire reactor. In this paper we present the results on the role of the hydrogen dilution and filament temperature on the film's structure, composition, morphology and transport properties. The film's structure changes from honeycomb-like to a granular needle shape as the filament temperature changes from about 2000°C and hydrogen dilution 87%, to values above 2100°C and hydrogen dilution 90%, respectively. The nanocrystalline silicon-based films produced have optical gaps varying from 1.6 to 1.95 eV, with conductivities up to 0.2 S cm-1 and grain sizes (obtained by X-ray diffraction) in the range of 10-30 nm. © 1999 Elsevier Science B.V. All rights reserved.

This paper is concerned with the modelling and simulation of amorphous and microcrystalline silicon optoelectronic devices. The physical model and its mathematical formulation are extensively described. Its numerical reduction is also discussed together with the presentation of a computer program dedicated to the simulation of the electrical behaviour of such devices. This computer program, called ASCA (Amorphous Silicon Solar Cells Analysis), is capable of simulating, on one- and two-dimensional domains, the internal electrical behaviour of multi-layer structures, homojunctions and heterojunctions under simple or complex spectra illumination and externally applied biases. The applications of the simulator presented in this work are the analysis of μc/a-Si:H p-i-n photovoltaic cell in thermal equilibrium and illuminated by monochromatic light and the AMI.5 solar spectrum, with and without polarisation. We also study the appearance within the device of lateral components of the electric field and current density vectors when the illumination is not uniform. © 1999 IMACS/Elsevier Science B.V. All rights reserved.

Microcrystalline silicon is a two-phase material. Its composition can be interpreted as a series of grains of crystalline silicon imbedded in an amorphous silicon tissue, with a high concentration of dangling bonds in the transition regions. In this paper, results for the transport properties of a μc-Si:H p-i-n junction obtained by means of two-dimensional numerical simulation are reported. The role played by the boundary regions between the crystalline grains and the amorphous matrix is taken into account and these regions are treated similar to a heterojunction interface. The device is analyzed under AM1.5 illumination and the paper outlines the influence of the local electric field at the grain boundary transition regions on the internal electric configuration of the device and on the transport mechanism within the μc-Si:H intrinsic layer.

Microcrystalline silicon is a two-phase material. Its composition can be interpreted as grains of crystalline silicon imbedded in an amorphous silicon tissue, with a high concentration of danglind bonds in the transition regions. In this paper, results obtained by means of numerical simulations about the transport properties of a μc-Si:H p-i-n junction are reported. The role played by the boundary regions between the crystalline grains and the amorphous matrix is taken in account, and these regions are treated similarly to a heterojunction interface. The influence of the local electric field at the grains boundary transition regions on the internal electric configuration of the device is outlined under illumination and applied external bias. © 1999 Elsevier Science S.A. All rights reserved.

This paper reports the performances of metal/insulator/semiconductor devices, simultaneously sensitive to hydrogen and to the visible region of the spectrum. The sensors used in this work are based on glass/Cr/a-SiH(n+)/a-Si:H(i)/SiOx/Pd structures, where the amorphous silicon was deposited by conventional r.f. techniques and the oxide grown thermally (in air) or chemically (in hydrogen peroxide). The proposed sensors present a response of ∼ 3 orders of magnitude change in the saturation current when in the presence of 400 ppm of hydrogen and an open circuit voltage that decreases in the presence of hydrogen, with a maximum spectral response at 500 nm. These sensors were also compared with equivalent crystalline silicon devices whose oxides were prepared exactly in the same conditions as the ones used for the a-Si:H devices. © 1998 Elsevier Science B.V. All rights reserved.

Large-area thin-film position-sensitive detectors (TFPSDs) using the hydrogenated amorphous silicon (a-Si:H) technology are presented. The detection accuracy of these devices (lengths of about 80 mm) is better than ±0.5% of the value of the full scale of the sensor, the spatial resolution is better than ±20 μm, the non-linearities measured are below ±2% and the frequency response is in the range of a few kilohertz, compatible with the sampling frequency of most electromechanical assembling/control systems. The obtained results are quite promising regarding the application of these sensors to a wide variety of optical inspection systems. © 1998 Elsevier Science S.A. All rights reserved.

Large-area thin-film position-sensitive detectors (TFPSDs) using the hydrogenated amorphous silicon (a-Si:H) technology are presented. The detection accuracy of these devices (lengths of about 80 mm) is better than ±0.5% of the value of the full scale of the sensor, the spatial resolution is better than ±20 μm, the non-linearities measured are below ±2% and the frequency response is in the range of a few kilohertz, compatible with the sampling frequency of most electromechanical assembling/control systems. The obtained results are quite promising regarding the application of these sensors to a wide variety of optical inspection systems.

Hydrogenated amorphous silicon photochemical sensors based on Pd metal/insulator/semiconductor (Pd-MIS) structures were produced by plasma enhanced chemical vapour deposition (PECVD) with two different oxidized surfaces (thermal and chemical oxidation). The behaviour of dark and illuminated current-voltage characteristics in air and in the presence of a hydrogen atmosphere is explained by the changes induced by the gases adsorbed, in the work function of the metal, modifying the electrical properties of the interface. The photochemical sensors produced present more than two orders of magnitude variation on the reverse dark current in the presence of 400 ppm hydrogen. When the sensors are submitted to light it corresponds a decrease of 45% on the open circuit voltage.

We have developed a linear thin film position-sensitive detector with 128 elements, based on p.i.n. a-Si:H devices. The incorporation of this sensor into an optical inspection camera makes possible the acquisition of three-dimensional information of an object, using laser triangulation methods. The main advantages of this system, when compared with the conventional charge-coupled devices, are the low complexity of hardware and software used, and that the information can be continuously processed (analogic detection). In this paper, we present the most significant characteristics of the singular one-dimensional thin film position-sensitive detectors that form part of the linear array with 128 sensors. © 1998 Elsevier Science S.A.

Pd-metal/insulator/semiconductor based on hydrogenated amorphous silicon were produced by plasma enhanced chemical vapour deposition with two different oxidised surfaces: thermal in ambient air and chemical with hydrogen peroxide. The diode characteristics have been investigated using dark and light current as f(v) measurements in the temperature range from 300 K to 380 K, from which it was possible to infer the electron barrier height. The data obtained show that the incorporation of a thin insulator layer between the semiconductor and the metal improves the performances of the devices by preventing the formation of suicides at the interface. Apart from that we also show that the MIS structures with the thermal oxide presents 'better' performances than the ones with the chemical oxide due to the type of interface states and of the oxide charges associated with the interface between the insulator and the semiconductor. © 1998 Elsevier Science B.V. All rights reserved.

In this paper, we present results of the role of laser beam energy and shot density on the electro-optical and structural properties of undoped and doped recrystallized amorphous silicon thin films, generated by pulsed Nd-YAG laser (λ = 532 nm). The data reveal that the structure and electrical characteristics of the recrystallized thin films are mainly dependent on the energy and shot density of the laser beam, while the morphology of the obtained films are mainly governed by the number of shots used. The data also show that the electrical conductivity of undoped and doped recrystallized films can be varied up to 6 orders of magnitude, by the proper choice of the recrystallization conditions. Doped samples with conductivities in the amorphous states in the range of 10-5 Ω-1 cm-1 present, after recrystallization, conductivities of about 300 Ω-1 cm-1. The SEM micro-chemical analysis also shows that the obtained crystalline grains are constituted by pure silicon. © 1998 Elsevier Science S.A.

The behaviour of a microcrystalline p-i-n junction locally illuminated with monochromatic radiation (incident power of 50 mW/cm2) is analysed by means of numerical experiences. The model used for the two-dimensional analysis of the transport properties of a μc-Si:H p-i-n photo-detector is based on the simultaneous solution of the continuity equations for holes and electrons together with the Poisson's equation. The solution is found on a rectangular domain, taking into account the dimension perpendicular to the junction plane and one on the parallel plane. The lateral effects occurring within the structure, due to the non-uniformity of the illumination, are outlined. The results we present show that the potential profile has a linear variation from the illuminated to the dark neutral region. The lateral components of the electric field and of the current density vectors reveal to be mainly localised inside the doped layers.

Hydrogenated amorphous silicon photochemical sensors based on Pd-MIS structures were produced by Plasma Enhanced Chemical Vapor Deposition with two different oxidized surfaces (thermal and chemical oxidation). The behaviour of dark and illuminated current-voltage characteristics in air and in the presence of a hydrogen atmosphere is explained by the changes induced by the gases in the work function of the metal, modifying the electrical properties of the interface. The photochemical sensors produced present more than 2 orders of magnitude variation on the reverse dark current when in presence of 400 ppm hydrogen to which it corresponds a decrease of 45% on the open circuit voltage.

Microcrystalline p-i-n silicon devices are a prospective contender for application in large-area optoelectronics. In this paper we analyse the behaviour of a μc-Si:H p-i-n photodevice under non-uniform illumination. The effect of a spatially non-uniform illumination is to create lateral electric fields and current flows inside the structure. We present in this paper a numerical application of a complete bidimensional model describing the transport properties within the structure. The continuity equations forholes and electrons together with Poisson's equation are solved simultaneously along the two directions parallel and perpendicular to the junction. The results of simulating p-i-n μc-Si:H junctions under non-uniform illumination show that the generated lateral effects depend not only in intensity but also in direction on the wavelength of the incident radiation. © 1997 Elsevier Science S.A.

We report here about a computer simulation program, based on a comprehensive physical and numerical model of an a/μc-Si:H p-i-n device, applied to the 2D problem of describing the transport properties within the structure under non- uniform illumination. The continuity equations for holes and electrons together with Poisson's equation are solved simultaneously along the two directions parallel and perpendicular to the junction. The basic semiconductor equations are implemented with a recombination mechanism reflecting the microcrystalline structure of the different layers. The lateral effects occurring within the structure, due to the non-uniformity of the radiation are outlined. The simulation results obtained for different wavelengths of the incident light are compared and shown their dependence on the energy of the radiation. The results of simulating a p-i-n μc-Si:H junctions under non-uniform illumination is that the generated lateral effects depend not only in intensity but also in direction on the wavelength of the incident radiation. ©2004 Copyright SPIE - The International Society for Optical Engineering.

A Linear array Thin Film Position Sensitive Detector (LTFPSD) based on hydrogenated amorphous silicon (a-Si:H) is proposed for the first time, taking advantage of the optical properties presented by a-Si:H devices we have developed a LTFPSD with 128 integrated elements able to be used in 3D inspections/measurements. Each element consists on a 1D LTFPSD, based on a p.i.n. diode produced in a conventional PECVD system, where the doped layers are coated with thin resistive layers to establish the required device equipotentials. By proper incorporation of the LTFPSD into an optical inspection camera it will be possible to acquire information about an object/surface, through the optical cross- section method. The main advantages of this system, when compared with the conventional CCDs, are the low complexity of hardware and software used and that the information can be continuously processed (analogue detection).

This work presents the main static and dynamic performances showed by one dimensional thin film position sensitive detectors (1D TFPSD), based on a-Si:H technology, with a size of 80 mm × 5 mm. The results obtained show that the TFPSD is able to respond to light powers as low as 2μ W/cm2, presenting a detection accuracy, linearity and response frequency better than 10 μm, 2% and 2 KHz, respectively. These results are quite promising regarding the application of these sensors to a wide variety of optical inspection systems where continuous quality control is required.

A novel compact linear thin film position sensitive detector with 128 elements, based on p-i-n a-Si:H devices was developed. The proper incorporation of this sensor into an optical inspection camera makes possible the acquisition of three dimension information of an object, using laser triangulation methods. The main advantages of this system, when compared with the conventional charge-coupled devices, are the low complexity of hardware and software used and that the information can be continuously processed (analogue detection).

This paper presents a low-cost technology for the realisation of large-area thin film position-sensitive detectors using the a-Si:H technology. The obtained results are quite promising regarding the application of these sensors to a wide variety of optical inspection systems, such as: machine tool alignment and control; angle measuring; rotation monitoring; surface profiling; medical instrumentation; targeting; remote optical alignment; guidance systems; etc., to which automated inspection control is needed.

The aim of this work is to present an analytical model able to interpret the role of the thin collecting resistive layer on the static performances exhibited by 1D amorphous silicon hydrogenated p-i-n thin film position sensitive detectors. The data obtained show that the devices present a linearity and a spatial resolution, of respectively, better than 99% and 20 μm for a spatial detection limit of about 80 mm, highly dependent on the characteristics exhibited by the collecting resistive layer that should have sheet resistivities in the range of 10 to 103 Ω/sq, as predicted by the model proposed. © 1996 American Institute of Physics.

We present here a two-dimensional numerical simulation of a hydrogenated amorphous silicon p-i-n solar cell non-uniformly illuminated through the p-layer. This simulation is used to show the effect of the presence of dark regions in the illuminated surface on the electrical behaviour of the device. The continuity equations for holes and electrons together with Poisson's equation, implemented with a recombination mechanism reflecting the amorphous structure of the material, are solved using standard numerical techniques over a rectangular domain. The results obtained reveal the appearance of a lateral component of the electric field and current density vectors inside the structure. The effect of such components is a lateral carrier flow of electrons inside the intrinsic layer and of holes inside the p-layer, resulting in leakage of the transverse current collected at the contacts and an increase in the series resistance.

The aim of this work is to present the main optoelectronic characteristics of large area 1D position sensitive detectors based on amorphous silicon p-i-n diodes. From that, the device resolution, response time and detectivity are derived and discussed concerning the field of applications of the 1D thin film position sensitive detectors.

The aim of this work is to present the main optoelectronic characteristics of large-area one-dimensional position-sensitive detectors (1D TFPSDs) based on amorphous silicon (a-Si) p-i-n diodes. From that, the device resolution, response time and detectivity (defined as being the reciprocal of the noise equivalent power pattern) are derived and discussed concerning the field of applications of the 1D TFPSDs. © 1996.

A Linear array Thin Film Position Sensitive Detector (LTFPSD) based on hydrogenated amorphous silicon (a-Si:H) is proposed for the first time, taking advantage of the optical properties presented by a-Si:H devices we have developed a LTFPSD with 128 integrated elements able to be used in 3D inspections/measurements. Each element consists on an one-dimensional TFPSD, based on a p.i.n. diode produced in a conventional PECVD system, where the doped layers are coated with thin resistive layers to establish the required device equipotentials. By proper incorporation of the LTFPSD into an optical inspection camera it will be possible to acquire information about an object/surface, through the optical cross-section method. The main advantages of this system, when compared with the conventional CCDs, are the low complexity of hardware and software used and that the information can be continuously processed (analogue detection).

The introduction into a traditional p.i.n. structure of two defective buffer layers near the p/i and i/n interfaces can improve the device stability and efficiency through an enhancement of the electric field profile at the interfaces and a reduction of the available recombination bulk centers. The defectous layer (`i-layer'), grown at a higher power density, present a high density of the defects and acts as `gettering centers' able to tailor light induced defects under degradation conditions. If the i-layer density of states remains below 1016 eV-1 cm-3 and assuming a Gaussian distribution of defect states, the gettering center distribution will not affect significantly the carrier population but only its spatial distribution. We report here about a device numerical simulation that allows us to analyze the influence of the `i- layer' position, thickness and density of states on the a-Si:H solar cells performances. Results of some systematic simulation rom the ASCA program (Amorphous Solar Cell Analysis), and for different configurations will be presented.