Manuela Vieira

Simulation of hydrogenated amorphous and microcrystalline silicon optoelectronic devices

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.

THIN FILM POSITION SENSITIVE DETECTOR BASED ON AMORPHOUS SILICON P-I-N DIODE

The application of hydrogenated amorphous silicon (a–Si:H) to optoelectronic devices are now well established as a viable low cost technology and is presently receiving much interest. Taking advantage of the properties of a–Si:H based devices, single and dual axis large area (up to 80×80 mm2) thin film position sensitive detectors (TFPSD) based on a–Si:H p–i–n diodes have been developed, produced by plasma enhanced chemical vapor deposition. In this study, the main optoelectronic properties presented by the TFPSD as well as their behavior under operation conditions, concerning its linearity and signal to noise ratio, are reported.

Material properties, project design rules and performances of single and dual-axis a-Si:H large area position sensitive detectors

Abstract We have developed large area (up to 80mm×80mm) Thin Film Position Sensitive Detectors (TFPSD) based on hydrogenated amorphous silicon (a-Si:H). Although crystalline silicon PSDs have been realized and applied to optical systems, their detection area is small (less than 10mm×10mm), which implies the need of optical magnification systems for supporting their field of applications towards large area inspection systems, which does not happen by using a-Si:H devices. The key factors for the TFPSDs resolution are the thickness uniformity of the constituting layers, the geometry and the position of the contacts. In this paper we present data on single and dual-axis rectangular TFPSDs correlating, their performances with the different underlying lateral effects. For the single axis-detector, with two opposite extended contacts, the output photocurrent difference to sum ratio is a linear function of the position of a narrow incident light beam, even for low illumination levels (below 20 lux). For the dual-axis detector with extended contacts, at all four sides (except for small gaps at the vertices due to edge effects) an almost linear relation has been found between the incident light spot position along both axis and the corresponding output photocurrents.

A two-dimensional numerical simulation of a non-uniformly illuminated amorphous silicon solar cell

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 Poissons 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.

Large Area Position Sensitive Detector Based on Amorphous Silicon Technology

We have developed a rectangular dual-axis large area Position Sensitive Detector (PSD), with 5 cm × 5 cm detection area, based on PIN hydrogenated amorphous silicon (a-Si:H) technology, produced by Plasma Enhanced Chemical Vapor Deposition (PECVD). The metal contacts are located in the four edges of the detected area, two of them located on the back side of the ITO/PIN/A1 structure and the others two located in the front side. The key factors of the detectors resolution and linearity are the thickness uniformity of the different layers, the geometry and the contacts location. Besides that, edge effects on the sensors corner disturb the linearity of the detector. In this paper we present results concerning the linearity of the detector as well as its optoelectronic characteristics and the role of the i-layer thickness on the final sensor performances.

Transport in μc-Six:Cy:Oz:H films prepared by a TCDDC system

Abstract N- and p-type weakly absorbing and highly conductive microcrystalline thin μc-Si x :C y :O z :H films, have been produced by a TCDDC (Two Consecutive Decomposition and Deposition Chamber) system 1 . The optoelectronic and structural results show that we are in the presence of a mixed phase of Si microcrystals (c-islands) embedded in a-Si x :C y :O z :H (a-tissue). Based on that, we propose a model where transport mechanisms are explained by the potential fluctuations related to films heterogeneities. Thus, conduction is due to carriers that by tunneling or percolation “pass” or “go” trough the barriers and/or percolate randomly by the formed channels.

Surface-barrier Si-based photodetectors fabricated by the spray pyrolysis technique

Abstract The ITO/n−-n+ Si epitaxial surface-barrier photodiodes have been fabricated by the spray pyrolysis technique, where ITO is a thin transparent conducting film of tin-doped indium oxide. The study was performed on devices with thin and thick (8 and 25 μm respectively) high-resistivity epitaxial layers. Quantitative analysis of the current-voltage and capacitance-voltage characteristics reveals that these devices behave like ideal quasi-Schottky devices where the barrier is formed between the ITO film and the silicon high-resistivity epitaxial layer. The design details and performance measurements of the fast-response (the rise time of 2.5 ns at wavelength λ = 0.91 μm) photodiodes operated in non-saturated carrier velocity conditions at a low-voltage bias of 10 V and at a non-uniform light absorption are discussed. Special attention is given to the design of radiation-resistant (up to a neutron flux of 3 × 1014 neutrons cm−2) surface-barrier photodiodes at zero-voltage bias. An analysis of the changes in the spectral sensitivity of the neutron-irradiated photodiodes is also addressed in this study.

Tailoring defects on amorphous silicon pin devices

Abstract This paper deals with a new model and structure able to tailor defects in pin devices. The model assumes the usual density of states profile, including donor and acceptor like states inside the mobility gap and has the capability to simulate the transient and steady state device behavior. The new structure is based in two interfacial defectous layers, located at the junctions, acting as “gettering” centers to tailor the defects. The role of the interlayer and its thickness on device performances will be also discussed.

Transport properties of doped silicon oxycarbide microcrystalline films produced by spatial separation techniques

Abstract This paper presents results of the role of the oxygen partial pressure used during the deposition process on the transport properties exhibited by doped microcrystalline silicon oxycarbide films produced by a Two Consecutive Decomposition and Deposition Chamber system, where a spatial separation between the plasma and the growth regions is achieved. This paper also presents the interpretative models of the optoelectronic behaviour observed in these films (highly conductive and transparent with suitable properties for optoelectronic applications) as well as the interpretation of the growth process that leads to films microcrystallization.

Role of oxygen partial pressure on the properties of doped silicon oxycarbide microcrystalline layers produced by spatial separation techniques

The aim of this work is to present experimental data concerning the role of the oxygen partial pressure during the production process on the properties (structure, morphology, composition, and transport properties) exhibited by doped microcrystalline silicon oxycarbide films. The films were produced by a two consecutive decomposition and deposition chamber system, where a spatial separation between the plasma and the growth regions is achieved. The films produced by this technique are highly conductive and highly transparent with suitable properties for optoelectronic applications requiring wide band‐gap and low‐conductivity materials.