P. Louro

Image capture devices based on p-i-n silicon carbides for biometric applications

Abstract The laser scanned photodiode (LSP) image sensor is optimized and used as a biometric (fingerprint) reader. A laser light illuminates the fingerprint placed on a glass surface in front of the sensor. The reflecting light coming from the glass is projected onto the active surface. The image is converted directly into a proportional electric current using the LSP as fingerprint scanner. Results show that a trade-off between read-out parameters (fingerprint scanner) and the biometric sensing element structure (p–i–n structure) is needed to minimize the cross talk between the fingerprint ridges (dark regions) and the fingerprint valleys (illuminated regions). In the optimized configuration and under reverse bias the user-specific information minutiae present a good contrast and a spatial resolution of 20 μm . An increased light-to-dark sensitivity, a flux range of two orders of magnitude and a responsivity lower than 6.5 μW cm −2 were obtained under slightly reverse voltage.

Transport mechanism in high resistive silicon carbide heterostructures

Glass/ZnO:Al/p (Si x C 1-x :H)/i (Si:H)/n (Si x C 1-x :H)/Al (0 < x < 1) heterojunctions were produced by PE-CVD at low temperature. Junction properties, carrier transport and photogeneration are investigated from illuminated current- and capacitance-voltage characteristics and spectral response measurements, in dark and under different illumination conditions. For the heterojunction high series resistance around 10 6 Ω and atypical J-V characteristics are observed leading to poor fill factors. It was also observed that the responsivity decreases with the increase of the light bias intensity. For the homojunction, the behaviour is typical of a non-optimised p-i-n cell and the responsivity has a slight variation with the light bias conditions. A numerical simulation gives insight into the transport mechanism suggesting that the potential drop across the low-conductive a-SiC:H contact causes a significant change in the drift-diffusion balance inside the i-layer bulk. In the homojunction even at high light fluxes the transport process remains drift dominated.

Optical Readout in Pinpi'n and Pini'p Imagers: A Comparison

Optimized a-SiC:H multilayer devices based on two different tandem configurations (TCO/pinpin/TCO and TCO/pinip/TCO) are compared and tested for proper image recognition and color separation process using an optical readout technique. In both configurations the doped layers are based on a-SiC:H to increase image resolution and to prevent image blurring. To profit from the light filtering properties of the active absorbers, the intrinsic layer of the front diode (i layer) is based on a-SiC:H and the back one (ilayer) on a-Si:H. The effect of the applied voltage on the color selectivity is discussed. Results show that the relative spectral response curves demonstrate rather good separation between the red, green and blue basic colors. Combining the information obtained under positive and negative applied bias a colour image is acquired, using the same optical technique either in pinpin or in the pinip configuration, without colour filters or pixel architecture.

Influence of the transducer configuration on the p-i-n image sensor resolution

Amorphous ZnO:Al/ a-Si x C 1 x :H-p-i-n/Al optical imagers that use a small-signal scanning beam to read out the photogenerated carriers are presented. The effect of the image intensity on the sensor output characteristics (distortion, sensitivity and signal-to-noise ratio) are analysed for different sensor configurations (0.5 < x < 1). Results show that the sensitivity and the geometrical distortion are limited by the conductivity of the doped layers. A 75% image distortion reduction with a responsivity of 2 W/m 2 is obtained by decreasing the n-layer conductivity by one order of magnitude. An analysis of the image acquisition and representation is performed. A physical model supported by an electrical simulation gave insight into the methodology used for image representation.

Tailored Laser scanned photodiodes (LSP) for image recognition

A tailored ZnO:Al/a-p-i-n SiC:H/Al configuration for the laser scanned photodiode (LSP) imaging detector is proposed. The LSP utilizes light modulated depletion layers as detector and a laser beam for readout. When highly resistive a-SiC:H doped layers are used its higher optical gap when compared with the active layer are responsible by charge accumulation at the illuminated interfaces which blocks the carrier collection under illumination. Those insulator-like layers act as MIS structures that prevent excess signal charge from blooming to the nearby dark regions avoiding the image smearing. The optical-to-electrical transfer characteristics show reciprocity between light intensity and image signal intensity only limited by the doped layers composition. Data reveal that the sensitivity, the responsivity and the spatial resolution are limited by the cell configuration while the linearity depends on the light source flux used to map the image onto the sensor. By using tailored SiC:H/Si:H/SiC:H p-i-n heterostructures an increase in the image signal optimized to the blue is achieved with a responsivity of 0.2 mW/cm 2 and a spatial resolution of 20 µm.

LSP image sensors based on SiC heterostructures

A newly engineered ZnO:Al/a-p–i–n SixC1 � x:H/Al configuration for the laser scanner photodiode (LSP) imaging detector is proposed and the read-out parameters improved. The effect of the sensing element structure and light source flux are investigated and correlated with the sensor output characteristics. When wide band gap doped layers are used, data reveals an increased image signal optimised to the blue, a dynamic range of two orders of magnitude, a sensitivity of 6 mA/W and a responsivity of 170 m Wc m � 2 at 530 nm. The main output characteristics such as image responsivity, resolution, linearity and dynamic range were analysed under reverse, and forward bias and short circuit mode. Results show that in a wide range of incident light power, the sensor performance are optimised in short circuit mode. A trade-off between read-out parameters and the required sensor characteristics is needed to minimise the cross talk between dark and illuminated regions. # 2001 Elsevier Science B.V. All rights reserved.

A μc-Si:H P-I-N Imager For 2-D Pattern Recognition

A TCO/ μc-p-i-n Si:H/AI imager is presented and analyzed. The μc-p-i-n Si:H photodiode acts as a sensing element. Contacts are used as an electrical interface. The image is acquired by a scan-out process. Sampling is performed on a rectangular grid, and the read-out of the photogenerated charges is achieved by measuring simultaneously both transverse photovoltages at the coplanar electrodes. The image representation in gray-tones is obtained by using low level processing algorithms. Basic image processing algorithms are developed for image enhancement and restoration.

Memory effects in highly resistive p–i–n heterojunctions for optical applications

Large area p-i-n diode structures based on amorphous hydrogenated silicon can be used as single element image sensors where the information is read out by a scanning laser beam. A high sensitivity is reached with silicon-carbon alloy contact layers. The higher defect density in the large band gap material is usually a problem for efficient carrier collection in solar cell applications. When used as an image sensor, however, the charge stored in deep defects represents an easy way to realize short-term image storage. In the case of a p-(Si:H)/i-(Si:H)/n-(Si x C 1-x :H) sensor structure we have measured a memory effect of approximately 1% after several minutes of image projection. Metastable sensor degradation is observed in accordance with the Staebler-Wronski effect. Fast degradation of sensor performance - corresponding to 90% erasable image storage capability - was studied in an unalloyed structure using a Nd:YAG laser system. The response can be modeled by a stretched exponential decay with parameters depending on the laser pulse energy.

Carrier transport and photogeneration in large area p-i-n Si/SiC heterojunctions

Glass/ZnO:Al/p (Si x C 1−x :H)/i (Si:H)/n (Si x C 1−x :H)/Al (0 6 ω and atypical J-V characteristics are observed leading to poor fill factors, also it was observed that the responsivity decreases with the increase of the light bias intensity. For the homojunction the behaviour is typical of a non optimised p-i-n cell and the responsivity varies only slightly with the light bias conditions. A numerical simulation gives insight into the transport mechanism suggesting that in the heterojunctions and in dark conditions the transport mechanism depends almost exclusively on field-aided drift while under illumination it is dependent mainly on the diffusion of free carriers.

Simulation of localized surface plasmon in metallic nanoparticles embedded in amorphous silicon

We propose the development and realization of a plasmonic structure based on the LSP interaction of metal nanoparticles with an embedding matrix of amorphous silicon. This structure need to be usable as the basis for a sensor device applied in biomedical applications, after proper functionalization with selective antibodies. The final sensor structure needs to be low cost, compact and disposable. The study reported in this paper aims to analyze different materials for nanoparticles and embedding medium composition. Metals of interest for nanoparticles composition are Aluminum, Gold and Alumina. As a preliminary approach to this device, we study in this work the optical properties of metal nanoparticles embedded in an amorphous silicon matrix, as a function of size, aspect-ratio and metal type. Following an analysis based on the exact solution of the Mie theory, experimental measurements realized with arrays of metal nanoparticles are compared with the simulations.