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- Direct Color Sensor, Optical Amplifier and Demux Device Integrated on a Single Monolithic SiC PhotodetectorPublication . Vieira, Manuela; Louro, Paula; Vieira, Manuel; Costa, João; Fernandes, MiguelA pi'n/pin a-SiC:H voltage and optical bias controlled device is presented and its behavior as image and color sensor, optical amplifier and demux device is discussed. The design and the light source properties are correlated with the sensor output characteristics. Different readout techniques are used. When a low power monochromatic scanner readout the generated carriers the transducer recognizes a color pattern projected on it acting as a direct color and image sensor. Scan speeds up to 10(4) lines per second are achieved without degradation in the resolution. If the photocurrent generated by different monochromatic pulsed channels is readout directly, the information is demultiplexed. Results show that it is possible to decode the information from three simultaneous color channels without bit errors at bit rates per channel higher than 4000 bps. Finally, when triggered by light of appropriated wavelength, it can amplify or suppress the generated photocurrent working as an optical amplifier (C) 2009 Published by Elsevier Ltd.
- Laser-scanned p-i-n photodiode (LSP) for image detectionPublication . Vieira, Manuela; Fernandes, Miguel; Martins, João; Louro, Paula; Maçarico, António Filipe Ruas Trindade; Schwarz, Reinhard; Schubert, Markus B.Amorphous and microcrystalline glass/ZnO:Al/p(a-Si:H)/i(a-Si:H)/n(a-Si1 C :H)/Al imagers with different n-layer resistivities were produced by plasma-enhanced chemical vapor deposition technique (PE-CVD). The transducer is a simple, large area p-i-n photodiode; an image projected onto the sensing element leads to spatially confined depletion regions that can be readout by scanning the photodiode with a low-power modulated laser beam. The essence of the scheme is the analog readout and the absence of semiconductor arrays or electrode potential manipulations to transfer the information coming from the transducer. The effect of the image intensity on the sensor output characteristics (sensitivity, linearity, blooming, resolution, and signal-tonoise ratio) are analyzed for different material composition. The results show that the responsivity and the spatial resolution are limited by the conductivity of the doped layers. An enhancement of one order of magnitude in the image intensity and on the spatial resolution is achieved with a responsivity of 0.2 mW/cm2 by decreasing the -layer conductivity by the same amount. In a 4 4 cm2 laser-scanned photodiode (LSP) sensor, the resolution was less than 100 m and the signal-to-noise (S/N) ratio was about 32 dB. Aphysical model supported by electrical simulation gives insight into the methodology used for image representation.
- Stacked photo-sensing devices based on SiC alloys A non-pixelled architecture for imagers and demultiplexing devicesPublication . Vieira, Manuela; Louro, Paula; Fernandes, Miguel; Fantoni, Alessandro; Vieira, Manuel Augusto; Costa, JoãoIn this review paper different designs based on stacked p-i'-n-p-i-n heterojunctions are presented and compared with the single p-i-n sensing structures. The imagers utilise self-field induced depletion layers for light detection and a modulated laser beam for sequential readout. The effect of the sensing element structure, cell configurations (single or tandem), and light source properties (intensity and wavelength) are correlated with the sensor output characteristics (light-to-dark sensivity, spatial resolution, linearity and S/N ratio). The readout frequency is optimized showing that scans speeds up to 104 lines per second can be achieved without degradation in the resolution. Multilayered p-i'-n-p-i-n heterostructures can also be used as wavelength-division multiplexing /demultiplexing devices in the visible range. Here the sensor element faces the modulated light from different input colour channels, each one with a specific wavelength and bit rate. By reading out the photocurrent at appropriated applied bias, the information is multiplexed or demultiplexed and can be transmitted or recovered again. Electrical models are present to support the sensing methodologies.
- New p-i-n Si : H imager configuration for spatial resolution improvementPublication . Vieira, Manuela; Fernandes, Miguel; Martins, João; Louro, Paula; Maçarico, António Filipe Ruas da Trindade; Schwarz, Reinhard; Schubert, Markus B.Amorphous glass/ZnO-Al/p(a-Si:H)/i(a-Si:H)/n(a-Si1-xCx:H)/Al imagers with different n-layer resistivities were produced by plasma enhanced chemical vapour deposition technique (PE-CVD). An image is projected onto the sensing element and leads to spatially confined depletion regions that can be readout by scanning the photodiode with a low-power modulated laser beam. The essence of the scheme is the analog readout, and the absence of semiconductor arrays or electrode potential manipulations to transfer the information coming from the transducer. The influence of the intensity of the optical image projected onto the sensor surface is correlated with the sensor output characteristics (sensitivity, linearity blooming, resolution and signal-to-noise ratio) are analysed for different material compositions (0.5 < x < 1). The results show that the responsivity and the spatial resolution are limited by the conductivity of the doped layers. An enhancement of one order of magnitude in the image intensity signal and on the spatial resolution are achieved at 0.2 mW cm(-2) light flux by decreasing the n-layer conductivity by the same amount. A physical model supported by electrical simulation gives insight into the image-sensing technique used.
- Optical demultiplexer device operating in the visible spectrumPublication . Louro, Paula; Vieira, Manuela; Vieira, Manuel Augusto; Amaral, S.; Costa, João; Fernandes, MiguelIn this paper, we present results on the use of multilayered a-SiC:H heterostructures as a device for wavelength-division demultiplexing of optical signals. These devices are useful in optical communications applications that use the wavelength division multiplexing technique to encode multiple signals into the same transmission medium. The device is composed of two stacked p-i-n photodiodes, both optimized for the selective collection of photo generated carriers. Band gap engineering was used to adjust the photogeneration and recombination rate profiles of the intrinsic absorber regions of each photodiode to short and long wavelength absorption in the visible spectrum. The photocurrent signal using different input optical channels was analyzed at reverse and forward bias and under steady state illumination. A demux algorithm based on the voltage controlled sensitivity of the device was proposed and tested. An electrical model of the WDM device is presented and supported by the solution of the respective circuit equations.