A. Maçarico

Laser-scanned p-i-n photodiode (LSP) for image detection

Amorphous and microcrystalline glass/ZnO:Al/p(a-Si:H)/i(a-Si:H)/n(a-Si 1 - x C x :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-to-noise 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/cm 2 by decreasing the n-layer conductivity by the same amount. In a 4 x 4 cm 2 laser-scanned photodiode (LSP) sensor, the resolution was less than 100 μm and the signal-to-noise (S/N) ratio was about 32 dB. A physical model supported by electrical simulation gives insight into the methodology used for image representation.

A two-dimensional visible/infrared detector based on μc-Si:H p–i–n structures

We present preliminary results on the analysis and application of an optical detector for imaging light contours. The sensor has a two-dimensional structure and uses a TCO/μc-p-i-n Si:H configuration as the active device and two lateral ohmic contacts to convert the projected light image into a lateral output photovoltage. A steady-state light pattern was used to simulate the image, in addition to a smallest intensity scanner beam which moved across the device creating an ac output signal. This signal depends on the position, intensity and homogeneity of the light pattern. A bidimensional model to explain the sensor performance is presented based on a small-signal ac circuit analysis.

Image processing in a μc-Si:H p–i–n image transducer

Abstract A two-dimensional p–i–n imager based on μc-Si:H material is analysed. The basic building block for the sensor element is a transparent conductive oxide (TCO)/μc-p–i–n Si:H photodiode with front metal contacts and a TCO back contact. A scan-out process based on the photovoltage signal induced by a modulated HeNe laser is used to acquire the image. An analysis of the geometric image distortion and image restoration is given. Basic image processing algorithms are applied for image enhancement and pattern recognition.

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.

MICROCRYSTALLINE SILICON THIN FILMS FOR OPTICAL APPLICATIONS

Abstract Microcrystalline hydrogenated silicon thin films, deposited by the cyclic CVD method, were used for the production of bidimensional detectors. The optical detectors use a standard TCO/ μ c-p-i-n Si:H configuration as active device and two lateral ohmic contacts. Based on the lateral photovoltaic effect developed in the p-i-n structure under local illumination, we propose to develop a new sensing method for the recognition of a light pattern projected onto a p-i-n optical sensor. It was used strong spatially fixed light spots to simulate an image and a weak chopped light scanning-beam to make its recognition. The induced ac component of the lateral photovoltage was found to be dependent on the image position and intensity. A small signal circuit analysis and simulation is presented. Applications for image detection are discussed.

New p-i-n Si : H imager configuration for spatial resolution improvement

Amorphous glass/ZnO:Al/p(a-Si:H)/i(a-Si:H)/n(a-Si 1-x C x :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 < I). 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.

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.

VIS/NIR detector based on μc-Si:H p–i–n structures

Abstract The spectral response and the photocurrent delivered by entirely microcrystalline p–i–n-Si:H detectors are analyzed under different applied bias and light illumination conditions. The devices consist of a ZnO covered glass substrate, followed by a p + /i/n + structure, and an Al top contact. The light is incident either through the glass or through the rear part of the device. The spectral range depends on the illumination side. Under rear illumination the spectral response is extended beyond 1000 nm and has a maximum near 700 nm with a good rejection of the blue spectrum. If the light is incident through the glass the same infrared response is observed, however, in the visible range the maximum is shifted to the blue region. Numerical modeling of the VIS/NIR detector, choosing appropriate band discontinuities near the grain boundaries and interfaces complements the study and gives insight into the internal physical processes. We suggest that transport will proceed on parallel paths along the amorphous, the crystalline, or both phases depending on the wavelength. The enhanced sensitivity to the red/infrared region is then related with the operation of the crystalline-like diode while the spectral responsivity in the ‘blue’ region is ascribed mainly to the amorphous-like photodiode.

Analog readout image sensor based on p–i–n hydrogenated amorphous silicon

Amorphous glass/ZnO:Al/p(a-Si: H)/i(a-Si: H)/n(a-Si 1-x C x :H))/Al imagers with different n-layer resistivities were produced by plasma enhanced chemical vapour deposition technique (PE-CVD). The image is projected onto the active surface of the sensor and defines itself 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 analogue 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-to-noise ratio) are analysed for different material composition (0.5 < x < 1). The results show that the response and the spatial resolution are limited by the conductivity of the doped layers. An enhancement of 75% in the image resolution is achieved with responsivity of 0.2 mW/cm 2 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.

A 3-phase model for VIS/NIR μC-Si:H p–i–n detectors

Abstract The spectral response and the photocurrent delivered by entirely microcrystalline p–i–n-Si:H detectors are analysed under different applied bias and light illumination conditions. The spectral response and the internal collection depend not only on the energy range but also on the illumination side. Under 〈p〉- and 〈n〉-side irradiation, the internal collection characteristics have an atypical shape. It is high for applied bias and lower than the open circuit voltage, shows a steep decrease near the open circuit voltage (higher under 〈n〉-side illumination) and levels off for higher voltages. Additionally, the numerical modeling of the VIS/NIR detector, based on the band discontinuities near the grain boundaries and interfaces, complements the study and gives insight into the internal physical process.