J. Martins

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.

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.

Electrical simulation of a p-i-n image sensor

Abstract We have modelled a p–i–n image sensor under local illumination through a two-dimensional non-linear circuit. The sensor is described as an array of photodiodes interconnected through lateral resistors, which model the sheet resistance of the doped layers. Under small-signal analysis, each photodiode is modelled by a current-controlled resistor proportional to the inverse of the photocurrent. A SPICE based simulator is used to analyse the sensor output characteristics. Several configurations and contact geometries are analysed for the image transducer. The image responsivity, the spatial resolution and the image distortion are modelled by changing the ratio between the transversal and the lateral resistors or the acquisition points. Results show that the geometry and location of the contacts affect the distortion of the restored image. The conductivity of the doped layers and the light flux illumination influences the image resolution and accuracy. The simulated and experimental results were found in a good agreement.

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.

Photocurrent Profile in a-SiC:H Monolithic Tandem Pinpin and Pinip Photodiodes

We present in this paper results about the analysis of photocurrent and spectral response in a-SiC:H/ a-Si:H pinpin and pinip structures. Our experiments and analysis reveal the photocurrent profile to have a strong nonlinear dependence on the externally applied bias and on the light absorption profile, i.e. on the incident light wavelength and intensity. Our interpretation points out the cause of such effect to a self biasing of the junctions under certain unbalanced light generation of carriers and to an asymmetric reaction of the internal electric fields to the externally imposed bias. The possibility to relate such a behavior to the light intensity and wavelength indicates realistic hypothesis of using these structures and this effect for color recognition sensors. We present results about the experimental characterization of the structures and numerical simulations obtained with the program ASCA. Considerations about electrical field profiles and inversion layers will be taken into account to explain the optical and voltage bias dependence of the spectral response. Our results show that in both structures the application of an external electrical bias (forward or reverse) mainly influences the field distribution within the less photo excited sub-cell.

Light Filtering Properties in a-SiC:H Multilayer Structures: A SPICE model

A SPICE model of a-SiC:H/ a-Si:H pin/pin detector with voltage controlled spectral sensitivity is presented. The equivalent electric circuit able to describe the behavior of the multilayer structure under non-uniform illumination is composed of two series connected diodes, representing the pin structures, with two nonlinear current sources in parallel, representing the photogeneration for different steady state RGB illumination, with their value depending also on the thickness of the absorber layer and the built-in potential of the diodes. This device represents the 1D model of the LSP (Laser Scanned Photodiode) and may be interconnected in a 2D array trough resistors, modeling the high resistivity of the a-SiC:H layers. Electrical simulations were performed for different transducer configurations and illumination conditions and compared with the experimental data. The influence of the lateral and transverse resistors and built-in potential of the diodes on sensor parameters like spatial resolution, and signal amplitude are analyzed. A physical model supported by the electrical simulation gives insight into the methodology used for image representation and color discrimination.

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.