Dynamic Response of Non-Pixeled Amorphous Silicon Based Image Sensors

AbstractLarge area hydrogenated amorphous silicon p-i-n structures with low conductivity doped layers were proposed as single element image sensors. This work is focused on the analysis of the dynamic behavior of the sensor. Additionally some sensor parameters like maximum scanning speed, from which depends the maximum achievable frame rate are presented and discussed.In order to evaluate the sensor response to a time varying light excitation the sensor was locally illuminated with a focused chopped light source and the generated photocurrent was measured under different load conditions. Results show that the sensor is mainly capacitive and a signal rise time of approximately 100 νs was measured under a 1 kΩ load. A model for the sensor was created from the experimental data and was used to simulate the dynamic behavior of the sensor. The simulation results obtained are in good agreement with the experimental ones.As conclusion one can expect a trade off between the frame rate and the number of pixels. A frame rate higher than 10 fps was achieved for 100×100 pixels readout without a significant degradation in the image quality.</jats:p

Two-dimensional a-Si:H/a-SiC:H n-i-p sensor array with ITO/a-SiNx antireflection coating

AbstractThis paper presents a two-dimensional a–Si:H/a-SiC:H n–i–p photodiode array with switching diode readout, developed specifically for fluorescence-based bio-assays. Both device structure and fabrication processing has enabled enhancement of the external quantum efficiency of the encapsulated device up to 80%, reduction of the photodiode leakage down to 10 pA/cm2 at -1V reverse bias, and increase of the rectification current ratio of the switching diodes up to 109. The critical fabrication issues associated with deposition of device-quality materials, tailoring of defects at the i–p interface, device patterning with dry etching, junction passivation, and contact formation will be discussed. Both sensing and switching diodes were characterized. While the observed dark current in the photodiodes at low reverse bias voltages is primarily due to carrier emission from deep states in the a–Si:H bulk, the leakage in the small switching diodes stems from peripheral defects along junction sidewalls. Optical losses in the photodiodes with ITO/a–SiNx:H antireflection coating were evaluated using numerical modeling, and the calculated transmission spectra correlated well with the spectral response characteristics. Measurements of the charge transfer time and output linearity demonstrated the efficiency of the single-switching diode readout configuration. The response of the array to optical excitation was also investigated. The observed long term retardation in the signal rise and decay at illumination levels less than 1010 photons/cm2-s can be associated with charge trapping in the undoped layer.</jats:p

Image Acquisition Using Non-Pixeled Amorphous Silicon Based Sensors

AbstractWe suggest to enhance the performance of image acquisition systems based on large area amorphous silicon based sensors by optimizing the readout parameters such as the intensity and cross-section of scanner beam, acquisition time and bias conditions. The main output device characteristics as image responsivity, signal to noise ratio and spatial resolution were analyzed in open circuit, short circuit and photodiode modes. The result show that the highest signal to noise ratio and best dark to bright ratio can be achieved in short circuit mode.It was shown that the sensor resolution is related to the basic device parameters and, in practice, limited by the acquisition time and scanning beam properties. The scanning beam spot size limits the resolution due to the overlapping of dark and illuminated zones leading to a blurring effect on the final image and a consequent degradation in the resolution.</jats:p

Effect of a-SiC:H Film Composition on the Performance of Large Area Optical Sensors

AbstractA series of large area single layers and heterojunction cells in the assembly glass/ZnO:Al/p (SixC1-x:H)/i (Si:H)/n (SixC1-x:H)/Al (0&lt;x&lt;1) were produced by PE-CVD at low temperature. Junction properties, carrier transport and photogeneration are investigated from dark and illuminated current-voltage and capacitance-voltage characteristics. For the heterojunction cells Atypical J-V characteristics under different illumination conditions are observed leading to poor fill factors. High serial resistances around 106 Ω are also measured. These experimental results were used as a basis for the numerical simulation of the energy band diagram, and the electrical field distribution of the whole structures. Further comparison with the sensor performance gave satisfactory agreement.Results show that the conduction band offset is the most limiting parameter for the optimal collection of the photogenerated carriers. As the optical gap increases and the conductivity of the doped layers decreases, the transport mechanism changes from a drift to a diffusion-limited process.</jats:p

Charge Carrier Transport in a-Si:H/a-SiC:H Heterojunction with Blocking Layer

AbstractThis paper presents a one-dimensional numerical simulation of the charge carrier transport and photogeneration within a p-i-n (a-Si:H) homojunction and a p(a-SiC:H)/i (a-Si:H)/n (a-SiC:H) heterojunction with weakly-doped n-layers. A good matching between the simulated J-V characteristics and the corresponding experimental curves has been achieved for both configurations. By analysing the simulated band diagrams, electric fielddistributions, the electron and hole current densities, and the free carrier population profiles we conclude that in short-circuit mode the carrier transport is different in the homojunction and heterojunction due to band offsets.</jats:p

Analysis of the Bias Dependent Spectral Response of a-SiC:H p-i-n Photodiode

AbstractThe bias voltage dependent spectral response (with and without steady state bias light) and the current voltage dependence has been simulated and compared to experimentally obtained values. Results show that in the heterostructures the bias voltage influences differently the field and the diffusion part of the photocurrent. The interchange between primary and secondary photocurrent (i. e. between generator and load device operation) is explained by the interaction of the field and the diffusion components of the photocurrent. A field reversal that depends on the light bias conditions (wavelength and intensity) explains the photocurrent reversal. The field reversal leads to the collapse of the diode regime (primary photocurrent) launches surface recombination at the p-i and i-n interfaces which is responsible for a double-injection regime (secondary photocurrent). Considerations about conduction band offsets, electrical field profiles and inversion layers will be taken into account to explain the optical and voltage bias dependence of the spectral response.</jats:p

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

ABSTRACTGlass/ZnO:Al/p (SixC1−x:H)/i (Si:H)/n (SixC1−x:H)/Al (0&lt;x&lt;1) heterojunction cells 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 cells high series resistance around 106 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.</jats:p