Photocurrent Profile in a-SiC:H Monolithic Tandem Pinpin and Pinip Photodiodes
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0989-A18-12
Photocurrent Profile in a-SiC:H Monolithic Tandem Pinpin and Pinip Photodiodes Alessandro Fantoni, Manuela Vieira, and Yuri Vygranenko DEETC, ISEL, Rua Conselheiro Emidio Navarro, Lisbon, 1949-014, Portugal ABSTRACT 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. INTRODUCTION Optical properties of amorphous silicon (a-Si:H, a-SiC:H) p-i-n structures allow to identify and develop good sensors for image pattern recognition and color filtering. This has been an important topic of research in the field of sensing applications [1, 2, 3]. Work on characterization and modeling of image sensor electrical behavior [4] led the way to the development of multilayer structures of interest for image and color sensor application: p-i-n structures for pattern detection of B/W images through the LSP (Laser Scanned Photodiode) thechnique [5]; and tandem structures for additional light wavelength discrimination [6, 7]. Light filtering properties, namely light wavelength discrimination, depend on the structure of the sensor, mainly carbon doping, thickness of each pin cell absorbing layer, and the selected sequence of the cells in the multilayer structure. A numerical ASCA simulation (Amorphous Silicon solar Cell Analysis [8]) procedure was performed for different sensor configurations. Adjusting the values of some parameters or the type of the simulation analysis, it was possible to explain some of the observed singularities of the sensor, aiming to explain color detection, light-to-dark sensitivity and image pattern recognition. The ASCA simulations show a strong dependence of the device effective functioning on the optical characteristic (absorption coefficient and optical gap) of the materials used, leading to a introduction of a-SiC:H layers into the stacked structure in order to improve RGB color screening.
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