Controlling the Lateral Photoeffect in a-Si:H Heterojunction Structures: The Influence of the Band Offset Analysed Throu
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CONTROLLING THE LATERAL PHOTOEFFECT IN a-Si:H HETEROJUNCTION STRUCTURES: THE INFLUENCE OF THE BAND OFFSET ANALYSED THROUGH A NUMERICAL SIMULATION a,b a A.Fantoni , M. Fernandes , P. Louro a, R. Schwarz a , M.Vieira a a) ISEL-DEEC, Rua Conselhero Navarro, 1900 Lisbon, Portugal b) UNINOVA, Quinta da Torre, 2825 Monte de Caparica, Portugal ABSTRACT When an a-Si:H p-i-n structure is locally illuminated by a light spot, the non uniformity of light causes the appearance of a gradient in the carrier concentration between the illuminated and the dark zone. Carrier start to flow in agreement with such gradients, and when equilibrium is reached, the lateral diffusion process is counterbalanced by the appearance of a lateral component of the electric field vector in addition to the transverse usual one. The lateral fields act as a gate for the lateral flow of the carriers and small lateral currents appears at the transition region between the illuminated and the dark zone. Such lateral photoeffect depends on the incident light wavelength, light intensity and on the device operation condition (mainly the applied bias). The introduction of carbon in the doped layers modifies the intensity and the extension of these lateral effects through the potential barriers deriving from the band banding at the interfaces. We have used the 2D numerical simulator ASCA to analyze the behavior of an a-Si:H p-i-n structure under local illumination with the goal of observing the appearance of the lateral components of the electric field and current density vectors. Different homo and heterojunctions have been simulated, outlining how the band offset at the interfaces influences the induced lateral photoeffect and aiming to explain how a correct device design and engineering can, depending on the foreseen application, alternatively enhance or reduce the intensity of such lateral effects. INTRODUCTION It is widely well known, and reported by any school manual, that when a semiconductor junction absorbs a photon flux and an excess of minority carriers is generated, the system energy distribution changes and the system is no longer in equilibrium. A transverse photovoltage appears between the two sides of the junction and a transverse electric current begins to flow between the two collecting contacts. When the illumination intensity is not uniformly distributed over the incident surface, the system energy redistribution and its departure from the thermodynamic equilibrium is consequently non-uniformly distributed within the structure. The non-uniformity of the illumination leads to the appearance, within the device, of lateral components of the electric field and of the current density in addition to the usual transverse ones. The inhomogeneous illumination of the semiconductor junction front side can be caused, among other reasons, by casual shadows projected on the device, by finger grid contacts or can be intentionally applied aiming to profit from the lateral effects. In fact, if a concentrated light spot illuminates the junction we assist to th
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