A-Si:H Ambipolar Diffusion Length and Effective Lifetime Measured by Flying Spot Technique (FST)
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A-Si:H AMBIPOLAR DIFFUSION LENGTH AND EFFECTIVE LIFETIME MEASURED BY FLYING SPOT TECHNIQUE (FST) M. VIEIRA, R. MARTINS, E. FORTUNATO, F. SOARES AND L. GUIMARAES. Faculdade de Ci~ncias e Tecnologia da Universidade Nova de Lisboa Quinta da Torre, 2825 Monte da Caparica, Portugal. ABSTRACT The determination of the ambipolar diffusion length, L*, and the effective lifetime, -*, in p/i and a-Si:H Schottky barriers (ITO/p/a-Si:H/Al-Si; Cr/a-Si:H/Cr/Ag) have been determined by Flying Spot Technique, FST. This technique consists in the transient analysis of the photocurrent/photopotential induced by a laser beam that moves perpendicularly to the structure with a constant motion ratio, at different velocities. Taking into account the competition between the diffusion/drift velocities of the excess carriers and the velocity of the flying spot, it is possible to solve the transport equations and to compute separately L* and t*', from the asymmetrical distribution responses. INTRODUCTION The analysis of the diffusion length in a-Si:H films (and their alloys) and its correlation with the density of states, DOS, is important when we want to use such materials in device applications, such as solar cells. Commonly, diffusion lengths in a-Si:H have been evaluated from Surface Photo Voltage, SPV, (1,2] and from Steady State Photocarrier Grating Technique, SSPG, [3,4). All these techniques are based on the analysis of the steady photovoltage and photocurrent. In this paper we report a simple new technique for the determination of the ambipolar diffusion length, FST (Flying Spot Technique), where instead of a steady light beam, a beam with a constant motion ratio is used to generate the carriers. So, taking into account the competition between the diffusion/drift velocities of the additional carriers and the velocity of its generation, it is possible to determine separately L* and T* from the asymmetrical distribution response of the photovoltage induced during the spot movement. PRINCIPLES OF THE FLYING SPOT TECHNIQUE Introduction The effect of illuminating a semiconductor junction is, as known, the appearance of a photovoltage effect between the two sides. If the illumination is nonuniform, an additional effect arises resulting in the development of a photovoltage parallel to the junction (lateral), in addition to the transverse one.
Mat. Res. Soc. Symp. Proc. Vol. 219. ©1991 Materials Research Society
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Lateral photoeffect When a thin transparent metal/semiconductor or p/i structure is illuminated perpendicularly with monochromatic localized light, a lateral photovoltage is developed in addition to the transverse one [5], depending on the position of the light spot. light
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B V
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Figure 1: Charge movement and potential for the lateral photoeffect.
At any point B (Fig.l) not close to the illuminated point, A, a deviation from equilibrium will appear, resulting in a lateral field (El). The potential V(y) characterizes the effect of the lateral field, and it depends on the posit
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