Study of the electrical behavior in Intermediate Band-Si junctions
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Study of the electrical behavior in Intermediate Band-Si junctions D. Pastor1,2,3*, J. Olea2,3, A. del Prado1,3, E. García-Hemme1,3, R. García-Hernansanz1,3, L. González-Pariente1, I. Mártil 1,3 and G. González-Díaz1,3. 1 Dpto. Física Aplicada III (Electricidad y Electrónica), Facultad de Ciencias Físicas,Universidad Complutense de Madrid, Spain 2 Instituto de Energía Solar, Escuela Técnica Superior de Ingenieros de Telecomunicación, Universidad Politécnica de Madrid, Spain 3 CEI Campus Moncloa, UCM-UPM, Madrid, Spain * Electronic mail: [email protected] ABSTRACT In this study we analyze the electrical behavior of a junction formed by an ultraheavily Ti implanted Si layer processed by a Pulsed Laser Melting (PLM) and the non implanted Si substrate. This electrical behavior exhibits an electrical decoupling effect in this bilayer that we have associated to an Intermediate Band (IB) formation in the Ti supersaturated Si layer. Time-of-flight secondary ion mass spectrometry (ToFSIMS) measurements show a Ti depth profile with concentrations well above the theoretical limit required to the IB formation. Sheet resistance and Hall mobility measurements in the van der Pauw configuration of these bilayers exhibit a clear dependence with the different measurement currents introduced (1μA-1mA). We find that the electrical transport properties measured present an electrical decoupling effect in the bilayer as function of the temperature. The dependence of this effect with the injected current could be explained in terms of an additional current flow in the junction from the substrate to the IB layer and in terms of the voltage dependence in the junction with the measurement current. INTRODUCTION In the last few years, an increasing effort has been made to improve the efficiency of the solar cell technology. The intermediate band solar cell (IBSC) has been proposed as one of the most promising candidates to increase the photovoltaic efficiency in the third generation of solar cells [1]. An intermediate band (IB) semiconductor presents a new electronic energy band of allowed states between the conventional conduction band and valence band. This IB could permit sub-band gap photon absorption by means of valence-to-IB and IB-to-conduction band transitions. This mechanism could overcome the Schockley-Queisser thermodynamical efficiency limit (30-40%) for single junction solar cells [2]. Deep-level impurities are known to act generally as a non-radiative recombination centers that reduce carrier lifetime and the efficiency of the solar cell. However, the introduction of an impurity concentration above the Mott limit (5.9x1019 cm-3) could form an IB, which would reduce this non radiative recombination [3]. Commonly, the Mott limit is several orders of magnitude above the solid solubility limit of deep level impurities in semiconductors. Therefore, the combination of two non equilibrium fabrication processes: ion implantation followed by pulsed laser melting (PLM) has been used to achieve these concentrations with high crystal quality
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