Characterization of amorphous/crystalline silicon interfaces from electrical measurements
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1066-A04-01
Characterization of amorphous/crystalline silicon interfaces from electrical measurements J. P. Kleider1, and A. S. Gudovskikh2 1 Laboratoire de Génie Electrique de Paris, CNRS UMR8507; SUPELEC; Univ Paris-Sud; UPMC Univ Paris 06, 11 rue Joliot-Curie, Gif sur Yvette, F-91192, France 2 St.-Petersburg Physics and Technology Centre for Research and Education of the RAS, Hlopina str. 8/3, St.-Petersburg, 194021, Russian Federation ABSTRACT Electrical techniques based on capacitance and conductance measurements are powerful tools for interface characterization in semiconductor heterostructures. Here we detail their application to the study of the heterointerface between hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si). The main parameters governing the device applications are the conduction and valence band mismatch, and the density of interface states. The presence of a high interface states density can be revealed by capacitance versus temperature and frequency measurements. However, for very high quality interfaces that are required for instance to reach high conversion efficiencies in solar cells, the usual measurements performed in the dark and at zero or reverse bias are not sensitive enough. We show that the sensivity to interface states can be enhanced by performing capacitance measurements under illumination and at a forward bias close or equal to the open-circuit voltage. In this case, the measured capacitance is determined by the diffusion of free carriers in c-Si and limited by recombination at the interface. Regarding the determination of band offsets, the method using a plot of the inverse square capacitance as a function of bias to determine the diffusion potential from the intercept of the extrapolated linear region is shown to lead to errors even in the absence of any interface charge. This is due to the presence of a strong inversion layer in c-Si at the interface, the effect of which has been ignored so far in the literature. The presence of this strong inversion layer is evidenced from planar conductance measurements on (n) a-Si:H/(p) c-Si structures. We emphasize that these measurements are very sensitive to details of the band structure profile. In particular, it is shown that the temperature dependence of the sheet electron density allows the determination of the conduction band offset between a-Si:H and c-Si with a good precision: ∆EC = EC a-Si:H – EC c-Si = 0.15 ± 0.04 eV.
INTRODUCTION Silicon heterojunction solar cells combining crystalline and thin film silicon technologies are attracting a lot of attention. This is because very high efficiencies (above 20%) have been demonstrated, while using low temperature deposition of doped silicon thin films to form the junctions and the back surface field reduces the cost and energy consumption during fabrication [1, 2]. As far as the physics is concerned, however, the precise description and characterization of the heterojunctions is still a matter of debate. In particular, there is still a lack of knowledge on the in
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