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Defect removal after low temperature annealing of boron implantations by emitter etch-back for silicon solar cells

: Müller, R.; Moldovan, A.; Schiller, C.; Benick, J.


Physica status solidi. Rapid research letters 9 (2015), No.1, pp.32-35
ISSN: 1862-6254
ISSN: 1862-6270
Bundesministerium für Umwelt, Naturschutz, Bau und Reaktorsicherheit BMUB
0325292; ForTeS
Journal Article
Fraunhofer ISE ()
Solarzellen - Entwicklung und Charakterisierung; Silicium-Photovoltaik; Dotierung und Diffusion; Emitter; defects; implantation

Ion implantation offers new possibilities for silicon solar cell production, e.g. single side doping that can be structured in-situ with shadow masks. While phosphorus implantations can easily be annealed at low temperature, the annealing of boron implantations is challenging. In this study, we use low energy implantations of boron (1 keV and 5 keV) with a projected range of 5.6 nm and 21.2 nm that form defects causing charge carrier recombination after a low temperature anneal (950 degrees C, 30 min). An ozone-based wet chemical etching step is applied to remove this near surface damage. With increasing chemical etch-back the electrical quality (i.e. emitter saturation current density, J(0e)) improves continuously. The calculated limit for J(0e) was reached with an abrasion of 35 nm for 1 keV and 85 nm for 5 keV implantations, showing that the relevant defects causing charge carrier recombination are located very close to the surface, corresponding to the as-implanted profile depth. This emitter etch-back allows for the fabrication of defect free boron doping profiles with good sheet resistance uniformity (standard deviation <2%). With the resulting characteristics (sheet resistance <100 /sq, surface doping concentration >5 x 10(19) cm(-3), J(0e) < 30 fA/cm(2)), these boron profiles are well suited for silicon solar cells.