Electrical studies of Fe-related defect complexes in silicon
Appears in the following Collection
- Fysisk institutt 
AbstractIron (Fe) is an important impurity in solar-grade silicon which contributes substantially in degrading the efficiency of solar cells. The degradation is mainly caused by the Fe atoms situating at an unperturbed tetrahedral interstitial sites (Fei) in the silicon crystal, consequently acting as a recombination center. By altering the position and the neighbouring environment at which the Fe atoms reside, there are opportunities in minimizing or neutralizing the electrical activity of Fe. Furthermore, utilizing the high mobility of Fe, one can increase the performance of a device by accumulating the Fe atoms from critical regions into regions where Fe can be tolerated. These approaches can help in realizing high efficient solar cells based on cheap and highly Fe-contaminated silicon. In this work, we have investigated the interaction between Fe and defects relevant to solar cells, using mainly electrical characterization methods such as capacitance-voltage measurement, deep level transient spectroscopy and admittance spectroscopy.
From the study of potential hydrogen passivation of Fe, hydrogen was introduced through wet chemical etching and further driven to a defined region. Using depth profiles, it is found that incorporation of hydrogen stimulates the dissociation of the iron-boron (Fe-B) pair, releasing and resulting in the unwanted Fei. At the same time, no passivation of Fe by hydrogen has been observed.
On the investigation of the mechanism of phosphorus gettering of metal impurities, vacancies have been generated through proton-irradiation. The resulting irradiation-induced defects were examined for reactions with Fe after heat treatments. Based on the evolution of defect concentrations by isochronal annealings, it is found that Fe interacts with the divacancy and the vacancy-oxygen complexes, forming deep levels of 0.28 eV and 0.34 eV above the valence band edge (EV), respectively.
In the search for substitutional Fe to investigate its electrical activity and thermal stability, measurements were performed around the projected range of Fe-implantations after rapid thermal annealing. A shallow acceptor is uncovered with an energy level position of EV +0.06 eV and a defect concentration closely following the calculated concentration of the Fe-implantation dose. However, chemical analysis with secondary ion-mass spectrometry shows out-diffusion of Fe from the region around the projected range after annealing. This suggests that the formation of the shallow acceptor is only assisted/promoted by Fe without Fe being a part of the final complex.
List of papers. Papers I, II, IV and VI are removed from the thesis due to copyright restrictions.
Paper I Electrically active centers introduced in p-type Si by rapid thermal processing. C.K. Tang, E. Lund, E.V. Monakhov, J. Mayandi, A. Holt and B.G. Svensson. Phys. Status Solidi C 8, No. 3, 725-728 (2011) doi:10.1002/pssc.201000263
Paper II Hydrogen-induced dissociation of the Fe-B pair in boron-doped ptype silicon. C.K. Tang, L. Vines, B.G. Svensson and E.V. Monakhov. Solid State Phenomena. Vol. 178-179, 183-187 (2011) doi:10.4028/www.scientific.net/SSP.178-179.183
Paper III Interaction between hydrogen and the Fe-B pair in boron-doped ptype silicon. C.K. Tang, L. Vines, B.G. Svensson and E.V. Monakhov. Appl. Phys. Lett. 99, 052106 (2011) doi:10.1063/1.3619848 Copyright 2011 American Institute of Physics
Paper IV Deep level transient spectroscopy on proton-irradiated Fe-contaminated p-type silicon. C.K. Tang, L. Vines, B.G. Svensson and E.V. Monakhov. Phys. Status Solidi C, In production (2012) doi:10.1002/pssc.201200163 Copyright 2013 American Institute of Physics
Paper V Divacancy-iron complexes in silicon. C.K. Tang, L. Vines, V. P. Markevich, B.G. Svensson and E.V. Monakhov. J. Appl. Phys. 113, 044503 (2013) doi:10.1063/1.4788695
Paper VI Iron-assisted formation of a shallow acceptor in p-type silicon. C.K. Tang, L. Vines, B.G. Svensson and E.V. Monakhov. Submitted to Phys. Status Solidi B (2012)