(Invited) Deep-Level Analysis of Passivation of Transition Metal Impurities in Silicon

Abstract

Experimental results on the interactions of transition metal (TM) impurities with hydrogen and on passivation by the displacement of the metals from interstitial to substitutional sites in crystalline silicon will be presented in this talk. Iron, titanium and vanadium from the 3d series, and Zr and Mo from the 4d series have been studied. The results have been obtained by means of deep level transient spectroscopy (DLTS), high resolution Laplace DLTS, minority carrier transient spectroscopy and secondary ion mass spectroscopy. The 3d metals are the most important TM series in relation to recombination in silicon. The most recombination active site is with the 3d metal in an interstitial position. It is found that in the presence of excess vacancies 3d TMs tend to move to substitutional sites and are quite stable in this position with no or much reduced recombination activity and much reduced diffusion. The experimental results on passivation of TMs with hydrogen show that in general the reactions of H atoms with TM impurities are much more effective in n-type material than in p-type Si. These findings can be easily explained considering charge states of the interacting species. Essentially the initial H-TM reaction is driven coulombically at equilibrium conditions. We have managed to promote reactions of hydrogen atoms with Fe, Mo and V impurities in p-type silicon by changing charge states of the interacting defects either by shifting the Fermi level upon the application of reverse bias to Schottky barrier diodes or by illumination of the contaminated and hydrogenated Si samples with the use of light emitting diodes with photon energies above the Si band gap energy. It has been found that the binding of hydrogen to the metal atoms, in all cases we have examined, is rather weak … of the order of 2 eV. This implies that the passivated fraction of the TMs resulting from the reaction of hydrogen with metals in bulk silicon is diminishingly small at device processing temperatures. Based on the results obtained we try to understand some of the previously published data on lifetime improvement in Si crystals by hydrogenation. In some of the commercially important cases, particularly those reporting on multi-crystalline slices, this is far from simple as other important effects such as enhanced diffusion, precipitation and gettering can take place during hydrogenation simultaneously with hydrogen-metal reactions. It appears that in some cases hydrogen-metal reactions does not seem to be the most important processes in enhancing the minority carrier lifetime of the silicon.