Evidence for Molybdenum-Hydrogen Bonding in p-Type Silicon upon Annealing under Illumination

Transition metals (TMs) are common contaminants in solar silicon produced by block casting or using lower grade polysilicon feedstock. Gettering can effectively remove certain metals, but slow diffusing TMs can remain and act as highly efficient recombination centers. One method to reduce their recombination activity is hydrogen passivation, however, this is generally ineffective in p-type silicon, in which hydrogen and TMs are typically both in positive charge states. Recent studies have suggested that under illumination, the charge state of hydrogen in silicon can be altered, and its bonding with certain defects can be enhanced. In this work, deep level transient spectroscopy (DLTS), Laplace DLTS, and capacitance-voltage measurements are used to study TM-H bonding in p-type silicon intentionally contaminated with Mo and hydrogenated via remote H plasma. It is found that heat-treatments at moderate temperatures (<250 °C) under illumination with an LED at 850 nm result in a reduction in the concentration of electrically active Mo, with no similar reduction seen in samples annealed in the dark. The passivated fraction of Mo can be recovered after dark annealing at temperatures >200 °C. The results obtained provide evidence that upon heat-treatments with illumination Mo-H bonding in p-type Si occurs. Possible mechanisms of the observed effects are discussed.