Molybdenum nano‐precipitates in silicon: A TEM and DLTS study

The quest for low cost solar cells has resulted in attempts to use lower purity silicon rather than conventional electronic grade material. The metallic content of such ‘solar silicon’ is usually much higher than electronic grade material and this compromises the efficiency of photovoltaic conversion via a variety of recombination mechanisms. Molybdenum is a common metallic contaminant in Si which is difficult to getter because of its low diffusivity. It is known that Mo incorporates into the Si lattice as single interstitial atoms, introducing a deep donor level into the gap. In this work, we show that Mo in silicon can also form nano-precipitates which may represent a new general class of efficient recombination centres. We have observed the Mo precipitates by TEM as 5–10 nm clusters in Si samples into which molybdenum was introduced during growth or by ion implantation and annealing. In general, such precipitates can form in Si samples containing metals with low diffusivity during high-temperature treatments and subsequent cooling. We report the carrier capture and emission properties of Mo nano-precipitates, which show that these could be very effective recombination centres for minority carriers in p-type Si. In particular we present data and analysis of minority carrier charge exchange at these nano-dimension metallic defects.