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

Abstract

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.

Recombination via transition metals in solar silicon: The significance of hydrogen–metal reactions and lattice sites of metal atoms

“…Molybdenum is arguably the most potentially harmful metallic impurity considered in this work, seen to significantly reduce carrier lifetime and correspondingly cell efficiency at lower concentrations than Ti and V . It is also practically impossible to getter and has not been observed to form large precipitates, although recent work has shown nanoscale precipitates of Mo can form and act as very efficient recombination centers.…”

Vanadium in silicon: Lattice positions and electronic properties

“…The DLTS signal from nano-precipitates appears to be independent of the metal species but dependent on the size to a small degree, but quite distinctive and so easily identifiable. 20 In the heat-treated vanadium-implanted samples, we see no DLTS signals characteristic of the larger precipitates but in some p-type samples DLTS evidence of nano-precipitates has been found. The key issue that we need to consider is whether the difference in chemical concentration (from SIMS) and concentration of V i (from DLTS) is due to the migration of V i to substitutional sites or to nanoprecipitates.…”

“…These are almost impossible to detect by TEM, as they tend to be very small ($5 nm) and at low metal concentrations do not have a high areal density. 20,21 However, they are easily detected by DLTS and, as in the case of larger precipitates, have a very distinctive but different carrier capture behaviour compared to the larger metal particles. The DLTS signal from nano-precipitates appears to be independent of the metal species but dependent on the size to a small degree, but quite distinctive and so easily identifiable.…”

Deep Levels in W-Doped Czochralski Silicon

“…Upon natural cooling from the annealing temperature, a supersaturation of W-related species is created, which is the driving force for precipitation. It is well-known that the surface is a favorable precipitation site for metal impurities, 3,23 so that the broad midgap band could correspond to some form of W-related clusters/precipitates. Alternatively, as suggested in Ref.…”