Semiconducting iron silicide thin films on silicon (111) with large Hall mobility and low residual electron concentration

Unprecedented Hall mobility, electron concentration and photoconductivity are demonstrated in semiconducting beta-FeSi2 thin films prepared on Si(111) surfaces by co-sputtering of iron and silicon followed by post-anneal. Characterization of the silicide as a function of the initial temperature and post-treatment shows that annealing temperatures above 800 degrees C are needed to obtain single phase beta-FeSi2. Reactive deposition on substrates heated at 850 degrees C leads to textured films. Majority carriers are electrons in all these unintentionally doped films. Hall concentrations between 1.8 x 10(17) and 5 x 10(15) electrons cm(-3) and respective Hall mobilities from 290 to 900 cm(2) V-1 s(-1) are measured at room temperature, involving two different conduction band minima in these two extreme cases. Only deep centres exist in the samples having the lower carrier concentration. In such a situation, raw data must be corrected for the substrate contribution to extract values which are relevant for the beta-FeSi2 film alone. Photoconductivity also takes place in these samples: at 80 K, it shows a maximum value at the direct band gap of beta-FeSi2 while at 296 K a step still appears at the same energy. Such results are a consequence of the important decrease of the residual impurity concentration in comparison to values previously published.