Lattice contraction due to boron doping in silicon

In modern electronic devices, strain is used to increase carrier mobility. It is thus mandatory to know precisely the effect of doping on the lattice parameter of silicon. However, there are many experimental biases which prevent one from measuring this effect with high accuracy. For this reason, we have designed and fabricated a step-like structure consisting of five 50 nm-thick Si layers of increasing substitutional boron concentrations. Then, we have used Dark Field Electron Holography, a Transmission Electron Microscopy based technique, to measure the strain in these pseudomorphic and defect-free layers. Using Finite Element Modelling, we show that the observed out-of-plane strains can be reproduced by assuming that the expansion coefficient of boron is about − 6.5 × 10-24 cm3. This value is slightly larger (in absolute value) than those previously reported. It is otherwise about 20% larger than predicted from the size mismatch between B and Si atoms.