Evaluation of Strain-Relaxation of Carbon-Doped Silicon Nanowires and Its Crystal Orientation Dependence Using X-Ray Diffraction Reciprocal Space Mapping

Carbon-doped silicon (Si:C) thin films with C concentrations of 0.60% and 0.83% were fabricated into nanowires, and the lattice strain relaxation with shrinking the nanowire width, W, was evaluated in detail by x-ray reciprocal lattice space mapping (RSM) measurements. The obtained RSM profiles showed a right-downward distribution. From the RSM profiles, we considered that the lattice relaxation of the Si:C nanowires progressed slowly from the nanowires/substrate interfaces to the nanowire top surfaces. Then, we assumed the lattice strain of the Si:C thin films to be 100% and derived the average lattice strain relaxation of the Si:C nanowires from the RSM profiles. To derive the lattice relaxation, we summed the RSM profiles in the q_x or q_z directions, respectively, and calculated the average in-plane and out-of-plane lattice parameters. The obtained average lattice strain relaxation became larger with shrinking W, and progressed rapidly at W = 200 nm. Thus, we considered that the large strain relaxation occurs in the region of approximately 100 nm from the edge of the nanowires. In addition, the lattice strain relaxation was smaller for Si:C nanowires fabricated with their long side along the [100] direction than for Si:C nanowires along the [110] direction. We considered this difference of strain relaxation might be due to the crystallographic orientation dependence of Young’s modulus.