Nanomechanical properties of multilayered amorphous carbon structures

A possible route toward reducing the intrinsic compressive stress in as-grown amorphous carbon films on Si substrates, with a high fraction of tetrahedral bonding, is by forming multilayered $a-\mathrm{C}$ structures composed of layers dense and rich in ${\mathrm{sp}}^3$ sites alternated by layers rich in ${\mathrm{sp}}^2$ geometries, a type of an amorphous superlattice. We present here a combined theoretical and experimental effort to investigate the stability, stress, and elastic properties of this type of $a-\mathrm{C}$ material. Our theoretical approach is based on Monte Carlo simulations within an empirical potential scheme, while the experimental part consists of spectroscopic ellipsometry, x-ray reflectivity, stress, and nanoindentation measurements in films prepared by magnetron sputtering. Our central result is that the average stress in the multilayered structures is nearly eliminated through layer-by-layer stress compensation, yet the fraction of ${\mathrm{sp}}^3$ sites in the dense regions remains high, sustained by the overwhelmingly compressive local stresses. The ${\mathrm{sp}}^3$-rich layers are stable both against a moderate increase of the width of the low-density layers, as well as under thermal annealing. The elastic moduli of the multilayered films are comparable with those of single-layer films. This, in conjuction with their low stress, makes them suitable for mechanical purposes.