Quantum stress in nanofilms
14 Sep 2012
Read heads in hard drives, lasers in DVD players, transistors on computer chips, and many other components all contain ultrathin films of metal or semiconductor materials. Stresses arise in thin films during their manufacture.
These influence the optical and magnetic properties of the components, but also cause defects in crystal lattices, and in the end, lead to component failure. As researchers in the department of Eric Mittemeijer at the Max Planck Institute for Intelligent Systems in Stuttgart have now established, enormous stresses in the films are created by a quantum-mechanical mechanism that has been unknown until now, based on an effect by the name of quantum confinement.
This effect can cause stresses equivalent to one thousand times standard atmospheric pressure, dependent of thickness. Knowledge of this could be helpful in controlling the optical and mechanical properties of thin-film systems and increase their mechanical stability. Additionally, very sensitive sensors might also be developed on the basis of this knowledge.
Films of metal, semiconductor materials or ceramics can be grown today one atomic layer at a time onto crystalline substrates such as silicon. Despite this atomic precision, defects invariably arise in crystal lattices of films only a few nanometres thick; sometimes only one atom is missing in a lattice where one should actually be. These kinds of lattice defects can impair the efficiency of solar cells or semiconductor lasers.
One reason for this are stresses that arise in the film. Up to now, the main reason for these stresses was considered to be the growth of the film on a different material, so that the crystal lattice of the film did not coincide with that of the substrate. The atomic separations in the film were correspondingly contracted or expanded, with a compressive or tensile stress developing.
Materials scientists working with Eric Mittemeijer, director at the Max Planck Institute for Intelligent Systems in Stuttgart, have now discovered an additional mechanism that is able to create enormous stress in the ultrathin films.
