Observation of body centered cubic Cu in Cu/Nb nanolayered composites

The deposition of thin alternating layers of Cu and Nb on Si(100) substrates has been studied by transmission electron microscopy as a function of layer thickness. For layer thickness above 25 Å, there is a strong texture orientation relationship with the close packed planes of fcc Cu parallel to close packed planes of bcc Nb, forming the so-called “Kurdjumov-Sachs” orientation relationship. However, at thicknesses of under 12 Å, the Cu is constrained to grow as a slightly distorted bcc structure. It is thought that, when it reaches a critical thickness between 12 and 20 Å, the bcc Cu loses coherency and transforms martensitically to the fcc phase, resulting in the observed Kurdjumov–Sachs orientation relationship. Electron energy loss spectroscopy observations indicate a difference of 2 eV in the L3 edge suggesting that the Fermi energy is lower in the constrained bcc form of Cu than in the equilibrium fcc structure.