Atomic Mechanism of Thin Film Destruction at High Temperatures. Molecular Dynamics Method
The atomic mechanism of destruction of an ultrathin aluminum film consisting of five atomic layers is investigated by the method of molecular dynamics, and factors pointing to the beginning of this process are revealed. Special attention is paid to the low-frequency part of the vibrational spectrum of the system. In this connection, a previous symmetry analysis of atomic displacements is performed, which allows independent calculation of the frequency spectra corresponding to the bending and compression vibrations of the film. Thus, it is possible to clearly determine to which branch of the phonon spectrum the considered vibrations with a given frequency V belong, and to find the wavelength λ of the corresponding phonon. After determining the temperature range (0 - 600 K) in which the film remains stable for a time of 0.6 ns, the symmetry approach is used to perform a frequency analysis of the vibrational spectrum, starting from a minimum frequency Vmin = 0.0166 THz. Based on the obtained temperature dependences of the low-frequency spectrum, the changes in the film size along the z axis and its shape, a possible scenario for the destruction of thin films in MD simulation is proposed. At the initial stage, with increasing temperature, in the system there arises a “retarded” bending mode corresponding to the wave of maximum length whose vibration period is greater than the time of existence of the stationary state (in our case, more than 0.6 ns). Then a constant in time increase of the amplitude of this mode is observed, which results in a decrease of its length to a critical value, at which there arises an attractive fluctuation interaction between atoms located on opposite slopes of the wave crest, after which the process develops exponentially, leading to the film destruction. Thus, it is shown that low-frequency bending vibrations corresponding to the wave with the maximum length in the system under consideration play a significant role in the process of film destruction.