  This study was aimed to investigate the thickness dependent morphological
changes of Mn films deposited on GaAs substrates by thermal evaporation
technique. Ni films were deposited under same conditions to perform comparative
study of the morphological changes with respect to the Mn films. The scanning
electron microscopy and atomic force microscopy studies revealed ripple-shaped
structure of Mn film with good periodicity, while Ni film only exhibited small
granules deposited throughout the surface. The influence of the thickness of
the Mn film in producing the ripple structure was clearly observed. In
addition, the annealing time was considered as the major parameter to control
the ordering of the ripple structure. X-ray diffraction pattern indicated the
formation of different phases of Ga-Mn and Mn-As due to diffusion of atoms
during annealing. A model for the creation of stress-driven microstructure is
proposed which indicates that Mn thin films grow on GaAs substrates in three
stages: in the primary stage, the growth occurs via two-dimensional nucleation
process; as the thickness increases, the stress is released by the film via
creation of additional surface roughness which produce ripples; and finally an
island-like growth occurs because of the non-uniform distribution of stress
along the surface of the film.
