Phase transition and surface morphology of MnAs/GaAs(001) studied within situvariable-temperature scanning tunneling microscopy

Abstract

The MnAs phase transition from the hexagonal ferromagnetic $\ensuremath{\alpha}$ to the orthorhombic paramagnetic $\ensuremath{\beta}$ phase has been investigated in situ by variable-temperature scanning tunneling microscopy (STM) as a function of epilayer thickness. The $\ensuremath{\alpha}\text{\ensuremath{-}}\ensuremath{\beta}$ phase coexistence leads to the formation of a self-organized stripes pattern of alternating $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ regions. The morphology evolution of the $\ensuremath{\alpha}\text{\ensuremath{-}}\ensuremath{\beta}$ periodic array of domains has been imaged in detail. The period and corrugation of this pattern are linear functions of the epilayer thickness with a domain periodicity nearly five times larger than film thickness. Also, STM local imaging through the phase-coexistence region $(10--45\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C})$ shows unambiguously the absence of mass transport during the transition. The self-organization of $\ensuremath{\alpha}\text{\ensuremath{-}}\ensuremath{\beta}$ stripes is consistent with an elastic-energy equilibrium state of the heteroepitaxial system at each temperature, as previously proposed for the origin of the modulated structure [V. M. Kaganer et al., Phys. Rev. B 66, 045305 (2002)]. Independently of self-organized $\ensuremath{\alpha}\text{\ensuremath{-}}\ensuremath{\beta}$ regions, the surface displays anisotropic mounds that are elongated along MnAs $a$ axis. This facetting process leads to a peculiar, highly anisotropic surface with oriented facets and submicron periodic modulation along the hexagonal $c$ axis. Smoother surfaces with larger terraces are obtained following postgrowth annealing. These results suggest that a careful control of the growth temperature and annealing procedure can be used to tailor the surface morphology for specific applications requiring anisotropic templates.