Structural and magnetic properties of Ge1-xMnx thin films grown on Ge (001) substrates

We investigate the structural and magneto-optical properties of Mn-doped Ge (Ge1-xMnx) films with self-organized nanocolumns, grown on Ge (001) substrates by molecular beam epitaxy (MBE), in which the substrate temperature (TS) and growth rate (RG) are varied. Transmission electron microscopy (TEM) observations and magnetic circular dichroism (MCD) measurements reveal that Mn-rich nanocolumnar precipitation is formed in the Ge1-xMnx films grown at TS ≤ 100 deg. C, with keeping the size and spacing. At higher TS (≥150 deg. C), ferromagnetic Mn5Ge3 clusters are formed. It is also found that the Mn distribution in the Ge1-xMnx films can be controlled: By lowering TS or increasing RG, the Mn content xnc in the nanocolumns decreases and Mn atoms are more distributed into the Ge matrix, and eventually the magnetic properties are changed. The formation of the nanocolumns is explained by the spinodal decomposition in the layer-by-layer growth mode. We analyzed the periodicity and Mn content xnc of nanocolumns by using the Cahn-Hilliard equation.