Surface characterization of MnxGe1-x and CryMnxGe1-x-y dilute magnetic semiconductors
We have used x-ray photoemission electron microscopy (XPEEM) and x-ray absorption spectroscopy (XAS) to characterize MnxGe1-x and CryMnxGe1-x-y films grown by molecular beam epitaxy. The surface layers of the as-grown films probed by XPEEM present segregation of Mn-rich phases. XAS using both total electron yield and fluorescence yield detection shows that the films are heavily oxidized after exposure to air. Etching in HF and HCl can be used to reduce oxidation, but inhomogeneities in the surface composition might not be completely eliminated depending on the Mn concentration. X-ray magnetic circular dichroism (XMCD) measurements reveal that neither the etched nor the as-grown films present remanent ferromagnetic behavior down to a temperature of 5 K within the probing depth of the fluorescence yield (similar to 20 nm). Mn is paramagnetic in both the oxidized and etched samples, with an increased tendency to order magnetically toward the interior of the films. Cr in CryMnxGe1-x-y possesses a paramagnetic moment only in the oxidized form. A comparison of the XAS line shapes obtained in the present study with those of Mn impurities deposited on Ge and GaAs surfaces demonstrates that the interpretation of XAS spectra of Mn-doped dilute magnetic semiconductors in the literature is often affected by residual oxidation.