In recent years, there has been a growing interest in synthesizing diluted magnetic semiconductrs (DMS), obtained by substituting 3d transition metal ions into nonmagnetic semiconductors. Among the various systems, which have been largely studied such as (Ga,Mn)As or II-VI DMS, Mn-doped Ge is of particular interest since it offers a direct route for integrating magnetism with existing silicon technology [1]. A result of particular interest is the recent observation of a new high-temperature ferromagnetic phase, consisting of nano-columns of ~4 nm in diameter and 10 nm in spacing [2]. To complete this work, we have combined STM with TEM to investigate the initial stages of Mn deposition on Ge(001) surfaces. The Ge deposition was carried out at two substrate temperatures: at 80 °C, a temperature which is usually used for growing MnGe DMS, and at 250 °C, temperature at which intermetallic precipitates have tendency to be formed. For Mn deposition at 80 °C, two distinct kinds of Mn islands are observed even for Mn coverage smaller than a monolayer. Big islands have an average size of ~4 nm and small islands have size of 1-2 nm. When the Mn coverage increases, coarsening between small islands with newly deposited adatoms occurs, giving rise to the formation of mono-sized islands. Interestingly, these islands have an average size of 4 nm and are separated by a spacing of 10 nm. High-resolution TEM analyses indicate that those islands are epitaxial, defect free and perfectly coherent with the Ge substrate. For a Mn deposition carried out at 250 °C, only one kind of islands is observed and they appear to be pinned at the surface steps. Nanoscale chemical analysis of the island composition by means of energy dispersive X-ray spectrometry and magnetic characterizations of samples are now in process.

[1] Y.D. Park et al., Science 295 (2002) 651;

[2] M. Jamet et al., French patent n° 06/01149 (2006); Accepted in Nature Materials 2006.

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1. On the formation of magnetic nanocomposites and impurity complexes in GaN doped with Fe and Mn
2. High Curie temperature ferromagnetism in self-organized Ge_1‑xMn_x nano-columns
3. Selective epitaxial growth of Ge/Si quantum dots on patterned Si(001) substrate by ultra high vacuum chemical vapor deposition