Samuel Tardif

Interface-driven phase separation in multifunctional materials: the case of GeMn ferromagnetic semiconductor

We use extensive first principle simulations to show the major role played by interfaces in the mechanism of phase separation observed in semiconductor multifunctional materials. We make an analogy with the precipitation sequence observed in over-saturated AlCu alloys, and replace the Guinier-Preston zones in this new context. A new class of materials, the

Strain and correlation of self-organized Ge1 xMnx nanocolumns embedded in Ge (001)

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Structure and magnetism of Ge3Mn5 clusters

phases, is proposed to understand the formation of the coherent precipitates observed in the GeMn system. The interplay between formation and interface energies is analyzed for these phases and for the structures usually considered in the literature. The existence of the alpha phases is assessed with both theoretical and experimental arguments.

Raman-strain relations in highly strained Ge: Uniaxial ⟨100⟩, ⟨110⟩ and biaxial (001) stress

We report on the structural properties of Ge_(1-x)Mn_x layers grown by molecular beam epitaxy. In these layers, nanocolumns with a high Mn content are embedded in an almost-pure Ge matrix. We have used grazing-incidence X-ray scattering, atomic force and transmission electron microscopy to study the structural properties of the columns. We demonstrate how the elastic deformation of the matrix (as calculated using atomistic simulations) around the columns, as well as the average inter-column distance can account for the shape of the diffusion around Bragg peaks.

Exchange bias in GeMn nanocolumns: The role of surface oxidation

We have grown Ge3Mn5 clusters by codepositing germanium and manganese atoms on Ge(001) substrates using low temperature molecular beam epitaxy and further annealing the films at high temperature. Clusters are spherical and randomly distributed in the germanium film in epitaxial relationship with the diamond lattice. They exhibit a broad size distribution. By performing a careful x-ray diffraction analysis, we could find that 97% of Ge3Mn5 clusters have their c-axis perpendicular to the film plane while 3% exhibit in-plane c-axis. We could also show a slight in-plane distortion of the Ge3Mn5 lattice leading to a reduction of uniaxial magnetic anisotropy. These observations are well confirmed by complementary superconducting quantum interference device and electron paramagnetic resonance measurements.

From diluted magnetic semiconductors to self-organized nanocolumns of GeMn in germanium

The application of high values of strain to Ge considerably improves its light emission properties and can even turn it into a direct band gap semiconductor. Raman spectroscopy is routinely used for strain measurements. Typical Raman-strain relationships that are used for Ge were defined up to ∼1% strain using phonon deformation potential theory. In this work, we have studied this relationship at higher strain levels by calculating and measuring the Raman spectral shift-strain relations in several different strain configurations. Since differences were shown between the usual phonon deformation potential theory and ab-initio calculations, we highlight the need for experimental calibrations. We have then measured the strain in highly strained Ge micro-bridges and micro-crosses using Raman spectroscopy performed in tandem with synchrotron based micro-diffraction. High values of strain are reported, which enable the calibration of the Raman-strain relations up to 1.8% of in plane strain for the (001) biaxial...

Interface water diffusion in silicon direct bonding

We report on the exchange biasing of self-assembled ferromagnetic GeMn nanocolumns by GeMn-oxide caps. The x-ray absorption spectroscopy analysis of this surface oxide shows a multiplet fine structure that is typical of the Mn2+ valence state in MnO. A magnetization hysteresis shift |HE|∼100 Oe and a coercivity enhancement ΔHc∼70 Oe have been obtained upon cooling (300–5 K) in a magnetic field as low as 0.25 T. This exchange bias is attributed to the interface coupling between the ferromagnetic nanocolumns and the antiferromagnetic MnO-like caps. The effect enhancement is achieved by depositing a MnO layer on the GeMn nanocolumns.

X-ray magnetic circular dichroism in (Ge,Mn) compounds: experiments and modeling

While achieving high Curie temperatures (above room temperature) in diluted magnetic semiconductors remains a challenge in the case of well controlled homogeneous alloys, several systems characterized by a strongly inhomogeneous incorporation of the magnetic component appear as promising. Incorporation of manganese into germanium drastically alters the growth conditions, and in certain conditions of low temperature Molecular Beam Epitaxy it leads to the formation of well organized nano-columns of a Mn-rich material, with a crystalline structure in epitaxial relationship with the Mn-poor Germanium matrix. A strong interaction between the Mn atoms in these nanocolums is demonstrated by x-ray absorption spectroscopy, giving rise to a ferromagnetic character as observed through magnetometry and x-ray magnetic circular dichroism. Most interesting, the magneto-transport characteristics of the whole structure strongly depend on the magnetic configuration of the nano-columns.

(Ge,Mn): A ferromagnetic semiconductor for spin injection in silicon

The kinetics of water diffusion through the gap formed by the direct bonding of two silicon wafers is studied using two different techniques. X-ray reflectivity is able to monitor the interface density changes associated with the water front progression. The water intake is also revealed through the defect creation upon annealing, creating a rim-like pattern whose extent also gives the water diffusion law. At room temperature, the kinetics observed by either technique are consistent with the Lucas-Washburn law for diffusion through a gap width smaller than 1 nm, excluding any significant no-slip layer thickness.

Cavity mode analysis of highly strained direct bandgap germanium micro-bridge cavities

X-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD) spectra at the L