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International audience; We have studied the stability of manganese germanide thin films grown on Ge(111) substrates by three different growth methods: solid phase epitaxy, reactive deposition epitaxy and co-deposition. By combining X-ray diffraction and magnetic measurements, we demonstrate that we can form either Mn5Ge3 or Mn11Ge8 thin films depending on the growth processes. In the case of solid phase epitaxy, we explain the phase formation sequence by taking into consideration the kinetic and thermodynamic processes involved. Tuning phase formation of the manganese germanide thin films was determined using in situ X-ray diffraction and the effect of the thin film thickness on the Mn5Ge3 thermal stability was investigated.

In this paper, we report investigations concerning the fabrication of a diluted Ge (Mn) solution using solid state Mn diffusion, and Mn/Ge reactive diffusion for spintronic applications. The study of Mn diffusion shows that the quasi-totality of the incorporated Mn atoms occupies Ge substitutional sites and probably exhibits two negative elementary charges. The solubility limit of Mn in Ge is comprised between 0.7 and 0.9 % (T  600 °C). We show that substitutional Mn atoms are not ferromagnetic in Ge and consequently that Ge (Mn) diluted magnetic semiconductor can not be produced. Beside the ferromagnetic signal from Mn5Ge3, ferromagnetic signals detected in the samples could be always attributed to surface or bulk Mn-Ge clusters. Furthermore, we show that the CMOS Salicide process is potentially applicable to Mn5Ge3 nanolayer fabrication on Ge for spintronic applications. During Mn (thin-film)/Ge reaction, Mn5Ge3 is the first phase to form, being thermally stable up to 310 °C and exhibiting ferromagnetic properties up to TC ~ 300 K.

International audience

In situ experiments based on X-ray diffraction, sheet resistance and differential scanning calorimetry were performed on Pt thin films deposited on Si either undoped or doped with As (n type) or B (p type). In all cases, these measurements show a sequential formation of Pt 2 Si followed by PtSi phases. Simulations of the kinetics show that the growth is controlled by both the interface and the diffusion through the growing silicide. A slowdown of the formation of Pt 2 Si has been observed at the end of Pt consumption; this might be due to the accumulation of impurities in the Pt film. The in situ measurements show a delay between the end of the growth of Pt 2 Si and the formation of PtSi that is related to the strain relaxation in Pt 2 Si: i.e. the growth of PtSi starts when most of the strain in Pt 2 Si is relaxed. Anisotropy in the magnitude of strain value and gradient was found for two grain orientations of Pt 2 Si. The dopants (B and As) do not significantly influence the silicide formation properties (phase sequence, strain and sheet resistance). Boron addition has almost no influence on the formation kinetics while Arsenic slightly slows it down. The sheet resistance of Pt 2 Si is lower than that of PtSi, which remains stable until 800 °C.

International audience; This communication describes the development of optimized metallic contacts on Si for thermoelectric applications. Thin solid films of Ni and Pt with the same thickness, were deposited on Si substrates. Two silicides were formed in a vacuum chamber and were studied. The EDX spectroscopy and electron microscopy have supported the presence of silicides in the surface of the samples. The thermoelectric study demonstrated that silicides could play a vital role in the enhancement of the electricity generated by thermoelectric materials that are made of Si. Pt silicide was found to be better candidate than three other metallic contacts (Pt, Ni and Ni silicide), but a comparison with other silicides is needed in the future, to get the best electronic contact on thermoelectric materials.

The possible fabrication of spintronic devices based on the Mn–Ge binary system supports extensive investigations on the magnetic properties of Mn–Ge structures. However, the global magnetic signal of a given sample is sometime complex due to the coexistence of several objects of unknown magnetic properties. We report the existence of ferromagnetic Mn–Ge nanoclusters (2–4 nm) containing only 4–6% Mn with a Curie temperature of ∼43.5 K, usually attributed to an 1.2% Mn-rich diluted magnetic semiconductuor.

Deposition of 0.5 monolayer of Mn by molecular beam epitaxy on the surface of a Ge(0 0 1) substrate, and annealing, allowed the fabrication of a cluster-free Ge(Mn) diluted solution. Electronic spin resonance (ESR) was used to study the magnetic properties of this solution. These measurements, combined with secondary ion mass spectrometry, atomic force microscopy, and Auger electron spectroscopy, show that the detected ferromagnetic signal is due to surface islands, while Mn atoms on Ge substitutional sites gives no detectable ESR signal.

We have combined numerous characterization techniques to investigate the growth of tensile-strained and n -doped Ge films on Si(001) substrates by means of solid-source molecular-beam epitaxy. The Ge growth was carried out using a two-step growth method: a low-temperature growth to produce strain relaxed and smooth buffer layers, followed by a high-temperature growth to get high crystalline quality Ge layers. It is shown that the Ge/Si Stranski–Krastanov growth mode can be completely suppressed when the growth is performed at substrate temperatures ranging between 260 °C and 300 °C. X-ray diffraction measurements indicate that the Ge films grown at temperatures of 700–770 °C are tensile-strained with typical values lying in the range of 0.22–0.24%. Cyclic annealing allows further increase in the tensile strain up to 0.30%, which represents the highest value ever reported in the Ge/Si system. n -Doping of Ge was carried out using a GaP decomposition source. It is shown that heavy n -doping levels are obtained at low substrate temperatures (210–250 °C). For a GaP source temperature of 725 °C and a substrate temperature of 210 °C, a phosphorus concentration of about 10 19 cm − 3 can be obtained. Photoluminescence measurements reveal an intensity enhancement of about 16 times of the direct band gap emission and display a redshift of 25 meV that can be attributed to band gap narrowing due to a high n -doping level. Finally, we discuss about growth strategies allowing optimizing the Ge growth/doping process for optoelectronic applications.

Interesting results have been reported concerning the magnetic properties of the Mn5Ge3 compound, opening the road to possibly create spin injectors in Ge. However, a process compatible with the Complementary Metal Oxide Semiconductor technology, allowing to produce a Mn5Ge3 layer on the active regions of Ge-based transistors has not been well established yet. Here, we report on the solid state reaction between a 50 nm-thick Mn film and amorphous Ge, aiming to investigate a similar process than the one (Salicide) used for contact production in the standard Si technology. In situ X-ray diffraction combined with ex situ structural and magnetic characterizations were used to identify and study phase formation during the Mn/Ge reaction.

The diffusion of a half monolayer of Mn deposited by molecular beam epitaxy on a Ge(0 0 1) substrate was studied via secondary ion mass spectrometry. Mn diffused in Ge under extrinsic conditions, exhibiting a solubility of 0.7–0.9%. All the Mn atoms were activated, occupying Ge substitutional sites and exhibiting a negative charge, in agreement with semiconductor doping theory. The diffusion mechanism being vacancy ( V )-mediated, the formation of Mn– V pairs is suggested. Mn surface desorption occurred for temperatures >600 °C.

An alternative solution for producing logic devices in microelectronics is spintronics (SPIN TRansport electrONICS). It relies on the fact that in a magnetic layer, the electrical current can be spin polarized. To fabricate such components, a material whose electronic properties depend on its magnetic state is needed. The Mn-Ge system presents a lot of phases with different magnetic properties, which can be used for spintronics. The most interesting phase among the Mn-Ge system is Mn5Ge3 because of its stability at high temperatures, its Curie temperature which is close to room temperature and its ability of injecting spin-polarized electrons into semiconductors. In this paper, we have combined Reflection High-Energy Electron Diffraction (RHEED) and X-ray Diffraction (XRD), to study the sequence of formation of MnxGey phases during reactive diffusion of both a 50 nm and a 210 nm thick Mn films deposited by Molecular-Beam Epitaxy (MBE) on Ge (111).

The Mn5Ge3 compound, thanks to its room-temperature ferromagnetism, metallic character and ability to epitaxially grow on germanium, acts as a potential candidate for spin injection into group-IV semiconductors. Understanding and controlling Ge overgrowth behaviour on Mn5Ge3/Ge heterostructures represents a crucial step to realize Ge/Mn5Ge3/Ge multilayers for numerous spintronic applications. Here, we have combined structural and morphological characterizations with magnetic analyses to study the mechanisms of Ge overgrowth on epitaxial Mn5Ge3 layers in the temperature range of 450-550 degrees C. It is found that deposited Ge instantly reacts with Mn to form a Mn5Ge3 surface layer, which, acting as a surfactant, continuously floats upwards from the growing surface to a distance larger than 70 nm. New Ge layers are successively formed underneath, allowing such a floating Mn5Ge3 surface layer to be stabilized by epitaxy. These observations can be considered as a typical example in which the stabilization of metastable thin films by epitaxy can overcome thermodynamic equilibrium. We have also investigated the effect of carbon adsorption on the top of the Mn5Ge3 layer prior to Ge deposition to control the Mn:Ge reaction. It is shown that adsorbed carbon effectively reduces the out-diffusion of Mn from Mn5Ge3, allowing Ge layers to stack up on top of Mn5Ge3. However, at temperatures of 450-550 degrees C, carbon may react with Mn to form manganese carbides and the resulting Ge overlayers are found to change their orientation from the (111) plane to the (001) plane, which has a higher surface energy. Finally, a strategy to realize Ge/Mn5Ge3/Ge multilayers will be addressed.

In recent years, spintronics whose principle is based on controlling the spin of electrons in semiconductor layers is presented as a complementary or even an alternative solution for production of logic devices in microelectronics. It relies on the fact that electric current in a magnetic layer can be spin polarized. Manufacture of such components is based on the use of materials or heterostructures whose electronic properties depend on their magnetic state. The magnetic Mn-Ge system is interesting because of its possible development at high Curie temperature and its integration on Si substrate. Among all of the Mn-Ge phase compounds of the diagram, Mn5Ge3 seems the most interesting one for spintronics applications: it is a stable and ferromagnetic phase at room temperature. In this paper, we present first results of the study, by Reflection High Energy Electron Diffraction (RHEED), X-ray diffraction (XRD) and Atomic Force Microscopy (AFM), of the sequence of formation of the MnxGey phases in the case of reaction of a nanometric-thick Mn film (200nm) deposited by MBE on Ge (111).