Guido Langouche

Co silicide formation on SiGeC/Si and SiGe/Si layers

The reaction of Co with epitaxial SiGeC/Si layers is investigated and compared to the reaction of Co with SiGe/Si layers. The sequence of phase formation is the same as the reaction of Co with monocrystalline Si, however, cobalt disilicide is formed at much higher temperatures. The presence of C further delays the disilicide formation, as a result of C accumulation at the silicide/substrate interface during the reaction, which blocks the Co diffusion paths. The CoSi2 layers thus formed exhibit a preferential (h00) orientation. The slow supply of Co atoms to the silicide/Si interface, due to the blocking of Co diffusion paths by Ge and C, is believed to be the reason for this epitaxial alignment.

Concentration-controlled phase selection of silicide formation during reactive deposition

Slow (low-rate) reactive deposition of a metal onto a Si substrate can result in direct formation of a metal disilicide, thereby skipping the metal-rich phases in the formation sequence. These observations have been explained thermodynamically by using the effective heat of formation model. As a result of this concentration-controlled phase selection, it is possible to form disilicides, such as CoSi2, NiSi2, or β-FeSi2 at much lower growth temperatures than possible in conventional solid-phase reaction of a metal layer deposited onto Si at room temperature (i.e., lower than the nucleation temperature). Moreover, epitaxial growth of CoSi2/Si(100), which is not possible by solid-phase reaction, becomes achievable when depositing Co atoms sufficiently slowly onto a heated Si substrate.

Thin film growth of semiconducting Mg2Si by codeposition

Ultrahigh vacuum evaporation of magnesium onto a hot silicon substrate (⩾200 °C), with the intention of forming a Mg2Si thin film by reaction, does not result in any accumulation of magnesium or its silicide. On the other hand, codeposition of magnesium with silicon at 200 °C, using a magnesium-rich flux ratio, gives a stoichiometric Mg2Si film which can be grown several hundreds of nm thick. The number of magnesium atoms which condense is equal to twice the number of silicon atoms which were deposited; all the silicon condenses while the excess magnesium in the flux desorbs. The Mg2Si layers thus obtained are polycrystalline with a (111) texture. From the surface roughness analysis, a self-affine growth mode with a roughness exponent equal to 1 is deduced.

Emission channeling studies of Pr in GaN

We report on the lattice location of Pr in thin film, single-crystalline hexagonal GaN using the emission channeling technique. The angular distribution of β− particles emitted by the radioactive isotope 143Pr was monitored by a position-sensitive electron detector following 60 keV room temperature implantation of the precursor isotope 143Cs at a dose of 1×1013 cm−2 and annealing up to 900 °C. Our experiments provide direct evidence that Pr is thermally stable at substitutional Ga sites.

Growth mechanism and optical properties of semiconducting Mg 2 Si thin films

Abstract Reactive deposition of magnesium onto a hot silicon substrate (200–500°C) does not result in any accumulation of magnesium or its silicide — the condensation coefficient of magnesium being zero. On the other hand, co deposition of magnesium with silicon at 200°C, using a magnesium-rich flux ratio, gives a stoichiometric Mg 2 Si film. The number of magnesium atoms which condense is equal to twice the number of silicon atoms which were deposited. The Mg 2 Si layers are polycrystalline with a (111) texture. These stoichiometric silicide films still show a tendency to sublimate; whereas capping with an oxide results in extensive intermixing during annealing. The Mg 2 Si films thus obtained exhibit optical transparency at sufficiently long wavelength, and an absorption edge. Extraction of the absorption coefficient from the data, and analysis of its energy dependence suggest an indirect bandgap of ∼0.74 eV, plus direct transitions at ∼0.83 and ∼0.99 eV.

Epilayer-Induced Structural Transition to b.c.c. Co during Epitaxial Growth of Co/Fe Superlattices

Molecular Beam Epitaxy was used to grow single-crystal b.c.c. Co/Fe multilayers on GaAs(10). The epitaxial relation between Co, Fe and GaAs has been determined during growth with Reflection High-Energy Electron Diffraction (RHEED). The presence of metastable b.c.c. Co was confirmed by 59Co NMR experiments. The thickness limit for the stability of the b.c.c phase established from the ex situ NMR study in a sandwiched structure is found to be twice the value determined from the in situ RHEED observed on an open surface. This shows that the Fe epilayer causes recrystallisation of the top atomic layers of Co to the b.c.c. structure.

Electron emission channeling with position-sensitive detectors

Abstract Electron emission channeling allows direct lattice location studies of low doses of radioactive atoms implanted in single crystals. For that purpose the anisotropic emission yield of conversion electrons from the crystal surface is measured, most conveniently by use of position-sensitive detectors. We discuss characteristic features of this method, including quantitative data analysis procedures, which are achieved by fitting simulated two-dimensional emission distributions for different lattice sites to the experimental patterns. The capabilities of this approach are illustrated by the case of rare earth atoms (Er, Tm, Yb) in Si, where we were able to do lattice location experiments down to implanted doses which are 150 times lower compared to previous RBS studies.

Low‐temperature anneal of the divacancy in p‐type silicon: A transformation from V2 to VxOy complexes?

Deep level transient spectroscopy of electron irradiated p‐type silicon reveals a defect level at Ev+0.19 eV, which during anneal treatments at 200 °C gradually transforms into a band with Ev+0.24 eV. Both energy levels however, are reported in literature to be the donor level of the divacancy. In the present study it is proposed that during the low‐temperature anneal the divacancy interacts with oxygen, forming a V2O complex. During heat treatments at temperatures in the range between 250 and 450 °C a further shift of the deep level to higher energy positions is observed which might be related with other vacancy‐oxygen complexes.

Aligned and twinned orientations in epitaxial CoSi2 layers

Heteroepitaxial CoSi2 layers have been made by ion beam synthesis and solid phase epitaxy in Si〈111〉 substrates. Using the x‐ray rocking curves of the asymmetric (331) reflections we are able to determine very accurately the relative amount of aligned (type A) and twinned (type B) CoSi2 in samples with different thicknesses. It is shown that for epilayers thinner than 360 A, the relative amount of type A CoSi2 decreases from 100% to 30%.

Paramagnetism in Mn/Fe implanted ZnO

Prompted by the generally poor understanding of the nature of magnetic phenomena in 3d-metal doped ZnO, we have undertaken on-line F57e Mossbauer spectroscopy on ZnO single crystals in an external magnetic field of 0.6 T, following the implantation of radioactive M57n ions at room temperature. The Mossbauer spectra of the dilute Fe impurities are dominated by sextets whose angular dependence rules out an ordered magnetic state (which had been previously proposed) but are well accounted for on the basis of Fe3+ paramagnetic centers on substitutional Zn sites with unusually long relaxation times (>20 ns).