Résultats exploitables
We have produced Ge single crystals with extremely high isotopic purity and different isotopic compositions. The 70Ge produced, with 99.9% purity, is one of the most perfect crystals ever made, with a concentration of electrically active impurities lower than 1011 cm-3. We also produced 73Ge, 74Ge, and 76Ge. We have developed an original method of silicon production starting from SiF4 but without using Al or K. We have obtained 28Si with 99.7% of purity. Using a centrifuge cascade we continue the accumulation of 28Si with 99.9% enrichment. We have grown Si single crystals of different isotopic compositions by means of liquid phase epitaxy (LPE). We have also grown, by molecular beam epitaxy, GaAs crystals with several isotopic compositions of Ga. We have prepared a large CdS crystal made of isotopically pure 114Cd, and a crystal of 116CdSe, for inelastic neutron scattering experiments.
Thermal conductivity experiments have been carried out in Si and Ge with different techniques. They show a sharp increase of the thermal conductivity when increasing the isotopic purity of the samples. A theoretical analysis corroborates the importance of the sample geometry, surface cleaning and the effect of dislocations in samples with extremely pure isotopic compositions. Thermal treatments improve the thermal conductivity of those samples.
We have performed inelastic neutron scattering (INS) measurements of the phonon dispersion relations of 114CdS, a prototype of wurtzite-type crystal, for the first time. They could not be performed up to now because natural Cd contains a high proportion of 113Cd, which has a large neutron absorption cross-section. We have successfully reproduced the neutron data with ab intio LDA pseudopotential calculations. We have performed similar measurements for 116CdSe. We have also investigated, by INS, the phonon self-energies (real and imaginary parts) due to isotopic disorder in a 70Ge0.576Ge0.5 crystal (and also in natural Ge). An elaborate theoretical study of these self energies has been carried out.
Reflectivity measurements have been made on GaAs crystal alloys, where the term "alloy" refers to an admixture of isotopically pure Ga crystals (mostly 69Ga and 71Ga). The measurements have allowed us to analyze the variation of the energy gap of GaAs with the change in isotopic mass. A theoretical description, based on an empirical pseudopotential method, performed in Valencia, describes quantitatively well the observed changes in the energy gap and at the same time allows a complete study of the effect of isotopic mass variation on the phonon density of states and electron-phonon interaction in these materials. Investigations of the dependence of electronic gaps on isotopic mass have been started for other tetrahedral semiconductors. Measurements of the effects of isotopic mass on the self-energies of the E1 and E1 + Δ1 electronic interband transitions of germanium have been completed.
We have developed a novel and highly accurate technique to investigate the dependence of lattice parameters on isotopic masses, a rather fundamental question which has received, thus far, little attention. The method, based on x-ray standing waves, has been successfully applied to germanium.
We have started a systematic investigation of the anharmonic and isotope disorder processes leading to phonon self-energies in zincblende-type materials. An understanding of the various mechanisms and their systematics has been reached.
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