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Analyses d’hétérostructures de semiconducteurs II-VI par sonde atomique tomographique et microscopie électronique en transmission

Jeudi 26 novembre 2015 14:00 - Duree : 2 heures
Lieu : Phelma – salle P014 - 23 rue des martyrs 38000 Grenoble

Orateur : Soutenance de Thèse de Bastien BONEF (CEA Grenoble - DSM/INAC/SP2M/NPSC)

This work addresses the problem of atomic scale structural and chemical characterization of II-VI based semiconductor heterostructures. A Correlative study with atom probe tomography and transmission electron microscopy is used to reveal the structure and composition of interfaces in ZnTe/CdSe superlattices and to improve their growth process. The atomic structure and the spatial distribution of chromium in the magnetic semiconductor (Cd,Cr)Te are also analysed.

Quantitative chemical data can be obtained with the atom probe on ZnTe or CdSe materials only when the experimental parameters of the instrument are properly optimized. The mass spectra also need to be correctly indexed. Experimental studies reveal that with the application of a low voltage on the tip and the use of a moderate laser power around 2.5 nJ with a green wavelength (515 nm), the measured composition in ZnTe and CdSe are close to stoichiometry between cations and anions. It is shown that a reliable parameter indicating optimum evaporation conditions is the charge ration between cations, which has to be set around 0.06 for Zn++/Z+ and around 0.35 for Cd++/Cd, independantly of the tip shape or the atom probe model. The use of these parameters allows lowering the loss in the detection of ions due to differences in their evaporation field. However, the application of a UV laser (343 nm) at low power will enhance the accuracy of the 3D reconstruction of both materials. Before the evaporation of superlattices, it is therefore compulsory to define the objectives of the experiment first.

Structural studies of ZnTe/CdSe superlattices reveal that interfaces are mainly composed of ZnSe-type bonding. The chemistry is obtained by high-resolution Z-contrast images, composition profiles measurements obtain by the zeta-factor analysis in EDX, and by the detection of ZnSe molecular ions in the atom probe tomography mass spectra. Many samples are investigated to highlight the ability of Zn and Se to bind together, instead of Cd and Te. Growth conditions are improved by taking this information into account to enforce the formation of CdTe based interfaces. However, the formation of ZnSe bonds seems inevitable and it lowers the possibility to finally obtain CdTe interfaces.

Atom probe tomography, correlated with EDX chemical mapping, reveal the clustering of Cr in rich regions of a few nanometers in the magnetic semiconductor (Cd,Cr)Te. Both techniques failed to provide a reliable composition of these Cr-rich regions, but they evidence a change in their shape with the increase of Cr concentration.

Contact : carmelo.castagna@cea.fr

Discipline évènement : (Physique)
Entité organisatrice : (INAC/SP2M)
Nature évènement : (Soutenance de thèse)
Site de l'évènement : Polygone scientifique

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