Thesis presented November 26, 2015
Abstract:
This PhD work addresses the problem of atomic scale structural characterization of II-VI based heterostructures. The correlative use of atom probe tomography and transmission electron microscopy reveals the structure and composition of interfaces in ZnTe/CdSe superlattices to improve their growth condition. The atomic structure and the atomic Cr distribution are also revealed in (Cd,Cr)Te diluted magnetic semiconductor.
When experimental parameters set in the atom probe are optimized, quantitative data can be obtain on both ZnTe and CdSe semiconductors with this technique. Compositions are obtained with the mass spectrum and it has to be correctly indexed. Experimental studies reveal that with the application of a low voltage on the tip and a moderate laser power around 2.5 nJ with a green laser (515 nm), the measured composition in ZnTe and CdSe are close to the stoichiometry between cations and anions. Setting the cations ratio Zn++/Zn+ around 0.06 et Cd++/Cd+ around 0.35 during the evaporation of the field is a reliable way to reach the optimum evaporation condition for different tips and in different atom probes. Those parameters are responsible for lowering the loss in the detection of the ions due to their different evaporation field. However, the application of a low laser power in UV (343 nm) will enhance the spatial resolution of the atom probe and the 3D reconstruction of both semiconductors. Before the evaporation of the superlattices, it is therefore compulsory to define the objectives of the experiment first.
Structural studies of ZnTe/CdSe superlattices reveal that interfaces are composed of ZnSe. Their chemistry is obtain by high resolution Z-contrast images, composition profiles obtain by the zeta-factor method in EDX and by the presence of ZnSe molecular ions in the atom probe tomography mass spectrum. Many samples are investigated to highlight the ability of Zn and Se to bind together instead of Cd and Te. Growth condition are improved by taking this information into account and to force the formation of CdTe based interfaces. Despite the growth precaution, ZnSe bonds seem inevitable and it lowers the possibility to finally obtain CdTe interfaces.
Atom probe tomography studies correlated with EDX chemical mapping reveal the gathering of Cr in rich region off a few nanometers in the diluted magnetic semiconductor CdCrTe. Both techniques are not reliable to get the composition of this Cr riche regions but they reveal a change in their shapes with the increase of Cr concentration in different samples.
Keywords: Clusters, Optoelectronics, Atom probe tomography, Transmission electron microscopy, II-VI semi-Conductors
On-line thesis.