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Technology and physical properties of the nanocomposite films of tellurides and manganites


Author: Şapoval Oleg
Degree:doctor of physics and mathematics
Speciality: 01.04.07 - Condensed matter physics
Year:2012
Scientific adviser: Valeriu Canţer
doctor habilitat, professor, Institute of the Electronic Engineering and Nanotechnologies
Institution: Institute of the Electronic Engineering and Nanotechnologies
Scientific council: DH 24-01.04.07
Institute of the Electronic Engineering and Nanotechnologies

Status

The thesis was presented on the 12 September, 2012
Approved by NCAA on the 15 November, 2012

Abstract

Adobe PDF document1.29 Mb / in romanian
Adobe PDF document0.78 Mb / in russian

Keywords

Epitaxial layers, nanocomposites, superlattices, strain relaxation, tellurides, manganites, complex oxides, simulation of x-ray difraction, magnetotransport, atomic force and transmission microscopy.

Summary

The originality of the workconsists in design of the new nanostructured functional materials on the base of tellurides and manganites. The problem of the strain relaxation due to thermal expansion coefficient of materials mismatch was investigated in multilayered structures of tellurides grown on silicon substrates. The lateral and vertical epitaxial nanostructures of manganites were investigated in combination with different phases of the complex and simple oxides.

Scientific novelty of the research
It has been demonstrated that the glide of dislocations in the main glide system of compounds A4B6 takes place in multilayered PbTe-SnTe structures in spite of presence numerous heterointerfaces and lattice mismatch about 2.06%. Fully relaxed individual layers of 40 nm PbTe and 70 nm CdTe don’t degrade and are able to stand stress originated from thermal expiation coefficient mismatch in multilayered structures grown on silicon substrates during the heating-cooling process.

The strain originated from BaTiO3 substrate is sustained in multilayered structure on the base of functional manganite La0.67Ca0.33MnO3 by introducing of intermediate BaTiO3 layers. The shift of parameters of the magnetic and metal-insulator transitions is in line with behavior of correlated polarons in La0.67Ca0.33MnO3.

Two different nanocrystalline epitaxial coupled phases of perovskite (LaSr(Ca)MnO3) and rock salt (MgO) (or wurtzite (ZnO)) grow during deposition from aerosol containing single solution of precursors of La, Sr(Ca), Mn and Mg(Zn). It has been demonstrated that magnetotransport properties of La1-xSrxMnO3/ MgO nanocomposite films are governed by spin polarized carriers tunneling through modified by MgO grain boundary of the La1-xSrxMnO3.

The presence of La, Sr and Mn impurities involving in the technological process wasn’t detected in nanocrystalline ZnO domain grown in La1-xSrxMnO3 matrix.

The practical significance of the results
The tailoring of multilayered structures (up to 500 periods) composed from individual layers of tellurides of 2-100 nm thickness and epitaxial grown on silicon substrate was elaborated. The ability to relax stress caused by mismatch of thermal expansion coefficients of the functional material and the substrate is the basis for the creation of devices based on them. The method of preparation of thermoelectric cooler based on PbTe-SnTe multilayered structure and monolithically integrated with silicon substrate was patent (Brevet de Invenţie MD Nr. 249. 2010).

The metalorganic aerosol deposition technique was designed for preparation of complex oxides multilayered structures in open atmosphere conditions. The construction of multichannel nozzle (Patent (international) WO 2009/083194 A1 (2009) allows growing oxides layers from three independent sources of solutions in parallel or serial modes.

The growth technique was elaborated for preparations of nanocrystalline ferromagnetic La1-xSrxMnO3 domains surrounded by thin insulating tunnel second phase MgO layers. The enhanced low field magnetoresistance demonstrated by nanocomposites suggests the base for high sensitive magnetic field sensors.