This thesis provides a combination of experimental and computational approaches to investigation of the structure-related mechanical behavior of pure and deliberately defected GaAs, InAs, GaSb semiconductor crystals as well as amorphous and nanocrystalline BiFeO3-based multiferroic materials. A similar approach is frequently used to examine the role of strain and defects to program discrete energy levels in quantum dots, lasers, photoelectrons, or to provide a sound evaluation of mechanical properties of nanomaterials. The mechanical response of the nanomaterials are described by parameters which range from a stress-strain relation to a fundamental description of the atomistic interactions. The mechanical properties of GaAs, GaSb and BiFeO3 materials within the framework of a nanoindentation experiment are elaborated on. Essential material parameters like Young's modulus, hardness, "true hardness" or yield strength defined as the elastic-plastic transition - which is caused either by phase transition or nucleation of defects in the studied materials - are addressed throughout the thesis. The nanoindentation results of BiFeO3 material obtained by atomic layer deposition are in line with the data obtained by means of other methods, such as Scanning Electron Microscopy, Energy Dispersive X-ray Spectroscopy, X-ray Diffraction and X-ray Photoelectron Spectroscopy. These results clearly demonstrate that nanoindentation is a convenient method to reveal the mechanical dependence of the stages of BFO film crystallization and finally the BiFeO3 structure formation. Molecular Dynamics and ab initio approaches are used to study the elastic and electronic properties of InAs and GaAs crystals containing various amounts of vacancies. The introduction of any vacancies into a crystal structure results in a linear decrease in elastic constants and lattice-parameters that causes the effect known as "the softening of material". Furthermore, the projected density of states analysis indicated the donor-like character of VGa, VIn vacancies and acceptor-like characteristics of VAs, respectively. Two different kinds of interatomic potential were used, namely the parameterized empirical potential of the Abell-Tersoff type and the pseudo-potential; the former was found to provide more suitable results. The successful modeling of elastic behavior of the defected structures provided an expanded insight into the problem of the impact of silicon admixture on the onset of plastic deformation in GaAs crystals under nanoindentation testing. It was found that an introduction of Si dopants resulted in the decrease of contact pressure at the elastic-plastic transition (pop-in event) and reduction of the pressure of GaAs-I - GaAs-II phase transformation suggesting there is a non-dislocation characteristic to the elastic-plastic transition in this material.
|Julkaisun otsikon käännös||Towards III-V compound semiconductors and multiferroics equipped with nanoindentation and atomistic calculations|
|Tila||Julkaistu - 2016|
|OKM-julkaisutyyppi||G5 Tohtorinväitöskirja (artikkeli)|