StatusThe thesis was presented on the 17 November, 2017
Approved by NCAA on the 11 May, 2018
Abstract– 1.17 Mb / in romanian
8.37 Mb /
The thesis is written in Romanian and contains several sections: introduction, 4 chapters, general conclusions and recommendations, bibliography of 273 titles, 5 annexes, 151 text pages, 106 figures and 4 tables. The results are published in 22 scientific papers. Keywords: zinc oxide, nanostructures, luminescence, laser, random laser, stimulated emission, Fabry–Pérot modes, guided modes, whispering gallery modes, refractive index. Field of study: nanotechnology and novel functional nanomaterials. The aim of the work is to develop technological processes for growing zinc oxide (ZnO) nanostructures with relevant optical properties to ensure stimulated emission and morphologies that ensure the formation of resonators with tailored properties and clarify the laser effect mechanism and the emission modes depending on the technologies used and the properties of the manufactured structures.
Objectives: development of ZnO micro- and nanostructures by metal organic chemical vapor deposition (MOCVD), carbothermal transport, chemical vapor deposition at low pressures (LPCVD), and electrochemical and thermal treatment; investigation of the influence of the corresponding technological parameters on the morphology and optical properties of the materials grown; study of the influence of nanostructuring on the radiation hardness of ZnO layers; identification of radiative recombination channels in micro- and nanostructured ZnO and appraisal of the possibilities of their application as laser gain media; determining the types of laser modes, resonator quality, lasing threshold in the micro- and nanostructures produced depending on the structure morphology and technological methods applied.
Novelty and scientific originality. Growth mechanisms of nanodots, nanorods, microand nanotetrapods, hexagonal micro- and nanodiscs, various microstructures, dense layers and layers of porous ZnO grown by MOCVD, LPCVD, carbothermal transport, and electrochemical and thermal treatment were determined. Radiative recombination channels in the grown ZnO structures were also determined. The high optical quality of the structures was shown to sustain lasing. In these structures, Fabry–Pérot modes were identified as well as guided modes, whispering gallery modes and random lasing with a quality factor of up to 3000. An effective instrument for the study of the refractive index of ZnO as a function of temperature by analyzing Fabry–Pérot mode positions in microtetrapodes has been proposed. Greater radiation hardness to heavy ion irradiation was demonstrated for nanostructured ZnO layers as compared to bulk ZnO and nanostructured GaN layers. The solved scientific problem is the identification of channels of radiative recombination in ZnO structures and development of nanolasers and microlasers with the quality factor, type and mode structure tailored by the morphology, shape and dimensions of grown structures.
Theoretical significance and practical value of the work. Growth mechanisms of ZnO structures, radiative recombination channels, mode type and structure of laser emissions were determined depending on the morphology, shape and dimensions of the structures obtained by different technological means. The developed technology broaden the design possibilities of resonators allowing the production of microlasers based on nanowires, microdiscs, microtetrapodes and microstructures assembled from these elements for use in optoelectronic microcircuits, photonic systems, identification and security systems. Carbothermal method offers the possibility to produce high quality optoelectronic devices at a low cost. Devices based on ZnO nanostructures can be used at higher levels of radiation. Heavy ion irradiation followed by annealing is a new method of increasing the optical quality.
Under consideration  :