The main objective and the major value of this work is to provide a combined setup of theoretical and experimental techniques to simulate and measure a number of properties altered and/or induced by defects in three representative and technologically relevant oxides: zinc oxide (ZnO), zirconium dioxide (ZrO2) and magnesium oxide (MgO). Zinc oxide and zirconium dioxide are wide-gap semiconductors with a vast set of technological applications generally related to the inclusion of defects and impurities and spanning from optoelectronics to spintronics devices and photocatalysis. They are the subjects of Part II and Part III, respectively. A detailed description of their structure and chemical-physical properties is provided in the respective background sections. Part II deals with the main subject of this work: bulk and surface zinc oxide properties. After synthesis, ZnO commonly presents n-type conductivity and in Chapter 3 some of the most common donor defects (specifically: hydrogen interstitial and substitutional to oxygen, zinc interstitial and oxygen vacancy) have been investigated in terms of thermodynamic and optical transition energy levels. The good agreement with existing experimental data provides a powerful validation of the computational method presented in Sec. 2.1.5.1. In Chapter 4 copper doping of ZnO has been analyzed under different perspectives, spanning from the presence of donor and acceptor states, to magnetic interactions, to cluster tendency and to the interaction with oxygen vacancies. In particular, in conjunction with HREELS experiments, for the first time the inclusion of copper donor impurities in bulk ZnO has been observed which, on the basis of our calculations, we have assigned to interstitial copper species. In Chapter 5, the most promising shallow acceptor candidate for p-type doping of ZnO, i.e. nitrogen substitutional to oxygen, is investigated. First, nitrogen has been observed in polycrystalline sample and characterized in terms of hyperfine and quadrupolar coupling constants through a combined EPR and theoretical study. Secondly, the nitrogen doping process has been analysed upon the sputtering with ammonia of the mixed-terminated ZnO (10 0) surface through TDS experiments. The effectiveness of the doping process and the fundamental role of post-treatment oxidation were proved. The defective states within the optical gap have been estimated using HREELS measurements with a 66 eV electron primary energy. The computation of the transition energy levels have then demonstrated that nitrogen acts as a deep acceptor species in ZnO, disappointing the hopes to achieve p-type conductivity through nitrogen-doping. In order to get more insight in the nitrogen doping mechanism, the reactivity of ZnO single crystal and powders towards ammonia has been addressed by a combined theoretical and HREEL and UHV-FTIR study, in Chapter 6. Different coverages have been considered and, while at low concentration only molecularly adsorbed species have been observed, at a full monolayer coverage the repulsive steric interactions between adsorbates have been shown to induce the formation of an ordered adlayer with (2 x 1) periodicity, presenting alternating molecular NH3 and singly deprotonated NH2 moieties adsorbed on cationic sites. Part III is focused on ZrO2, specifically the tetragonal polymorph which is commonly stabilized by impurities. In particular, here the interest is related to the titanium doping of the material, which was experimentally found to induce a large red shift of the optical absorption edge. In Chapter 7 we proposed a rationalization of this experimental observation based on the computation of the optical transition levels of Ti species. The possible interaction with oxygen vacancies has also been considered. In Part IV the nitrogen doping of MgO, recently proposed as potential route to achieve room temperature ferromagnetism has been investigated. This work was divided in two parts. In Chapter 8 the electronic structure and the spin properties of nitrogen impurities have been theoretically studied, considering also possible charge transfers with magnesium or oxygen vacancies. Finally, in Chapter 9 we show how a trapped N2‒ radical anion in the polycrystalline material has been identified and characterized through a combined EPR and DFT study.

(2012). A combined computational and experimental study of spectroscopic evidences by dopants and defects in semiconducting and insulating oxides. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2012).

A combined computational and experimental study of spectroscopic evidences by dopants and defects in semiconducting and insulating oxides

GALLINO, FEDERICO
2012

Abstract

The main objective and the major value of this work is to provide a combined setup of theoretical and experimental techniques to simulate and measure a number of properties altered and/or induced by defects in three representative and technologically relevant oxides: zinc oxide (ZnO), zirconium dioxide (ZrO2) and magnesium oxide (MgO). Zinc oxide and zirconium dioxide are wide-gap semiconductors with a vast set of technological applications generally related to the inclusion of defects and impurities and spanning from optoelectronics to spintronics devices and photocatalysis. They are the subjects of Part II and Part III, respectively. A detailed description of their structure and chemical-physical properties is provided in the respective background sections. Part II deals with the main subject of this work: bulk and surface zinc oxide properties. After synthesis, ZnO commonly presents n-type conductivity and in Chapter 3 some of the most common donor defects (specifically: hydrogen interstitial and substitutional to oxygen, zinc interstitial and oxygen vacancy) have been investigated in terms of thermodynamic and optical transition energy levels. The good agreement with existing experimental data provides a powerful validation of the computational method presented in Sec. 2.1.5.1. In Chapter 4 copper doping of ZnO has been analyzed under different perspectives, spanning from the presence of donor and acceptor states, to magnetic interactions, to cluster tendency and to the interaction with oxygen vacancies. In particular, in conjunction with HREELS experiments, for the first time the inclusion of copper donor impurities in bulk ZnO has been observed which, on the basis of our calculations, we have assigned to interstitial copper species. In Chapter 5, the most promising shallow acceptor candidate for p-type doping of ZnO, i.e. nitrogen substitutional to oxygen, is investigated. First, nitrogen has been observed in polycrystalline sample and characterized in terms of hyperfine and quadrupolar coupling constants through a combined EPR and theoretical study. Secondly, the nitrogen doping process has been analysed upon the sputtering with ammonia of the mixed-terminated ZnO (10 0) surface through TDS experiments. The effectiveness of the doping process and the fundamental role of post-treatment oxidation were proved. The defective states within the optical gap have been estimated using HREELS measurements with a 66 eV electron primary energy. The computation of the transition energy levels have then demonstrated that nitrogen acts as a deep acceptor species in ZnO, disappointing the hopes to achieve p-type conductivity through nitrogen-doping. In order to get more insight in the nitrogen doping mechanism, the reactivity of ZnO single crystal and powders towards ammonia has been addressed by a combined theoretical and HREEL and UHV-FTIR study, in Chapter 6. Different coverages have been considered and, while at low concentration only molecularly adsorbed species have been observed, at a full monolayer coverage the repulsive steric interactions between adsorbates have been shown to induce the formation of an ordered adlayer with (2 x 1) periodicity, presenting alternating molecular NH3 and singly deprotonated NH2 moieties adsorbed on cationic sites. Part III is focused on ZrO2, specifically the tetragonal polymorph which is commonly stabilized by impurities. In particular, here the interest is related to the titanium doping of the material, which was experimentally found to induce a large red shift of the optical absorption edge. In Chapter 7 we proposed a rationalization of this experimental observation based on the computation of the optical transition levels of Ti species. The possible interaction with oxygen vacancies has also been considered. In Part IV the nitrogen doping of MgO, recently proposed as potential route to achieve room temperature ferromagnetism has been investigated. This work was divided in two parts. In Chapter 8 the electronic structure and the spin properties of nitrogen impurities have been theoretically studied, considering also possible charge transfers with magnesium or oxygen vacancies. Finally, in Chapter 9 we show how a trapped N2‒ radical anion in the polycrystalline material has been identified and characterized through a combined EPR and DFT study.
DI VALENTIN, CRISTIANA
YUEMIN, WANG
ZnO, ZrO2, MgO, DFT, HREELS, EPR, IR, TDS, transition levels, nitrogen-doping, copper-doping, ammonia, titanium-doping, B3LYP, CRYSTAL, p-type doping
CHIM/03 - CHIMICA GENERALE E INORGANICA
English
13-gen-2012
Scuola di dottorato di Scienze
SCIENZA DEI MATERIALI - 08R
24
2010/2011
Thesis presented for the degree of Doctor Europaeus and For the degree of Doctor Europaeus in Materials Science of the Network Physics and Chemistry of Advanced Materials (PCAM).
open
(2012). A combined computational and experimental study of spectroscopic evidences by dopants and defects in semiconducting and insulating oxides. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2012).
File in questo prodotto:
File Dimensione Formato  
Phd_unimib_725138.pdf

accesso aperto

Tipologia di allegato: Doctoral thesis
Dimensione 7.09 MB
Formato Adobe PDF
7.09 MB Adobe PDF Visualizza/Apri

I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.

Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/10281/28403
Citazioni
  • Scopus ND
  • ???jsp.display-item.citation.isi??? ND
Social impact