Access Type

Open Access Dissertation

Date of Award

1-1-2010

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Physics and Astronomy

First Advisor

Gavin Lawes

Abstract

ABSTRACT

Materials that simultaneously exhibit different physical properties provide a rich area of research leading to the development of new devices. For example, materials having a strong coupling between charge and spin degrees of freedom are essential to realizing a new class of devices referred to generally as spintronics. However, these multifunctional systems pose new scientific challenges in understanding the origin and mechanisms for cross-control of different functionalities. The core of this Ph.D. dissertation deals with multifunctional nitride and oxide compound semiconductors as well as multiferroic magnetic oxide systems by investigating structural, optical, electrical, magnetic, magnetodielectric and magnetoelectric properties.

Thin films of InN nitride compound semiconductors and closely related alloys have been investigated to understand the effects of intrinsic defects on the materials properties while considering possible applications of highly degenerate InN thin films. As grown rf sputtered InN films on c-axis (0001) sapphire exhibit highly degenerate n-type behaviour due to oxygen defects introduced during growth. The effect of oxygen in InN matrix has been further investigated by intentionally adding oxygen into the films. These studies confirm that oxygen is one of the main sources of donor electrons in degenerate InN. Above some critical concentration of oxygen, secondary phases of In2O3 and In-O-N complexes were formed. It was also possible to tune the carrier concentration to produce changes in the plasmon frequency, which varied from 0.45 eV to 0.8 eV. This characteristic energy scale suggests that these highly degenerate InN thin films could be used for thermophotovoltaic cells, optical filters, and other IR electro-optic applications.

To probe the magnetism in transition metal doped InN system, In0.98Cr0.02N and In0.95Cr0.05N thin films were fabricated. Our results suggest that these films develop ferromagnetic order above room temperature with a spin polarization of ∼ % ± 5 % , suggesting strong correlation between electron carriers and observed ferromagnetism.

Another In-based multifunctional material that has been explored is defect-rich In2O3. This system exhibits numerous interesting properties such as being simultaneously transparent and electrically conducting and above room temperature ferromagnetism together with semiconducting properties. The oxygen stoichiometry in In2O3 plays a crucial role in determining its optical, electronic, and magnetic properties. The effect of oxygen vacancies on different physical properties has been investigated. Our results suggest that as grown, nearly stoichiometric In2O3 thin films exhibit strong photopersistent current with very long carrier lifetime. Heat treatment under reduced oxygen environment creates oxygen vacancies in these films, producing electron donors. Thus vacuum annealed In2O3 becomes a highly degenerate n-type conductor. Oxygen deficient In2O3 can be used as transparent conducting oxide without any further doping, which allows the conductivity to be switched reversibly by thermal annealing in air or vacuum. In addition highly oxygen deficient In2O3 films exhibit ferromagnetism above room temperature.

We have also investigated oxide based magnetoelectric multiferroics which show simultaneous magnetic and ferroelectric properties. This study included detailed investigations of YMnO3, Ni3V2O8 and FeVO4, where we have investigated FeVO4 a new multiferroic system in the vanadate family. The main focus of this project was to understand the microscopic origin of the magnetoelectric coupling and cross-control of different ferroic order parameters in these system. We have synthesized bulk Ni3V2O8 and FeVO4 ceramics and characterized the thermal, magnetic, dielectric and magnetodielectric response of these samples in bulk form. To understand the cross-control of magnetic and ferroelectric order parameter we deposited thin films of Ni3V2O8 and FeVO4 systems and investigated their multiferroic properties using dielectric spectroscopy. We have demonstrated the direct control of multiferroic transition temperature with the applied external electric and magnetic fields. These investigations confirm the strong magnetoelectric coupling between different ferroic order parameters in such multifunctional multiferroic systems.

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