Electical properties of pure and Co-substituted ZnO thin films investigated by UV-assisted SPM experiments

Abstract

ZnO is an intrinsic n-type, wide band-gap (3.4 eV at 0 K), semiconductor which has been attracted the interest of the scientific community for several decades. More recently, ZnO has caught a renewed interest due to the improvements in the epitaxial growth techniques as well as to the possibility of p-type conductivity and ferromagnetic behaviour as a consequence of cation doping. In addition to this, as it shows much stronger electric polarization effects than other wide-gap semiconductors, such as GaN and SiC, ZnO has a great potential for manufacturing energy-harvesting systems. Moreover, as ZnO is a semiconducting piezoelectric material, it is already widely used in electro-mechanical systems for making smart sensors and nano-actuators and in communications for surface acoustic wave and thin film bulk acoustic wave resonator devices. However, the potential applications of ZnO as well as of various transition-metal oxides, e.g. TiO2, V2O5, and SnO2, is affected by their electronic structure and surface chemistry. In particular, the material band structure can be properly designed and tuned by electron doping or atom substitution, whereas the stabilization of surface defects, chemistry and reactivity is still a largely unexplored field. In this work the effect of Co-substitution on the electronic, electromechanical and surface properties of ZnO thin films (50 nm in thickness) has been deeply investigated. The substitution of Zn with Co atoms ensures no electrical doping, since the valence number of Zn and Co is the same. However, a 5% content of Co-substitution in ZnO has been previously shown to be enough to affect the electron conductivity of ZnO and to induce ferromagnetism in the semiconducting oxide. This behaviour has been theoretically addressed to the capability of Co to affect ZnO electronic band structure, by bonding the oxygen vacancies (typical ZnO intrinsic defects), introducing additional spin-polarized electronic levels close to the conduction band edge. Being spatially localized around the Co-complexes, these levels may alter the concentration of free carriers in intrinsically n-type ZnO, hence its bulk conductivity. These effects of Co-substitution on magnetic and conductive properties of ZnO have been recently studied by means of “bulk” experimental (X-ray absorption near edge structure, Hall measurements, magnetization measurements, etc.) and theoretical investigation techniques (DFT, calculation for isolated defect in a crystal matrix, etc.). On the contrary, in the case of our nano-sized thin films (50 nm in thickness) it is of fundamental importance to distinguish between “bulk” effect of Co-substitution and surface properties as well as eventual high defect concentration. In this scenario, scanning probe microscopy based experiments, such as conductive atomic force microscopy, electrostatic force microscopy, Kelvin probe force microscopy and piezo-response force microscopy, have been used to study the effect of Co-substitution on ZnO surface reactivity, work function, piezoelectricity and charge storage. The complexity of the observed phenomena required a deep study in different environmental conditions, such as in air and ultra-high vacuum, dark and under monochromatic UV light irradiation. In this thesis, it will be demonstrated that Co-substitution (5% content) deeply affect ZnO work function, leading to a 410 meV downward shift of the Fermi level, towards the valence band, and surface reactivity, inhibiting the adsorption of reactive molecular species at the surface. On the contrary, it does not affect ZnO piezoelectricity and charge storage phenomenon. These findings will be discussed in the framework of existing theoretical model. [edited by Author]