Application of biased scanning probe microscopy techniques for multifunctional characterization of BiFeO3 and ZnO thin films

Abstract

This research work is focused on advanced characterization of multifunctional thin film materials by using biased scanning probe microscopy techniques. The first material which is characterized in this study is BFO, a well-studied multiferroic and the other material is ZnO, one of the potential future materials for advanced electronic applications. Scanning probe microscopy techniques, Piezoresponse Force Microscopy (PFM) and Kelvin Probe Force Microscopy (KPFM) are used in this work to characterize these materials for its multifunctional behavior. PFM technique is used for the ferroelectric domain imaging and switching, and for dc biased writing. However, KPFM is used to study the surface potential and charge transportation behaviors. Firstly, undoped BFO thin films were studied for the coupling effect of mechanical stress and magnetic field on its electrical properties. The results indicate that there is change in the ferroelectric domain and its switching behavior under the coupling effects of mechanical stress and magnetic field. This study is very useful in device designing and application if BFO is selected as a material. In addition the effects of magnesium (Mg) doping on Bi0.9La0.1FeO3 properties were also studied. The domain switching results suggests that switching became easier after Mg doping. It is also noticed that the Mg doping enhanced the information storage capabilities in the Bi0.9La0.1FeO3 thin films. KPFM study results revealed the presence and migration of oxygen vacancy in the doped sample when electric field was applied. Secondly, ZnO is studied for the effect of copper doping on its ferroelectric properties. It is found that copper gives rise to the ferroelectric-like behavior observed in doped ZnO. Only positive bias could switch the polarization while there is no effect of negative bias on polarization switching. A time-dependent study on polarization relaxation process concluded that the switched polarization can last even longer than 65 hrs. This study indicates that copper doped zinc oxide can be used for future data-storage application, if the positive dc bias is used for writing the information and negative to erase. In another study on ZnO thin film, the charge storage possibility in Cu and Co codoped ZnO thin films were characterized. The surface potential results under an unbiased condition show that the contact between the conductive tip (Pt-coated) and codoped ZnO surface has changed to Ohmic from the original Schottky contact in undoped ZnO. Therefore, more quantity of charge (both positive and negative) can store in the thin film sample. In addition, the codoped ZnO film has higher resistivity compare to the single element doped ZnO, which basically give rise to the polarization in the material. When the dc bias is applied on the sample surface, more charge could store as polarization and injected charge rather than the surface charge. This led to the long lasting stability of the bipolar charge in Cu and Co co-doped ZnO thin film. Finally, undoped ZnO thin films were investigated based on the contact engineering. A ferroelectric-like behavior is observed under certain combination of condition in ZnO. Some of the important conditions are: the top and bottom electrode (Pt found best), oxygen partial pressure, film deposition temperature, film thickness and the bias voltage. It is found that a 240 nm thick film, with Pt as bottom electrode, deposited under medium partial pressure and fully crystalline structure shows ferroelectric-like behavior. Therefore undoped ZnO also have some possibility for information storage application.