Interfacial Characterization Study Of Ferroelectric/AlGaN/GaN Semiconductor Heterostructured Thin Films

Interface is of importance for the functional oxide/semiconductor heterostructured films. The heterostructured films has not only the field effect and the transport properties of charge carriers, but also the fruitful properties of oxides, such as superconducting, ferromagnetic, ferroelectric, piezoelectric, and optioelectic functions. One of the current research focuses is to combine these versatile properties with field effect at the interface. This may induce new phenomena and new effects. Recent years have evident a great advance of the growth method for the function oxide/semiconductor hetero-structured films. However, the further development of the function oxide/semiconductor films is hindered by the limited understanding of their interface properties, especially the lack of efficient characterization methods.Therefore, in this thesis we focus on the characterization study of the interfaces of ferroelectric/AlGaN/GaN heterostructured thin films, including AlGaN/GaN, ferroelectric/semiconductor (F/S), and ferroelectric domain wall (DW) interfaces. The nanoscale characterization of the two-dimensional electron gas (2DEG) at AlGaN/GaN interfaces, quantitative evaluation of interface trap states at F/S interfaces, and the kinetics of DW interfaces are systematically studied to understand the relationship between macroscopic properties of ferroelectric/AlGaN/GaN and these interface.For AlGaN/GaN interfaces, the nanoscale characterization of the 2DEG are successfully demonstrated by the home-build nanoscale capacitance spectroscopy (NCS) based on the probe of an atomic force microscopy. The capacitance resolution of NCS is about 10 aF (10x10-18 F). A current measuring method is proposed to determine the contact radius of the probe, and helps our NCS quantitatively characterize the local density of 2DEG. By using NCS, the effect of ultra-thin (~ 2 nm) MgO and TiO2 buffers on the properties of 2DEG is studied. It is found that MgO is superior to TiO2 for preserving the original properties of 2DEG when dielectric oxides are epitaxially grown on the AlGaN/GaN heterostructures.For F/S interfaces, the trap states are studied by the conductance method. A model for the effective conductance are proposed to separate the contributions of the trap states at the AlGaN/GaN interace and the ferroelectric/AlGaN interface. Using this model, two different trap states, including interface traps and bulk traps are found in the LiNbO3/AlGaN/GaN structure, and the effect of buffers of PZT/AlGaN/GaN on the interface trap density are systematically studied. It is found that LiNbO3/AlGaN/GaN has lower trap densities, and TiO2/MgO buffers are good candidates for reducing taps densities and leakage currents.In order to characterize the nanoscale DW interfaces and switching processes simultaneously, an electrode-piezoresponse force microscopy (PFM) was developed by combining a PFM with the pulse switching current measurement. Using the electrode-PFM, a comparative study was performed on PZT/SRO/STO, PZT/SRO/TiO2/GaN structures. By imaging the motion of DW interfaces during the polarization switching, a region-by-region nucleation-coalescence process are found in the PZT/SRO/TiO2/GaN structure. This process differs significantly from that of PZT/SRO/STO, which obey classical nucleation-limited-switching model. These characterization methods provide new tools for the further studies of ferroelectric oxide/semiconductor heterostructures.