Epitaxial Integration And Performance Of Ferroelectric Thin Films With GaN

Integrated ferroelectrics and related devices based on the integration of ferroelectric thin films with semiconductors are the frontiers of microelectronics,condensed matter physics and materials science.The third-generation wide band semiconductor GaN has attracted extensive attention due to its excellent semiconductor characteristics of wide bandgap,high thermal conductivity,high breakdown field strength and high saturated electron drift velocity.Thereinto,it has been an important research direction to construct GaN-based integrated ferroelectric structures by using ferroelectric gate instead of traditional dielectric gate and develop enhanced GaN-based high electron mobility transistors.Therefore,the epitaxial integration of ferroelectric thin films with semiconductor substrates has become a research hotspot in international academia and industry.BaTiO₃?BTO?,as a perovskite lead-free ferroelectric material with excellent performance,is regarded as one of the most promising materials for field effect transistor insulated gates owing to its high dielectric constant and low dielectric loss.In recent years,ferroelectric properties have been found in Hf_0.5Zr_0.5O₂?HZO?.The unconventional HfO₂-based ferroelectric thin films have brought new opportunities for the research and application of integrated ferroelectric devices due to their unique physical characteristics such as high compatibility with the CMOS process and extremely thin physical thickness of the films.However,there still exist great challenges in the epitaxial integration of ferroelectric thin films with GaN,such as the large lattice mismatch,different crystal structures and incompatible growth environment.It is difficult to achieve the high-performance epitaxial ferroelectric thin films on GaN substrates.To this end,this thesis has inserted suitable buffer layers to promote the high-quality epitaxial integration of BTO and HZO ferroelectric thin films with GaN substrates by pulsed laser deposition?PLD?.The epitaxial growth mechanisms of BTO and HZO ferroelectric thin films on GaN have been studied.The relationships between the microstructure and electrical properties of BTO and HZO epitaxial thin films have been explained.The detailed research contents and results are as follows:1.High-quality perovskite?111?BaTiO₃?BTO?ferroelectric thin films have been epitaxially grown on wurtzite?0002?GaN substrates with the rationally designed SrTiO₃?STO?/TiN buffer layers.Particularly,TiN thin films with excellent conductivity are served as the bottom electrodes.By introducing STO/TiN buffer layers,lattice mismatch between?111?BTO and?0002?GaN is decreased from+12.5%to-2.5%,which promotes epitaxial growth of?111?BTO thin films on?0002?GaN substrates.According to the epitaxial relationship model,the epitaxial relationship of the heterostructures is?111?[11?0]BTO//?111?[11?0]STO//?111?[11?0]TiN//?0002?[112?0]GaN.STO/TiN buffer layers facilitate the epitaxial growth of BTO thin films,leading to significantly improved crystallinity and morphology of perovskite BTO thin films on wurtzite GaN substrates.Moreover,the epitaxial BTO perovskite ferroelectric thin films on wurtzite GaN substrates possess the favorable ferroelectric properties with the remnant polarization of 12.97?C/cm².2.Highly?111?-oriented HZO thin films have been epitaxially grown on?0002?GaN substrates using TiN as buffer layers and bottom electrodes.HZO thin films have exhibited ferroelectric properties.According to the lattice matching mechanism,the epitaxial relationship of HZO/TiN/GaN heterostructures is?111?[11?0]HZO//?111?[11?0]TiN//?0002?[112?0]GaN.The influences of the temperature and oxygen partial pressure on the epitaxy growth of HZO thin films also have been studied.The results show that high temperature and high oxygen partial pressure are beneficial to the crystallinities,surface morphologies and ferroelectric properties of HZO epitaxial films.The ferroelectric polarization of HZO thin films have modulation effect on the two-dimensional electron gas?2DEG?at the interface of AlGaN/GaN heterostructures.