Effect Of GaN Based Substrate On The Propeteries Of BTO Thin Film Electrocaloric Effect

The electrocaloric（EC）effect of ferroelectric films has broad application prospects in many fileds such as sensors,electronic components and military.Barium titanate（BTO）is a typical environment-friendly lead-free ABO₃perovskite material.It is widely used in industrial production because of its high dielectric constant and low dielectric loss and many excellent ferroelectric materials can be obtained by doping BTO.Ga N is a third-generation semiconductor material,with a hexagonal wurtzite structure at room temperature（RT）and strong spontaneous polarization and has advantages of a large bandgap,good thermal conductivity,and corrosion resistance.In optoelectronics,high-power and high-frequency devices have a very wide range of applications.Meanwhile,the hexagonal structure of Ga N substrate can easily trigger the growth of rhombohedral phase in the upper ferroelectric film,which will significantly affect the negative EC effect of the thin film.Nevertheless,there is an inherent limitation:the rhombohedral phase only appears at the Ga N and ferroelectric thin film interface.In order to achieve a larger and wider negative EC effect,it is necessary to further control the structure of the material.Based on this background,high-quality lead-free ferroelectric thin film Mn-doped Ba TiO₃（Mn-BTO）were spin-coated on Pt（111）/TiO_x/SiO₂/Si（100）substrates by the sol-gel method.The surface morphology,energy storage and electrical properties were systematically studied.In order to improve the EC effect of Mn-BTO thin films,the structural simulation results show that the Si N/p-Ga N composite substrate is conducive to the formation of a large number of rhombohedral phases in the upper thin film.We prepared Si N/p-Ga N composite substrate to regulate the structure of upper ferroelectric thin films by MOCVD,and spin-coated high-quality Mn-BTO thin films on Si N/p-Ga N composite substrate by the sol-gel method.The electrical and dielectric properties of the thin films were systematically studied.The main research contents and achievements of this work are as follows:（1）Mn-BTO thin films spin-coated on Pt（111）/TiO_x/SiO₂/Si（100）substrates show excellent energy storage properties after annealing.The thin film with citric acid has better energy storage density～10.0 J/cm^-3 and energy storage efficiency～70.7%.The energy storage density and efficiency of the thin film without citric acid are 9.0 J/cm^-3 and 59.5%,respectively.All thin films exhibit an ultra-wide working temperature range(FWHM_ΔT>131 K)of the negative EC effect.Under the external electric field of 20 V,the thin film without citric acid has greater negative EC effect,ΔT and correspondingΔS are-3.03 K and 3.5JK^-1 kg^-1,respectively.However,ΔT and correspondingΔS of the thin film with citric acid are–0.8 K and 1.1 JK^-1 kg^-1,respectively.（2）The structural simulation calculation reveals that the free energy of structure of Si N/p-Ga N/Rhombohedral-phase Mn-BTO is smaller than the structure of p-Ga N/Rhombohedral-phase Mn-BTO,which suggests that more rhombohedral phases can be constructed by Si N/p-Ga N composite substrate.Under the external electric field of 20 V,the negative EC effect(ΔT_max～-0.80K,FWHM_ΔT～136 K)of the Mn-BTO thin film can be greatly improved(ΔT_max～-2.94 K,FWHM_ΔT～186 K)by constructing nano-scale rhombohedral phases over the Si N/p-Ga N composite substrate instead of the Pt（111）/TiO_x/SiO₂/Si（100）substrate.HRTEM test reveals that there are a large number of rhombohedral phases in the middle of the thin film,which greatly improve the negative EC effect,and verify the results of structural simulation calculation.