Preparation And Study On The Electrical And Optical Properties Of Hybrid Improper Ferroelectric Ca₃Ti₂O₇ Thin Films

In recent years,Hybrid improper ferroelectric materials with perovskite R-P structures have received extensive attention.The general formula of the Ruddlesden-Popper series perovskite structure is Ca_n+1Ti_nO_3n+1,and the molecular formula of Ca₃Ti₂O₇(CTO)is obtained with n=2.The ferroelectricity of CTO is obtained due to the rotation and tilt of the oxygen octahedron.This opens new avenues for room-temperature multiferroic materials.In this paper,we prepare high-quality CTO thin films and investigate their physical properties at room temperature.In this work,we prepare the CTO thin films on Nb-doped SrTiO₃substrates by pulsed laser deposition.By changing the pressure of high-purity oxygen(10Pa,20Pa,30Pa,40Pa,50Pa,60Pa,70Pa)and different annealing temperatures(600?,650?,700?,750?,800?)during the preparation process,to control the quality of the CTO film.It is found that the CTO films with Nb-SrTiO₃substrate are epitaxially grown in both in-plane and out-of-plane directions,and the surface roughness is the smallest when the oxygen pressure is 40Pa.The CTO film has ferroelectricity at room temperature and when the oxygen pressure is 40Pa,the remanent polarization value of the CTO film is the smallest(0.78?C/cm~2),the leakage current density is the smallest,and the electrical performance is the best.When the annealing temperature is700?,the remanent polarization value of the CTO film is the smallest,the leakage current density is the smallest,and the ferroelectric performance is the best.Secondly,in this paper,we prepare the CTO thin films on Si substrates by pulsed laser deposition,and use a annealing in different temperature(600?,650?,700?,750?,800?)to explore their optical properties.The results show that the refractive index and extinction coefficient of the CTO thin film samples increase with the increase of the annealing temperature in air.The transmittance of CTO films is not significantly affected by the annealing temperature.By comparing the data before and after annealing,the band gap is significantly reduced.Then,we prepare the CTO thin films on LaAlO₃(LAO)substrates by pulsed laser deposition with deposition times of 8 min,16 min and 32 min,and the effects of different preparation times on the optical properties of CTO films were explored.It is found that the thickness of the sample increases linearly with the increase of the deposition time.Refractive index and extinction coefficient change with thickness.The possible reason is the influence of oxygen vacancies and defects.Finally,CTO films were prepared on SrTiO₃(STO),LAO,and Si substrates by pulsed laser deposition,and the effects of different substrates on the optical properties of CTO films were explored.The results show that the Si substrate CTO film has a larger refractive index,and the CTO film prepared on the LAO and STO substrates has a smaller refractive index,of which STO is the smallest.The minimum band gap value of silicon substrate film is 3.5e V,the band gap value of LAO substrate is 4.12e V,and the band gap value of STO substrate is 4.52e V.The possible reason is that the stress in different substrate is different,resulting in slightly different optical properties.