Preparation And Properties Of Bi₄Ti₃O₁₂ Thin Films

With the rapid development of integrated circuit technology,the research on the properties of semiconductor device materials is more and more in-depth.Bi₄Ti₃O₁₂(BIT)with bismuth lamellar structure has attracted more and more researchers'attention for its excellent ferroelectric properties,high Curie temperature,good photoelectric properties,strong fatigue resistance and anisotropy.Bi₄Ti₃O₁₂has excellent tailoring and designability due to its unique crystal structure.Moreover,compared with traditional perovskite ferroelectric materials such as barium titanate and lead zirconate titanate,Bi₄Ti₃O₁₂has a relatively low band gap of about 3.0eV,which enables its absorption of sunlight to be extended to the visible region.These unique characteristics make Bi₄Ti₃O₁₂widely used in information storage technology,optoelectronic devices and sensors.Based on the above research background,this thesis mainly prepared Bi₄Ti₃O₁₂thin film devices by sol-gel method,and studied their resistance characteristics,photovoltaic effect and photodiode characteristics in depth.The main research contents of this thesis are as follows:Firstly,Au/BIT/LNO/Si structures were prepared by sol-gel method to study the memristor properties of Bi₄Ti₃O₁₂ferroelectric thin films.This is the first demonstration that the device has repeatable and stable bipolar resistance switch(RS)characteristics.Devices annealed in air have switching ratios of about 10~2at voltages 0.1 and-0.1V.After 100consecutive test cycles,RS performance did not degrade significantly.We also discuss the factors affecting the RS behavior of the device.Through the study of RS characteristics of annealed devices and XPS analysis,we found that the properties of RS are closely related to the existence of oxygen vacancies.The RS effect of the device annealed in air is obviously higher than that of the device annealed in oxygen.According to the slope fitting,the conduction mechanism of the device is ohmic conduction and space charge limited current(SCLC).In this study,BIT ferroelectric thin film was successfully applied to RS layer of memristors for the first time.In addition,the conductive wire theory is proposed to fully explain the relationship between RS behavior and oxygen vacancy,which provides meaningful inspiration for the design of high-quality random access resistance memory.Secondly,BIT/TiO₂composite films were successfully prepared on ITO transparent conductive substrate by sol-gel method.The microstructure,physical properties,photoelectric response and the effect of polarization on the optical response of the composite films were investigated.The current-voltage and current-time characteristics show that the composite films have better photoelectric properties than pure Bi₄Ti₃O₁₂and TiO₂films.The energy band variation of heterojunction is used to explain the conduction mechanism of composite films enhancing photovoltaic performance.In addition,the effects of external bias and polarization manipulation on the photoelectric characteristics of composite films are also discussed.The results show that the positive bias voltage can improve the photoelectric response,while the negative bias voltage can reduce the photoelectric response.At the same time,the negative polarization operation obviously improves the photoelectric performance,while the positive polarization operation not only reduces the photocurrent,but also reverses the photocurrent direction.This tunable photovoltaic effect is due to the coupling effect between the ferroelectric depolarization field and the electric field inside the heterojunction interface.This study provides an idea for improving and controlling ferroelectric photovoltaic performance of composite films,which can be applied to new solar energy conversion technology.Finally,a novel Au/BIT/P-Si photodiode was prepared by sol-gel method.The photodiode has excellent ultraviolet(UV)controlled photodiode characteristics.The device has a large conducting photocurrent under UV irradiation,which can reach up to 0.6 mA at a bias voltage of 8 V and UV intensity of 30.5 mW/cm~2.At a given positive voltage,the photocurrent of the photodiode increases significantly with the increase of ultraviolet light intensity.On the basis of experiments,we study the band variation of photodiode and the conduction mechanism of carrier movement under ultraviolet irradiation and bias.