【作者】 李昊；

【导师】 何枫；

【作者基本信息】 哈尔滨工业大学 ， 电子信息（专业学位）， 2024， 硕士

【摘要】 铋氧硒（Bi₂O₂Se）由于其优异的特性如高载流子迁移率,优异的空气稳定性和简单的制造工艺使得其在电子学、光电子学和铁电学方面的应用潜力而引起了人们的广泛关注。超快光谱是一种强大的实验技术,可以高精度地研究材料中的载流子动力学过程,通过深入了解其载流子的激发、传输和弛豫过程,揭示其光电特性。这对于评估该材料在电子学、光电子学和铁电学领域的应用潜力至关重要。目前,只有少数基于超快光谱的实验研究来阐明Bi₂O₂Se薄膜中的载流子动力学。此外,在太阳能电池、光电探测器等功能器件中,材料的厚度会直接影响载流子的生成、传输和收集效率,进而影响器件的性能。因此,通过研究厚度对载流子动力学的影响,可以指导功能器件的设计与优化,提高器件的转换效率和性能稳定性。几个研究小组已经对不同厚度的薄膜中的超快时间尺度和相关机制进行了报告,但是关于厚度和影响的全面认识仍然缺乏。载流子动力学揭示了稳态测量无法获取的信息,如载流子的扩散、复合和输运的物理过程以及相应的时间尺度,通过研究载流子动力学,可以指导材料设计,使其在光电器件等领域具有更优异的性能。在这项工作中,我们通过使用拉曼光谱和泵浦-探测技术,对在云母衬底上采用化学气相沉积（CVD）方法制备的Bi₂O₂Se薄膜进行了系统研究,这些薄膜的厚度范围在22.44 nm到4.62 nm之间,分析了其厚度对拉曼光谱和超快载流子动力学的影响。结合慢弛豫时间的厚度依赖性和能流密度依赖性,我们证明了在低于材料损伤阈值的较薄（<8 nm）Bi₂O₂Se薄膜中,受衬底诱导的压缩应变和高泵浦能流密度的影响,存在隐藏的铁电相变。此外,这种转变可以在高泵浦能流密度下表现出来。我们的研究结果加深了对Bi₂O₂Se薄膜铁电相和其半导体特性的理解,为铁电转变的光电器件提供了潜在的应用。另外,我们还比较了使用水热法和化学气相沉积法制备的Bi₂O₂Se薄膜样品在结构和性能上的差异。利用水热法制备了新的样品,并采用了相同的表征手段和超快光谱研究方法进行对比,比较了这两种方法得到的样品在载流子弛豫过程中的表现。我们发现化学气相沉积法制备的样品的扩散系数比水热法大3.75倍,这归因于采用化学气相沉积法法制备的材料具有更少的缺陷,使电子-空穴的复合效率降低。另外,CVD法制备的Bi₂O₂Se薄膜具有较低的A_1g频率,这意味着其层间耦合能力更弱。这些发现为不同制备方法得到的Bi₂O₂Se薄膜在光电领域的应用提供了重要的指导。总之,这项研究为操纵铁电相变提供了新的途径,并且为铁电相变的光电器件提供了潜在的应用,深化了对Bi₂O₂Se薄膜铁电相和其半导体特性的理解。揭示了水热法和化学气相沉积法制备对Bi₂O₂Se薄膜结构和性能的影响。这对于优化生长方法、控制材料特性,并为二维半导体材料在光电器件等领域的应用提供理论指导具有重要意义。更多还原

【Abstract】 Bismuth Oxychalcogenide（Bi₂O₂Se）has attracted widespread attention due to its excellent characteristics such as high carrier mobility,outstanding air stability,and easy preparation process,holding great potential for applications in electronics,optoelectronics,and ferroelectricity.Ultrafast spectroscopy is a powerful method that delves into the excitation,transport,and relaxation processes of charge carriers,offering valuable insights into the photoelectric properties of materials.This technique,utilizing ultrashort laser pulses in the femtosecond to picosecond range,provides exceptional time resolution,enabling the observation and analysis of rapid electronic and optical responses within materials.This comprehensive understanding is crucial for assessing the material’s potential in various applications such as electronics,optoelectronics,and ferroelectricity.However,only a few experimental studies based on ultrafast spectroscopy have elucidated the carrier dynamics in Bi₂O₂Se thin films.Furthermore,the thickness of the material directly affects the generation,transport,and collection efficiency of carriers in functional devices such as solar cells and photodetectors,thus impacting device performance.Therefore,studying the influence of thickness on carrier dynamics can guide the design and optimization of functional devices,improving their conversion efficiency and performance stability.Several research groups have reported on the ultrafast timescale and related mechanisms of films with different thicknesses,but a comprehensive understanding of the relationship between thickness and its impact is still lacking.Carrier dynamics reveal information that cannot be obtained from steady-state measurements,such as the physical processes of carrier diffusion,recombination,and transport,as well as the corresponding timescales.By studying carrier dynamics,material design can be guided to achieve superior performance in areas such as optoelectronic devices.In this work,we systematically studied the influence of thickness on Raman spectra and ultrafast carrier dynamics in Bi₂O₂Se thin films prepared by chemical vapor deposition（CVD）on mica substrates with thickness ranging from 22.44 nm to 4.62 nm.Combining the thickness dependence and fluence dependence of the slow decay time,we demonstrated the presence of a hidden ferroelectric phase transition induced by substrate-induced compressive strain and high pump fluence in Bi₂O₂Se films that are thinner（<8 nm）than the material damage threshold.Furthermore,this transition can be observed under high pump fluence.Our findings deepen the understanding of the ferroelectric phase and semiconductor properties of Bi₂O₂Se thin films,providing potential applications for optoelectronic devices based on ferroelectric transitions.Additionally,the differences in structure and properties of Bi₂O₂Se thin film samples prepared by organic ions template assisted solution growth（OTG）approach and CVD methods were compared.New samples were prepared using the OTG approach,and the same characterization methods and ultrafast spectroscopy were employed to compare their performance with the samples obtained through CVD methods.It was observed that the diffusion coefficient of the CVD-prepared samples was 3.75 times higher than that of the OTG approach.This can be attributed to the lower defect density in the CVD-prepared materials,resulting in a reduced efficiency of electron-hole recombination.Additionally,the Bi₂O₂Se films prepared by the CVD method showed a lower A_1gfrequency,indicating weaker interlayer coupling ability compared to the films prepared by the OTG approach.These findings provide important guidance for the application of Bi₂O₂Se thin films obtained by different preparation methods in the field of optoelectronics.In conclusion,this study provides a new approach to manipulate ferroelectric phase transitions and offers potential applications for ferroelectric phase transition-based optoelectronic devices,deepening the understanding of the ferroelectric phase and semiconductor properties of Bi₂O₂Se thin films.The influence of OTG approach and CVD methods on the structure and properties of Bi₂O₂Se thin films was revealed.This is of great significance for optimizing growth methods,controlling material properties,and providing theoretical guidance for the application of two-dimensional semiconductor materials in optoelectronic devices and other fields.更多还原