碲化铋基热电薄膜的非平衡载流子输运和相干声子振荡的研究

Thermoelectric thin film is a kind of new and prospective device for energy conversion,which is used to develop and utilize waste heat and solar energy. It has become commercialized partly.However, there still exists one big defect, which is low thermoelectric conversion efficiency. In the cause of optimum structure of material and functional design of device, the relaxation dynamics of excited carriers and phonons need to be understood elaborately in order to improve the efficiency of energy conversion. In this proposal, Bismuth Telluride (Bi2Te3) film，a representative high-performance thermoelectric film materials at room temperature, with different component ratios and impurities are fabricated by magnetron sputtering and evaporation coating. Transient reflectivity experiment setup is combined two kind of technique, which are detection of weak signal and femtosecond time resolved spectrum The ultrafast dynamics of film is measured by use of high-sensitive transient reflectivity technique so as to study heat transport and dissipation of nonequilibrium carriers. The main focus of proposed research is to discuss the relaxation rate and coherent phonon of this thermoeletric film.With the change of Bi/Te component ratio and doping, scattering of impurity and resistor of interface are taken into account for the improvement of thermoelectric figure of merit. It is expected that novel results and explanation will be provided experimentally and theoretically for the fabrication of high-performance functional thermoelectric device.

热电薄膜是将热能转换为电能的一种新型能量转换器件，可实现废弃热能、太阳能开发利用，已部分商品化，但热电转换效率低是其核心问题。为能够通过热电薄膜结构优化和性能设计来提高转换效率，迫切需要对热电薄膜的载流子输运和声子弛豫动力学过程进行深入剖析。本项目针对室温下性能最好的热电转换材料-碲化铋(Bi2Te3)，使用机械合金化法配合磁控和真空溅射法合成多种掺杂组分Bi2Te3热电薄膜，将微弱信号检测技术与飞秒时间分辨光谱学手段结合搭建高灵敏度瞬态反射实验装置，观测系列热电薄膜的超快动力学过程，研究薄膜的非平衡载流子热输运和耗散过程，着重讨论载流子弛豫速率和相干声子振荡频率的变化特性。改变薄膜材料的Bi/Te比例或掺杂元素，揭示散射、掺杂和界面热阻等因素对薄膜热电优值的影响规律，为设计与研发热电转换性能优异的功能器件提供理论基础和技术支撑。

针对半导体材料多晶CdSe、Bi2Te3和CdSeTe量子点材料开展了飞秒瞬态反射或透射实验的研究，能够明显看到材料中的相干声子振荡。对于CdSe多晶发现随着泵浦能量变化，CdSe多晶材料的载流子动力学随着泵浦能量的变化，有着明显的变化，在5ps左右有一个明显的拐点处，瞬态反射率的符号发生变化。对于真空电子束蒸镀得到的Bi2Te3薄膜，厚度约为60 nm，发现明显的相干声子振荡，其中一个成分是~2.69 THz的振动，其是源于纵模光学声子A1g模式的振动。另一个是由于薄膜界面影响和激光泵浦能量变化相关的振荡模式，随着能量的增加能够明显看到振动峰位的移动，我们将这一现象归结为模式软化。所得到的数据结果与相同厚度的Bi薄膜的超快动力学和相干声子振荡进行比较。搭建了飞秒种子光作为激发光源的单色泵浦-探测实验装置，研究了CdSeTe纳米晶体的相干声子和电子衰减动力学。尺寸由7.8 nm增加到22.0 nm伴随着更长的上升沿时间和更长的寿命。在CdSeTe（R=3.9 nm）中观察到了明显的相干声子振荡，初步推断这一振荡来自于共振激发造成的Fröhlich耦合。尺寸为11.6 nm和22.0 nm的CdSeTe纳米晶体中发现寿命为纳秒量级的衰减，初步判断是由于热累积造成的温度升高。针对几种半导体材料开展的相干声子振荡研究有利于了解光与半导体相互作用的载流子和声子作用过程，为进一步了解半导体材料的光热效应提供测量技术和理论依据。在Optics Letters等杂志上发表科研论文三篇，修改和投稿SCI科研论文三篇，出版科研书籍2本，申请专利四项。