Nanoscopic detection of anisotropic heat transports

薄膜因其优异的电学、光学和机械性能，在广泛的技术应用中发挥着越来越重要的作用。由于这些薄膜的热性能有助于许多技术部件(例如微电子和热电子器件或MEMS)的基本功能，因此它们的热特性是最令人感兴趣的。尤其是薄膜器件的热导率测量在最近几年引起了人们的极大关注，因为器件的工作温度同时影响着器件的寿命和性能。然而，常用的评估导热系数的技术在空间分辨率或关于热传输的方向性分析方面都受到限制。导热系数通常被简单地视为标量性质。然而，热输运既可以是各向异性的，也可以是界面处的一些非线性贡献。本项目第一阶段已经证明，利用扫描热显微镜(STHM)可以成功地研究超薄膜中各向异性的跨平面和面内热输运。以前只能通过模拟才能获得的热传输特性，如Stefan-Boltzmann输运方程，首次得到了实验证明。在薄膜厚度明显大于平均自由声子路径长度的情况下，弹道输运机制已被证明，这与通常的宏观扩散描述相矛盾。因此，在项目的第二阶段，我们以最高的空间分辨率定量研究了非晶层和多晶层的静态和动态热输运特性与温度的关系。一方面，卤化铅钙钛矿薄膜在太阳能电池、LED和激光等领域具有重要的应用前景。最有利的是，通过适当选择阳离子和卤素，它们可以根据晶体结构、维度和晶体取向获得热导率。同样，热输运研究也可以在晶界上进行。另一方面，通过原子层沉积产生的层状结构，提供了在多层体系的界面上以及在从二维导热到三维导热的转变过程中的定向传热机制。因此，在未来器件的故障分析和可靠性调查方面将打开全新的创新视角。此外，对纳米系统的热物理考虑，到目前为止还只是从理论上进行研究，现在可以通过测量来探索和验证。最后，我们将发现经典热传导定律在低维系统中的局限性。

Thin films are becoming more and more important in a wide range of technical applications due to their outstanding electronic, optical, and mechanical properties. Since the thermal properties of these thin films contribute to the basic functionality of a number of technical components (e.g. micro- and thermo-electronic devices or MEMS), their thermal characteristics are of uttermost interest. Especially, thermal conductivity measurements on thin film devices have attracted significant attention within the recent years, because the device operating temperature influences both: lifetime and performance. However, commonly used techniques to assess thermal conductivity are limited either in spatial resolution or with regard to directional analysis of heat transport. The thermal conductivity is often simply regarded as a scalar property. Nevertheless, the heat transport can either be anisotropic or may have some nonlinear contributions at interfaces.It has already been demonstrated within the first phase of this project that anisotropic cross-plane and in-plane thermal transport in ultrathin films can be studied successfully with Scanning Thermal Microscopy (SThM). Heat transport characteristics, that were previously accessible only by simulations, e.g. the Stefan-Boltzmann transport equation, were evidenced experimentally for the first time. Ballistic transport mechanisms have been demonstrated at film thicknesses significantly larger than the mean free phonon path lengths, which is in contradiction to the usual macroscopic diffusive description.Therefore, in the second phase of the project, that we apply for, here, the static and dynamic thermal transport properties of amorphous and polycrystalline layers are quantitatively studied with highest spatial resolution in dependence on the temperature. On the one hand, thin film of lead-halide perovskites will be considered which are of great current interest for applications such as solar cells, LEDs and LASERs. Most favorably, they grant access to the thermal conductivity in dependence on the crystal structure, the dimensionality, and the crystal orientation by suitable choice of their cations and halogens. Likewise, heat transport investigations are accessible at grain boundaries, hereby. On the other hand, layered structures, produced by atomic layer deposition, provide access to the directed heat transfer mechanisms at interfaces of multilayer systems and at the transitions from two-dimensional to three-dimensional heat conduction. Thereby, entirely new and innovative perspectives on failure analyses and reliability investigations of prospective devices will open up. In addition, thermo-physical considerations on nano-systems which were as of yet only studied theoretically, can now be explored and verified by measurements. Finally, limits of classical heat conduction laws at low dimensional systems will be discovered.