Qian Xin, Ding Zhiwei, Shin Jungwoo, Schmidt Aaron J, Chen Gang
Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
Rev Sci Instrum. 2020 Jun 1;91(6):064903. doi: 10.1063/5.0003770.
Measuring anisotropic thermal conductivity has always been a challenging task in thermal metrology. Although recent developments of pump-probe thermoreflectance techniques such as variable spot sizes, offset pump-probe beams, and elliptical beams have enabled the measurement of anisotropic thermal conductivity, a metal film transducer enabled for the absorption of the modulated pump laser beam and the detection of the thermoreflectance signal. However, the existence of the transducer would cause in-plane heat spreading, suppressing the measurement sensitivity to the in-plane thermal conductivity. In addition, the transducer film also adds complexity to data processing, since it requires careful calibration or fitting to determine extra parameters such as the film thickness and conductivity, and interface conductance between the transducer and the sample. In this work, we discussed the methodology for measuring in-plane thermal conductivity of layered semiconductors and semimetals without any transducer layer. We show that the removal of transducer results in the dominantly large sensitivity to in-plane thermal conductivity compared with other parameters, such as cross-plane thermal conductivity and the absorption depth of the laser beams. Transducerless frequency-domain thermoreflectance (FDTR) measurements are performed on three reference layered-materials, highly ordered pyrolytic graphite, molybdenum disulfide (MoS), and bismuth selenide (BiSe) and demonstrated using the analytical thermal model that the measured in-plane thermal conductivity showed much-improved accuracy compared with conventional FDTR measurement with a transducer.
在热计量学中，测量各向异性热导率一直是一项具有挑战性的任务。尽管诸如可变光斑尺寸、偏移泵浦 - 探测光束和椭圆光束等泵浦 - 探测热反射技术的最新发展使得各向异性热导率的测量成为可能，但需要一个能够吸收调制泵浦激光束并检测热反射信号的金属薄膜换能器。然而，换能器的存在会导致面内热扩散，从而抑制对面内热导率的测量灵敏度。此外，换能器薄膜也增加了数据处理的复杂性，因为它需要仔细校准或拟合以确定诸如薄膜厚度、电导率以及换能器与样品之间的界面电导等额外参数。在这项工作中，我们讨论了在没有任何换能器层的情况下测量层状半导体和半金属面内热导率的方法。我们表明，与其他参数（如 cross - plane 热导率和激光束的吸收深度）相比，去除换能器会导致对面内热导率具有显著更高的灵敏度。对三种参考层状材料——高度有序热解石墨、二硫化钼（MoS）和硒化铋（BiSe）进行了无换能器频域热反射（FDTR）测量，并使用解析热模型证明，与使用换能器的传统 FDTR 测量相比，所测量的面内热导率的准确性有了很大提高。
