In-plane Thermal Conductivity Measurement with Nanosecond Grating Imaging Technique

Abstract

We develop a nanosecond grating imaging (NGI) technique to measure inplane thermal transport properties in bulk and thin-film samples. Based on nanosecond time-domain thermoreflectance (ns-TDTR), NGI incorporates a photomask with periodic metal strips patterned on a transparent dielectric substrate to generate grating images of pump and probe lasers on the sample surface, which induces heat conduction along both cross-and inplane directions. Analytical and numerical models have been developed to extract thermal conductivities in both bulk and thin-film samples from NGI measurements. This newly developed technique is used to determine thickness-dependent in-plane thermal conductivities (κ x) in Cu nano-films, which agree well with the electron thermal conductivity values converted from four-point electrical conductivity measurements using the Wiedemamn-Franz law, as well as previously reported experimental values. The κ x measured with NGI in an 8 nm x 8 nm GaAs/AlAs superlattice (SL) is about 10.2 W/m⋅K, larger than the cross-plane thermal conductivity (8.8 W/m⋅K), indicating the anisotropic thermal transport in the SL structure. The uncertainty of the measured κ x is about 25% in the Cu film and less than 5% in SL. Sensitivity analysis suggests that, with the careful selection of proper substrate and interface resistance, the uncertainty of κ x in Cu nanofilms can be as low as 5%, showing the potential of the NGI technique to determine κ x in thin films with improved accuracy. By simply installing a photomask into ns-TDTR, NGI provides a convenient, fast, and cost-effective method to measure the in-plane thermal conductivities in a wide range of structures and materials.

Picosecond transient thermoreflectance for thermal conductivity characterization

“…For TDTR, techniques such as offset laser beams [44] and varying pump or probe laser spot size [11] have been implemented. For ps-TDTR, our recently developed grating imaging technique [9], [45] can be integrated with ease. As regard to the thermal model to extract thermal properties from experimental data, for ps-TDTR we use finite difference method to solve the simple thermal diffusion model numerically.…”

“…In the past decade, noncontact thermoreflectance techniques have been invented in various forms, such as time-domain thermoreflectance (TDTR) [1], [2], frequency-domain thermoreflectance (FDTR) [3]- [5], transient thermoreflectance (TTR) [6], transient thermal grating (TG) [7], [8] and transient grating imaging [9]. Even though different in experimental configurations and analytical models, the common feature of these techniques is the use of a strong pump laser as the heating source to elevate the surface temperature and a weak probe laser to monitor the surface temperature change.…”

“…(4) Unlike the nanosecond laser TTR that is mainly sensitive to thermal conductivity in bulk materials, this ps-TTR system can measure thermal conductivity in nanostructures as well as interfacial thermal conductance. A grating imaging technique, recently developed in our group [9] and similar to the heterodyned transient grating technique [7], [8], can be incorporated into this ps-TTR system for in-plane thermal conductivity measurement. With this newly developed ps-TTR system, we firstly examine thermal conductivities in three bulk materials, including Si, GaAs, and sapphire; then tested thin-film samples including MoS2 thin-films over a wide range of thicknesses, and digital alloy samples of GaAs/InAs.…”

Recent Advances in Solution‐processed Inorganic Thermoelectric Thin Films

“…The commonly used techniques for the thermal conductivity measurement of thin films are the 3ω method, time-domain thermoreflectance (TDTR), transient thermal grating (TTG), and four-prove thermal measurement method. [52] Since most of these methods are suited for measuring the through-plane thermal conductivity of thin films, the measurement of in-plane thermal conductivity is challenging or time-consuming. For instances, the 3ω method could be used to evaluate the lateral thermal transport properties of films in theory, but it requires the fabrication of long-suspended films.…”