Jia Yang

Characterizing the thermal properties of MoS₂ is important for the design of electronic devices based on this material. We used frequency domain thermoreflectance to study the cross-plane thermal transport in mechanically exfoliated MoS₂ samples supported on SiO₂ and muscovite mica substrates. The thickness of MoS₂ ranged between one and five layers, and the MoS₂ layers were sandwiched between a metal layer and the substrate. In the case of mica, heat transport into the substrate remained the same whether or not a monolayer of MoS₂ was present, whereas, for SiO₂, heat transport was reduced by surface roughness. We observed a significant improvement in heat transport across monolayer MoS₂ as compared to few layer MoS₂. For MoS₂ on SiO₂, the effective thermal interface conductance was improved by more than three times if a monolayer was used. For MoS₂ on mica, the thermal interface conductance was approximately two times better for monolayer MoS₂. This implies that monolayer MoS₂ has superior thermal properties and can be used in electronic devices as an intermediate layer between two materials. Additionally, we also report on the measurement of anisotropic thermal conductivity in bulk MoS₂ and mica.

Optical pump-probe techniques based on thermoreflectance, such as time domain thermoreflectance and frequency domain thermoreflectance (FDTR), have been widely used to characterize the thermal conductivity of thin films and the thermal conductance across interfaces. These techniques typically use a transducer layer to absorb the pump light and improve the thermoreflectance signal. The transducer, however, complicates the interpretation of the measured signal because the approximation that all the energy from the pump beam is deposited at the transducer surface is not always accurate. In this paper, we consider the effect of laser absorption in the top layer of a multilayer sample, and derive an analytical solution for the thermoreflectance signal in the diffusion regime based on volumetric heating. We analyze the measurement sensitivity to the pump absorption depth for transducers with different thermal conductivities, and investigate the additional effect of probe laser penetration depth on the measured signal. We validate our model using FDTR measurements on 490 nm thick amorphous silicon films deposited on fused silica and silicon substrates.

We derive a generally applicable formula to calculate the precision of multi-parameter measurements that apply least squares algorithms. This formula, which accounts for experimental noise and uncertainty in the controlled model parameters, is then used to analyze the uncertainty of thermal property measurements with pump-probe thermoreflectance techniques. We compare the uncertainty of time domain thermoreflectance and frequency domain thermoreflectance (FDTR) when measuring bulk materials and thin films, considering simultaneous measurements of various combinations of thermal properties, including thermal conductivity, heat capacity, and thermal boundary conductance. We validate the uncertainty analysis using Monte Carlo simulations on data from FDTR measurements of an 80 nm gold film on fused silica.

Silicon carbide is used as a substrate for high-power GaN devices because of its closely matched lattice spacing with GaN and its high thermal conductivity. In these devices, thermal resistance at the GaN–SiC interface is a bottleneck to heat flow, making this property an important factor in device design. In this letter, we report the first measurements of the thermal boundary conductance of epitaxial GaN grown directly on SiC without a transition layer. We find that the thermal boundary conductance increases from approximately 230 MW/m²K at 300 K to 330 MW/m²K at 600 K. Our measured values are in good qualitative agreement with the diffuse mismatch model for thermal boundary conductance and are in good quantitative agreement when we include a correction factor based on the ratio of Debye temperatures of the two materials. We also report the thermal conductivity of the GaN film, the thermal conductivity of 4H-SiC, and the thermal boundary conductance between Ni and GaN.

Suspended graphene has the highest measured thermal conductivity of any material at room temperature. However, when graphene is supported by a substrate or encased between two materials, basal-plane heat transfer is suppressed by phonon interactions at the interfaces. We have used frequency domain thermoreflectance to create thermal conductance maps of graphene contacts, obtaining simultaneous measurements of the basal-plane thermal conductivity and cross-plane thermal boundary conductance for 1–7 graphitic layers encased between titanium and silicon dioxide. We find that the basal-plane thermal conductivity is similar to that of graphene supported on silicon dioxide. Our results have implications for heat transfer in two-dimensional material systems, and are relevant for applications such as graphene transistors and other nanoelectronic devices.

A thermal property microscopy technique based on frequency domain thermoreflectance (FDTR) is presented. In FDTR, a periodically modulated laser locally heats a sample while a second probe beam monitors the surface reflectivity, which is related to the thermal properties of the sample with an analytical model. Here, we extend FDTR into an imaging technique capable of producing micrometer-scale maps of several thermophysical properties simultaneously. Thermal phase images are recorded at multiple frequencies chosen for maximum sensitivity to thermal properties of interest according to a thermal model of the sample. The phase versus frequency curves are then fit point-by-point to obtain quantitative thermal property images of various combinations of thermal properties in multilayer samples, including the in-plane and cross-plane thermal conductivities, heat capacity, thermal interface conductance, and film thickness. An FDTR microscope based on two continuous-wave lasers is described, and a sensitivity analysis of the technique to different thermal properties is carried out. As a demonstration, we image ∼3 nm of patterned titanium under 100 nm of gold on a silicon substrate, and simultaneously create maps of the thermal interface conductance and substrate thermal conductivity. Results confirm the potential of our technique for imaging and quantifying thermal properties of buried layers, indicating its utility for mapping thermal properties in integrated circuits.