Yee Kan Koh

Heat Conduction across Monolayer and Few-Layer Graphenes

We report the thermal conductance G of Au/Ti/graphene/SiO(2) interfaces (graphene layers 1 ≤ n ≤ 10) typical of graphene transistor contacts. We find G ≈ 25 MW m(-2) K(-1) at room temperature, four times smaller than the thermal conductance of a Au/Ti/SiO(2) interface, even when n = 1. We attribute this reduction to the thermal resistance of Au/Ti/graphene and graphene/SiO(2) interfaces acting in series. The temperature dependence of G from 50 ≤ T ≤ 500 K also indicates that heat is predominantly carried by phonons through these interfaces. Our findings suggest that metal contacts can limit not only electrical transport but also thermal dissipation from submicrometer graphene devices.

Comparison of the 3ω method and time-domain thermoreflectance for measurements of the cross-plane thermal conductivity of epitaxial semiconductors

The 3ω technique and time-domain thermoreflectance (TDTR) are two experimental methods capable of measuring the cross-plane thermal conductivity of thin films. We compare the cross-plane thermal conductivity measured by the 3ω method and TDTR on epitaxial (In0.52Al0.48)x(In0.53Ga0.47)1−xAs alloy layers with embedded ErAs nanoparticles. Thermal conductivities measured by TDTR at low modulation frequencies (∼1 MHz) are typically in good agreement with thermal conductivities measured by the 3ω method. We discuss the accuracy and limitations of both methods and provide guidelines for estimating uncertainties for each approach.

Reliably Counting Atomic Planes of Few- Layer Graphene (n > 4)

We demonstrate a reliable technique for counting atomic planes (n) of few-layer graphene (FLG) on SiO(2)/Si substrates by Raman spectroscopy. Our approach is based on measuring the ratio of the integrated intensity of the G graphene peak and the optical phonon peak of Si, I(G)/I(Si), and is particularly useful in the range n > 4 where few methods exist. We compare our results with atomic force microscopy (AFM) measurements and Fresnel equation calculations. Then, we apply our method to unambiguously identify n of FLG devices on SiO(2) and find that the mobility (μ ≈ 2000 cm(2) V(-1) s(-1)) is independent of layer thickness for n > 4. Our findings suggest that electrical transport in gated FLG devices is dominated by carriers near the FLG/SiO(2) interface and is thus limited by the environment, even for n > 4.

Two-tint pump-probe measurements using a femtosecond laser oscillator and sharp-edged optical filters

We describe a simple approach for rejecting unwanted scattered light in two types of time-resolved pump-probe measurements, time-domain thermoreflectance (TDTR) and time-resolved incoherent anti-Stokes Raman scattering (TRIARS). Sharp edged optical filters are used to create spectrally distinct pump and probe beams from the broad spectral output of a femtosecond Ti:sapphire laser oscillator. For TDTR, the diffusely scattered pump light is then blocked by a third optical filter. For TRIARS, depolarized scattering created by the pump is shifted in frequency by approximately 250 cm(-1) relative to the polarized scattering created by the probe; therefore, spectral features created by the pump and probe scattering can be easily distinguished.

Lattice thermal conductivity of nanostructured thermoelectric materials based on PbTe

We report the through-thickness lattice thermal conductivity Λl of (PbTe)1−x/(PbSe)x nanodot superlattices (NDSLs) over a wide range of periods 5 nm≤h≤50 nm, compositions 0.15≤x≤0.25, growth temperatures 550 K≤Tg≤620 K, and growth rates 1 μm h−1≤R≤4 μm h−1. All of our measurements approach Λl of bulk homogenous PbTe1−xSex alloys with the same average composition. For 5 nm≤h≤50 nm, Λl is independent of h; a result we attribute to short mean-free paths of phonons in PbTe and small acoustic impedance mismatch between PbTe/PbSe. We alloyed the PbTe layers of four NDSLs with SnTe up to a mole fraction y=18%; Λl is reduced by <25%.

Thermal conductivity of (Zr,W)N/ScN metal/semiconductor multilayers and superlattices

The cross-plane thermal conductivities of metal/semiconductor multilayers and epitaxial superlattices have been measured as a function of period by time-domain thermoreflectance at room temperature. (001)-oriented ZrN (metal)/ScN (semiconductor) multilayers and (Zr,W)N/ScN epitaxial superlattices with the rocksalt crystal structure were grown on (001)MgO substrates by reactive magnetron sputtering. A distinct minimum in thermal conductivity at a period of ∼6 nm is observed for ZrN/ScN multilayers. The minimum thermal conductivity of 5.25 W/m K is a factor of ∼2.7 smaller than the mean of the thermal conductivities (including only the lattice contributions) of the values measured for films of the constituent materials, and approximately equal to the lattice component of the thermal conductivity of a Zr0.65Sc0.35N alloy film (∼5 W/m K). Alloying the ZrN layers with WNx reduces the lattice mismatch, yielding epitaxial (Zr,W)N/ScN superlattices. The addition of WNx also reduces the thermal conductivity to ∼2 ...

Thermal Conductance of InAs Nanowire Composites

The ability to measure and understand heat flow in nanowire composites is crucial for applications ranging from high-speed electronics to thermoelectrics. Here we demonstrate the measurement of the thermal conductance of nanowire composites consisting of regular arrays of InAs nanowires embedded in PMMA using time-domain thermoreflectance (TDTR). On the basis of a proposed model for heat flow in the composite, we can, as a consistency check, extract the thermal conductivity Lambda of the InAs nanowires and find Lambda(NW) = 5.3 +/- 1.5 W m(-1) K(-1), in good agreement with theory and previous measurements of individual nanowires.

Thermoreflectance of metal transducers for time-domain thermoreflectance

We report measurements of the temperature dependence of the optical reflectivity, i.e., the thermoreflectance dR/dT, of 18 metallic elements at two laser wavelengths commonly used in ultrafast pump-probe experiments, 1.55 μm and 785 nm. The thermoreflectance is determined using time-domain thermoreflectance combined with measurements of the laser power and spot size and comparisons between the data and quantitative modeling of the temperature evolution at the surface of the sample. At a laser wavelength of 1.55 μm, four elements within this set of samples, Nb, Re, Ta, and V, have dR/dT comparable to or larger than 0.6×10−4 K−1. At a laser wavelength of 785 nm, the highest thermoreflectance is found in Al and Ta, dR/dT≈2.1×10−4 K−1 and 2.2×10−4 K−1, respectively. Alloying Au with 5% Pd increases the optical absorption by a factor of 3 and the thermoreflectance by a factor of 2.

Temperature Dependence of Anisotropic Thermal-Conductivity Tensor of Bulk Black Phosphorus

The anisotropic thermal-conductivity tensor of bulk black phosphorus (BP) for 80 ≤T ≤ 300 K is reported. Despite the anisotropy, phonons are predominantly scattered by Umklapp processes in all the crystallographic orientations. It is also found that the phonon mean-free-paths of BP are rather long (up to 1 µm) in the through-plane direction.

Role of low-energy phonons with mean-free-paths >0.8 μm in heat conduction in silicon

Despite recent progress in the first-principles calculations and measurements of phonon mean-free-paths (MFPs), contribution of low-energy phonons to heat conduction in silicon is still inconclusive, as exemplified by the discrepancies between different first-principles calculations. Here we investigate the contribution of low-energy phonons with MFP>0.8 um by accurately measuring the cross-plane thermal conductivity of crystalline silicon films by time-domain thermoreflectance (TDTR), over a wide range of film thickness 1-10 um and temperature 100-300 K. We employ a dual-frequency TDTR approach to improve the accuracy of our cross-plane thermal conductivity measurements. We find from our cross-plane thermal conductivity measurements that phonons with MFP>0.8 um contribute 53 W/m-K (37%) to heat conduction in Si at 300 K while phonons with MFP>3 um contribute 523 W/m-K (61%) at 100 K, >20% lower than the first-principles predictions by Lindsay et al. of 68 W/m-K (47%) and 695 W/m-K (77%), respectively. Using a relaxation times approximation (RTA) model, we demonstrate that macroscopic damping (e.g., Akhiesers damping) eliminates the contribution of phonons with mean-free-paths >30 um at 300 K, which contributes 15 W/m-K (10%) to heat conduction in Si according to Lindsay et al. Thus we propose that omission of the macroscopic damping for low-energy phonons in the first-principles calculations could be one of the possible explanations for the observed discrepancy between our measurements and calculations by Lindsay et al. Our work provides an important benchmark for future measurements and calculations of the distribution of phonon mean-free-paths in crystalline silicon.