Gilles Pernot

Precise control of thermal conductivity at the nanoscale through individual phonon-scattering barriers

The ability to precisely control the thermal conductivity (kappa) of a material is fundamental in the development of on-chip heat management or energy conversion applications. Nanostructuring permits a marked reduction of kappa of single-crystalline materials, as recently demonstrated for silicon nanowires. However, silicon-based nanostructured materials with extremely low kappa are not limited to nanowires. By engineering a set of individual phonon-scattering nanodot barriers we have accurately tailored the thermal conductivity of a single-crystalline SiGe material in spatially defined regions as short as approximately 15 nm. Single-barrier thermal resistances between 2 and 4 x 10(-9) m(2) K W(-1) were attained, resulting in a room-temperature kappa down to about 0.9 W m(-1) K(-1), in multilayered structures with as little as five barriers. Such low thermal conductivity is compatible with a totally diffuse mismatch model for the barriers, and it is well below the amorphous limit. The results are in agreement with atomistic Greens function simulations.

Heterodyne picosecond thermoreflectance applied to nanoscale thermal metrology

Picosecond thermoreflectance is an unprecedented powerful technique for nanoscale heat transfer analysis and metrology, but different sources of artifacts were reported in the literature making this technique difficult to use for long delay (several ns) thermal analysis. We present in this paper a new heterodyne picosecond thermoreflectance (HPTR) technique. As it uses two slightly frequency shifted lasers instead of a mechanical translation stage, it is possible to avoid all artifacts leading to erroneous thermal parameter identifications. The principle and set-up are described as well as the model. The signal delivered by the HPTR experiment is calculated for each excitation configurations, modulating or not the pump beam. We demonstrate the accuracy of the technique in the identification of the thermal conductivity of a 50 nm thick SiO2 layer. Then, we discuss the role of the modulation frequency for nanoscale heat transfer analysis.

Superdiffusive heat conduction in semiconductor alloys. II. Truncated Lévy formalism for experimental analysis

evy dynamics with fractal dimension α< 2. Here, we present a framework that enables full three-dimensional experimental analysis by retaining all essential physics of the quasiballistic BTE dynamics phenomenologically. A stochastic process with just two fitting parameters describes the transition from pure L´ evy superdiffusion as short length and time scales to regular Fourier diffusion. The model provides accurate fits to time domain thermoreflectance raw experimental data over the full modulation frequency range without requiring any “effective” thermal parameters and without any ap rioriknowledge of microscopic phonon scattering mechanisms. Identified α values for InGaAs and SiGe match ab initio BTE predictions within a few percent. Our results provide experimental evidence of fractal L´ evy heat conduction in semiconductor alloys. The formalism additionally indicates that the transient temperature inside the material differs significantly from Fourier theory and can lead to improved thermal characterization of nanoscale devices and material interfaces.

Growth and characterization of TbAs:GaAs nanocomposites

Recently, there has been interest in semimetallic rare earth monopnictide nanoparticles epitaxially embedded in III-V semiconductors due to the drastic changes brought about in these materials’ electrical and thermal properties. The properties of terbium codeposited with gallium arsenide by molecular beam epitaxy are discussed here. These new materials were characterized by x-ray diffraction, Rutherford backscattering spectrometry, resistivity measurements, photoluminescence, time-domain thermoreflectance thermal conductivity measurements, optical absorption spectroscopy, and plan-view high-angle annular dark-field scanning transmission electron microscopy. Results revealed successful formation of randomly distributed nanoparticles with an average diameter of ∼1.5 nm, reduction of thermal conductivity by a factor of about 5, and consistency with theoretical predictions of mid-band-gap Fermi level pinning and behavior of past similar materials. The success of these TbAs:GaAs materials will lead the way for...

Thermoelectric properties of epitaxial TbAs:InGaAs nanocomposites

InGaAs lattice-matched to InP was grown by molecular beam epitaxy with randomly distributed TbAs nanoparticles for thermoelectric power generation applications. TbAs:InGaAs is expected to have a large thermoelectric figure of merit, ZT, particularly at high temperatures, owing to energy band alignment between the nanoparticles and their surrounding matrix. Here, the room temperature thermoelectric properties were measured as a function of TbAs concentration, revealing a maximum thermoelectric power factor of 2.38 W/mK2 and ZT of 0.19 with 0.2% TbAs. Trends in the thermoelectric properties closely resemble those found in comparable ErAs:InGaAs nanocomposite materials. However, nanoparticles were not observed by scanning transmission electron microscopy in the highest ZT TbAs:InGaAs sample, unlike the highest ZT ErAs:InGaAs sample (0.2% ErAs) and two higher concentration TbAs:InGaAs samples examined. Consistent with expectations concerning the positioning of the Fermi level in these materials, ZT was enhanced by TbAs incorporation largely due to a high Seebeck coefficient, whereas ErAs provided InGaAs with higher conductivity but a lower Seebeck coefficient than that of TbAs:InGaAs. Thermal conductivity was reduced significantly from that of intrinsic thin-film InGaAs only with TbAs concentrations greater than ∼1.7%.

Titanium-based silicide quantum dot superlattices for thermoelectrics applications.

Ti-based silicide quantum dot superlattices (QDSLs) are grown by reduced-pressure chemical vapor deposition. They are made of titanium-based silicide nanodots scattered in an n-doped SiGe matrix. This is the first time that such nanostructured materials have been grown in both monocrystalline and polycrystalline QDSLs. We studied their crystallographic structures and chemical properties, as well as the size and the density of the quantum dots. The thermoelectric properties of the QDSLs are measured and compared to equivalent SiGe thin films to evaluate the influence of the nanodots. Our studies revealed an increase in their thermoelectric properties-specifically, up to a trifold increase in the power factor, with a decrease in the thermal conductivity-making them very good candidates for further thermoelectric applications in cooling or energy-harvesting fields.

Designing low thermal conductivity of RuO2 for thermoelectric applications

We have applied Umklapp phonon-phonon and phonon-defect scattering to calculate the thermal conductivity of unalloyed as well as Fe- and La-alloyed RuO2 (P42/mnm). These models are computationally efficient and parameter free as they are supported by density functional theory. We predict an order of magnitude drop in the thermal conductivity upon alloying, which is beneficial for thermoelectric applications as it increases the figure of merit. Thermal conductivity data obtained by thermoreflectance on magnetron sputtered thin films are consistent with the calculations. The here employed research strategy may also be beneficial for designing phases that require manipulation of entangled properties.

Thermoreflectance temperature measurement with millimeter wave.

GigaHertz (GHz) thermoreflectance technique is developed to measure the transient temperature of metal and semiconductor materials located behind an opaque surface. The principle is based on the synchronous detection, using a commercial THz pyrometer, of a modulated millimeter wave (at 110 GHz) reflected by the sample hidden behind a shield layer. Measurements were performed on aluminum, copper, and silicon bulks hidden by a 5 cm thick Teflon plate. We report the first measurement of the thermoreflectance coefficient which exhibits a value 100 times higher at 2.8 mm radiation than those measured at visible wavelengths for both metallic and semiconductor materials. This giant thermoreflectance coefficient κ, close to 10(-3) K(-1) versus 10(-5) K(-1) for the visible domain, is very promising for future thermoreflectance applications.

Nanoscale Thermal Transport Studied With Heterodyne Picosecond Thermoreflectance

We present in this paper a new pump-probe thermore-flectance technique, which is called heterodyne as it uses two slightly frequency shifted lasers instead of a mechanical translation stage as used in the homodyne classical technique. The great advantage of the heterodyne technique is to avoid many artifacts leading to erroneous thermal parameter identifications. The principle and set-up are described as well as the model. Then, after presenting the identification procedure, it has been applied to the study of nanometric SiO2 layer.Copyright

Ballistic heat transport and associated frequency dependence of thermal conductivity in semiconductor alloys

Pump-probe time-domain thermoreflectance is commonly used for thermal characterisation of thin films. Lock-in detection at the pump modulation frequency provides in-phase and out-of-phase components of the temperature oscillations. The thermal conductivity is then obtained by fitting this signal to a diffuse heat spreading model. In 2007, Koh and Cahill reported a significant reduction of thermal conductivity with modulation frequency in semiconductor alloys. They attributed this effect to the assumption that phonons with mean free path longer than the thermal penetration length do not contribute to the measured conductivity. The effect has been successfully reproduced but remained poorly understood. We propose a model that incorporates ballistic transport as internal heat source for the diffusive channel. The results show frequency dependent behaviour similar to the experimental data, even for a given mean free path (MFP). Moreover, the ballistic single pulse response shows accelerated decays compared to the the diffuse one but yet the extracted apparent conductivity is reduced at high modulation frequencies. This strongly suggests that the reduction of apparent conductivity can be mostly attributed to pulse accumulation effects and the fitting procedure.