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SnSe is semiconductor with various applications in optoelectronic devices, but its synthesis still facing challenges. Here, an original approach to tune the morphology, the optical properties and microstructure of SnSe films is proposed, based on the change of pulse duration during electrodeposition. The syntheses were carried out at a unique applied potential of −0.55 V vs AgCl/Ag with pulse durations (tON) ranging from 50 ms to 500 ms. The microstructure was systematically studied with SEM, TEM, XRD and XPS characterizations. First, the formation of SnSe of compact films without pinholes is demonstrated, with no significant change of composition (Se/Sn ratio close to 1:1). All the films are crystalized according to the Pnma orthorhombic phase with a preferential growth direction perpendicular to the (1 1 1) direction. The increase of pulse duration from 50 to 500 ms reduces the crystallinity of film, but less Se and SnO2 by-products are formed. Finally, the absorption coefficient of the films was extracted from ellipsometric measurements and evaluated in the near-edge region to evaluate the optical bandgaps. The results confirmed a slight change in the optical bandgap, from 1 to 1.1 eV. This work opens new alternative to tune physico-chemical properties of SnSe films.

International audience; In this paper, we demonstrate that it is possible to combine two computational approaches, usually used in very different contexts, namely, the Green-Kubo formalism and statistical approaches, to solve the Boltzmann transport equation to calculate the thermal conductivity of semiconductors. In this framework, first the phonon transport is solved by the Monte Carlo method according to the Debye-Callaway or Holland formalisms, then the flux autocorrelation is calculated. This new approach has been implemented to calculate the properties of Si, Ge, GaN, and C (diamond) in an extended temperature range, from 50 to 600 K. The simulation results are compared with those given by experimentation, with very good agreement.

The characterization of material thermal properties at nanoscales remains a challenge even if progress was achieved in developing outstanding characterization techniques like scanning thermal microscopy (SThM). In the present work, we propose a detailed procedure based on the combined use of a SThM probe characterization and its Finite Element Method (FEM) modeling to recover in operando 3ω measurements achieved under high vacuum. This approach is based on a two-step methodology: (i) a fine description of the probe's electrical and frequency behaviors in “out of contact” mode to determine the intrinsic parameters of the SThM tip and (ii) a minimization of the free parameter of our model, i.e., the contact thermal resistance, by comparing 3ω measurements with the simulations of the probe operating “in contact mode.” Such an approach allows us to measure thermal interface resistances between the tip and the surface. We applied our methodology to three different materials with known thermal properties: Si, SiO2 bulk materials, and a gold thin film. In addition, the FEM modeling provides insights into SThM thermal probes sensitivity, as a function of probe/sample interface resistance and the contact area to measure material thermal conductivity paving the way to quantitative SThM measurements.

The present work addresses the problem of thermal conductivity simulation in ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ and SnSe thermoelectric nanostructures. It first details phonon lifetime calculation in both thermoelectric compounds assuming polynomial dispersion properties and the Debye-Callaway model for the relaxation-time approximation. For both materials, distinct crystallographic directions are considered, i.e., $\mathrm{\ensuremath{\Gamma}}\ensuremath{-}Z$ (trigonal axis) and $\mathrm{\ensuremath{\Gamma}}\ensuremath{-}X$ (basal plane) for ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ and $\mathrm{\ensuremath{\Gamma}}\ensuremath{-}X$ ($a$ axis), $\mathrm{\ensuremath{\Gamma}}\ensuremath{-}Y$ ($b$ axis), and $\mathrm{\ensuremath{\Gamma}}\ensuremath{-}Z$ ($c$ axis) for SnSe. On this basis, the lifetime model is parametrized and bulk thermal conductivity is computed through the resolution of the phonon Boltzmann transport equation with a Monte Carlo method. Gr\"uneisen parameter and mass-disorder lifetime are adjusted to fit experimental temperature dependence. The second part of the study addresses the calculation of thermal conductivity for these two thermoelectric materials in the case of thin films (cross-plane case) and nanowires. The main goals of this work are to provide a fully parametric description of heat transport in ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ and SnSe nanofilms and nanowires showing reliable behavior on an extended size range, from 20 nm to $2\phantom{\rule{0.0pt}{0ex}}\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$, and large temperature range (100--500 K for ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$; 200--800 K for SnSe). Comparisons to bulk thermal conductivity calculations and measurements as well as to recent investigations on nanowires, demonstrate the effectiveness of the proposed methodology to deal with nanostructured ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ and SnSe.

In this work we present a molecular dynamics investigation of thermal transport in a silica-gallium nitride nanocomposite. A surprising enhancement of the thermal conductivity for crystalline volume fractions larger than 5% is found, which cannot be predicted by an effective medium approach, not even including percolation effects, the model systematically leading to an underestimation of the effective thermal conductivity. The behavior can instead be reproduced if an effective volume fraction twice larger than the real one is assumed, which translates into a percolation effect surprisingly stronger than the usual one. Such a scenario can be understood in terms of a phonon tunneling between inclusions, enhanced by the iso-orientation of all particles. Indeed, if a misorientation is introduced, the thermal conductivity strongly decreases. We also show that a percolating nanocomposite clearly stands in a different position than other nanocomposites, where thermal transport is dominated by the interface scattering and where parameters such as the interface density play a major role, differently from our case.

Thermoelectric transport properties of InGaAs with a high concentration of embedded semimetallic TbAs nanoparticles were studied in a 300–600 K temperature range. The TbAs concentrations studied were over 1% in a regime where coherent multiple scattering could affect electron transport. Scattering by ionized embedded nanoparticles, which are approximately one nanometer in diameter, is significantly different from scattering by shallow ionized impurities. The Seebeck coefficient and electrical conductivity were characterized as a function of temperature. The coefficients showed unusual behavior for a typical semiconductor, both growing with temperature. However, drastic worsening of the electrical conductivity compared with Si-doped InGaAs was also observed. The measured room temperature thermal conductivities of TbAs:InGaAs nanocomposites were lower than those of Si-doped InGaAs. Nonetheless, the thermoelectric performance was still governed by the electrical conductivity, and no enhancement of the thermoelectric figure of merit ZT was achieved at room temperature.

Nearly all experimental observations of quasiballistic heat flow are interpreted using Fourier theory with modified thermal conductivity. Detailed Boltzmann transport equation (BTE) analysis, however, reveals that the quasi-ballistic motion of thermal energy in semiconductor alloys is no longer Brownian but instead exhibits L\'evy dynamics with fractal dimension $\ensuremath{\alpha}l2$. 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 a priori knowledge of microscopic phonon scattering mechanisms. Identified $\ensuremath{\alpha}$ 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.

International audience; 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.

International audience; A High-Throughput Time-Domain ThermoReflectance (HT-TDTR) technique was developed to perform fast thermal conductivity measurements with minimum user actions required. This new setup is based on a heterodyne picosecond thermoreflectance system. The use of two different laser oscillators has been proven to reduce the acquisition time by two orders of magnitude and avoid the experimental artefacts usually induced by moving the elements present in TDTR systems. An amplitude modulation associated to a lock-in detection scheme is included to maintain a high sensitivity to thermal properties.We demonstrate the capabilities of the HT-TDTR setup to perform high-throughput thermal analysis by mapping thermal conductivity and interface resistances of a ternary thin film silicide library FexSiyGe100-x-y (20 < x, y < 80) deposited by wedge-type multi-layer method on a 100 mm diametersapphire wafer offering more than 300 analysis areas of different ternary alloy compositions.

Ge was deposited as thickness gradient films at temperatures up to 800C by direct current (DC) and high power pulsed magnetron sputtering (HPPMS). Structural characterization shows increased crystallization with increasing substrate temperature and film thickness. Thermal conductivity was measured by a novel high-throughput time-domain thermo-reflectance method. Thermo-electrical properties correlate to the degree of crystallization. Conductivities increase with increasing substrate temperature up to 500C. For higher temperatures the trend reverses. A room temperature deposited/ annealed film displays smaller crystallites (10 nm) and lower thermal conductivity (5Wm-1 K-1) compared to 25Wm-1K-1 for hot DC deposition. Compared to DC, HPPMS films show higher thermal conductivities up to 45Wm-1K-1.

International audience; 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.

International audience; Thermal interface (Kapitza) resistance expresses how hard it is for heat to flow across material junctions inside multilayer structures. This quantity plays a crucial role in the thermal performance of nanoscale devices but is still poorly understood. Here we show that conventional Fourier-based metrology overestimates metal/semiconductor resistances by up to threefold due to misinterpretation of ballistic heat flow modes. We achieve improved identification and a different physical insight with a truncated Lévy formalism. This approach properly distinguishes interfacial dynamics from nearby quasiballistic heat flow suppression in the semiconductor. Unlike conventionally extracted values, interface resistances obtained with our new approach are independent of laser modulation frequency, as physically appropriate, and much more closely approach theoretical predictions.

International audience; We investigate thermal transport in high-quality Al0.1Ga0.9N thin films grown using plasma-assisted molecular beam epitaxy by time-domain thermoreflectance (TDTR) in the 100 K–500K temperature range. The apparent thermal conductivity at 300K and 500K drops by 30% when the laser modulation frequency is increased from 0.8MHz to 10MHz. Tempered Lévy analysis of the quasi-ballistic heat conduction reveals superdiffusion exponents a=1.70+-0.06 at room temperature and a=1.83+-0.16 at 500 K. We describe limitations in concurrent extraction of other model parameters and also discuss the impact of boundary scattering in the 100 K–200K temperature range.

International audience; We present experimental observations of picosecond Joule heating inside the gold metallization of ErAs:GaAs photoconductive switches. Femtosecond laser time-domain thermoreflectance is employed to resolve the fast thermal dynamics in the central switch electrode during generation and transmission of rf electrical pulses. Sharp features in the thermoreflectance signal scaling quadratically with the bias/pulse voltage reveal Joule heating with durations 5 ps inside the gold metal. The temporal shape and rise time of the signals is in excellent agreement with the theoretical pulse wave form and subpicosecond carrier lifetime of the active medium. Probing different locations on the electrode shows the propagation, attenuation, and dispersion of the electrical pulses along the coplanar waveguide structure. Overall, the presented photothermal experiments demonstrate great potential to reveal the internal dynamics of photoconductive switches, characterize transmission lines, and study ultrafast heating in metals.

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.

International audience; 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 SiO(2) layer. Then, we discuss the role of the modulation frequency for nanoscale heat transfer analysis. (C) 2011 American Institute of Physics. [doi:10.1063/1.3665129]

Nanostructuring has opened new ways to increase the thermoelectric performance of a host of materials, mainly by decreasing their thermal conductivity κ while preserving the Seebeck coefficient S and electrical conductivity σ. The thermoelectric properties of degenerated polycrystalline silicon films with nanocavities (NCs) have been studied as a function of annealing temperature upon isochronous annealings in argon carried out every 50°C in the range 500 – 1000°C which were used to modify the shape of the NCs. We found that presence of the NCs had no negative effect on the electronic properties of the system. The measured values of S and σ were close to those previously reported for the blank polycrystalline silicon films with the same doping level. The thermal conductivity was also found to be close to the value measured on the blank sample, about half of the reported value in polycrystals. This led to a power factor of 15.2 mWm-1K-2 and a figure of merit of 0.18 at 300 K.

Network Identification by Deconvolution (NID) method is used to capture the heat cumulative effect in the homodyne configuration of the Pump-Probe Transient Thermoreflectance (PPTTR) experiment. This cumulative effect is very important in the interpretation of the PPTTR which is becoming widely used for the extraction of thin film thermal conductivity. We show that this intrinsic behavior can be introduced as a cumulative effect weight function in the time constant spectrum of the structure under study. We show how the main features of this weight function change when we change the laser repetition rate and/or the laser pump beam modulation frequency, and how these changes may affect the extraction of the thermal properties of the sample under study, particularly the thermal conductivity and the interface thermal resistance. Numerical simulations of the PPTTR experiment are used to validate the application of NID method. Limitations of the method will also be discussed.

Over the past three decades, ultrashort laser pulses have been demonstrated to be a very powerful tool to investigate materials properties at the nanoscale. A key driving force is the high-time resolution required to study heat transfer across interfaces and in thin films. The Time-Domain Thermoreflectance (TDTR) is now widely used. This optical technique offers an interesting alternative to electrical approaches such as the 3ω method. We present a complete study of the TDTR signals. We investigate the influence of the modulation frequency on the measured signals and we show how this experimental parameter could be set to enhance or reduce the sensitivity to a specific thermal parameter. The dependence of the measured “apparent” thermal conductivity of the thin film as a function of the modulation frequency is discussed. Results are applied to investigate thermal properties of a series of InGaAs samples with embedded ErAs nanoparticles.

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 Green's function simulations.

There are several classes of coherent phonons whose generation mechanisms are completely different: impulsive stimulated Raman scattering, Brillouin oscillations, and coherent longitudinal-acoustic-phonon Bragg reflection. All of these are investigated in Si/ SiGe superlattices using a picosecond ultrasonics technique at room temperature. Bragg reflection for a single vibrational mode is related to gaps in the phonon-dispersion relation at the center and the boundary of the folded Brillouin zone. The two lowest minigaps are observed at 283 and 527 GHz. Theoretical calculations of phonon-dispersion relation and phonon reflection rate are given to explain experimental results.

International audience; 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.

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%.

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...

We describe a new ultrafast imaging technique based on the use of two new generation Ytterbium lasers emitting at 1030 nm at 50 MHz repletion rate. Ultrafast acquisition is achieved by slightly shifting the repetition rate of the "pump" and the "probe" beams. In that conditions a single shot response is acquired in 1ms that allows sweeping the surface of the device or the material and obtain movies of the reflectivity field of the surface. This technique allows filming the reflectance changes of a material at very high speed typically 1 Tera frame per second during 20ns. We will show applications of this technique in acoustic imaging of surface waves and non destructive evaluation of microelectronic materials.

This PhD-thesis work focuses on the characterization of heat transport in nanomaterials by scanning thermal microscopy (SThM). This technique is used to measure the thermal properties of micro-objects at the scale of a few tens of nanometers. Achieving quantitative measurements at these scales remains a challenge due to the complexity of the heat exchanges at the interface between the probe and the sample. To address these issues, we have developed an experimental bench to control the environment and thus control the heat transfer modes. We have also set up a frequency approach (3ω) for the thermal study. The experimental signals obtained on this bench were compared and analyzed using detailed finite element modeling of the SThM tip. In this framework we have demonstrated the reliability of our bench to study materials with a thermal conductivity between 0.1 and 100 W/m.K., Ce travail de thèse porte sur la caractérisation du transport de la chaleur dans les nanomatériaux par microscopie thermique à balayage (SThM). Cette technique est utilisée pour la mesure des propriétés thermiques de micro-objets à l’échelle de quelques dizaines de nanomètres. Parvenir à des mesures quantitatives à ces échelles demeure un défi du fait de la complexité des échanges à l’interface entre la sonde et l’échantillon. Pour répondre à ces problématiques, nous avons développé un banc expérimental permettant de contrôler l’environnement et ainsi contrôler les modes de transfert. Nous avons également mis en place une approche fréquentielle (3ω) pour l’étude thermique. Les signaux expérimentaux obtenus sur ce banc ont été comparés et analysés à l’aide d’une modélisation détaillée de la pointe SThM par éléments finis. Dans ce cadre nous avons démontré la fiabilité du banc pour étudier des matériaux qui ont une conductivité thermique comprise entre 0.1 et 100 W/m.K.

International audience; Energy harvesting is certainly one of the most exciting challenge in modern physics. Theoretical calculations, material engineering and novel experimental characterization techniques have risen to fully explore the nanoscale limit and new materials with tailored properties. I The purpose of this work is to present the current state-of-progress of the thermal metrology plateform developed at LEMTA and more specifically the scanning thermal microscope (SThM). We applied this SThM technique to the study of thermoelectric-based Bi2Te3 nanowires.