Phonon heat transport in semiconductors containing mesoscale nano-inclusions – MESOPHON

There are three major work-packages (WPs) in the Mesophon project; i) advanced elaboration on thin Ge films with embedded nanoparticles, ii) thermal transport properties measurements in thin films, iii) modeling and numerical simulation, at different length scales, of phonon transport in such media.

For elaboration, Ge:Mn nano-structured thin films are grown by molecular beam epitaxy. When exceeding concentration of about ~ 1% in Ge, Mn atoms segregate in a Mn rich columnar phase during the growth. By this mean nano-columns of sizes from 1 to 5 nm can be grown with various concentrations. In a second stage, annealing post-treatment is used to modify samples structuring, colums collapsing in spherical inclusions. The size of these nano-objects can spread over 1 to 20 nm and their concentration depends on experimental parameters (Mn concentration, growth temperature, annealing temperature, etc.). Chemical composition, crystallographic phase of the columns and inclusions are extensively studied by TEM, EXAFS, GISAXS etc, using state of the art characterization tools.

For the characterization stage, 3-omega measurements will then be carried out on Ge:Mn having various nano-inclusion size and spatial distributions. Preliminary measurements done near room temperature have shown that inclusions affect significantly the phonon flow. When decreasing the temperature the dominant phonon wavelength will increase until it reaches the mean inclusion size. Striking effects are expected on the thermal conductivity.

For modeling and simulation, ab-initio calculations will be carried out to address DOS, phonon dispersion relations and lifetimes of both the Ge:Mn clusters and the host Ge matrix. Then molecular dynamic is used to evaluate, interface Kapitza resistance and transmission/reflection parameters. Eventually, Monte Carlo simulation will be used to model structures similar as the experimental ones, using input from previous calculations.

At INAC the growth of Ge:Mn alloys by molecular beam epitaxy has been done. We have grown thin Ge:Mn films on different substrates and with Mn contents ranging from 8 to 14%. The thickness of the grown layers is about 200 to 250 nm.
Those layers were then characterized by transmission electron microscopy. At this step, the formation of the nano-inclusions was acknowledged. Furthermore, the TEM structural characterization allowed us to evaluate the structural quality of the inclusions. We observe that all the inclusions are of perfect crystalline quality. Those samples were eventually made available for characterization at Néel Institute.

At Institut Néel, we have mostly worked on the instrumentations and experimental technique of the 3 omega technique. This technique is designed to probe thermal conductivties of very thin films deposited on a substrate. This technique can be very sensitive but is also under severe constraints if one wants to extract with a sufficient level of confidence the thermal properties. We have decided to measure reference samples (like sapphire, SiN on Si, GeTec on Si) to make a definitive proof of the validity of that technique to obtain thermal properties.
We have also secured the ability of doing thermal measurement over a wide temperature range (from 30K to 300K).

At LEMTA we have studied the electronic and phononic properties of Ge:Mn using ab initio calculations. Different stoichiometries and space groups were considered: Mn2Ge, Mn3Ge [Fm3m], Mn3Ge [Pm3m] and Mn5Ge3.
The electronic structure of these compounds reveals that all but Mn2Ge are magnetic materials. This unusual behavior can be explained from the local environment of Mn atoms that have 4 Ge and 4 Mn atoms as nearest neighbors. Phonon related properties were also computed. A similar frequency range is obtained for all compounds, and the computed lattice thermal conductivity is ranging from 3 to 10 W/m K at 300K. Mn3Ge [Pm3m] has the lowest lattice thermal conductivity.

Mesophon project aims to provide a comprehensive approach of the design of innovative nano-structured materials that can bring breakthroughs in heat energy recovery and management for several industrial purposes. To reach this ambitious objective, basic research involving experimental and numerical strategies is necessary to tackle such a complex problem.
Concerning elaboration technologies the project results will improve the knowledge on MBE and annealing techniques devoted to the preparation of nano-structured materials that contain inclusions. This could be valuable for the design and the fabrication of a broad range of new materials including nano-inclusions, for example “electron-crystal phonon-glass” thermoelectric materials.
Besides, characterization techniques that will be developed in this study are quite unique in the sense that they imply very low temperature environment. Probing and extracting very weak signals in these conditions is challenging but clearly in the field of competencies of the peoples from the Institute Néel. Devices (“Lab on chip”, MEMS & NEMS) that are going to be built in this framework could be useful to probe the thermal transport properties of a broad range of nanostructures like nanostructured thin films and nanowires.
Eventually, progresses in material numerical simulations are also expected and could be very useful to improve models and tools dedicated to phonon research field. These models are necessary for in-silico material design and to numerical screening in order to discover new compounds and alloys with optimal properties.

At this stage of the project, it can be claimed that the elaboration phase is well advanced as several expected materials have been produced. For characterization, robust calibration has been done and first measurement of thermal properties are under course. Finally, in simulation, first stage of computation (DFT) is nearly finished and phonon properties are available for MD and MC calculations.

Book chapter

* M. Verdier, K. Termentzidis, D. Lacroix, Modeling thermal transport in nano-porous semiconductors, in Submicron Porous Materials, Editor P. Bettotti, p. 253-284, Springer, February 22, 2017.

Publications in international journals

* M. Verdier, D. Lacroix, K. Termentzidis, Effect of the amorphization around spherical nano-pores on the thermal conductivity of nano-porous Silicon, Journal of Physics: Conferences Series, vol 785, 2016.

* Y. Han, L. Chaput, K. Termentzidis and D. Lacroix, Electronic and Phonon Transport Properties of Thermoelectric MnxGe1-x Compounds from First-principles Calculations, in preparation for publication in Physical Review B.

* J. Larroque, P. Dollfus, J. Saint Martin, D. Lacroix, L. Chaput, Ab Initio Monte Carlo calculations of the thermal conductivity at the micron scale, in preparation for publication in Phys Rev B or Applied Physics Letters.

International conferences

* D. Lacroix, Monte Carlo Modeling of Phonon Transport in Nanostructures, The Minerals, Metals & Materials Society, TMS annual meeting, San Diego - USA, February 26-March 2, 2017. Invited talk.

* L. Chaput, Ab Initio Calculations of the Lattice Thermal Conductivity and the Discovery of New Thermoelectric Materials, The Minerals, Metals & Materials Society, TMS annual meeting, San Diego - USA, February 26-March 2, 2017. Invited talk.

Poster

* J. Paterson, Y. Liu, D. Tainoff, M. Boukhari, J. Richard, A. Barski, P. Bayle-Guillemaud, E. Hadji, O. Bourgeois Phonon scattering in an epitaxial semiconductor by materials engineering at the nanoscale, Ecole Les Houches Son et Lumière, April 2017.

Mesophon project is devoted to the study of thermal transport in nano-materials where the characteristic sizes are comparable to the phonon wavelength Lph (100nm at 3K & 1nm at 300K). In order to reach this goal a multidisciplinary research project is proposed. It lies on the elaboration of specific nano-structures and on the study of their thermal properties by using state of the art measurement and modeling. The main asset of the project is to develop an original approach based on very low temperature measurements and numerical simulations to observe wave effects on phonon transport. In this low temperature limit, thermal transport in nano-structured semiconductor has been scarcely studied, as measurements are very challenging at these length scales. Nevertheless, recent experimental works pointed out significant contributions of intermediate phonon wavelength to heat transport at the nanoscale, indeed questioning existing textbook thermal models. Furthermore, there is an increasing interest for understanding phonon scattering in newly developed phononic devices that could be relevant for several applications, particularly in the fields of electronic and optic.

Within this collaborative research program, specialists in the fields of elaboration, thermal measurement, theory and numerical simulation will work together. The scientific program involves 3 workpackages that address specific issues.
• First, unique thin Ge films with Ge:Mn or Ge:Sn:Mn nano-inclusions will be grown at SiNaPS. Their design, concerning size and dispersion of clusters, make them the most suitable objects to be used as model materials to study wave effects due phonon-cluster interaction.
• Second, thermal properties measurements of the elaborated thin films, from 300K to 4K, will be held at Institut Néel. Through dedicated devices, thermal conductivity and phonon drag effect on Seebeck coefficient are going to be measured. Collected data will help to develop a theoretical model for phonon scattering by nano-inclusions as a function of their wavelength.
• Third, at the same time within the LEMTA, models and numerical simulations will be developed to appraise thermal transport properties and compare them with measurements. Feedback to the growing procedure will improve the targeted window in terms of size and dispersion of nano-inclusions. New numerical models targeting wave effect in the phonon transport will be investigated in order to appraise properties such as a phonon scattering phase function.

This work is strongly collaborative and interdependent. From the material viewpoint, thin films elaboration for new devices with a controlled tailoring of the nano-structures is very challenging. Thus, developing characterization tools (experiments, models and simulations) that can measure or predict optimal properties give the necessary feedback for improving materials. From the advanced characterization standpoint, high sensitivity metrology development requires model materials with well defined properties as well as theoretical and numerical model to extract and appraised reliability of measured properties. Lastly, in what concerns modeling and numerical simulation at the atomic and mesoscales, realistic materials as well as accurate properties measurements are extremely valuables to assess models and tools. In this frame, the Mesophon project will provide useful knowledge for several research communities.
Furthermore, as it is exposed in the detailed scientific document, preliminary studies including fabrication of Ge:Mn films, thermal conductivity measurements at 300K and simulations on nano-structured Ge were already conducted by the consortium members to appraise the project feasibility. The obtained promising results demonstrate that there are interesting scientific and technological challenges to tackle. Results of this study should foster the development of “phononic” science and its applications (thermal management, heat recovery, heat diodes, etc).