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Abstract
One of the main challenges nowadays concerning nanostructured materials is the understanding
of the heat transfer mechanisms, which are of the utmost relevance for many
specific applications. There are different methods to characterize thermal conductivity at
the nanoscale and in films, but in most cases, metrology, good resolution, fast time
acquisition, and sample preparation are the issues. In this chapter, we will discuss one of
the most fascinating techniques used for thermal characterization, the scanning thermal
microscopy (SThM), which can provide simultaneously topographic and thermal information
of the samples under study with nanometer resolution and with virtually no
sample preparation needed. This method is based on using a nanothermometer, which
can also be used as heater element, integrated into an atomic force microscope (AFM)
cantilever. The chapter will start with a historical introduction of the technique, followed
by the different kinds of probes and operation modes that can be used. Then, some of the
equations and heating models used to extract the thermal conductivity from these measurements
will be briefly discussed. Finally, different examples of actual measurements
performed on films will be shown. Most of these results deal with thermoelectric thin
films, where the thermal conductivity characterization is one of the most important
parameters to optimize their performance for real applications.
Keywords: scanning thermal microscopy, thermal probes, thermoelectric thin films,
thermal conductivity, local temperature measurements

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https://cdn.intechopen.com/pdfs/64049.pdf

64049.pdf

New Book Materials Science » "New Research on Silicon - Structure, Properties, Technology", book edited by Vitalyi Igorevich Talanin, ISBN 978-953-51-3160-1, Print ISBN 978-953-51-3159-5, Published: May 31, 2017 under CC BY 3.0 license. © The Author(s).

Chapter 8

Silicon‐Germanium (SiGe) Nanostructures for Thermoelectric Devices: Recent Advances and New Approaches to High Thermoelectric Efficiency

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Figure 1. (a) Total world energy consumption sorted by energy source between the period 1990 and 2040. Dotted lines for coal (black) and renewables (green) show the predicted effects of the USA Clean Power Plan (CPP) regulation. (b) World net electricity generation predictions sorted by energy source, for the period of 2012–2040. Both figures are reprinted with permission from Ref. [3]. Copyright 2016.

Figure 2. Timeline of some breakthrough or historical event in Si‐Ge in thermoelectric, photovoltaic cells and microelectronics. References in Table 1.

Figure 3. Schematic diagram that briefly summarizes some of the main strategies for the improvement of the figure of merit through the increase in the power factor and the decrease in the thermal conductivity. The graph shows the behavior of the Seebeck coefficient, electrical conductivity, and thermal conductivity versus carrier concentration. This figure is adapted from Ref. [50].

Figure 4. Scheme of the most used strategies for reducing thermal conductivity and their effect on phonon scattering. Grain boundaries scatter mid‐long wavelength phonons at their interfaces, while alloy atoms, dopants, defects, lattice vibrations, and nano‐inclusions scatter short‐wavelength phonons. The electrons, which are depicted as arrows in the figure, are supposedly not scattered and thus electrical conductivity is not altered.

Figure 5. One of the strategies that has been proven to be useful in improving thermoelectric performance is to reduce dimensionality. Here, different configurations that the silicon‐germanium has been fabricated at the nanometric scale to improve its thermoelectric properties are shown.

Figure 6. A summary of the latest reported measurements of different structures of SixGe1−x is presented, shows (a) Seebeck coefficient, (b) electrical conductivity, and (c) power factor reported for bulk, thin films, nanomeshes, nanowires, and nanotubes. (d) The thermal conductivity for different SixGe1−x nanostructures and bulk samples as a function of the alloy composition. This figure is adapted from Ref. [100].

Figure 7. Raman spectra of thin films deposited on gold/glass substrates: ex situ thermal treatment (in blue: a, b, c, and d) and in situ thermal treatment (in red: e, f, g, and h) for samples treated at RT, 300, 400, and 500°C, respectively. The expected vibrational bands (schematically represented) corresponding to Ge‐Ge, Si‐Si, and Si‐Ge bonds are marked on the figure. With permission from Ref. [92].

Figure 8. (a) and (b) present different measurements for the ex situ (a) and in situ (b) MIC fabricated films with 500°C treatment temperatures. On the right side, the optical image of the surface along with a Raman mapping of the surface is presented. In the left side, the different Raman spectra collected corresponding to Si0.8Ge0.2 (blue color), nano‐Si1−xGex (green color), and pure silicon (red color) are shown (note that for the in situ film, b), there is no evidence of silicon segregation). The inset at the left side presents SEM image of the film surface. (c) Synchrotron radiation SR‐GIXRD diffractograms measured at 1.3775 Å wavelength for ex situ (blue color) and in situ (red color) MIC fabricated films, with heat treatments at 500°C. The heights of the intensities in dotted lines correspond to the Si‐Ge phase intensity values given in the JCPDS 04‐016‐6750 data sheet. The inset shows the calculated lattice parameters for the Si‐Ge films. This figure is adapted from Ref. [92].

Figure 9. (a) Sketch and optical image of a porous alumina template and (b) the SiGe film nanomesh fabricated on top of it.

Figure 10. (a) Thermal conductivity (κ, red triangles) and electrical conductivity (σ, black spheres) and (b) Seebeck coefficient (S, blue squares) and figure of merit (zT, green spheres) plotted versus the pore diameter of the nanomesh. The transport properties obtained for a Si0.8Ge0.2 film grown under the same conditions are also plotted for comparison (inside the rectangle on the left of each graph, corresponding to continuous thin film). This figure is adapted from Ref. [100].

Figure 11. (a) X‐ray diffraction and (b) Raman spectra of a Si0.8Ge0.2 grown on nanomeshes with a pore diameter of 31 nm (black line) 137 nm (blue line) and 294 nm (red line). This figure is adapted from Ref. [100].

Figure 12. (a) SEM image of a Si0.8Ge0.2 nanomeshed film of 294 ± 5 nm pore size. (b) Topography image by AFM and (c) surface potential image by KPM. The uniformity in the contrast of the KPM image reveals homogeneity in the surface potential of the film. This figure is adapted from Ref. [100].

Thermoelectric lms on exible substrates are of interest for the integration of thermoelectric in wearable devices. In this work, copper selenide lms are achieved by a novel low-temperature technique, namely pulsed hybrid reactive magnetron sputtering (PHRMS). A brief introduction to the basic chemistry and physics involved during growth is included to explain its fundamentals. PHRMS is a single-step, room temperature (RT), fabrication process carried out in another ways conventional vacuum sputtering system. It does not require high-temperature post-annealing to obtain lms with great thermoelectric performance. It is, therefore, compatible with polymeric substrates like Kapton tape. Several sets of lms covering a large exploratory compositional range (from Cu/Se = 1 to 9) are deposited and their micro- structure and thermoelectric properties are analyzed at RT. Power factors as high as 1.1 mW m−1 K−2 in the in-plane direction and thermal conductivities as low as κ = 0.8 ± 0.1 W m−1 K−1 in the out-of-plane direction have been obtained for β-Cu2Se lms. Consequently, a gure of merit of 0.4 at RT can be estimated under the assumption that for this polycrystalline cubic phase no additional anisotropy in the thermoelectric properties is introduced by the planar con guration. Moreover, PHRMS is also industrially scalable and compatible with the in-line fabrication of other selenides.
Available from: https://www.researchgate.net/publication/317022491_Pulsed_Hybrid_Reactive_Magnetron_Sputtering_for_high_zT_Cu2Se_thermoelectric_films