SiOx Hybrid

In-Plane Thermal Conductivity of Radial and Planar Si/SiOx Hybrid Nanomembrane Superlattices Guodong Li,*,†,∥,# Milad Yarali,‡,# Alexandr Cocemasov,§,# Stefan Baunack,∥ Denis L. Nika,§ Vladimir M. Fomin,§,∥ Shivkant Singh,‡ Thomas Gemming,⊥ Feng Zhu,*,†,∥ Anastassios Mavrokefalos,*,‡ and Oliver G. Schmidt†,∥ †

Material Systems for Nanoelectronics, Technische Universität Chemnitz, 09107 Chemnitz, Germany Department of Mechanical Engineering, University of Houston, Houston, Texas 77204, United States § E. Pokatilov Laboratory of Physics and Engineering of Nanomaterials, Department of Physics and Engineering, Moldova State University, Chisinau, MD-2009, Republic of Moldova ∥ Institute for Integrative Nanosciences and ⊥Institute for Complex Materials, IFW Dresden, 01069 Dresden, Germany ‡

S Supporting Information *

ABSTRACT: Silicon, although widely used in modern electronic devices, has not yet been implemented in thermoelectric applications mainly due to its high thermal conductivity, κ, which leads to an extremely low thermoelectric energy conversion efficiency (figure of merit). Here, we present an approach to manage κ of Si thin-film-based nanoarchitectures through the formation of radial and planar Si/SiOx hybrid nanomembrane superlattices (HNMSLs). For the radial Si/SiOx HNMSLs with various numbers of windings (1, 2, and 5 windings), we observe a continuous reduction in κ with increasing number of windings. Meanwhile, the planar Si/SiOx HNMSL, which is fabricated by mechanically compressing a fivewindings rolled-up microtube, shows the smallest in-plane thermal conductivity among all the reported values for Si-based superlattices. A theoretical model proposed within the framework of the Born−von Karman lattice dynamics to quantitatively interpret the experimental data indicates that the thermal conductivity of Si/SiOx HNMSLs is to a great extent determined by the phonon processes in the SiOx layers. KEYWORDS: thermal conductivity, hybrid nanomembrane superlattice, strain-engineered rolling and compressing technique, silicon-based thermoelectrics, BvK lattice dynamics individual components to the electron and phonon transport.9 Hence, studies of thermal transport properties in some model hybrid material systems, such as Si/SiOx, are becoming increasingly important. To facilitate the implementation of Si in thermoelectrics, it is imperative to establish reliable nanofabrication methods to decrease or efficiently manage thermal conductivity, κ. Proven ways to achieve this are (i) the use of nanostructured bulk materials,10,11 in which multi-interfaces among the nanostructured constituents generate additional phonon scattering, and (ii) low-dimensional structures such as Si thin films,12−20 Si/Ge superlattices,21−24 and Si nanowires,25−28 where either

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deep understanding of phonon transport properties in nanoscale systems1−3 as well as bulk nanostructured materials4−6 is of fundamental importance in realizing high-performance thermoelectric devices for both energy harvesting and solid-state refrigeration. As dimensions of modern electronic devices continuously shrink, the size effect of nanostructures and interfacial scattering are increasingly dominating the electron and phonon transport properties. Based on the interfacial effects, ideas of carrier-pocket engineering7 and energy filtering8 have been proposed to manage the thermal transport, thereby enhancing the thermoelectric efficiency. In addition, types of heterostructures consisting of hybrid materials, namely, systems of inorganic/ organic, semiconductor/metal, or crystalline/amorphous heterostructures, show promising potential in tailoring the thermoelectric properties because of varying contributions of © 2017 American Chemical Society

Received: May 10, 2017 Accepted: August 3, 2017 Published: August 3, 2017 8215

DOI: 10.1021/acsnano.7b03219 ACS Nano 2017, 11, 8215−8222

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Figure 1. Rolled-up and compressed Si/SiOx HNMSLs. (a) Schematic diagram of a Si thin film sample as well as radial and planar Si/SiOx HNMSLs. (b) Optical microscopy image of as-rolled Si/SiOx tubes within a sample area of 2 × 2 mm2. (c) SEM image of one pair of as-rolled Si/SiOx tube with the rolling direction indicated by the arrows. (d−f) Magnified cross-sectional SEM images of as-rolled Si/SiOx tubes with various numbers of windings. (g) SEM image of a planar Si/SiOx HNMSL revealing a flat and smooth surface.

Figure 2. Suspended microdevices for thermal measurements and results. (a) Overview optical image of the suspended microdevice. (b, c) SEM images of representative devices with one planar sample (b) and one radial sample (c) transferred on top of the suspended microdevices. (d) Total, intrinsic, and contact thermal resistance of one-winding radial sample with a suspension length of 16 μm. (e) Experimental in-plane thermal conductivities of planar (black squares) and radial (red circles for one-winding, purple triangles for two-winding, and cyan diamonds for five-winding) Si/SiOx HNMSLs as a function of temperature.

and 5) are first measured, while three planar HNMSLs each incorporating stacked Si/SiOx nanomembranes are then selected to carry out comparative measurements. First, the inplane thermal conductivities of the radial Si/SiOx HNMSLs with various numbers of windings (1, 2, and 5) show a tremendous reduction relative to the bulk single-crystalline silicon in the temperature range from 300 to 400 K. Second, a continuous reduction in κ is detected with increasing number of windings, indicating thermal coupling between the layers of Si/ SiOx HNMSLs. Third, planar Si/SiOx HNMSLs, which are fabricated by mechanically compressing the five-windings radial tube, shows the lowest in-plane thermal conductivity among all the reported values of Si-based planar superlattices. A detailed

the phonon bands are changed or the interfacial phonon scattering is enhanced. Recently, some of us have developed a roll-up and compression (RuC) technique to fabricate types of Si/SiOx hybrid nanomembrane superlattices (HNMSLs) in either radial or planar geometry.29−31 In those structures, single-crystalline Si nanomembranes alternate with amorphous thin SiOx layers with well-defined interfaces, providing a hybrid proto-system to study both the thermal and electron transport. In this report, we investigate the in-plane thermal transport of both radial and planar Si/SiOx HNMSLs and demonstrate the evident reduction effect in thermal conductivity in these superlattice structures. By using a suspended microdevice,32,33 three types of tube configurations with diverse windings (1, 2, 8216


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of the samples and verify that no uncertainty arises in the inplane thermal conductivity measurement due to the interfacial thermal conductance. Figure 2e summarizes the in-plane thermal conductivities as a function of the temperature for all the radial and planar Si/SiOx HNMSL samples. For the one-winding device, in which the Si/ SiOx nanomembrane is around 24 nm thick, κ = 7.64 ± 0.6 W m−1 K−1 at 300 K. The large decrease in κ from the value of bulk single-crystalline Si (156 W m−1 K−1)36 is consistent with reported values for thin Si films with equivalent thickness14,18 (around 20 W m−1 K−1 for 20-nm-thick Si films) and originates from the decreased thickness of the film. We attribute the extra ∼60% reduction in κ of our one-winding sample to additional defect or dislocation scattering of the phonons in the strainrelaxed Si film (see more details in the following part of structure characterization). Such strong depression of the thermal conductivity has also been reported in Si nanowire systems, such as ∼7 W m−1 K−1 for 22 nm Si nanowire25 and 1.6 W m−1 K−1 for very rough 52 nm Si nanowires.26 For the two-winding device, κ = 6.2 ± 1.08 W m−1 K−1, while for the five-winding device, κ = 3.28 ± 0.18 W m−1 K−1 at 300 K. As the winding number of the Si/SiOx tube increases, the in-plane thermal conductivity decreases. The dependence of κ on the winding number of Si/SiOx tubes demonstrates the thermal coupling between Si layers. As for the planar sample, which consists of stacked planar hybrid membranes, κ is in the range of 4.4 to 5.7 W m−1 K−1 at temperatures from 200 to 400 K. In comparison to singlecrystalline silicon,36 the reported thermal conductivity value is a factor of 60 and 20 smaller at 200 and 400 K, respectively. At 300 K, the measured in-plane thermal conductivity of a planar Si/SiOx HNMSL is 5.3 W m−1 K−1, being 29 times smaller than the value of bulk single-crystalline silicon (156 W m−1 K−1). In addition, these values are on the same order of magnitude as those of nanopatterned Si thin-film phononic crystals.14−16,20 Furthermore, compared to the five-winding Si/SiOx tube, the thermal conductivities of the planar sample are about 60% enhanced. This can be understood by the fact that the thermal adhesion among different windings becomes better after mechanically compressing the radial Si/SiOx HNMSLs into the planar structures. Tight adhesion among different layers improves the phonon transport processes at the interfaces and hence the corresponding in-plane thermal conductivities. To further understand the phonon transport properties in the planar Si/SiOx HNMSLs, it is important to investigate the structural properties of mechanically bonded interfaces between the Si layers. Figure 3a shows a bright field transmission electron microscopy (TEM) image of the planar Si/SiOx HNMSL cross section. On top of the sample surface, a few hundred nanometers of Pt are deposited to prevent the Ga ions from damaging the sample during the FIB cutting process. As shown in the image, Si layers are mechanically bonded to each other with thin interfaces presumably consisting of two layers of oxide, one from each neighboring nanomembrane. Figure 3b and c show cross-sectional TEM images of the as-rolled fivewinding sample and the compressed planar sample, respectively. From Figure 3c it was deduced that the period in the Si/ SiOx HNMSL consists of 20.5 ± 0.5 nm silicon and approximately 2 nm oxide on each side (see Figure S5 in the Supporting Information for more details). High-resolution TEM images and selected area electron diffraction (SAED) proved that the Si layers are single crystalline both in the rolled and in the pressed state. For instance, Figure 3d shows a high-

structural characterization of the planar Si/SiOx HNMSL reveals single-crystalline/amorphous interfaces inside the superlattices. Furthermore, the phonon dispersion relations for planar Si/SiO2 HNMSLs and the thermal conductivity are calculated using a theoretical model developed within the framework of Born−von Karman (BvK) lattice dynamics. The agreement between theory and experiment implies that the thermal conductivity of Si/SiO2 HNMSLs is to a great extent determined by the phonon processes in the thin SiO2 layers. Our results open up an effective route for managing low thermal conductivities of Si-based materials. As schematically shown in Figure 1a, radial and planar Si/ SiOx HNMSLs are fabricated from a tensile-strained epitaxial silicon thin film, which is carefully deposited onto a Ge sacrificial layer on a Si(001) substrate by molecular beam epitaxy (MBE). To create rolled-up tubes, first the silicon thin film is lithographically patterned and etched into isolated mesas with an area of 300 × 400 μm2. Then, by selectively etching away the underneath Ge sacrificial layer in a solution of H2O2 (30 vol %) for specific time ranges at 80 °C, the thin Si film rolls up into one or multiwinding tubes due to the release of built-in tensile stress. In Figure 1b, an array of multiwinding Si tubes is realized on a sample size of 2 × 2 mm2 with a successful formation yield of the tubes up to 90%. Figure 1c shows a scanning electron microscopy (SEM) image of one pair of 300μm-long as-rolled Si tubes with a rolling distance around 40 μm. Figure 1d−f show high-magnification cross-sectional SEM images of as-rolled Si tubes with different numbers of windings. As shown in these images, the Si thin film has rolled downward toward the substrate surface and all Si windings are tightly joined together to form a monolithic and compact tube wall. Finally, the SEM image of a planar Si/SiOx HNMSL in Figure 1g reveals a flat and smooth surface.

RESULTS AND DISCUSSION The suspended microdevices for thermal transport measurements are shown in Figure 2a−c, consisting of two adjacent low-stress silicon nitride (SiNx) membranes, each of which is suspended by six SiNx beams over a through-substrate hole. To quantify the intrinsic thermal resistance of the HNMSLs and their contact thermal resistance with the membranes, multiple measurements with different suspended lengths varying from 8 to 16 μm between the membranes for the same HNMSLs were performed based on the procedures introduced in refs 34 and 35 (also see details of part II in the Supporting Information). Typically, Figure 2d shows each thermal resistance of a device with a suspended length of 16 μm for the one-winding Si/SiOx tube. In the whole measured temperature range, the contact thermal resistances are on the order of 105 K/W, being one magnitude smaller than the intrinsic thermal resistance. The intrinsic in-plane thermal conductivity of the samples is then converted from the intrinsic thermal conductance by taking into account the real sample dimensions based on a hollow cylinder structure (cross section area = π/4(outer diameter2 − inner diameter2)). The outer and inner diameters of each radial sample are listed in Table S1. Furthermore, we carried out heat transfer analysis for both types of Si/SiOx HNMSLs using finite element calculations (COMSOL), to verify that the interfacial thermal conductance (ITC) between the layers does not introduce any errors in our measurements (see part III in the Supporting Information for more details).The results indicate that for the expected range of ITC values at the SiOx−SiOx interface there is no temperature variation across the thickness 8217


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thermal transport in ultrathin Si nanomembranes show that the native oxide layer can lead to strong phonon scattering and dramatically reduces the thermal conductivity of Si nanomembranes.19 Therefore, for our Si/SiOx HNMSL system, on one hand, additional defects in the strain-relaxed singlecrystalline Si thin film introduce extra phonon scattering; on the other hand, amorphous oxide layers being periodically sandwiched between the single-crystalline Si layers are assumed to be more effective to restrain the phonon transport. To obtain a deeper insight into the phonon transport processes in HNMSLs, we propose a model simulating the phonon thermal conductivity of planar Si/SiOx HNMSLs in the framework of a linearized Boltzmann transport equation (BTE).39−41 For representing the amorphous SiOx layers, there have been several approaches reported.19,42−44 In our current model, we assume an SL period consisting of 20 nm single-crystalline Si and 2 nm amorphous SiO2 on each side, as shown schematically in Figure 4a. Within BvK lattice dynamics theory, we calculate the phonon dispersion relations in planar Si/SiO2 HNMSLs. The amorphous SiO2 layers are treated in the virtual crystal (VC) approximation, which has been extensively used for modeling vibrational and thermal properties of disordered materials and interfaces.42,43 A detailed description of the VC approximation and BvK model for Si/ SiO2 HNMSLs are presented in the Methods and part V of the Supporting Information, respectively. In Figure 4b and c we show the BvK phonon energy spectra for in-plane ωs(qx) and cross-plane ωs(qz) directions. For comparison, both in-plane and cross-plane phonon branches of bulk Si and SiO2 are shown together. Within the lattice dynamics approach, the size effect is included directly in the phonon spectrum. It is manifested in the appearance of a large number of quantized phonon branches with dispersions significantly different from those in the bulk case (see Figure 4b). The number of phonon branches is determined by the number of degrees of freedom in the unit cell; that is, it depends on the superlattice period. It is worth noting that for the in-plane direction the Brillouin zones of bulk materials and HNMSLs are almost the same size, while the Brillouin zone of the HNMSL for the cross-plane direction is ∼1/90th of the corresponding value for the in-plane case due to the zone folding. The average phonon group velocities are calculated over all (qx, qz) points in the Brillouin zone, and the results are plotted as a function of the phonon energy in Figure 4d. For a Si/SiO2 HNMSL, an average is taken for a particular component of the phonon group velocity, i.e., in-plane vx or cross-plane vz. From Figure 4d, the influence of the phonon confinement on phonon energy dispersion and group velocities in Si/SiO2 HNMSLs can be revealed. Importantly, there appear hybrid modes propagating in the whole structure, rather than separate vibrational modes in individual Si or SiO2 layers. In the framework of a linearized BTE approach (see parts VI−VIII in the Supporting Information for more details), the in-plane phonon thermal conductivity kx of a Si/SiO2 HNMSL is simulated and plotted as a function of temperature in Figure 4e. Theoretical calculations were performed for different values of the mean vibrational energy ⟨E⟩ = ℏ⟨ω⟩ in SiO2, namely, 34 meV (purple curve), 53 meV (black curve), and 70 meV (green curve). The experimental data points are denoted with red squares. The computed values of kx show a good agreement with experimental data for ⟨E⟩ = 53 meV. Within our model, we have also considered the effects of point defect scattering, as shown in Figure S8 of the Supporting

Figure 3. Cross-sectional transmission electron microscopy (TEM) images of Si/SiOx HNMSLs. (a) Bright field image of the planar sample. From bottom to top: Silicon substrate, Si/SiOx HNMSL, and a few hundred nanometers Pt. (b) Bright field image of the upper part of radial five-winding sample. (c) Bright field image of the central part of a planar Si/SiOx HNMSL. (d) High-resolution image showing the lower Si layers in the planar sample (substrate lower left). Here the width of the amorphous region is ca. 3.8 nm. The lattice fringes prove the single-crystalline nature of the Si layers. The inset shows the corresponding selected area electron diffraction pattern.

resolution TEM image of the planar HNMSL. The lattice fringes in the Si layers prove its single-crystalline nature, while the interfacial region is of amorphous nature. The inset in Figure 3d shows the SAED pattern corresponding to singlecrystalline Si, and the weak rings have their origin in the amorphous interlayers and amorphous residues from the FIB preparation. In addition, in all samples the image intensity inside the Si layers varies. The variation of image intensity within the Si layers could be due to defects, strain, or bending, which are introduced in the sample fabrication procedure. In other Si-based superlattices (SLs), such as Si/Ge or SiGe alloy SLs,21,23,37,38 the in-plane thermal conductivity decreases as the thickness of the SL period decreases. Up to now, the smallest in-plane thermal conductivity of Si/Ge SLs is 6 W m−1 K−1 at 300 K when the SL period is as thin as 4 nm.37 However, in our planar Si/SiOx HNMSLs, the in-plane thermal conductivity at 300 K is 5.3 W m−1 K−1. Given that one SL period in our Si/SiOx HNMSL is composed of 20 nm singlecrystalline Si and 2 nm native SiOx on each side, we have realized a much stronger reduction of the in-plane thermal conductivity but with larger system dimensions. Studies of 8218


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Figure 4. Theoretical model for planar Si/SiO2 HNMSLs. (a) Schematic structure of a planar Si/SiO2 HNMSL with an SL period consisting of 2 nm SiO2, 20 nm Si, and 2 nm SiO2. (b, c) Phonon energy spectra along the qx (in-plane) and qz (cross-plane) directions from the Brillouin = qmax = 11.57 nm−1; zone center for a Si/SiO2 HNMSL (black curves), bulk Si (red curves), and bulk SiO2 (green curves). For bulk Si, qmax x z = qmax = 11.64 nm−1; for the SL, qmax = 11.57 nm−1, qmax = 0.13 nm−1. (d) Average phonon group velocity as a function of for bulk SiO2, qmax x z x z the phonon energy for a Si/SiO2 HNMSL (black curve, in-plane component; purple curve, cross-plane component), bulk Si (red curve), and bulk SiO2 (green curve). (e) In-plane lattice thermal conductivity as a function of temperature in a Si/SiO2 HNMSL. Red solid squares denote the experimental data. Theoretical calculations for an HNMSL with amorphous SiO2 layers at different values of mean vibrational energy ⟨E⟩, 34 meV (purple curve), 53 meV (black curve), and 70 meV (green curve), are presented, along with those for an HNMSL with crystalline αquartz layers (gray curve).

surfaces, a silicon nanosize membrane with ultrathin native oxide layers on both sides19 and thin core−shell Si nanowires with amorphous shells,45 is consistent with our conclusion. In addition, as reported in ref 30, the cross-plane thermal transport of planar Si/SiOx HNMSLs was also significantly reduced, with a thermal conductivity of 0.7 ± 0.3 W m−1 K−1, being more than 2 orders of magnitude smaller than that of bulk single-crystalline silicon. On the basis of the developed theoretical model and under the same scattering mechanisms, we obtained the cross-plane thermal conductivity kz = 1.1 W m−1 K−1 at room temperature, which is in reasonable agreement with the experimental value. Speaking of the thermoelectric properties of Si/SiOx HNMSLs, the existence of SiOx layers will have certain impedance on the electrical transport. However, investigation on the electrical properties of this kind of structure has not been fully performed, due to the difficulty in realizing effective electrical contacts to the tubes with micrometer-scale diameters (around 2 μm).

Information. Point defect scattering in Si layers has a clear impact on thermal conductivity, namely, exclusion of point defect scattering from calculation for a Si/SiO2 HNMSL with 20 nm Si and 2 nm SiO2 leads to ∼45% rise of thermal conductivity in a wide temperature range. At the same time, the influence of Umklapp scattering in Si is very weak: at room temperature, exclusion of Umklapp scattering from calculation increases the thermal conductivity by less than 5%. While it is a clear evidence of the impact of point defects on thermal conductivity, the obtained values without point defect scattering are still approximately 3 times lower than those of 20 nm individual Si nanolayers. Therefore, our calculations imply the dominant role of phonon scattering in SiO2 layers in reducing the thermal conductivity of planar Si/SiO2 HNMSLs. In order to reveal the influence of the structural disorder of amorphous SiO2 on the thermal conductivity of a Si/SiO2 HNMSL, we have carried out calculations with crystalline SiO2 layers. As an example of crystalline SiO2, α-quartz is taken and the in-plane kx in a Si/α-quartz HNMSL is shown by the gray curve in Figure 4e. A 53% to 60% drop of thermal conductivity for temperatures between 100 and 400 K is seen when comparing HNMSLs with crystalline α-quartz layers and amorphous SiO2 layers (the case with ⟨E⟩ = 53 meV), respectively. Therefore, our calculations reveal a strong dependence of the lattice thermal conductivity in a Si/SiO2 HNMSL on the structural and vibrational properties of the SiO 2 layers. A significant suppression of the thermal conductivity in other Si-based nanostructures with amorphous

CONCLUSIONS In conclusion, based on a strain-engineered roll-up and compression technique, we have fabricated radial and planar Si/SiOx hybrid nanomembrane superlattices, in which singlecrystalline Si alternates with amorphous SiOx with well-defined interfaces. For the radial Si/SiOx HNMSLs, the in-plane thermal conductivity of a one-winding Si tube shows a tremendous reduction compared to both the bulk Si material 8219


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layer thicknesses for the simulation. The lamellae were prepared by focused-ion-beam (FIB) cutting and thinning (Zeiss NVision 40). FIB cutting was done with Ga+ ions impinging with 30 keV, and final polishing was done at 5 keV. TEM investigations were done in a Tecnai F30 (FEI) with an accelerating voltage of 300 kV. Theoretical Simulation. Within the VC approximation for amorphous SiO2 layers, we model vibrations in a Si/SiO2 HNMSL as mechanical vibrations around a rigid microscopic reference state in the form of a diamond-type crystal lattice. Specific features of the single-crystalline Si and amorphous SiO2 are included via their interatomic forces and scattering mechanisms for vibrational excitations. In particular, the SiO2 layer is characterized by the interatomic force constants and a scattering rate resulting from the diffusion of localized vibrations (“diffusons”) in amorphous SiO2.

and other Si thin-film systems of the same thickness. Interestingly, as the number of windings of Si tubes increases, the in-plane thermal conductivity drops, attributed to the thermal coupling between various windings. As for the planar Si/SiOx HNMSLs, the in-plane thermal conductivities are much smaller than those reported for Si-based superlattices with equivalent period thicknesses. In the framework of Born−von Karman lattice dynamics and the linearized Boltzmann transport equation, a theoretical model that considers a planar superlattice structure with a period 2 nm SiO2/20 nm Si/2 nm SiO2 is proposed to interpret the experimental data. The theoretical simulation fits our experimental data very well and implies that the thermal conductivity of the fabricated Si/SiOx HNMSLs is strongly affected by the phonon processes in the amorphous SiOx layers. The obtained thermal properties together with the roll-up and compression technique demonstrate a way for Si-based thermoelectric applications.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b03219. More detailed information about the thermal measurements, COMSOL simulation, TEM structure characterization, and BTE theoretical calculation (PDF)

METHODS Materials and Technique. As schematically shown in Figure 1a, first, a tensile-strained silicon film with a thickness of 24 nm is carefully deposited onto a 40 nm thick Ge sacrificial layer on Si(001) substrates at 300 °C by MBE. Then, the silicon thin film is lithographically patterned and etched by the reactive ion etching technique into isolated mesas with an area of 300 × 400 μm2. To create rolled-up tubes, the underneath Ge sacrificial layer is selectively etched away in a solution of H2O2 (30 vol %) for specific time ranges at 80 °C. Owing to the release of built-in tensile stress, the thin Si film rolls up into one or multiwinding tubes. During the etching procedure, thin SiOx layers (around 2 nm) are formed on each side of the Si nanomembrane due to oxidation by the H2O2 solution; that is, each winding of the tubes is composed of 2 nm amorphous SiOx, 20 nm single-crystalline Si, and 2 nm amorphous SiOx. By making use of a bonding machine (FINEPLACER lambda) with appropriate working parameters (pressing for 10 min with a pressure of 20 bar at a temperature of 300 °C), the as-rolled radial Si/SiOx superlattices are converted into planar Si/SiOx HNMSLs. Thermal Measurements. In order to measure the in-plane thermal conductivity precisely, at least two thermal devices are produced out of each above-mentioned Si/SiOx HNMSL configuration. The Si/SiOx HNMSLs are carefully cut into specific lengths, then picked up and transferred to a suspended microdevice using a sharp etched tungsten tip actuated by a micromanipulator under an optical microscope. The suspended microdevice shown in Figure 2a−c consists of two adjacent low-stress silicon nitride (SiNx) membranes, each of which is suspended by six SiNx beams over a through-substrate hole. A platinum serpentine line is patterned on each membrane, serving as a heater and resistance thermometer (RT). To obtain the sample’s thermal conductivity, one membrane is Joule heated, and heat conduction through the sample raises the temperature of the other membrane. The temperature rise in the two membranes is measured using the two Pt RTs. From the measured Joule heat and membrane temperatures, the thermal conductance of the sample can be obtained that includes the intrinsic thermal resistance of the suspended HNMSL segment between the two membranes and the contact thermal resistance between the HNMSLs and the membranes. To reduce the contact resistance, we covered both membranes with a proper amount of vacuum-compatible grease (Dow Corning highvacuum grease) using the tungsten tip before and after sample transferring as shown in Figure 2b and c. Then the samples are immediately loaded into a high-vacuum cryostat to perform the thermal transport measurement under various temperatures. TEM Characterization. To be consistent with the thermal measurements, thin lamella samples for TEM characterization were prepared out of the same parent radial (5 windings) and planar samples, from which the measured devices were produced. The purpose of the TEM investigation was to prove the structure of the stack (single-crystalline Si/amorphous SiOx) and to determine the

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Guodong Li: 0000-0002-1327-0005 Feng Zhu: 0000-0001-9175-3718 Author Contributions #

G. Li, M. Yarali, and A. Cocemasov contributed equally to this work. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS G.L., F.Z., and O.G.S. acknowledge financial support from the DFG project SPP 1386 (Grant No. 51471401) and Cfaed SiNW path (Grant No. 85048411). A.C. and D.N. acknowledge financial support from the Moldova government (Project No. 15.817.02.29F). M.Y., S.S., and A.M. acknowledge financial support from University of Houston. All authors appreciate Dr. D. Grimm, E. Pankenin, K. Manga, and the clean room staff at Institute for Integrative Nanosciences, IFW Dresden, for helpful discussions and experimental assistance. REFERENCES (1) Hicks, L.; Dresselhaus, M. Effect of Quantum-Well Structures on the Thermoelectric Figure of Merit. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 12727−12731. (2) Cahill, D. G.; Ford, W. K.; Goodson, K. E.; Mahan, G. D.; Majumdar, A.; Maris, H. J.; Merlin, R.; Phillpot, S. R. Nanoscale Thermal Transport. J. Appl. Phys. 2003, 93, 793−818. (3) Cahill, D. G.; Braun, P. V.; Chen, G.; Clarke, D. R.; Fan, S.; Goodson, K. E.; Keblinski, P.; King, W. P.; Mahan, G. D.; Majumdar, A.; Maris, H. J.; Phillpot, S. R.; Pop, E.; Shi, L. Nanoscale Thermal Transport. II. 2003−2012. Appl. Phys. Rev. 2014, 1, 11305. (4) Dresselhaus, M. S.; Chen, G.; Tang, M. Y.; Yang, R. G.; Lee, H.; Wang, D. Z.; Ren, Z. F.; Fleurial, J.-P.; Gogna, P. New Directions for Low-Dimensional Thermoelectric Materials. Adv. Mater. 2007, 19, 1043−1053. 8220


Article

ACS Nano (5) Snyder, G. J.; Toberer, E. S. Complex Thermoelectric Materials. Nat. Mater. 2008, 7, 105−114. (6) Poudel, B.; Hao, Q.; Ma, Y.; Lan, Y.; Minnich, A.; Yu, B.; Yan, X.; Wang, D.; Muto, A.; Vashaee, D.; Chen, X.; Liu, J.; Dresselhaus, M. S.; Chen, G.; Ren, Z. High-Thermoelectric Performance of Nanostructured Bismuth Antimony Telluride Bulk Alloys. Science 2008, 320, 634−638. (7) Koga, T.; Sun, X.; Cronin, S. B.; Dresselhaus, M. S. Carrier Pocket Engineering to Design Superior Thermoelectric Materials Using GaAs/AlAs Superlattices. Appl. Phys. Lett. 1998, 73, 2950−2952. (8) Humphrey, T. E.; O’Dwyer, M. F.; Linke, H. Power Optimization in Thermionic Devices. J. Phys. D: Appl. Phys. 2005, 38, 2051−2054. (9) Deneke, C.; Songmuang, R.; Jin-Phillipp, N. Y.; Schmidt, O. G. The Structure of Hybrid Radial Superlattices. J. Phys. D: Appl. Phys. 2009, 42, 103001. (10) Bux, S. K.; Blair, R. G.; Gogna, P. K.; Lee, H.; Chen, G.; Dresselhaus, M. S.; Kaner, R. B.; Fleurial, J. P. Nanostructured Bulk Silicon as an Effective Thermoelectric Material. Adv. Funct. Mater. 2009, 19, 2445−2452. (11) Schierning, G.; Theissmann, R.; Stein, N.; Petermann, N.; Becker, A.; Engenhorst, M.; Kessler, V.; Geller, M.; Beckel, A.; Wiggers, H.; Schmechel, R. Role of Oxygen on Microstructure and Thermoelectric Properties of Silicon Nanocomposites. J. Appl. Phys. 2011, 110, 0−9. (12) Asheghi, M.; Leung, Y. K.; Wong, S. S.; Goodson, K. E. PhononBoundary Scattering in Thin Silicon Layers. Appl. Phys. Lett. 1997, 71, 1798. (13) Liu, W.; Asheghi, M. Phonon-Boundary Scattering in Ultrathin Single-Crystal Silicon Layers. Appl. Phys. Lett. 2004, 84, 3819−3821. (14) Yu, J.-K.; Mitrovic, S.; Tham, D.; Varghese, J.; Heath, J. R. Reduction of Thermal Conductivity in Phononic Nanomesh Structures. Nat. Nanotechnol. 2010, 5, 718−721. (15) Tang, J.; Wang, H. T.; Lee, D. H.; Fardy, M.; Huo, Z.; Russell, T. P.; Yang, P. Holey Silicon as an Efficient Thermoelectric Material. Nano Lett. 2010, 10, 4279−4283. (16) Hopkins, P. E.; Reinke, C. M.; Su, M. F.; Olsson, R. H.; Shaner, E. A.; Leseman, Z. C.; Serrano, J. R.; Phinney, L. M.; El-Kady, I. Reduction in the Thermal Conductivity of Single Crystalline Silicon by Phononic Crystal Patterning. Nano Lett. 2011, 11, 107−112. (17) Cuffe, J.; Chavez, E.; Shchepetov, A.; Chapuis, P. O.; El Boudouti, E. H.; Alzina, F.; Kehoe, T.; Gomis-Bresco, J.; Dudek, D.; Pennec, Y.; Djafari-Rouhani, B.; Prunnila, M.; Ahopelto, J.; Torres, C. M. S. Phonons in Slow Motion: Dispersion Relations in Ultrathin Si Membranes. Nano Lett. 2012, 12, 3569−3573. (18) Ferrando-Villalba, P.; Lopeandia, A. F.; Abad, L.; Llobet, J.; Molina-Ruiz, M.; Garcia, G.; Gerbolès, M.; Alvarez, F. X.; Goñi, A. R.; Muñoz-Pascual, F. J.; Rodríguez-Viejo, J. In-Plane Thermal Conductivity of Sub-20 Nm Thick Suspended Mono-Crystalline Si Layers. Nanotechnology 2014, 25, 185402. (19) Neogi, S.; Reparaz, J. S.; Pereira, L. F. C.; Graczykowski, B.; Wagner, M. R.; Sledzinska, M.; Shchepetov, A.; Prunnila, M.; Ahopelto, J.; Sotomayor-Torres, C. M.; Donadio, D. Tuning Thermal Transport in Ultrathin Silicon Membranes by Surface Nanoscale Engineering. ACS Nano 2015, 9, 3820−3828. (20) Lim, J.; Wang, H.-T.; Tang, J.; Andrews, S. C.; So, H.; Lee, J.; Lee, D. H.; Russell, T. P.; Yang, P. Simultaneous Thermoelectric Property Measurement and Incoherent Phonon Transport in Holey Silicon. ACS Nano 2016, 10, 124−132. (21) Lee, S.-M.; Cahill, D. G.; Venkatasubramanian, R. Thermal Conductivity of Si−Ge Superlattices. Appl. Phys. Lett. 1997, 70, 2957. (22) Koga, T.; Cronin, S. B.; Dresselhaus, M. S.; Liu, J. L.; Wang, K. L. Experimental Proof-of-Principle Investigation of Enhanced Z[sub 3D]T in (001) Oriented Si/Ge Superlattices. Appl. Phys. Lett. 2000, 77, 1490. (23) Huxtable, S. T.; Abramson, A. R.; Tien, C. L.; Majumdar, A.; Labounty, C.; Fan, X.; Zeng, G.; Bowers, J. E.; Shakouri, A.; Croke, E. T. Thermal Conductivity of Si/SiGe and SiGe/SiGe Superlattices. Appl. Phys. Lett. 2002, 80, 1737−1739.

(24) Chen, P.; Katcho, N. A.; Feser, J. P.; Li, W.; Glaser, M.; Schmidt, O. G.; Cahill, D. G.; Mingo, N.; Rastelli, A. Role of SurfaceSegregation-Driven Intermixing on the Thermal Transport through Planar Si/Ge Superlattices. Phys. Rev. Lett. 2013, 111, 1−6. (25) Li, D.; Wu, Y.; Kim, P.; Shi, L.; Yang, P.; Majumdar, A. Thermal Conductivity of Individual Silicon Nanowires. Appl. Phys. Lett. 2003, 83, 2934. (26) Hochbaum, A. I.; Chen, R.; Delgado, R. D.; Liang, W.; Garnett, E. C.; Najarian, M.; Majumdar, A.; Yang, P. Enhanced Thermoelectric Performance of Rough Silicon Nanowires. Nature 2008, 451, 163− 167. (27) Boukai, A. I.; Bunimovich, Y.; Tahir-Kheli, J.; Yu, J.-K.; Goddard, W. a; Heath, J. R. Silicon Nanowires as Efficient Thermoelectric Materials. Nature 2008, 451, 168−171. (28) Jiang, J. W.; Yang, N.; Wang, B. S.; Rabczuk, T. Modulation of Thermal Conductivity in Kinked Silicon Nanowires: Phonon Interchanging and Pinching Effects. Nano Lett. 2013, 13, 1670−1674. (29) Songmuang, R.; Deneke, C.; Schmidt, O. G. Rolled-up Microand Nanotubes from Single-Material Thin Films. Appl. Phys. Lett. 2006, 89, 2004−2007. (30) Songmuang, R.; Rastelli, A.; Mendach, S.; Schmidt, O. G. SiOx/ Si Radial Superlattices and Microtube Optical Ring Resonators. Appl. Phys. Lett. 2007, 90, 2−5. (31) Grimm, D.; Wilson, R. B.; Teshome, B.; Gorantla, S.; Rümmeli, M. H.; Bublat, T.; Zallo, E.; Li, G.; Cahill, D. G.; Schmidt, O. G. Thermal Conductivity of Mechanically Joined Semiconducting/metal Nanomembrane Superlattices. Nano Lett. 2014, 14, 2387−2393. (32) Shi, L.; Li, D.; Yu, C.; Jang, W.; Kim, D.; Yao, Z.; Kim, P.; Majumdar, A. Measuring Thermal and Thermoelectric Properties of One-Dimensional Nanostructures Using a Microfabricated Device. J. Heat Transfer 2003, 125, 881. (33) Mavrokefalos, A.; Pettes, M. T.; Zhou, F.; Shi, L. Four-Probe Measurements of the in-Plane Thermoelectric Properties of Nanofilms. Rev. Sci. Instrum. 2007, 78, 0−6. (34) Yang, J.; Yang, Y.; Waltermire, S. W.; Gutu, T.; Zinn, A. A.; Xu, T. T.; Chen, Y.; Li, D. Measurement of the Intrinsic Thermal Conductivity of a Multiwalled Carbon Nanotube and Its Contact Thermal Resistance with the Substrate. Small 2011, 7, 2334−2340. (35) Lee, S.; Yang, F.; Suh, J.; Yang, S.; Lee, Y.; Li, G.; Sung Choe, H.; Suslu, A.; Chen, Y.; Ko, C.; Park, J.; Liu, K.; Li, J.; Hippalgaonkar, K.; Urban, J. J.; Tongay, S.; Wu, J. Anisotropic in-Plane Thermal Conductivity of Black Phosphorus Nanoribbons at Temperatures Higher than 100 K. Nat. Commun. 2015, 6, 8573. (36) Glassbrenner, C. J.; Slack, G. A. Thermal Conductivity of Silicon and Germanium from 300K to the Melting Point. Phys. Rev. 1964, 134, 134. (37) Liu, W. L.; Borca-Tasciuc, T.; Chen, G.; Liu, J. L.; Wang, K. L. Anisotropic Thermal Conductivity of Ge Quantum-Dot and Symmetrically Strained Si/Ge Superlattices. J. Nanosci. Nanotechnol. 2001, 1, 39−42. (38) Aksamija, Z.; Knezevic, I. Thermal Conductivity of Si1-xGex/Si 1-yGey Superlattices: Competition between Interfacial and Internal Scattering. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 88, 1−11. (39) Volz, S. G.; Chen, G. Molecular Dynamics Simulation of Thermal Conductivity of Silicon Nanowires. Appl. Phys. Lett. 1999, 75, 2056. (40) Mingo, N. Calculation of Si Nanowire Thermal Conductivity Using Complete Phonon Dispersion Relations. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 68, 113308. (41) Nika, D. L.; Cocemasov, A. I.; Isacova, C. I.; Balandin, A. A.; Fomin, V. M.; Schmidt, O. G. Suppression of Phonon Heat Conduction in Cross-Section-Modulated Nanowires. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 205439. (42) Abeles, B. Lattice Thermal Conductivity of Disordered Semiconductor Alloys at High Temperatures. Phys. Rev. 1963, 131, 1906−1911. (43) Beechem, T.; Graham, S.; Hopkins, P.; Norris, P. Role of Interface Disorder on Thermal Boundary Conductance Using a Virtual Crystal Approach. Appl. Phys. Lett. 2007, 90, 54104. 8221


Article

ACS Nano (44) Rahaman, O.; Mortazavi, B.; Rabczuk, T. A First-Principles Study on the Effect of Oxygen Content on the Structural and Electronic Properties of Silicon Suboxide as Anode Material for Lithium Ion Batteries. J. Power Sources 2016, 307, 657−664. (45) Donadio, D.; Galli, G. Atomistic Simulations of Heat Transport in Silicon Nanowires. Phys. Rev. Lett. 2009, 102, 195901.

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