Native surface oxide turns alloyed silicon membranes into nanophononic metamaterials with ultralow thermal conductivity

Abstract

A detailed understanding of the relation between microscopic structure and phonon propagation at the nanoscale is essential to design materials with desired phononic and thermal properties. Here we uncover a new mechanism of phonon interaction in surface oxidized membranes, i.e., native oxide layers interact with phonons in ultra-thin silicon membranes through local resonances. The local resonances reduce the low frequency phonon group velocities and shorten their mean free path. This effect opens up a new strategy for ultralow thermal conductivity design as it complements the scattering mechanism which scatters higher frequency modes effectively. The combination of native oxide layer and alloying with germanium in concentration as small as 5% reduces the thermal conductivity of silicon membranes to 100 time lower than the bulk. In addition, the resonance mechanism produced by native oxide surface layers is particularly effective for thermal condutivity reduction even at very low temperatures, at which only low frequency modes are populated.Controlling terahertz vibrations and heat transport in nanostructures has a broad impact on several applications, such as thermal management in micro-and nano-electronics, renewable energies harvesting, sensing, biomedical imaging and information and communication technologies [1][2][3][4][5][6][7][8]. Significant efforts have been made to understand and engineer heat transport in nanoscale silicon due to its natural abundance and technological relevance [9][10][11][12]. In the past decade researchers explored strategies to obtain silicon based materials with low thermal conductivity (TC) and unaltered electronic transport coefficients, so to achieve high thermoelectric figure of merit and enable silicon-based thermoelectric technology [11][12][13][14][15][16][17][18].From the earlier studies it was recognized that lowdimensional silicon nanostructures, such as nanowires, thin films and nano membranes feature a largely reduced TC, up to 50 times lower than that of bulk at room temperature. TC reduction becomes more prominent with the reduction of the characteristic dimension of the nanostructures [19][20][21][22]. Theory and experiments consistently show that surface disorder and the presence of disordered material at surfaces play a major role in determining the TC of nanostructures [12,[23][24][25]. However, a comprehensive understanding of the physical mechanisms underlying so large TC reduction is lacking. The effect of surface roughness and surface disorder on phonons has been so far interpreted in terms of phonon scattering [26][27][28][29][30], but scattering would not account for mean free path reduction of long-wavelength low-frequency modes. Recent theoretical work demonstrated that surface nanostructures, such as nanopillars at the surface of thin films or nanowires, can efficiently reduce TC through resonances, a mechanism that is intrinsically different from scattering [31,32]. Surface resonances alter directly phonon dispersion relations by hybridizing with prop...

Advances in the optimization of silicon-based thermoelectrics: a theory perspective

“…Contrary to the assumption of diffusive phonon scattering [59], spectral energy decomposition [60] and frequency-resolved calculations of phonon mfps [61] show that surface layers of amorphous materials mainly produce resonances that modify the dispersion relations of low-frequency propagating modes, reducing their group velocity. [62,63] These resonances are similar to those obtained by nanopatterning the surface of thin films, membranes or nanorods according to the recently proposed nanophononic metamaterials paradigm. [64,65,66,67,68] Such reduction of κ p is sufficient to utilize silicon membranes for sensing [69], but not for TE energy conversion, as the maximum ZT that can be achieved is 0.2 at room temperature [22].…”

“…Simulations predict that it may be possible to achieve even higher ZT by combining compatible optimization routes, e.g. alloying with nanostructuring [63,67,92] or nano-crystallinity with nanoporosity [93]. However, several design concepts proposed by modeling still wait for experimental verification.…”

“…This can be obtained by introducing "bulk" defects. Alloying with germanium, nanopores, twin boundaries, dislocations and heterostructures have been proposed as effective ways to further reduce κ p , [70,71,72,57,73,63] but their impact on charge transport coefficients still needs to be assessed. In particular, MD predicts that introducing 10% of Ge would decrease κ p for 10 nm membranes ten times leading to ZT > 1.…”

Phonon and heat transport control using pillar-based phononic crystals

“…Thus, the observed reduction in thermal conductivity was attributed to the surface roughness and the amorphous layer under the pillars. Indeed, amorphous layers at the surfaces are known to reduce the thermal conductivity of nanostructures [61,64–66] and can diffusely scatter phonons stronger than just surface roughness [67]. …”

Thermal Conductivity Reduction in a Nanophononic Metamaterial versus a Nanophononic Crystal: A Review and Comparative Analysis

“…for a survey of potential configurations using atomically disordered inclusions). The notion of a surface oxidized membrane acting as an NPM—with the disordered oxygen atoms covering the membrane surface acting as the nanoresonators—was proposed by Xiong et al NPMs in the form of branched nanoribbon materials composed of molybdenum disulfide (MoS 2 ) were studied by Liu et al Giri and Hopkins and Yang and co‐workers extended the NPM concept to carbon nanotubes and graphene sheets, respectively. Another intriguing NPM architecture is based on a graphene sheet with branching fullerene nanoresonators .…”