GaAs/GaP Superlattice Nanowires for Tailoring Phononic Properties at the Nanoscale: Implications for Thermal Engineering

The possibility to tune the functional properties of nanomaterials is key to their technological applications. Superlattices, i.e., periodic repetitions of two or more materials in one or more dimensions, are being explored for their potential as materials with tailor-made properties. Meanwhile, nanowires offer a myriad of possibilities to engineer systems at the nanoscale, as well as to combine materials that cannot be put together in conventional heterostructures due to the lattice mismatch. In this work, we investigate GaAs/GaP superlattices embedded in GaP nanowires and demonstrate the tunability of their phononic and optoelectronic properties by inelastic light scattering experiments corroborated by ab initio calculations. We observe clear modifications in the dispersion relation for both acoustic and optical phonons in the superlattices nanowires. We find that by controlling the superlattice periodicity, we can achieve tunability of the phonon frequencies. We also performed wavelength-dependent Raman microscopy on GaAs/GaP superlattice nanowires, and our results indicate a reduction in the electronic bandgap in the superlattice compared to the bulk counterpart. All of our experimental results are rationalized with the help of ab initio density functional perturbation theory (DFPT) calculations. This work sheds fresh insights into how material engineering at the nanoscale can tailor phonon dispersion and open pathways for thermal engineering.


Supporting information 2: Polarization dependent Raman scattering experiments
In figure S2(a), we present results on the µ-Raman measurements in the ̅ (, ) scattering configuration on the reference NW.For comparison we also present the theoretically calculated spectra of WZ GaP in figure S2(b).In Figure S2(a), we see a peak at 356.9 cm -1 which can be attributed to the  !" of WZ GaP 1 .
There is another peak at 303 cm -1 , which is due to the silicon substrate 2 .In Figure S2(b), we present the calculated spectrum of WZ GaP.The spectrum has two peaks at 356.8 cm -1 and at 362.7 cm -1 , which can be attributed to the  !" and TO of GaP, respectively.The presence of E2 H mode confirms the WZ crystal phase of the NWs.In the experimental spectrum, in the ̅ (, ) configuration, the resolution of the TO mode difficult which is why it is not used in the fitting in Figure S2 (a).
In Figure S3 (a), we present the results on the µ-Raman measurements in the ̅ (, ) and ̅ (, ) on SL NW with a period of 10 nm.In the ̅ (, ) scattering configuration, the most intense peak in the GaAs-like phonon modes region is at 276.9 cm -1 while in the GaP-like phonon modes region there is an intense peak at 357.6 cm -1 .In the ̅ (, ) scattering configuration, the most intense peak in the GaAs-like phonon modes region is at 269.4 cm -1 while in the GaP-like phonon modes region there is an intense peak at 351.6 cm-1.The overall intensity of the spectrum decreases in the ̅ (, ) as compared to ̅ (, ) configuration by a factor of about 2, possibly due also to the dielectric mismatch effect. 3In Figure S3 (b), we show the calculated Raman spectrum of GaAs/GaP SL with L=6.39 nm in both the polarization configuration.The intensities are normalized for ease of comparison.In Table S1 we list the computed Γ-point frequencies, setting a threshold intensity indicative of their experimental detectivity.
Table S2.Calculated acoustic phonon mode frequencies with frequencies less than 1 THz for peaks corresponding to q value close to 85 µm -1 from the dispersion relation in Figure S3 for SL with periodicity of 6.3 nm.

Period
Frequency (GHz) 6. 3   In Figure S6, we show the eigen displacement of the atoms of the Raman inactive phonon mode at 218 cm -1 , whose vibrations involve all atoms in the SL unit cell.The Ga atoms are represented by blue spheres, As by yellow spheres, and P will be green spheres.

Figure S2 .
Figure S2.(a) Polarized µ-Raman spectrum on a reference NW collected in the ̅ (, ) scattering configuration.The experimental data are shown in pink spheres, light green curves show the individual Lorentzian fitting, the red curve shows the cumulative fitting and dark blue line is the baseline.(b) Theoretically calculated spectrum of WZ GaP in black curve, with the full width at half maximum fixed at 5 cm -1 .The green curves show the individual Lorentzian fitting and the red curve shows the cumulative fitting.The dark blue line is the baseline.

Figure S3 .
Figure S3.(a) Polarization resolved µ-Raman spectra of a 10 nm SL NW.The red curve shows the data collected in the ̅ (, ) configuration and the black curve shows the data collected in the ̅ (, ); (b) Calculated Raman spectra of a GaAs/GaP SL of L=6.39 nm.The intensities are normalized.The red curve shows the ̅ (, ) configuration and black curves show the ̅ (, ).

Figure S6 .
Figure S6.The Eigen displacement of atoms for phonon mode at 218 cm -1 .The different coloured spheres represent the constituent atoms with blue as Ga, yellow as As, and green as P.