ABSTRACT
Nanowire geometry allows semiconductor heterostructures to be obtained that are not achievable in planar systems, as in, for example, axial superlattices made of large lattice mismatched materials. This provides a great opportunity to explore new optical transitions and vibrational properties resulting from the superstructure. Moreover, superlattice nanowires are expected to show improved thermoelectric properties, owing to the dominant role of surfaces and interfaces that can scatter phonons more effectively, reducing the lattice thermal conductivity. Here, we show the growth of long (up to 100 repetitions) GaAs/GaP superlattice nanowires with different periodicities, uniform layer thicknesses, and sharp interfaces, realized by means of Au-assisted chemical beam epitaxy. By optimizing the growth conditions, we obtained great control of the nanowire diameter, growth rate, and superlattice periodicity, offering a valuable degree of freedom for engineering photonic and phononic properties at the nanoscale. As a proof of concept, we analyzed a single type of superlattice nanowire with a well-defined periodicity and we observed room temperature optical emission and new phonon modes. Our results prove that high-quality GaAs/GaP superlattice nanowires have great potential for phononic and optoelectronic studies and applications.
ABSTRACT
Recently synthesized hexagonal group IV materials are a promising platform to realize efficient light emission that is closely integrated with electronics. A high crystal quality is essential to assess the intrinsic electronic and optical properties of these materials unaffected by structural defects. Here, we identify a previously unknown partial planar defect in materials with a type I3 basal stacking fault and investigate its structural and electronic properties. Electron microscopy and atomistic modeling are used to reconstruct and visualize this stacking fault and its terminating dislocations in the crystal. From band structure calculations coupled to photoluminescence measurements, we conclude that the I3 defect does not create states within the hex-Ge and hex-Si band gap. Therefore, the defect is not detrimental to the optoelectronic properties of the hex-SiGe materials family. Finally, highlighting the properties of this defect can be of great interest to the community of hex-III-Ns, where this defect is also present.
ABSTRACT
Recent advances in nanowire synthesis have enabled the realization of crystal phases that in bulk are attainable only under extreme conditions, i.e., high temperature and/or high pressure. For group IV semiconductors this means access to hexagonal-phase SixGe1-x nanostructures (with a 2H type of symmetry), which are predicted to have a direct band gap for x up to 0.5-0.6 and would allow the realization of easily processable optoelectronic devices. Exploiting the quasi-perfect lattice matching between GaAs and Ge, we synthesized hexagonal-phase GaAs-Ge and GaAs-SixGe1-x core-shell nanowires with x up to 0.59. By combining position-, polarization-, and excitation wavelength-dependent µ-Raman spectroscopy studies with first-principles calculations, we explore the full lattice dynamics of these materials. In particular, by obtaining frequency-composition calibration curves for the phonon modes, investigating the dependence of the phononic modes on the position along the nanowire, and exploiting resonant Raman conditions to unveil the coupling between lattice vibrations and electronic transitions, we lay the grounds for a deep understanding of the phononic properties of 2H-SixGe1-x nanostructured alloys and of their relationship with crystal quality, chemical composition, and electronic band structure.
