Similar to electron waves, the phonon states in semiconductors can undergo changes induced by external boundaries. However, despite strong scientific and practical importance, conclusive experimental evidence of confined acoustic phonon polarization branches in individual free-standing nanostructures is lacking. Here we report results of Brillouin—Mandelstam light scattering spectroscopy, which reveal multiple (up to ten) confined acoustic phonon polarization branches in GaAs nanowires with a diameter as large as 128 nm, at a length scale that exceeds the grey phonon mean-free path in this material by almost an order-of-magnitude. The dispersion modification and energy scaling with diameter in individual nanowires are in excellent agreement with theory. The phonon confinement effects result in a decrease in the phonon group velocity along the nanowire axis and changes in the phonon density of states. The obtained results can lead to more efficient nanoscale control of acoustic phonons, with benefits for nanoelectronic, thermoelectric and spintronic devices.

Growth and characterization of iron-doped semi-insulating InP buffer layers for Al-free GaInP/GaInAs high electron mobility transistors

The ability to control the optical field in the vicinity of an individual nano-object is an obvious stepping-stone in the tailoring of light-matter interactions at the nanoscale. Earlier reports on tailoring light fields in the vicinity of a nano-object have been restricted by their dependence on cumbersome optical or fabrication techniques, have relied mostly on in-plane electric field polarizations, and have been demonstrated only for bulk materials and structures with strong in-plane anisotropies. In addition, traditional methods for manipulating the longitudinal electric fields are significantly hindered by the lack of appropriate probes that can be used to unambiguously measure or calibrate the light coupling efficiency to nano-objects. Here, we demonstrate such a possibility for the specific case of optical second-harmonic generation (SHG). Our technique relies on spatial phase-shaping of a high-order laser beam to tailor the longitudinal fields at the beam focus and allows SHG from an individual and well-defined vertically-aligned GaAs nanowire to be manipulated on demand. Our technique is applicable to tailoring the efficiency of nonlinear emission on the nanoscale and to arbitrary polarization control at the beam focus in general.

We report a new phenomenon related to Al-induced carrier confinement at the interface in core-shell GaAs/Al(x)Ga(1-x)As nanowires grown using metal-organic vapor phase epitaxy with Au as catalyst. All Al(x)Ga(1-x)As shells strongly passivated the GaAs nanowires, but surprisingly the peak photoluminescence (PL) position and the intensity from the core were found to be a strong function of Al composition in the shell at low temperatures. Large and systematic red shifts of up to ~66 nm and broadening in the PL emission from the GaAs core were observed when the Al composition in the shell exceeded 3%. On the contrary, the phenomenon was observed to be considerably weaker at the room temperature. Cross-sectional transmission electron microscopy reveals Al segregation in the shell along six Al-rich radial bands displaying a 3-fold symmetry. Time-resolved PL measurements suggest the presence of indirect electron-hole transitions at the interface at higher Al composition. We discuss all possibilities including a simple shell-core-shell model using simulations where the density of interface traps increases with the Al content, thus creating a strong local electron confinement. The carrier confinement at the interface is most likely related to Al inhomogeneity and/or Al-induced traps. Our results suggest that a low Al composition in the shell is desirable in order to achieve ideal passivation in GaAs nanowires.

The Young's modulus of thin conical InP nanowires with either wurtzite or mixed "zinc blende/wurtzite" structures was measured. It has been shown that the value of Young's modulus obtained for wurtzite InP nanowires (E = 130 ± 30 GPa) was similar to the theoretically predicted value for the wurtzite InP material (E = 120 ± 10 GPa). The Young's modulus of mixed "zinc blende/wurtzite" InP nanowires (E = 65 ± 10 GPa) appeared to be 40% less than the theoretically predicted value for the zinc blende InP material (E = 110 GPa). An advanced method for measuring the Young's modulus of thin and flexible nanostructures is proposed. It consists of measuring the flexibility (the inverse of stiffness) profiles 1/k(x) by the scanning probe microscopy with precise control of loading force in nanonewton range followed by simulations.