S. Rubini

Self-catalyzed growth of GaAs nanowires on cleaved Si by molecular beam epitaxy

Self-assembled GaAs nanowires have been grown on Si by molecular beam epitaxy without the use of any outside metal catalyst. The growth occurs on Si facets obtained by the cleavage of Si(100) substrates. The growth has been obtained with or without Ga pre-deposition. In both cases two kinds of nanowires have been obtained. The wires of the first type clearly present a Ga droplet at their free end and have a lattice structure that is wurtzite for wide regions beneath the Ga droplet. The second type, in contrast, ends with pyramidally shaped GaAs and has a crystal lattice that is mainly zincblende with only a few and small wurtzite regions, if any. The Ga-ended nanowires are longer than the others and thinner on average. The experimental findings suggest that the two types of nanowires grow after different growth processes.

Low-temperature synthesis of ZnSe nanowires and nanosaws by catalyst- assisted molecular-beam epitaxy

Single-crystal ZnSe nanowires are grown on a prepatterned gold catalyst by molecular-beam epitaxy. Optimum selectivity and maximum nanowire densities are obtained for growth temperatures in the range 400–450°C, but gold-assisted growth is demonstrated for temperatures as low as 300°C. This suggests a diffusion process on/through the catalyst particle in the solid state, in contrast to the commonly assumed liquid phase growth models. Straight wires, as thin as 10nm, nucleate together with thicker and saw-like structures. A gold particle is always found at the tip in both cases.

Room temperature luminescent InGaAs/GaAs core-shell nanowires

InGaAs/GaAs core-shell nanowires have been grown by molecular beam epitaxy. The core-shell nanowires show room temperature photoluminescence. At low temperatures their luminescence intensity is two to three orders of magnitudes larger than that of parent InGaAs nanowires grown without external GaAs shell. The nanowires have been structurally characterized by scanning electron microscopy and transmission electron microscopy.

Photoluminescence of Mn-catalyzed GaAs nanowires grown by molecular beam epitaxy

We present photoluminescence spectra of GaAs nanowires and nanoleaves grown by molecular beam epitaxy using Mn as growth catalyst. At low temperature and low excitation intensity the spectra are characterized by several narrow peaks superimposed onto a broader band. The peak at the highest energy is located at 1.522 eV, i.e. 7 meV above the free exciton position in GaAs, irrespective of growth conditions and of the shape and size of the nanostructures. We suggest that this peak originates from electron–hole recombination in wurtzite-type GaAs.

Structural characterization of GaAs and InAs nanowires by means of Raman spectroscopy

We report Raman studies of GaAs and InAs nanowires (NWs) grown on SiO2 and GaAs surfaces by means of catalyst-assisted molecular beam epitaxy. We have investigated several tens of NWs grown using either Mn or Au as a catalyst. The LO and TO phonon lines of the NWs showed an energy downshift and a broadening as compared to the lines usually observed in the corresponding bulk materials. A doublet is sometimes observed in the LO region due to the observation of a signal attributed to the surface optical (SO) phonon. The energy position of the SO phonon agrees with the values expected considering the section diameter of the NWs. LO and TO downshifts are due to the presence of structural defects within the NWs. The larger the energy downshift, the smaller the dimension of the defect-free regions. The results demonstrate that different catalysts provide wires with comparable crystal quality. The measurements also point out that differences in defect density can be found in wires coming from the same batch indic...

Selective growth of ZnSe and ZnCdSe nanowires by molecular beam epitaxy

Controlled growth of ZnSe and ZnCdSe nanowires is demonstrated by molecular beam epitaxy using Au or Ag catalyst films in the temperature range 400–550 °C. The highest density of small-diameter (10 nm), highly-crystalline ZnSe nanowires is achieved by using Au at 400 °C. Direct growth onto transmission electron microscope grids clearly indicates a tip-growth regime. Pre-patterning of the catalyst film allows highly selective ZnSe deposition as probed by photoluminescence and Raman spectroscopy. In similar conditions, the addition of Cd vapour in the MBE reactor allows the synthesis of ZnCdSe ternary nanowires.

Self-catalyzed GaAs nanowire growth on Si-treated GaAs(100) substrates

Self-catalyzed GaAs nanowire growth was obtained by molecular beam epitaxy on GaAs(001) substrates after predeposition of subnanometer-thick Si layers. Two substrate preparation methods are presented, the first based on the epitaxial growth of Si on GaAs and subsequent exposure to atmosphere, and the second on the direct deposition of Si on epiready GaAs substrates. X-ray photoemission spectroscopy shows that both methods result in a thin Si oxide layer that promotes the growth of GaAs nanowires aligned along the 〈111〉 direction. High densities of nanowires were obtained at substrate temperatures between 620 and 680 °C. Systematic electron microscopy studies indicate that nanowire growth is associated with the formation of Ga nanoparticles on the substrate surface, which act as a catalyst in the vapor-liquid-solid growth mechanism frame. The majority of the nanowires have a pure zinc-blende structure, and their photoluminescence is dominated by a photoluminescence peak 3 to 5 meV in width and centered at ...

Growth of III-V semiconductor nanowires by molecular beam epitaxy

We present here the growth of GaAs, InAs and InGaAs nanowires by molecular beam epitaxy. The nanowires have been grown on different substrates [GaAs(001), GaAs(111), SiO2 and Si(111)] using gold as the growth catalyst. We show how the different substrates affect the results in terms of nanowire density and morphology. We also show that the growth temperature for the InGaAs nanowires has to be carefully chosen to obtain homogeneous alloys.

Interface composition and stacking fault density in II-VI/III-V heterostructures

The Zn/Se flux ratio employed during the early stages of molecular beam epitaxy of pseudomorphic ZnSe/GaAs(001) as well as lattice-matched ZnSe/In0.04Ga0.96As(001) heterostructures controls the density of the native stacking faults, which have been associated with the early degradation of blue-green lasers. In particular, the density of Shockley stacking fault pairs decreases by three to four orders of magnitude and that of Frank stacking faults by one order of magnitude in going from Zn-rich to Se-rich interfaces

Photoreflectance and reflectance investigation of deuterium-irradiated GaAsN

The effect of deuterium irradiation on the optical and strain properties of GaAsN∕GaAs heterostructures was investigated by photoreflectance and reflectance techniques. The strain occurring in as-grown and deuterated GaAsN layers is monitored and measured by means of photoreflectance spectroscopy, highlighting the strain inversion after irradiation. By combining static and modulated reflectance results, evidence is given that the deuterium-induced recovery of the GaAs band gap as well as the strain inversion in GaAsN layers are accompanied by a 0.4%–0.8% reduction of the refractive index in the 1.31 and 1.55μm spectral windows of interest for fiber optic communications. These results anticipate a single step process to an in-plane confinement of carriers and photons.