Kasama, Takeshi; Thuvander, M.; Siusys, A.; Gontard, L.C.; Kovács, A.; Yazdi, Sadegh; Duchamp, M.; Dunin-Borkowski, R.E.; Gustafsson, A.; Sadowski, J. Doping mechanisms of Mn in GaAs nanowires (NWs) that have been grown self-catalytically at 600 C by molecular beam epitaxy (MBE) are investigated using advanced electron microscopy techniques and atom probe tomography. Mn is found to be incorporated primarily in the form of non-magnetic tetragonal Ga 0.82 Mn 0.18 nanocrystals in Ga catalyst droplets at the ends of the NWs, while trace amounts of Mn (22 6 4 at. ppm) are also distributed randomly in the NW bodies without forming clusters or precipitates. The nanocrystals are likely to form after switching off the reaction in the MBE chamber, since they are partially embedded in neck regions of the NWs. The Ga 0.82 Mn 0.18 nanocrystals and the low Mn concentration in the NW bodies are insufficient to induce a ferromagnetic phase transition, suggesting that it is difficult to have high Mn contents in GaAs even in 1-D NW growth via the vapor-liquid-solid process. V C 2015 AIP Publishing LLC.

Self-catalyzed growth of axial GaAs1−xSbx nanowire (NW) arrays with bandgap tuning corresponding to the telecommunication wavelength of 1.3 µm poses a challenge, as the growth mechanism for axial configuration is primarily thermodynamically driven by the vapor-liquid-solid growth process. A systematic study carried out on the effects of group V/III beam equivalent (BEP) ratios and substrate temperature (Tsub) on the chemical composition in NWs and NW density revealed the efficacy of a two-step growth temperature sequence (initiating the growth at relatively higher Tsub = 620 °C and then continuing the growth at lower Tsub) as a promising approach for obtaining high-density NWs at higher Sb compositions. The dependence of the Sb composition in the NWs on the growth parameters investigated has been explained by an analytical relationship between the effective vapor composition and NW composition using relevant kinetic parameters. A two-step growth approach along with a gradual variation in Ga-BEP for offsetting the consumption of the droplets has been explored to realize long NWs with homogeneous Sb composition up to 34 at.% and photoluminescence emission reaching 1.3 µm at room temperature.

In this work we present a comprehensive study on the effects of Sb incorporation on the composition modulation, structural and optical properties of self-assisted axial GaAs 1−x Sb x nanowires of 2-6 μm in length grown on (111) Si substrate by molecular beam epitaxy. The Sb composition in the GaAs 1−x Sb x axial nanowire (NW) was varied from 2.8-16 at.%, as determined from energy dispersive x-ray spectroscopy. Lower Sb composition leads to thinner nanowires and inhomogeneous Sb composition distribution radially with a depleted Sb surface region inducing weak type-II optical emission, the presence of an additional peak at higher Bragg angle in the x-ray diffraction spectra and an electric-field-induced strong Raman LO mode. Higher Sb composition of 16 at.% leads to a more uniform Sb compositional distribution radially leading to type-I optical transitions exhibiting the lowest PL peak energy occurring at 1.13 eV. In addition, the high quality of these nanowires exhibiting pure zinc blende crystal structure, largely free of any planar defects, is borne out by high resolution transmission electron microscopy and selected area diffraction patterns. The shift and broadening of the Raman LO and TO modes reveal evidence of increased Sb incorporation in the nanowires. Significant improvement in optical characteristics was achieved by the incorporation of a Al 0.2 Ga 0.8 As passivating shell. The results are very promising and reveal the potential to further red shift the optical emission wavelength by fine tuning of the fluxes during growth.

In this work, we report on the p-i GaAsSb/AlGaAs nanowires (NWs) ensemble device exhibiting good spectral response up to 1.1 μm with a high responsivity of 311 A W, an external quantum efficiency of 6.1 × 10%, and a detectivity of 1.9 × 10 Jones at 633 nm. The high responsivity of the NWs has been attributed to in situ post-growth annealing of GaAsSb axial NWs in the ultra-high vacuum. The enabling growth technology is molecular beam epitaxy for the Ga-assisted epitaxial growth of these NWs on Si (111) substrates. Room temperature Raman spectra, as well as temperature dependent micro-photoluminescence peak analysis indicated suppression of band tail states and non-radiative channels due to annealing. A similar improvement in in situ annealed p-i GaAsSb NW ensemble with an AlGaAs passivating shell was inferred from a reduction in the Schottky barrier height as well as the NW resistance compared to the as-grown NW ensemble. These results demonstrate in situ annealing of nanowires to be an effective pathway for improving the optoelectronic properties of the NWs and the device thereof.

We report on in situ Te-doping in GaAs 1−x Sb x nanowires (NWs) grown via self-assisted molecular beam epitaxy. Enhanced Te incorporation in the NW at higher Te cell temperature was attested by the broadening of the x-ray diffraction peak and the presence of a strong coupled-LO phonon mode in the Raman spectra. Te-doping was estimated from the shift in the coupled-LO phonon mode to be ∼2.0×10 18 /cm 3 . The surfactant nature of the Te modulated the growth kinetics, which was manifested in an enhanced radial growth rate with improved photoluminescence (PL) characteristics at both room temperature (RT) and 4 K. No noticeable planar defects were observed as ascertained from the high-resolution transmission electron microscopy images and selected-area electron diffraction patterns. Finally, we demonstrate the experimental realization of a GaAs 1−x Sb x axial p-type/intrinsic/n-type (p-i-n) structure on a Si substrate with Te as the n-type dopant. The GaAs 1−x Sb x p-i-n NW structures exhibited rectifying current-voltage (I-V ) behavior. The dopant concentration and the transport parameters estimated from the PL spectra and I-V curve were found to be in good agreement.