4 research outputs found
Luminous Efficiency of Axial In<sub><i>x</i></sub>Ga<sub>1ā<i>x</i></sub>N/GaN Nanowire Heterostructures: Interplay of Polarization and Surface Potentials
Using
continuum elasticity theory and an eight-band <b>k</b>Ā·<b>p</b> formalism, we study the electronic properties
of GaN nanowires with axial In<sub><i>x</i></sub>Ga<sub>1ā<i>x</i></sub>N insertions. The three-dimensional
strain distribution in these insertions and the resulting distribution
of the polarization fields are fully taken into account. In addition,
we consider the presence of a surface potential originating from Fermi
level pinning at the sidewall surfaces of the nanowires. Our simulations
reveal an in-plane spatial separation of electrons and holes in the
case of weak piezoelectric potentials, which correspond to an In content
and layer thickness required for emission in the blue and violet spectral
range. These results explain the quenching of the photoluminescence
intensity experimentally observed for short emission wavelengths.
We devise and discuss strategies to overcome this problem
Photoelectrochemical Properties of (In,Ga)N Nanowires for Water Splitting Investigated by in Situ Electrochemical Mass Spectroscopy
We investigated the photoelectrochemical
properties of both n-
and p-type (In,Ga)N nanowires (NWs) for water splitting by in situ
electrochemical mass spectroscopy (EMS). All NWs were prepared by
plasma-assisted molecular beam epitaxy. Under illumination, the n-(In,Ga)ĀN
NWs exhibited an anodic photocurrent, however, no O<sub>2</sub> but
only N<sub>2</sub> evolution was detected by EMS, indicating that
the photocurrent was related to photocorrosion rather than water oxidation.
In contrast, the p-(In,Ga)N NWs showed a cathodic photocurrent under
illumination which was correlated with the evolution of H<sub>2</sub>. After photodeposition of Pt on such NWs, the photocurrent density
was significantly enhanced to 5 mA/cm<sup>2</sup> at a potential of
ā0.5 V/NHE under visible light irradiation of ā¼40 mW/cm<sup>2</sup>. Also, incident photon-to-current conversion efficiencies
of around 40% were obtained at ā0.45 V/NHE across the entire
visible spectral region. The stability of the NW photocathodes for
at least 60 min was verified by EMS. These results suggest that p-(In,Ga)ĀN
NWs are a promising basis for solar hydrogen production
Control over the Number Density and Diameter of GaAs Nanowires on Si(111) Mediated by Droplet Epitaxy
We
present a novel approach for the growth of GaAs nanowires (NWs) with
controllable number density and diameter, which consists of the combination
between droplet epitaxy (DE) and self-assisted NW growth. In our method,
GaAs islands are initially formed on Si(111) by DE and, subsequently,
GaAs NWs are selectively grown on their top facet, which acts as a
nucleation site. By DE, we can successfully tailor the number density
and diameter of the template of initial GaAs islands and the same
degree of control is transferred to the final GaAs NWs. We show how,
by a suitable choice of V/III flux ratio, a single NW can be accommodated
on top of each GaAs base island. By transmission electron microscopy,
as well as cathodo- and photoluminescence spectroscopy, we confirmed
the high structural and optical quality of GaAs NWs grown by our method.
We believe that this combined approach can be more generally applied
to the fabrication of different homo- or heteroepitaxial NWs, nucleated
on the top of predefined islands obtained by DE
Observation of Dielectrically Confined Excitons in Ultrathin GaN Nanowires up to Room Temperature
The realization of semiconductor
structures with stable excitons
at room temperature is crucial for the development of excitonics and
polaritonics. Quantum confinement has commonly been employed for enhancing
excitonic effects in semiconductor heterostructures. Dielectric confinement,
which gives rises to much stronger enhancement, has proven to be more
difficult to achieve because of the rapid nonradiative surface/interface
recombination in hybrid dielectric-semiconductor structures. Here,
we demonstrate intense excitonic emission from bare GaN nanowires
with diameters down to 6 nm. The large dielectric mismatch between
the nanowires and vacuum greatly enhances the Coulomb interaction,
with the thinnest nanowires showing the strongest dielectric confinement
and the highest radiative efficiency at room temperature. In situ
monitoring of the fabrication of these structures allows one to accurately
control the degree of dielectric enhancement. These ultrathin nanowires
may constitute the basis for the fabrication of advanced low-dimensional
structures with an unprecedented degree of confinement