33 research outputs found
Processing and adsorption control in ZnO single nanowire photodetectors
ZnO single nanowire photodetectors have been measured in different ambient conditions in order to understand and control adsorption processes on the surface. A decrease in the conductivity has been observed as a function of time when the nanowires are exposed to air, due to adsorbed O2/H2O species at the nanowire surface. In order to have a device with stable characteristics in time, thermal desorption has been used to recover the original conductivity followed by PMMA coating of the exposed nanowire surface
Growth of Nitrogen-Doped Mg~xZn~1~-~xO for Use in Visible Rejection Photodetectors
Improvement in the Schottky behavior of metal (Au) contacts with Mg0.01Zn0.99O and Mg0.01Zn0.99O:N thin films were observed by treating the films with hydrogen peroxide (H2O2) (dipping of samples in H2O2 at 100 _C for 3 min). Contacts formed on untreated film showed Ohmic behavior in the current-voltage (I-V ) measurements. The H2O2 treatment led to a smooth surface morphology for the films and resulted in Schottky contact of Au fabricated on the treated films with barrier heights of 0.82≈0.85 eV. The absolute current density at a reverse bias of 3 V was 1≈6 × 10−6 A/cm2 for Au contacts on H2O2-treated films. The treated films showed lower electron concentration than the untreated films due to removal of the relatively high conducting top layers of the thin films. A metal-semiconductor-metal (MSM) detector was fabricated using a Mg0.05Zn0.95O:N film and was characterized for its spectral response
Optical properties and microstructure of 2.02-3.30 eV ZnCdO nanowires: Effect of thermal annealing
International audienceZnCdO nanowires with up to 45% Cd are demonstrated showing room temperature photoluminescence (PL) down to 2.02 eV and a radiative efficiency similar to that of ZnO nanowires. Analysis of the microstructure in individual nanowires confirms the presence of a single wurtzite phase even at the highest Cd contents, with a homogeneous distribution of Cd both in the longitudinal and transverse directions. Thermal annealing at 550 C yields an overall improvement of the PL, which is blue-shifted as a result of the homogeneous decrease of Cd throughout the nanowire, but the single wurtzite structure is fully maintained
Flat-band ferromagnetism in quantum dot superlattices
Possibility of flat-band ferromagnetism in quantum dot arrays is
theoretically discussed. By using a quantum dot as a building block, quantum
dot superlattices are possible. We consider dot arrays on Lieb and kagome
lattices known to exhibit flat band ferromagnetism. By performing an exact
diagonalization of the Hubbard Hamiltonian, we calculate the energy difference
between the ferromagnetic ground state and the paramagnetic excited state, and
discuss the stability of the ferromagnetism against the second nearest neighbor
transfer. We calculate the dot-size dependence of the energy difference in a
dot model and estimate the transition temperature of the
ferromagnetic-paramagnetic transition which is found to be accessible within
the present fabrication technology. We point out advantages of semiconductor
ferromagnets and suggest other interesting possibilities of electronic
properties in quantum dot superlattices.Comment: 15 pages, 7 figures (low resolution). High-resolution figures are
available at
http://www.brl.ntt.co.jp/people/tamura/Research/PublicationPapers.htm
Coherent Stranski-Krastanov growth in 1+1 dimensions with anharmonic interactions: An equilibrium study
The formation of coherently strained three-dimensional islands on top of the
wetting layer in Stranski-Krastanov mode of growth is considered in a model in
1+1 dimensions accounting for the anharmonicity and non-convexity of the real
interatomic forces. It is shown that coherent 3D islands can be expected to
form in compressed rather than in expanded overlayers beyond a critical lattice
misfit. In the latter case the classical Stranski-Krastanov growth is expected
to occur because the misfit dislocations can become energetically favored at
smaller island sizes. The thermodynamic reason for coherent 3D islanding is the
incomplete wetting owing to the weaker adhesion of the edge atoms. Monolayer
height islands with a critical size appear as necessary precursors of the 3D
islands. The latter explains the experimentally observed narrow size
distribution of the 3D islands. The 2D-3D transformation takes place by
consecutive rearrangements of mono- to bilayer, bi- to trilayer islands, etc.,
after exceeding the corresponding critical sizes. The rearrangements are
initiated by nucleation events each next one requiring to overcome a lower
energetic barrier. The model is in good qualitative agreement with available
experimental observations.Comment: 12 pages text, 15 figures, Accepted in Phys.Rev.B, Vol.61, No2
Carrier capture and relaxation in Stranski-Krastanow InxGa1-xAs/GaAs(311)B quantum dots
We have investigated the structure and optical properties of In0.6Ga0.4As/GaAs(311)B quantum dots (QD's) formed by the Stranski-Krastanow growth mode during metal-organic chemical-vapor deposition. We find that (311)B QD structures display a higher energy QD luminescence emission and a stronger wetting-layer emission than (100) QD's of similar diameter and density. Temperature-dependent photoluminescence (PL) measurements reveal shallow QD confinement energies and strong interaction between neighboring quantum dots. Longer PL rise times of the ground-state emission of (311)B QD's compared to (100) QD's are ascribed to the effect of differing numbers, energies, and level spacings of QD confined states on intersublevel relaxation mechanisms at low-carrier excitation densities
Effect of Interfacial Bonds on the Morphology of InAs QDs Grown on GaAs (311) B and (100) Substrates
The morphology and transition thickness (tc) for InAs quantum dots (QDs) grown on GaAs (311) B and (100) substrates were investigated. The morphology varies with the composition of buffer layer and substrate orientation. Andtcdecreased when the thin InGaAs was used as a buffer layer instead of the GaAs layer on (311) B substrates. For InAs/(In)GaAs QDs grown on high miller index surfaces, both the morphology andtccan be influenced by the interfacial bonds configuration. This indicates that buffer layer design with appropriate interfacial bonds provides an approach to adjust the morphologies of QDs grown on high miller surfaces
Schottky contact on ZnO nano-columnar film with H2O2 treatment
The surface treatment with boiling hydrogen peroxide (H2O2) solution on the surface of ZnO nano-columnar film was investigated. Field emission-SEM and TEM analysis revealed that amorphous ZnO2 layer covers the ZnO nano-column surface through the H2O2 treatment at 100 degrees C for 1 min. X-ray photoemission spectroscopy (XPS) has been conducted on the H2O2 treated ZnO surface. The surface exhibits high resistive conductivity after the H2O2 treatment, suggesting that the treatment promotes a compensation effect. We demonstrate that dramatic improvement in the rectifying behavior on the Schottky diodes can be achieved by inserting a ZnO2 interface layer between the Pt Schottky electrode and the ZnO nano-column film. The ZnO2 interface layer promotes surface passivation and suppresses the surface leakage current. This is expected to increase the Schottky barrier height to 0.78 eV. The H2O2 treated Schottky diode showed five orders of magnitude in current rectification between forward and reverse bias at 3 V. (C) 2011 American Institute of Physics.publishe