Magnetic characterization of ZnO doped with vanadium

The magnetic properties of vanadium doped ZnO nanorods formed by different growth methods were examined. The samples under investigation were either grown by metallorganic vapour-phase epitaxy (MOVPE) with in situ incorporation of the V atoms or by aqueous chemical growth (ACG) in combination with an ion-implantation process. The V concentration for all samples was less than 2 at.%.Field-cooled SQUID measurements only reveal a weak ferromagnetic signal still in a very sensitive measuring range. However, the MFM micrographs obtained at room temperature show a clear magnetic contrast and complex structures, the images are in good agreement with the phase images expected for vertically aligned magnetic dipoles. This is a strong indication for a ferromagnetic behaviour at room temperature.     

Vapour transport growth of ZnO nanorods

The fabrication of low-dimensional ZnO structures has attracted enormous attention as such nanostructures are expected to pave the way for many interesting applications in optoelectronics, spin electronics gas sensor technology and biomedicine. Many reported fabrication methods, especially for ZnO nanorods are mostly based on catalyst-assisted growth techniques that employ metal-organic sources and other contaminating agents like graphite to grow ZnO nanorods at relatively high temperatures. We report on catalyst-free vapour-phase epitaxy growth of ZnO nanorods on 6H-SiC and (11-20)Al₂O₃ using purely elemental sources at relatively low temperatures and growth pressure. ZnO nanorods with widths of 80–900 nm and lengths of up to 12 μm were obtained. Nanorod density on the order of 10⁹ cm^-2 with homogenous luminescence and high purity was also noted. PACS 81.10.Bk; 81.15.Kk; 81.16.Hc; 78.55.Et; 81.05.Dz; 81.07.Bc     

Effect of the external electric field on the kinetics of recombination of photoexcited carriers in a ZnSe/BeTe type II heterostructure

JETP Letters - The kinetics of the radiative recombination of photoexcited electrons and holes for a spatially direct transition in a ZnSe/BeTe type II heterostructure in an external electric field...     

Luminescence polarization and spontaneous lowering of symmetry caused by magnetic-polaron formation in semimagnetic-semiconductor quantum wells

An experimental and theoretical study of the magnetic polaron states of two-dimensional excitons in quantum wells based on semimagnetic semiconductors (Cd,Mn)Te is reported. It is shown that magnetic-polaron formation in in-plane magnetic fields leads to a lowering of the system symmetry, provided the fields are not too strong. The magnetic moment of the polaron thus formed is not parallel to the external magnetic field and contains a component normal to the quantum-well plane. This spontaneous lowering of the symmetry results in a change of the polarization characteristics of the luminescence from magnetic polaron states and in a weakening (compared to the three-dimensional case) in the efficiency of magnetic field-induced polaron suppression. Fiz. Tverd. Tela (St. Petersburg) 39, 2079–2084 (November 1997)     

Picosecond carrier relaxation in type-II ZnSe/BeTe heterostructures

Relaxation of photoexcited carriers in the course of the formation of spatially separated layers of electrons and holes in type-II ZnSe/BeTe heterostructures has been studied based on a high time resolution investigation of fast luminescence kinetics. The hole escape times τ from the ZnSe layer have been measured in structures with different ZnSe layer thicknesses (τ = 2.5, 7.5, and 23 ps for thicknesses d = 10, 15, and 20 nm, respectively). It is shown that the increase in the time τ can be explained by the fact that the escape rate of photoexcited holes from the lowest above-barrier level in the ZnSe layer into the BeTe layer decreases as the thickness of the ZnSe layer increases.     

Evidence for the quantum birth of our universe

We present evidence for a nonsingular origin of the Universe with intial conditions determined by quantum physics and relativistic gravity. In particular, we establish that the present temperature of the microwave background and the present density of the Universe agree well with our predictions from these intial conditions, after evolution to the present age using the Einstein-Friedmann equation. Remarkably, the quantum origin for the Universe naturally allows its evolution at exactly the critical density. We also discuss the consequences of these results to some fundamental aspects of quantum physics in the early Universe.     

The fast multipole method and Fourier convolution for the solution of acoustic scattering on regular volumetric grids

The fast multipole method (FMM) is applied to the solution of large-scale, three-dimensional acoustic scattering problems involving inhomogeneous objects defined on a regular grid. The grid arrangement is especially well suited to applications in which the scattering geometry is not known a priori and is reconstructed on a regular grid using iterative inverse scattering algorithms or other imaging techniques. The regular structure of unknown scattering elements facilitates a dramatic reduction in the amount of storage and computation required for the FMM, both of which scale linearly with the number of scattering elements. In particular, the use of fast Fourier transforms to compute Green’s function convolutions required for neighboring interactions lowers the often-significant cost of finest-level FMM computations and helps mitigate the dependence of FMM cost on finest-level box size. Numerical results demonstrate the efficiency of the composite method as the number of scattering elements in each finest-level box is increased.     

Electrodeposition of ZnO nanorods for device application

We report the electrochemical growth of zinc oxide nanorods in a zinc nitrate/hexamethylenetetramine solution at 70 °C. High-density vertical nanorods were grown on Au films on silicon substrates with a texture coefficient better than 99.9%. By varying the reactant concentration the diameter can be varied between 100 and 250 nm, with corresponding lengths of 1 to 4 μm. Furthermore, this approach was used for the selective growth on Ti/Au strip conductors ordered in an interdigitated structure on an insulating substrate. We achieved the growth of ZnO nanorods between neighbouring strip conductors bridging the gap between them. In this configuration the nanorods are already contacted and electrical measurements can be directly performed. First I–V measurements show a good conductivity of the as-grown nanorods and the resistance could be estimated to be 0.1 Ω cm. Under UV illumination the ZnO nanorods demonstrate a photoconductivity, but only after annealing the sample at 300 °C in N₂. PACS 61.05.cp; 73.63.-b; 78.55.Et; 81.15.Pq; 82.45.Yz