E. Janik

ZnTe nanowires grown on GaAs(100) substrates by molecular beam epitaxy

ZnTe nanowires with an average diameter of about 30nm and lengths above 1μm were grown on GaAs(100) substrate by molecular beam epitaxy. The growth process was based on the Au-catalyzed vapor-liquid-solid mechanism. A thin gold layer (3–20A thick) annealed in high vacuum prior to the nanowire growth was used as a source of catalytic nanoparticles. The nanowires are inclined about 55° to the (100) substrate surface normal. They have a zinc-blende crystal structure and their growth axis is ⟨111⟩.

Designing quantum dots for solotronics

Solotronics, optoelectronics based on solitary dopants, is an emerging field of research and technology reaching the ultimate limit of miniaturization. It aims at exploiting quantum properties of individual ions or defects embedded in a semiconductor matrix. It has already been shown that optical control of a magnetic ion spin is feasible using the carriers confined in a quantum dot. However, a serious obstacle was the quenching of the exciton luminescence by magnetic impurities. Here we show, by photoluminescence studies on thus-far-unexplored individual CdTe dots with a single cobalt ion and CdSe dots with a single manganese ion, that even if energetically allowed, nonradiative exciton recombination through single-magnetic-ion intra-ionic transitions is negligible in such zero-dimensional structures. This opens solotronics for a wide range of as yet unconsidered systems. On the basis of results of our single-spin relaxation experiments and on the material trends, we identify optimal magnetic-ion quantum dot systems for implementation of a single-ion-based spin memory.

Structural properties of cubic MnTe layers grown by MBE

Abstract We report on the growth of zinc-blende MnTe on (001) GaAs substrates by MBE. Layers with thickness up to 8 μm were grown. The growth was performed for various ratios of Mn and Te fluxes. For high values of the Te Mn flux ratio, Raman scattering spectra showed a presence of tellurium lines corresponding to crystalline tellurium. For low flux ratios the presence of Te precipitates was less evident. These results were confirmed by X-ray diffraction studies. The lattice parameter of cubic MnTe and its temperature dependence were determined by X-ray diffraction. From the analysis of the rocking curves, for various Bragg reflections, the fluctuations of the lattice parameter and mosaicity were separated out.

GaAs : Mn nanowires grown by molecular beam epitaxy of (Ga,Mn)As at MnAs segregation conditions

GaAs:Mn nanowires were obtained on GaAs(001) and GaAs(111)B substrates by molecular beam epitaxial growth of (Ga,Mn)As at conditions leading to MnAs phase separation. Their density is proportional to the density of catalyzing MnAs nanoislands, which can be controlled by the Mn flux and/or the substrate temperature. After deposition corresponding to a 200 nm thick (Ga,Mn)As layer the nanowires are around 700 nm long. Their shapes are tapered, with typical diameters around 30 nm at the base and 7 nm at the tip. The wires grow along the 111 direction, i.e., along the surface normal on GaAs(111)B and inclined on GaAs(001). In the latter case they tend to form branches. Being rooted in the ferromagnetic semiconductor (Ga,Mn)As, the nanowires combine one-dimensional properties with the magnetic properties of (Ga,Mn)As and provide natural, self-assembled structures for nanospintronics.

ZnTe–ZnO core–shell radial heterostructures grown by the combination of molecular beam epitaxy and atomic layer deposition

ZnTe-ZnO core-shell radial heterostructures were grown using a new method of combining molecular beam epitaxy (MBE) and atomic layer deposition (ALD). Zinc telluride nanowires (core) were grown on a GaAs substrate using gold catalyzed vapor-liquid-solid mechanism. An atomic layer deposition technique using diethyl zinc and deionized water as precursors was applied for zinc oxide shell formation. The core-shell ZnTe-ZnO heterostructures thus obtained were characterized by scanning electron microscopy, transmission electron microscopy, x-ray diffraction and photoluminescence measurements.

Giant Spin Splitting in Optically Active ZnMnTe/ZnMgTe Core/Shell Nanowires

An enhancement of the Zeeman splitting as a result of the incorporation of paramagnetic Mn ions in ZnMnTe/ZnMgTe core/shell nanowires is reported. The studied structures are grown by gold-catalyst assisted molecular beam epitaxy. The near band edge emission of these structures, conspicuously absent in the case of uncoated ZnMnTe nanowires, is activated by the presence of ZnMgTe coating. Giant Zeeman splitting of this emission is studied in ensembles of nanowires with various average Mn concentrations of the order of a few percent, as well as in individual nanowires. Thus, we show convincingly that a strong spin sp-d coupling is indeed present in these structures.

Zn1−xMnxTe Diluted Magnetic Semiconductor Nanowires Grown by Molecular Beam Epitaxy

It is shown that the growth of II-VI diluted magnetic semiconductor nanowires is possible by the catalytically enhanced molecular beam epitaxy (MBE). Zn(1-x)MnxTe NWs with manganese content up to x=0.60 were produced by this method. X-ray diffraction, Raman spectroscopy, and temperature dependent photoluminescence measurements confirm the incorporation of Mn(2+) ions in the cation substitutional sites of the ZnTe matrix of the NWs.

Optical injection of spin-polarized carriers across a strongly mismatched heterostructure

Abstract We observed an effective injection of spin-polarized carriers from II–VI Zn 0.97 Mn 0.03 Te diluted magnetic semiconductor spin aligning layer into III–V-based, GaAs-based quantum well structure. By using circular polarized excitation and detection, we demonstrate that the injection of spin-polarized carriers indeed proceeds through the II–VI/III–V interface in spite of a huge lattice mismatch (∼7.8%) which is decorated by a great number of dislocations. This indicates that spins are quite robust and maintain their polarization memory even after passing through a dense array of misfit dislocations.

Semimagnetic self-organized Cd1−xMnxTe quantum dots generated by postgrowth thermal annealing

We demonstrate the possibility to fabricate self-organized semimagnetic quantum dots by the growth of two-dimensional layers and subsequent thermal annealing. In particular, CdTe/CdMgTe quantum-well samples containing four narrow MnTe barriers are characterized by optical methods. After annealing well above the growth temperature, from time-resolved photoluminescence (PL), PL experiments under selective excitation, as well as micro-PL measurements, we find clear evidence for the presence of quantum dots. For a sample with a average Mn content of 5%, the dot luminescence shows a redshift of 18 meV under an applied magnetic field of 4 T, due to the giant Zeeman effect.

Disorder suppression and precise conductance quantization in constrictions of PbTe quantum wells

Conductance quantization was measured in submicron constrictions of PbTe, patterned into narrow,12 nm wide quantum wells deposited between Pb