Ivan Isakov

Mobility Enhancement by Sb-mediated Minimisation of Stacking Fault Density in InAs Nanowires Grown on Silicon

We report the growth of InAs(1-x)Sb(x) nanowires (0 ≤ x ≤ 0.15) grown by catalyst-free molecular beam epitaxy on silicon (111) substrates. We observed a sharp decrease of stacking fault density in the InAs(1-x)Sb(x) nanowire crystal structure with increasing antimony content. This decrease leads to a significant increase in the field-effect mobility, this being more than three times greater at room temperature for InAs0.85Sb0.15 nanowires than InAs nanowires.

Minimization of the contact resistance between InAs nanowires and metallic contacts

We investigate different processes for optimizing the formation of Ohmic contacts to InAs nanowires. The nanowires are grown via molecular beam epitaxy without the use of metal catalysts. Metallic contacts are attached to the nanowires by using an electron beam lithography process. Before deposition of the contacts, the InAs nanowires are treated either by wet etching in an ammonium polysulfide (NH(4))(2)S(x) solution or by an argon milling process in order to remove a surface oxide layer. Two-point electrical measurements show that the resistance of the ammonium polysulfide-treated nanowires is two orders of magnitude lower than that of the untreated nanowires. The nanowires that are treated by the argon milling process show a resistance which is more than an order of magnitude lower than that of those treated with ammonium polysulfide. Four-point measurements allow us to extract an upper bound of 1.4 × 10(-7) Ω cm(2) for the contact resistivity of metallic contacts on nanowires treated by the argon milling process.

Plasma and Ion Beam Injection into an FRC

Experiments on the transverse injection of intense (5–20 A/cm2), wide cross-section (10-cm), neutralized, ∼100-eV H+ plasma and 100-keV H+ ion beams into a preformed B-field reversed configuration (FRC) are described. The FRC background plasma temperature was ∼5 eV with densities of ∼1013 cm−3. In contrast to earlier experiments, the background plasma was generated by separate plasma gun arrays. For the startup of the FRC, a betatron-type “slow” coaxial source was used. Injection of the plasma beam into the preformed FRC resulted in a 30–40% increase of the FRC lifetime and the amplitude of the reversed magnetic field. As for the ion beam injection experiment into the preformed FRC, there was evidence of beam capture within the configuration.

Effect of lithographically-induced strain relaxation on the magnetic domain configuration in microfabricated epitaxially grown Fe81Ga19

We investigate the role of lithographically-induced strain relaxation in a micron-scaled device fabricated from epitaxial thin films of the magnetostrictive alloy Fe81Ga19. The strain relaxation due to lithographic patterning induces a magnetic anisotropy that competes with the magnetocrystalline and shape induced anisotropies to play a crucial role in stabilising a flux-closing domain pattern. We use magnetic imaging, micromagnetic calculations and linear elastic modelling to investigate a region close to the edges of an etched structure. This highly-strained edge region has a significant influence on the magnetic domain configuration due to an induced magnetic anisotropy resulting from the inverse magnetostriction effect. We investigate the competition between the strain-induced and shape-induced anisotropy energies, and the resultant stable domain configurations, as the width of the bar is reduced to the nanoscale range. Understanding this behaviour will be important when designing hybrid magneto-electric spintronic devices based on highly magnetostrictive materials.

Current-mode deep level transient spectroscopy of a semiconductor nanowire field-effect transistor

One of the main limiting factors in the carrier mobility in semiconductor nanowires is the presence of deep trap levels. While deep-level transient spectroscopy (DLTS) has proved to be a powerful tool in analysing traps in bulk semiconductors, this technique is ineffective for the characterisation of nanowires due to their very small capacitance. Here, we introduce a new technique for measuring the spectrum of deep traps in nanowires. In current-mode DLTS (“I-DLTS”), the temperature-dependence of the transient current through a nanowire field-effect transistor in response to an applied gate voltage pulse is measured. We demonstrate the applicability of I-DLTS to determine the activation energy and capture cross-sections of several deep defect states in zinc oxide nanowires. In addition to characterising deep defect states, we show that I-DLTS can be used to measure the surface barrier height in semiconductor nanowires.

Observation of coherent electron transport in self-catalysed InAs and InAs1- xSbx nanowires grown on silicon

We report the observation of phase coherent transport in catalyst-free InAs and InAs1–xSbx nanowires grown by molecular beam epitaxy on silicon (111) substrates. We investigate three different methods to gain information on the phase coherence length of the nanowires: first through the study of universal conductance fluctuations as a function of both magnetic field and gate voltage and then through localisation effects. The analysis of these different quantum effects gave consistent results and a phase-coherence length in the hundred nanometre range was extracted for all nanowires below 10 K. This demonstrates the potential of catalyst-free nanowires as building blocks for future quantum electronics devices directly integrated with silicon circuits.

Generation and transport of a low energy intense ion beam

This paper describes experiments on the formation and transport, in vacuum and plasma, of a low-energy (70-120 keV), high-intensity (10-30 A/cm/sup 2/), long-pulse (0.5-1/spl mu/s) H/sup +/ ion beam. The beam was generated in a magnetically insulated diode with an applied radial B-field and active hydrogen-puff plasma source at the anode. The combination of a ballistic focusing large area anode (250 cm/sup 2/) with a post-cathode toroidal magnetic lens and straight transport solenoid section provided beam transport to a distance of >1 m with an overall efficiency of /spl ges/ 50%. Two-dimensional single-particle computer simulations of the ions trajectory in the lens/solenoid system supported optimization of the lens and solenoid parameters.