Development of nanowire devices with quantum functionalities

Silicon has dominated the microelectronics industry for the last 50 years. With its zero nuclear spin isotope (28Si) and low spin orbit coupling, it is believed that silicon can become an excellent host material for an entirely new generation of devices that operate under the laws of quantum mechanics [1}. Semiconductor nanowires however, offer huge potential as the next building blocks of nano-devices due to their one-dimensional structure and properties [2]. We describe a fabrication process to prepare doped vapor-liquid-solid (VLS) grown silicon nanowire samples in a 2- and 4-terminal measurement setup for electrical characterisation.Comment: 2 pages Optoelectronic and Microelectronic Materials & Devices (COMMAD), 2014 Conferenc

Dirac-screening stabilized surface-state transport in a topological insulator

We report magnetotransport studies on a gated strained HgTe device. This material is a threedimensional topological insulator and exclusively shows surface state transport. Remarkably, the Landau level dispersion and the accuracy of the Hall quantization remain unchanged over a wide density range ($3 \times 10^{11} cm^{-2} < n < 1 \times 10^{12} cm^{-2}$). This implies that even at large carrier densities the transport is surface state dominated, where bulk transport would have been expected to coexist already. Moreover, the density dependence of the Dirac-type quantum Hall effect allows to identify the contributions from the individual surfaces. A $k \cdot p$ model can describe the experiments, but only when assuming a steep band bending across the regions where the topological surface states are contained. This steep potential originates from the specific screening properties of Dirac systems and causes the gate voltage to influence the position of the Dirac points rather than that of the Fermi level.Comment: 12 pages 4 figure

Top down fabricated silicon nanowires with quantum functionalities

© 2018 Dr. Michael Herbert StuiberThis Ph.D thesis is concerned with the design, fabrication and mea- surement of silicon-on-insulator based nanowire devices that exhibit quantum effects in their operation. The devices were all formed us- ing top-down fabrication techniques in conjunction with ion implan- tation doping and/or silicidation. Devices studied include ring structures that exhibit Aharonov-Bohm oscillations, Esaki diodes and hybrid superconducting / semicon- ducting devices that reveal interesting magnetic-field dependent transport features at cryogenic temperatures. The research was performed within the Centre of Excellence for Quantum Computation and Communication Technology (CQC2T) with an over- arching aim of developing devices and fabrication protocols that can be useful for creating some of the future building blocks needed in quantum information hardware and quantum sensor development. In a combined research effort with our collaborators at Namlab in Dresden, Germany, a top down fabrication approach for phosphorous or erbium/oxygen implanted sili- con nanowires with diffused nickel contacts was developed. Several different device designs where realised such as conventional 4-terminal structures, Hall-bars and ring structures. The main focus was on design and fabrication of 0-dimensional systems, so called quantum dots, accomplished by confining a small nanowire segment in between two silicided segments (NixSiy). These devices allow study of fundamental physics of impurities in confined systems and quantisation effects like single electron tunneling (SET) for future quantum computation applications. Further insights in NixSiy was gained by investigating fully silicided 4-terminal ring structures at cryogenic temperatures. Periodic oscillations in the magneto-resistance data taken with a constant AC current of I = 1 μA were found. A fast Fourier transformation (FFT) confirmed the assumption that those features are caused by the changing flux through the enclosed ring area of the device, as predicted by the Aharonov-Bohm Effect. A second peak could be identified with the Altshuler-Aronov- Spivak oscillations. A comparison of both peak heights allowed the phase coherence length lφ to be estimated which was in good agreement with values presented in the literature. In addition to the oscillations in the magneto-resistance data, a resistance drop around zero magnetic field was present which could be identified with the effect of weak anti-localisation. A fit-function was used to model the collected data and de- termine important values of the system such as the spin orbit breaking length ls.o. and phase breaking length lφ which confirm the already existing values in this work and literature. Additional ring structures where characterised, degenerately implanted (either p- or n-type) and top down fabricated, to determine similarities and differences towards the silicided ones. 4-terminal AC characterisation measurements with a constant current of I = 1 μA at cryogenic temperatures revealed no Aharonov-Bohm effect. Neverthe- less, periodic and symmetric features in the magneto-resistance data for temperatures below the critical temperature of aluminum, Tcrit = 1.19 K, have been found for both devices. The features observed in the magneto-resistance data are sensitive to temper- ature and magnetic fields (parallel B|| and perpendicular B⊥) and disappear at higher magnetic fields (≈ 200 mT) or temperatures (≈ 1.2 K). This behaviour strongly sug- gests that the cause of the observed features can be found in magnetic flux quantisation related to Andreev reflections at the superconductor / metal interfaces. However, other effects like quantum phase slips can not be ruled out at this stage. Finally, the versatility of ion implantation and the chosen top down approach to fab- ricate sharp degenerately implanted p/n junctions, so called Esaki diodes, to observe electron tunneling or negative differential resistance was investigated. Multiple over- lay exposures and two ion implantation steps allowed the formation of a p/n junction nanowire diode. Room temperature I/V characterisation measurements of the de- generately implanted diode revealed a non linear behaviour which showed a negative differential resistance like behaviour for small forward biases. Those devices offer po- tential applications as building blocks for future complimentary metal semiconductor oxide (CMOS) compatible tunnel diodes. Main fabrication tools used where electron beam evaporation, electron beam lithog- raphy (EBL) and reactive ion etching (RIE). Characterisation involved room tem- perature and cryogenic magneto-resistance measurements with AC and DC signals of Hall-bar and van der Pauw devices as well as standard material characterisation such as Rutherford backscattering (RBS), Raman characterisation, atomic force microscopy (AFM), focused ion beam (FIB) and scanning electron microscopy (SEM). For testing purposes many devices have been fabricated to identify critical tool parameters for nanofabrication such as dose parametric tests for EBL

Low-noise diamond-based D.C. nano-SQUIDs

Nanoscale superconducting quantum interference devices (nano-SQUIDs) with Dayem bridge junctions and a physical loop size of 50 nm have been engineered in boron-doped nanocrystalline diamond films using precision Ne-ion beam milling. In an unshunted device, the nonhysteretic operation can be maintained in an applied field exceeding 0.1 T with a high flux-to-voltage transfer function, giving a low flux noise at 1 kHz and a concurrent spin sensitivity of . At elevated magnetic fields, up to 2 T, flux modulation of the nano-SQUID output voltage is maintained but with an increase in period, attributed to an additional phase bias induced on the nano-SQUID loop by up to 16 vortices per period penetrating the nano-SQUID electrodes

The PanEDM neutron electric dipole moment experiment at the ILL

The neutron's permanent electric dipole moment dn is constrained to below 3 × 10−26e cm (90% C.L.) [1, 2], by experiments using ultracold neutrons (UCN). We plan to improve this limit by an order of magnitude or more with PanEDM, the first experiment exploiting the ILL's new UCN source SuperSUN. SuperSUN is expected to provide a high density of UCN with energies below 80 neV, implying extended statistical reach with respect to existing sources, for experiments that rely on long storage or spin-precession times. Systematic errors in PanEDM are strongly suppressed by passive magnetic shielding, with magnetic field and gradient drifts at the single fT level. A holding-field homogeneity on the order of 10−4 is achieved in low residual fields, via a high static damping factor and built-in coil system. No comagnetometer is needed for the first order-of-magnitude improvement in dn, thanks to high magnetic stability and an assortment of sensors outside the UCN storage volumes. PanEDM will be commissioned and upgraded in parallel with SuperSUN, to take full advantage of the source's output in each phase. Commissioning is ongoing in 2019, and a new limit in the mid 10−27e cm range should be possible with two full reactor cycles of data in the commissioned apparatus

The PanEDM Neutron Electric Dipole Moment Experiment at the ILL

International audienceThe neutron's permanent electric dipole moment dn is constrained to below 3 × 10−26e cm (90% C.L.) [1, 2], by experiments using ultracold neutrons (UCN). We plan to improve this limit by an order of magnitude or more with PanEDM, the first experiment exploiting the ILL's new UCN source SuperSUN. SuperSUN is expected to provide a high density of UCN with energies below 80 neV, implying extended statistical reach with respect to existing sources, for experiments that rely on long storage or spin-precession times. Systematic errors in PanEDM are strongly suppressed by passive magnetic shielding, with magnetic field and gradient drifts at the single fT level. A holding-field homogeneity on the order of 10−4 is achieved in low residual fields, via a high static damping factor and built-in coil system. No comagnetometer is needed for the first order-of-magnitude improvement in dn, thanks to high magnetic stability and an assortment of sensors outside the UCN storage volumes. PanEDM will be commissioned and upgraded in parallel with SuperSUN, to take full advantage of the source's output in each phase. Commissioning is ongoing in 2019, and a new limit in the mid 10−27e cm range should be possible with two full reactor cycles of data in the commissioned apparatus